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Topic 6 Notes - Human Physiology
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Topic 6: Human Physiology (NOTES)
Topic 6.1 – Digestion and absorption
Topic 6.2 – The blood system
Topic 6.3 – Defence against infectious disease
Topic 6.4 – Gas exchange
Topic 6.5 – Neurons and synapses
Topic 6.6 – Hormones, homeostasis and reproduction
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Topic 6.1 – Digestion and absorption
The human digestive system
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The parts of the human digestive system are as follows:
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Mouth: Ingestion and chewing of food particles. Saliva is secreted in the mouth which contains
the enzyme salivary amylase. This enzymes breaks down amylose (starch).
Esophagus: Swallowing the chewed food.
Stomach: Firstly, the stomach secretes pepsin which starts the digestion of proteins into
polypeptides and amino acids. Secondly, it secretes hydrochloric acid which kills bacteria and
other harmful organisms preventing food poisoning and it also provides the optimum pH for the
enzyme pepsin to work in (pH 1.5 - 2). Mucus is also secreted to protect the linings of the
stomach wall against the acid.
Liver: It secretes bile.
Gall bladder: Stored the bile produced by the liver.
Pancreas: Secretes digestive enzymes which are released in the small intestine.
Small intestine: The final stages of digestion occur here. The intestinal wall secretes enzymes
and it also receives enzymes from the pancreas. The main function of the small intestine is the
absorption of the digested food particles. It possesses many villi which increase the surface area
for absorption of these food particles.
Large intestine: The material that has not been digested from the small intestine moves into the
large intestine, and water is absorbed. This produces solid faeces which are then egested
through the anus.
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Structure of the small intestine
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The parts of the tissue layers in transverse sections of the small intestine are as follows:
Digestion in the small intestine
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The contraction of circular and longitudinal muscle of the small intestine mixes the food with
enzymes and moves it along the gut.
 The muscle contraction occurs in the form of waves as the food passes along the
intestine.
 The contraction of circular and longitudinal muscle (smooth muscles) of the small
intestine helps mechanically mix the food with enzymes and moves the semi-digested
food along the gut in a process known as peristalsis.
 The food passes through the small intestine very slowly to allow for maximum digestion
and absorption of nutrients into the bloodstream.
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The pancreas secretes enzymes into the lumen of the small intestine.
 Enzymes are also known as biological catalysts that speed up the rate of reaction in
chemical digestion.
 They catalyze hydrolysis (catabolic) reactions.
 A variety of macromolecules are acted upon by these enzymes, including starch,
proteins, glycogen and lipids.
 The pancreas secretes 3 types of enzymes as part of its pancreatic juice.
 The optimal pH for these enzymes is around 7-8 as the pancreatic juice is slightly
alkaline (basic).
 The three enzymes are;
1. Endopeptidases – breaks the peptide bond between amino acids in polypeptides.
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2. Lipases – catalyzes the hydrolysis of lipids (fats and oils)
3. Amylases – digestion of starch.
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Enzymes digest most macromolecules in food into monomers.
 The breakdown is catalyzed by means of catabolic reactions.
 The table shows the types of enzymes secreted by the body;
Enzyme
Source
Substrate
pH
Products
Amylase
Salivary glands &
pancreas
Amylose (starch)
7-8
Maltose
Dextrinase
Small intestine
Dextrins
7-8
Glucose
Maltase
Small intestine
Maltose
7-8
Glucose
Lipase
Pancreas
Lipids
7-8
Fatty acids &
glycerol
Endopeptidase
Pancreas
Proteins
7-8
Smaller peptides
Protease (pepsin)
Stomach
Proteins
1-2
Amino acids
Digestion of starch
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Starch is used by plants as a food reserve, and is a polysaccharide consisting of a lo g hai of α
(alpha) glucose molecules.
Starch consists of two types of molecules: Amylose which linear/unbranched and Amylopectin
which is branched.
Amylose 1,4 bonds can be broken down by amylase to form the disaccharide maltose.
The enzyme amylase cannot break down the 1,6 bonds in amylopectin, which results in the
formation of larger fragments known as dextrins.
These dextrins are further broken down by the enzyme dextrinase into glucose.
Similarly, maltase acts on maltose and converts it to glucose.
These glucose molecules are then absorbed by the villi and the microvilli using protein pumps
(active transport mechanism).
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Intestinal villi
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Villi are the finger-like projections that compose the surface of the small intestine and make it
look highly folded.
Villi increase the surface area of epithelium over which absorption is carried out.
Villi absorb monomers formed by digestion as well as mineral ions and vitamins.
The parts of a villi are as follows:
 The outermost layer of the villi is a single layer of epithelial cells to allow particles to
easily diffuse across a short distance into the blood.
 The villi have microvilli which are small hair-like projections that aid to increase surface
area.
 A dense network of capillaries situated very close to the epithelium to allow maximum
amount of nutrients to diffuse into the bloodstream.
 Along the middle is the lacteal, which is involved in the direct absorption of fats/lipids.
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Methods of absorption
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Different methods of membrane transport are required to absorb different nutrients. They are
as follows:
1. Simple diffusion: Nutrients pass down the concentration gradient between phospholipids in
the membrane.
2. Facilitated diffusion: Nutrients pass down the concentration gradient through specific
protein channels in the membranes.
3. Active transport: Nutrients are pumped against concentration gradient by pump proteins
across the membrane.
4. Endocytosis: Small droplets of fluid are passed through the membranes in the form of
vesicles. This is also known as pinocytosis.
Dialysis tubing
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Dialysis tubing can be used to model absorption by the epithelium of the intestine.
The membrane of the dialysis tube is only permeable to small sized particles.
The solvent outside the tube is tested at intervals to see if the particles within the tube have
diffused out across the membrane.
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Topic 6.2 – The blood system
William Harvey and blood circulation
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William Harvey discovered in the 17th century how the blood circulates around our body.
Prior to this, the theories of how the circulation of the blood in the body took place, were made
by a Greek philosopher named Galen.
He had theorized that blood was produced in the liver, and then is pumped out by the heart and
is consumed in the other organs of the body.
Gale ’s theories ere so ell-established at that time, due to which Harvey had to face
widespread criticism with his discovery.
Harvey demonstrated experiments and provided evidence for supporting his own theory.
Harvey demonstrated for the first time that the arteries and veins circulate blood through the
whole body.
He also showed that blood flow through the vessels is unidirectional with valves to prevent
backflow, and also that the rate of flow through major vessels is far too high for blood to be
consumed in the body after being pumped out by the heart.
Harvey also sho ed that the heart’s eat produ es a o sta t ir ulatio of lood through the
whole body.
He also predicted the presence of numerous fine vessels (capillaries) that linked arteries to veins
in the tissue of the body.
Double circulation
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The circulation in humans discovered by Harvey is doublecirculation: there are separate circulations for the lungs
(pulmonary circulation) and for other organs (systemic
circulation).
Pulmonary circulation proceeds from the right ventricle to
the lungs (via pulmonary artery) and returns to the left
atrium of the heart (via pulmonary vein).
Whereas, systemic circulation goes from the left ventricle to
the rest of the body (via aorta) and back to the right atrium
(via vena cava).
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Blood vessels
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Blood vessels are tubes within the body that carry blood.
There are three main types of blood vessels:
1. Arteries:
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Carry blood away from the heart to tissues around the body
They must be able to withstand the high pressure and high blood volume created when
the ventricles contract.
Consists of a very thick wall of smooth muscle tissue making them strong and elastic in
nature, and withstand high pressures.
The thick smooth muscles can be used to help regulate blood pressure by changing the
diameter of the lumen between pumping cycles.
A narrow lumen to help maintain high pressures.
Most arteries carry oxygenated blood (except pulmonary artery).
2. Veins:
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Transport and return blood back to the heart from the capillaries in tissues.
Collect blood at low pressure from the tissues of the body.
Thin layers of tissue with few or no elastic fibres.
Thin outer coat, as the pressure is low, so there is no danger to bursting.
Thin walled allows veins to be pressed flat by neighboring vessels to assist blood flow.
Wide lumen allows large amounts of blood to slowly return to the heart at low pressure.
Valves are present at intervals in veins to prevent backflow of blood.
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3. Capillaries:
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Thin walls consists of a single layer of cells, for allowing the diffusion distance across it
to be very small.
Capillaries have permeable walls that allow exchange of materials between cells in the
tissue and the blood in the capillary.
Very narrow lumen (about 10 µm), so that they can fit in small spaces.
Highly branched networks of capillaries increase the surface area, maximizing the
amount of nutrients and gases that can move in and out of the capillaries.
There are pores between the cells which allows the blood plasma to be leaked out to
make up tissue fluid.
Phagocytes can also squeeze through the pores out of the capillaries.
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Structure of the heart
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Aorta: Carries oxygenated blood away from the heart.
Vena cava: Carries deoxygenated blood to the heart.
Pulmonary artery: Carried deoxygenated blood away from the heart to the lungs.
Pulmonary vein: Carried oxygenated blood to the heart from lungs.
Atrium: Upper chambers of the heart.
Ventricles: Lower chambers of the heart
Atrio-ventricular (AV) valves: Valves between the atria and ventricles of the heart.
Semi-lunar valves: Valves between the ventricles and the major arteries.
The cardiac cycle
The beating of the heart consists of a cycle of actions as follows:
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Atrial systole:
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The walls of the atria contract.
Pressure increases in the left and right atria, and the blood is pushed down into the
ventricles (e.g. left atrium into left ventricle).
Ventricle walls relaxed, therefore the pressure is low.
Atrio-ventricular valves are open and semi-lunar valves are shut.
Ventricles fill with blood.
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Ventricular Systole:
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Walls of the ventricles contract powerfully.
Pressure in the ventricles increases rapidly.
Atrio-ventricular valves close preventing backflow to the atria.
The semi-lunar valves open.
Blood is pumped out of the ventricles into the arteries through the semi-lunar valves.
Pressure in the aorta increases.
The atria start to refill by collecting blood from the veins.
Atrial and Ventricular Diastole:
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Muscles in the walls of the ventricles strop contracting and relax.
The semi-lunar valves close.
Since the pressure drops in the atria, blood flows into the right atrium via the vena cava
and into the left atrium from the pulmonary veins.
Atrio-ventricular valves open as the pressure in the ventricles drops below the pressure
in the atria, and blood enters into the ventricles from the atria.
Pressure in the aorta drops, but remains quite high throughout this cardiac cycle due to
the elastic and muscle fibres in the walls.
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Control of the heart rate
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The walls of the heart are made of cardiac muscles, which can contract on its own without being
stimulated by a nerve (myogenic contraction).
The heart beat is initiated by a group of specialized muscle cells in the right atrium which is
collectively called the sinoatrial node (SA).
As these cells initiate each heartbeat, the SA node is called the pacemaker as they control the
rate of a hu a ’s heart eat.
The sinoatrial node sends out an electrical signal that stimulates contraction as it is propagated
through the walls of the atria and then the walls of the ventricles.
The electrical signal reaches another node called the AV node.
These impulses are conducted by tiny bundles of muscle fibres called Purkinje Fibres
Messages can be carried out to the SA node by nerves, as well as hormones;
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The heart rate can be increased or decreased by impulses brought to the heart through
two nerves from the medulla of the brain (one nerve speeds up the rate, while the other
nerve slows it down).
Epinephrine increases the heart rate to prepare for vigorous physical activity.
Coronary heart disease
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Coronary arteries are blood vessels that deliver oxygen and nutrient rich blood to the cardiac
muscles of the heart to allow it to pump blood efficiently.
Coronary heart disease is caused by fatty plaque building up in the lining of the coronary
arteries, which get occluded (narrower).
With more severity, blood flow to the cardiac muscles get restricted leading to chest pain.
The fatty plaque may eventually get hardened due to deposition of minerals on it.
If the cardiac tissue is deprived of oxygen, it will eventually lead to a heart attack.
Some of the factors linked with coronary heart disease are as follows:
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High blood cholesterol levels
Smoking
High blood sugar levels (diabetics)
Genetic factors
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Topic 6.3 – Defence against infectious disease
Pathogen & Infection barriers
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Pathogen: An organism or virus that causes a disease.
A pathogen can enter the body by various means and cause an infection, but the body forms a
barrier to prevent the entry of these pathogens.
The primary defence mechanism against pathogens are the skin and mucous membranes. These
are examples of non-specific immunity.
1. Skin:
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Skin form a tough physical barrier against infection from pathogens.
The outermost epidermal layer of skin are constantly replaced. These dead cells provide
effective protection against foreign pathogens.
The sebaceous glands in skin also secretes sebum (containing lactic acid) to lubricate the
skin. This lowers the pH of the skin, which effectively helps inhibit bacterial growth.
2. Mucous membranes:
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Mucous membranes are soft areas of skin that line the surfaces of the nasal cavity,
trachea, vagina, urethra, bronchi, and bronchioles.
Though they do not form a physical barrier, the sticky mucous trap foreign particles and
pathogens.
The mucous contains lysozymes, a type of enzyme that can kill pathogens.
Also, trapped pathogens can also be expelled out of the body through the mouth or
nose, or swallowed and destroyed by the high acidity of the stomach.
Blood clotting
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Blood clotting is the phenomenon in which cuts in the skin or
broken blood vessels are repaired and sealed to prevent
excessive blood loss and prevent entry of pathogens.
When a blood vessel breaks or cut, a blood platelets collect
at the site of the damaged blood vessel forming a platelet
plug (semi-solid blood clot).
Platelets have an important role in blood clotting.
These are small cell fragments present in the bloodstream
along with the red and the white blood cells.
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The process of blood clotting involves a series of steps (cascade of reactions) as follows;
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When the skin is cut, platelets and the damaged tissue release chemical factors called
clotting factors.
These clotting factors convert the protein prothrombin to its active form thrombin
(enzyme) which is a catalyst in the next reaction.
The enzyme thrombin converts another soluble protein fibrinogen into the insoluble
fibrous form i.e. fibrin.
Fibrin forms a mesh across wounds or at the point of the broken vessel further trapping
other platelets sealing up the damaged vessel and forming a semi-solid clot.
On exposure to air, the clot dries forming a protective scab that remains till the wound
heals completely.
Once the wound has completely healed, the blood clot dissolves in the blood.
The cascade results in the rapid conversion of fibrinogen to fibrin by thrombin, and
ensures that clotting only happens when it is needed.
Blood clots in coronary arteries
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Atherosclerosis is a disease characterized by the
deposition of plaques of fatty material on the
inner walls of the arteries.
The arteries become damaged and coarsened, and
if the deposits of plaque in arteries rupture, it can
result in the formation of blood clots.
In people suffering from coronary heart disease,
sometimes blood clots (coronary thrombosis)
form in the coronary arteries.
This may lead to complete blockage of the artery.
If the arteries are blocked, that part of the heart becomes deprived of oxygen and vital
nutrients.
The heart thus stops beating in a coordinated way, and is termed as a heart attack.
This uncoordinated contraction of cardiac muscle is known as fibrillation.
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Causes of coronary heart disease (CHD)
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The four main causes of CHD are as follows:
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Obesity & lack of exercise
Diabetes
Hypertension (high blood pressure)
Smoking
Phagocytes
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The main types of phagocytic leucocytes (white blood cells) are called macrophages.
These phagocytes are found in the blood and ingest pathogens.
They do so by recognising pathogens and engulfing them by the process of endocytosis.
Pathogens are recognized as non-self cells by the structure of their protein coat.
Enzymes within the phagocytes called lysosomes then digest the pathogens.
Phagocytes can ingest pathogens in the blood but also within body tissue as they can squeeze
through the pores of capillaries and into these tissues.
Ingestion of pathogens by phagocytic white blood cells gives non-specific immunity (because it
cannot distinguish between pathogens) to diseases.
Antigens & Antibodies
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Production of antibodies by lymphocytes in response to particular pathogens gives specific
immunity.
Antigens are defined as being proteins, glycoproteins or macromolecules present on the surface
of the cell membrane of the pathogen that are recognized by a specific antibody, to stimulate
the immune response.
When a pathogen enters the bloodstream, the specific antigen on the surface of the membrane
is identified as being non-self or foreign.
This stimulates a specific immune response in which antibodies are produced that are specific
for only that particular antigen.
B-lymphocytes are the white blood cells that produce antibodies.
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A lymphocyte can produce one type of antibody; so a vast diversity of lymphocytes are needed
to be able to respond to millions of foreign antigens.
Once an antigen binds to the surface of a lymphocyte, the B-lymphocytes become active and are
stimulated to divide by mitosis to produce a large amounts of clones of themselves.
These active B-lymphocytes that are formed are known as plasma cells.
These plasma cells produce and release mass amounts of same antibody into the bloodstream.
These antibodies bind to the antigens on the foreign pathogens, and aid their destruction.
The pathogens can be destroyed by the antibodies and other white blood cells by a variety of
different methods.
After an infection has been cleared from the body, most of the lymphocytes used to produce
the antibodies disappear, but some of them persists. These are memory cells.
These cells can quickly reproduce to form a clone of plasma cells if a pathogen carrying a specific
antigen is re-encountered (allowing long-term immunity).
HIV & the Immune system
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HIV (human immunodeficiency virus) is a retrovirus that causes
AIDS, a condition in humans where the immune system is
susceptible to infections by a variety of pathogens that would
normally be controlled easily.
HIV targets and destroys a type of lymphocyte which has a vital
role in antibody production.
This will result in the loss of ability to produce antibodies
needed to fight off infection from invading pathogens.
The inability of the immune system to fight off diseases is what
eventually causes death.
Methods of transmission of HIV:
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Through exchange of bodily fluids via sexual contact between individuals.
Across the placenta from a mother to a baby, or even in milk during breast-feeding.
Blood transfusion of infected blood (or with blood products such as Factor VII).
Sharing of needles by intravenous drug abusers.
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Antibiotics
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Antibiotics are a type of drug or chemical that kill or inhibits the growth of microorganisms;
mainly bacteria.
Antibiotics block processes that occur in prokaryotic cells but not in eukaryotic cells.
There are many differences between human and bacterial cells, and each antibiotic blocks one
of the processes in bacteria, without causing any harm to humans.
Viruses lack a metabolism and cannot therefore be treated with antibiotics.
Since viruses lack their own metabolism, they have to use the chemical processes of a cell from
a host that they infect.
So harming the virus cells would mean trying to harm the host cell.
Some strains of bacteria have evolved with genes that confer resistance to antibiotics and some
strains of bacteria have multiple resistance (e.g. S. aureus)
Penicillin
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A scientist named Alexander Fleming originally discovered penicillin in the year 1928.
Then two scientists, Florey and Chain managed to develop a method of producing the penicillin
in liquid cultures and purifying the penicillin in these cultures.
Their first test was on mice infected with Streptococcus bacteria which would cause death in the
mice if left untreated.
Four mice that were given penicillin recovered, but the untreated mice died.
They also carried out human trials on five individuals all of whom were cured.
After this breakthrough, penicillin became the first antibiotic to be widely produced and used by
pharmaceutical companies.
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Topic 6.4 – Gas exchange
Ventilation
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The cells in the tissue of the body need oxygen for carrying out cellular respiration.
This cell respiration happens in the cytoplasm and the mitochondria of a cell, and involves the
release of energy in the form of ATP.
Therefore, humans need a system to keep a fresh supply of O2 and to get rid of excess CO2.
This process of swapping one gas for another is called as gas exchange.
In humans, the ventilation system provides a fresh supply of O2 in the alveoli, allowing the
oxygen to diffuse into the blood capillaries surrounding them.
This diffused oxygen is then transported to all the tissues in the body.
The product of respiration CO2 in the tissues is transported to the lungs by the blood, where it is
exhaled out into the atmosphere after diffusion into the alveoli.
A steep concentration gradient exists across the alveoli where gases are exchanged.
The blood arriving at the alveoli via the pulmonary arteries, arterioles and then capillaries, is rich
in CO2, and due to the concentration gradient between the capillaries and the alveoli, CO2
rapidly diffuses out of the alveoli.
Whereas the air in the lungs is rich in O2 as compared to the blood, and so oxygen diffuses
across the alveoli into the bloodstream.
The ventilation system diagram shows that air is carried from the alveoli in the trachea, bronchi
and bronchioles.
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Mechanism of ventilation
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Pressure changes in the thorax force air in and out of the lungs.
These pressure changes help in ventilation and are caused by the action of muscle contractions.
The process of ventilation involves the following two phenomenon: Inspiration (inhalation) and
Expiration (exhalation).
Different muscles are required for both these processes, as muscles can only do work when they
contract. These muscles which cause opposite movement from each other are known as
antagonistic muscles.
Inhalation
Exhalation
1.
External intercostal muscles contract
Internal intercostal muscles contract
2.
Ribcage moves up and out
Ribcage moves down and in
3.
Diaphragm contract, moves down and
becomes flat
Abdominal muscles contract, pushing
diaphragm up in a dome shape
4.
Thorax volume increases
Thorax volume decreases
5.
Pressure drops inside the thorax
Pressure increases inside the thorax
6.
Movement of air into the lungs from outside
the body
Movement of air out of the body from the
lungs
Alveoli adaptations for gas exchange
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Air is carried to the lungs in the trachea
and bronchi and then to the alveoli in
bronchioles.
The alveoli are tiny clusters of air sacs,
which carry out gas exchange.
Although each alveoli is very small, the
lungs contain hundreds of millions of
alveoli.
The four properties of an alveoli that
make it efficient for gas exchange are;
1. A large total surface area for
diffusion of gases
2. Permeable to both oxygen and
carbon dioxide
3. Surface is moist, to allow the
oxygen to dissolve easily
4. Very thin so that the diffusion distance is small
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Type 1 pneumocytes:
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Type 2 pneumocytes:
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Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas
exchange.
Most of the alveolar wall consists of a single layer of these cells.
Since the alveoli are surrounded by capillaries that are also only one cell thick, O2 and CO2
have a very short distance to diffuse, and this adaptation allows for a rapid rate of gas
exchange.
Type II pneumocytes secrete a solution containing surfactant that creates a moist surface
inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing
surface tension.
The moist inner alveolar surface also allows for the gases to dissolve.
Blood capillaries:
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

A dense network of blood capillaries surround the alveolus.
These capillaries contain high concentrations of CO2 and low concentrations of O2.
The walls of the capillaries are also thin, and consists of a single layer of cells only to allow
for efficient diffusion of gases (O2 diffuses from the air into the blood and CO2 diffuses in the
opposite direction).
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Lung cancer

The causes of lung cancer are as follows:
1. Smoking – It is the main cause of lung cancer, with nearly 90% of the lung cancer cases
caused by smoking. Tobacco smoke contains a wide number of carcinogens that can cause
tumours to develop. There is a very strong correlation with the number of cigarettes
smoked in a day and the incidence of lung cancer. Also, exhaled breath from smokers can
pass carcinogens on to others (passive smoking).
2. Air pollution – A minor cause of lung cancer is due to air pollution arising from exhaust
fumes containing nitrogen oxides, fumes from diesel engines and smoke from burning
carbon compounds such as wood or coal.
3. Radon gas – In some regions, this radioactive gas can leak out of certain rocks such as
granite, accumulating in poorly ventilated houses.
4. Asbestos and silica – Dust from these materials if deposited in the lungs can cause cancer.


Lung cancer is a very serious disease and the consequences can be severe, even fatal especially
if the cancer is not recognized early on.
Some of the consequences of lung cancer are:







Fatigue
Chest pain
Breathing difficulty
Persistent cough
Coughing up blood
Appetite loss
Weight loss
Emphysema


Emphysema is a respiratory disease that is largely linked to smoking.
The cause of emphysema are:



Smoking
Air pollution
The consequences of this disease are as follows;



There is loss of elasticity of the alveoli in the lungs, resulting in the destruction of lung tissue
over a period of time.
This results in shortness of breath
The surface area for gas exchange is reduced, so that oxygen saturation of the blood falls
making exercise more and more difficult.
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
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Cilia lining the airways are damaged, so mucus builds up in the lungs causing infections.
Phagocytes prevent lung infections and produce a hydrolytic enzyme called elastase.
I a s oker’s lu g, there are a large u er of phago tes hi h results i a larger a ou t
of this enzyme being released and eventually causing a complete breakdown of the alveolus
walls.
The tiny alveoli are progressively replaced by larger air sacs containing thicker less
permeable walls (less total surface area).
Monitoring ventilation in humans



The process of ventilation can be monitored by means of ventilation rate or tidal volume.
Ventilation rate is the number of inhalations or exhalations per minute. The ventilation rate can
be obtained by means of data-logging.
Tidal volume is the volume of air taken in or out with each inhalation or exhalation. Tidal
volumes are measured using a spirometer.
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Topic 6.5 – Neurons and synapses
Structure of neurons
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Neurons are specialized cells for the conduction of nerve impulses, and act as the fundamental
unit of the nervous system.
These cells carry messages at high speed in the form of electrical impulses.
The nerve fibres have a myelin sheath with small gaps called the nodes of Ranvier.
These allow the nerve impulses to jump from node to node, speeding up the transmission and is
known as saltatory conduction.
Resting potential
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The electrical potential across the plasma membrane of a neuron when it is not conducting an
impulse is known as resting potential.
The sodium-potassium pumps maintains the gradient of the resting potential (-70 mV) by means
of active transport.
They pumps Na+ ions out and K+ ions in, by active transport across the membranes.
The presence of chloride and other ions inside results in a net negative charge.
This makes the inside of the membrane relatively negative as compared to the outside.
Action potential
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The depolarization and repolarization of the electrical potential across the plasma membrane of
a cell, as an electrical impulse passes along it is termed as action potential.
These ions move by means of facilitated diffusion across the membranes.
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
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Na+ and K+ channels in nerve cells are voltage-gated, meaning they can open and close
depending on the voltage across the membrane.
If the potential across the membrane rises from -70mV to -50mV, the sodium channels open and
as a result Na+ ions diffuse inside.
The entry of these Na+ ions causes the inside of the neuron to develop a net positive charge as
compared to the outside, and this is known as depolarization.
The reversal of membrane polarity also activates voltage-gated potassium channels, causing K+
to exit the neuron via facilitated diffusion.
The exit of K+ causes the membrane potential to become negative again, and this is called
repolarization as the potential across the membrane is restored.
Propagation of nerve impulses
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A nerve impulse is an action potential that travels along the neuron from one end to another.
Propagation of nerve impulses is the result of local currents that cause each successive part of
the axon to reach the threshold potential.
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
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Whenever a part of the axon reaches the threshold potential (-50mV), an action potential is
generated, which triggers an action potential in the next part. This is propagation of nerve
impulse.
It occurs due to diffusion of Na+ ions between a region with an action potential and the next
region which is still at the resting potential.
Local currents is the phenomenon when Na+ ions diffuse along the axon, both inside and outside
the membrane.
It causes the voltage across the membranes to change from -70mV (resting potential) to -50mV
(threshold potential), which later results in an action potential.
Oscilloscope traces

The following are the parts of an oscilloscope trace;
1. The axon membrane is at resting potential and rises to the threshold potential, either due to
local currents or binding of a neurotransmitter.
2. Depolarization of the membrane (Na+ ions diffusing inside).
3. Repolarization of the membrane (K+ ions diffusing out).
4. Active transport of Na+ ions out, and K+ ion inside occurs by the action of the sodiumpotassium pumps.
5. Membrane returns to its resting potential, therefore another action potential can occur.
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Synapses
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A synapse is a junction between 2 neurons or between neurons and other cells, that permits a
neuron to pass an electrical or chemical signal to another cell.
At a synapse, the plasma membrane of the presynaptic neuron is closely related to the plasma
membrane of the postsynaptic neuron.
Between the two there is a narrow fluid filled gap called the synaptic cleft.
Chemical signals called neurotransmitters pass from the presynaptic neuron to the post synaptic
neuron, and they always pass in the same direction.
The synapses function in the following way;
8. The receptors on the post-synaptic neuron are transmitted-gated Na+ channels which open
and let Na+ into the postsynaptic neuron when the neurotransmitters bind.
9. As these ions enter the postsynaptic neuron they cause its membrane to depolarize.
10. This depolarization results in an action potential which passes down the postsynaptic
neuron.
11. The neurotransmitters in the synaptic cleft are then rapidly broken down to prevent
continuous synaptic transmission, and the calcium ions are pumped back into the synaptic
cleft from inside the presynaptic neuron.
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Acetylcholine
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Many of the synapses make use of the neurotransmitter acetylcholine.
These synapses are termed as cholinergic synapses.
The pre-synaptic neuron secretes acetylcholine into the synaptic cleft, which diffuses across the
synapse and later binds to the receptors on the membrane of post-synaptic neuron.
The acetylcholine is broken down by the enzyme cholinesterase in the synaptic cleft, giving
acetyl group and choline.
This choline is reabsorbed by the pre-synaptic neuron.
Neonicotinoid pesticides
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Neonicotinoid pesticides bind to the acetylcholine receptors present on the membrane of the
post-synaptic neuron.
Thus, the cholinergic synapses in insects are affected by these pesticides.
The enzyme cholinesterase cannot break these down, and thus they remain bound to the
receptors, inhibiting the binding of acetylcholine.
Thus, synaptic transmission is blocked, which eventually leads to the death of the pests.
However, along with the targeted pests, other insects like honeybees which are not the
intended target of these pesticides also die.
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Topic 6.6 – Hormones, homeostasis and reproduction
Regulating blood glucose concentration
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
Blood glucose concentration is carefully monitored by negative feedback mechanisms.
Depending on the concentration of blood glucose, the pancreas secrete the hormones insulin or
glucagon to regulate this glucose.
When the blood glucose levels are high:
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

After consuming a meal, the blood glucose level rises suddenly.
This is detected by the receptors of the pancreas which release insulin.
This insulin is secreted by the beta (ß) cells of the pancreas.
It stimulates the absorption of glucose from the blood into muscles and liver cells, and thus
allowing it to convert glucose into glycogen.
This in turn causes the blood glucose levels to lower back to normal.
When the blood glucose levels are low:

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

Receptors in the pancreas sense when the blood glucose level is too low.
Glucagon is secreted by the alpha (ᾳ) cells of the pancreas into the bloodstream.
This stimulates the liver to breakdown glycogen (stored in liver cells) into glucose which is
released into the bloodstream.
This raises the blood glucose levels back to normal.
Diabetes
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
The control of blood glucose levels does not work efficiently in some people, and the
concentration can rise or fall beyond normal limits. This condition is known as diabetes.
There are 2 forms of diabetes:
1. Type I diabetes:


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


It is an autoimmune disease characterized by the inability of the pancreas to produce
insulin in sufficient amounts.
This type of diabetes usually develops during childhood.
The β- ells of the pa reas that produ e i suli are atta ked a d destro ed o e’s
own immune system.
Thus, the body loses the ability to take up the glucose in the blood into its cells and
convert glucose into glycogen.
Diet alone cannot by itself control type I diabetes.
People having type I diabetes must take insulin shots or injections often before meals.
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2. Type II diabetes:
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It is the most common form of diabetes.
Type II diabetes is considered late onset as it occurs later on in life.
It occurs when the insulin receptors (target cells) in body cells lose their ability to
respond to insulin.
The pancreas still produces insulin, thus insulin injections are not needed.
Type II diabetes is usually a result of obesity due to over-eating, diet rich in fat and low
in fibre, age, lack of exercise or genetic factors affecting metabolism.
Diabetes II can be controlled by lifestyle and diet changes (low carbohydrate intake).
Thyroxin
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Thyroxin is secreted by the thyroid gland to regulate the metabolic rate and help control body
temperature.
The chemical structure of thyroxin contains iodine; therefore, prolonged deficiency to iodine in
the diet prevents the synthesis of thyroxin.
Thyro i is i porta t i the regulatio of the od ’s eta oli rate, a d all the od ells are its
targets.
But the prime targets of this hormone are the liver, brain and the muscles.
The od ’s eta oli rate is the a ou t of e erg used up ( ata oli a d anabolic reactions
combined), and a higher metabolic rate supports more protein synthesis, growth and generation
of more body heat.
As th ro i auses a i rease i the od ’s eta oli rate, there is a i rease i the od
temperature.
Thus, when the surrounding temperature drops more thyroxin is secreted by the thyroid gland
resulting in increased respiration rate of adipose tissue, shivering, metabolic rate and
vasoconstriction of the blood vessels.
When external temperature falls
When external temperature rises
1.
Hypothalamus detects reduced body
temperature
Hypothalamus detects raised body
temperature
2.
Thyroid gland secretes more thyroxin
Thyroid gland secretes less thyroxin
3.
Metabolic rate is increased
Metabolic rate is decreased
4.
Vasoconstriction of skin arterioles
Vasodilation of skin arterioles
5.
Increased respiration in brown adipose
tissue (BAT)
Decreased respiration in brown adipose
tissue (BAT)
6.
Shivering
-
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Leptin
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Leptin is secreted by cells in adipose (fat) tissue and acts on the hypothalamus of the brain to
inhibit appetite.
Leptin is a hormone made by cells of the adipose tissue that helps to regulate energy balance by
inhibiting or suppressing hunger.
Its action is on the receptors of the hypothalamus in the brain to regulate appetite in order to
achieve energy homeostasis.
The concentration of leptin in the blood is controlled by food intake and the amount of adipose
tissue in the body.
If the amount of adipose tissue in an individual increases, then their concentrations of leptin
also increases.
Leptin binds to receptors in the membranes of the hypothalamus cells causing long term
appetite inhibition and reduced food intake.
In the case of obese individuals sensitivity to leptin is decreased, resulting in an inability to
recognize when they are full.
Leptin & clinical obesity
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Studies have shown that mice containing a recessive allele (ob) produce a different version of
the leptin hormone.
This led into severe obesity in these mice, because the signal that tells the brain that they are
full did ’t ork a
ore.
The discovery of how mice become obese because of the lack of the hormone leptin and the
subsequent treatment of the mice with leptin injections, led to human trials to reduce obesity.
However, the trials conducted with humans did not yield similar results, as the physiology of
humans is much different then mice.
By carrying out extensive trials, it was then determined that most of the obesity cases were
caused by a change in the receptor protein for leptin, not in the production of leptin as most
humans have quite a high leptin concentration.
The side effects of leptin injections were skin irritation and swelling. Also, since leptin is a shortlived protein it was needed to be injected several times in a day.
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Melatonin
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Melatonin is secreted by the pineal gland in the brain to control circadian rhythms.
The secretion of melatonin is controlled by cells in the hypothalamus.
Ganglion cells in the retina detect whether it is light or dark and send impulses to the SCN in the
hypothalamus, which control secretion of melatonin from pineal gland.
Melatonin helps control sleep and wake cycles (circadian rhythms).
Melatonin secretion increases in the evening and drops to a low level at dawn.
Natural melatonin levels slowly declines with age, thus explaining how sleep patterns becomes
irregular as we grow older.
Very small amounts of melatonin are also found in foods such as meats, grains, fruits, and
vegetables.
Melatonin & jet-lag
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
The od ’s circadian rhythms are disrupted by travelling rapidly between time zones.
The symptoms often experienced together are known as jet lag, and are as follows:

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Sleep disturbance
Headaches
Fatigue
Irritability
The SCN of the hypothalamus and the pineal gland continue to set the circadian rhythm of the
place the person is departing from, rather than the destination.
Therefore, when a person lands in another country of a different time zone, they feel sleepy in
the day and awake at night.
Jet lag will only last a few days, as the body adjusts to the new times when the light is detected
by the cells in the retina during a different time period.
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Sex determination
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Embryos are formed by the fusion of an egg and a sperm, and the embryonic gonads that are
formed could become either ovaries or testes.
The presence of a gene called the SRY gene on the Y chromosome, causes the embryonic gonads
to become testes and begin secreting testosterone.
SRY codes for a protein called TDF (testis-determining factor) that stimulates the expression of
other genes located on the Y chromosome that cause testis development.
If there are two X chromosomes, TDF is not produced and so the gonads develop as ovaries, and
the fetus becomes a female.
Male reproductive system

The parts of the male reproductive system are as follows:

Penis – Has erectile tissue that can get enlarged/hardened for allowing penetration into the
vagina and promoting copulation.

Sperm duct (Vas deferens) – Transfers sperm during ejaculation.

Seminal vesicle – Secretes a fluid containing proteins.

Prostate gland – Secretes a fluid and helps the sperm to swim.

Epididymis – Stores sperm until ejaculation.
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
Scrotum – Holds testes outside the body, to keep and maintain it at a lower temperature than
the core body temperature.

Testis – Produces sperm and testosterone.

Urethra – Transfers semen during ejaculation, and urine during urination.
Female reproductive system

The parts of the female reproductive system are as follows:

Ovary – Produces eggs, estrogen and progesterone.

Oviduct – Collect eggs at ovulation, provides fertilization site for the embryo.

Cervix – Protects the fetus during pregnancy, dilates to provide a birth canal.

Vagina – Stimulates penis to cause ejaculation, and provides a birth canal.

Uterus – Provides protection, food, oxygen and removal of waste products from the fetus during
pregnancy.

Vulva – Protects the internal parts of the female reproductive system.
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The menstrual cycle
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In females who are not pregnant, between puberty and menopause there occurs a regular
monthly cycle known as the menstrual cycle.
The menstrual cycle is controlled by both, negative and positive feedback mechanisms involving
ovarian hormones (estrogen and progesterone) and pituitary hormones (LH and FSH).
Negative feedback: A rise in the levels feeds back to decrease production and reduce the level.
Similarly, a fall in the level of the hormone feeds back to increase production and raise the level.
Negative feedback has a stabilizing effect because a change in level always causes the opposite
change.
Positive feedback: It leads to a sudden rises or falls, because an increase causes further increase
and a decrease in levels causes further decrease.
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
The steps in the menstrual cycle are as follows:
1. The pituitary gland secretes the hormone FSH and its levels start to rise. This stimulates the
development of follicles (containing an oocyte and follicular fluid). FSH also stimulates the
follicle cells to secret estrogen.
2. Estrogen then causes the follicle cells to make more FSH receptors so that these can respond
more strongly to the FSH, which boosts estrogen production.
3. This is positive feedback and causes the estrogen levels to increase and stimulate the repair and
thickening of the endometrium (uterus lining).
4. Estrogen levels rise to a peak and by doing so it stimulates LH secretion from the pituitary gland.
5. LH then increases to its peak and results in ovulation (release of egg from the follicle).
6. LH then stimulates the follicle cells to secrete less estrogen and more progesterone. After
ovulation has occurred, LH stimulated the follicle to develop into the corpus luteum.
7. The corpus luteum then starts to secrete high amounts of the hormone progesterone. This
prepares the uterine lining for an embryo.
8. The high levels of estrogen and progesterone then start to inhibit FSH and LH (negative feedback
because FSH and LH stimulated estrogen and progesterone secretion).
9. If no embryo has been formed, the levels of estrogen and progesterone fall. This stimulates
menstruation (break down of the uterine lining). When the levels of these two hormones are
low enough FSH and LH start to be secreted again.
10. FSH levels rise once again, starting a new menstrual cycle.
Roles of hormones

Testosterone:

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
Estrogen:

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
Produced in the developing testes in the fetus of males.
Stimulates the development of pre-natal genitalia.
Stimulates the development of male secondary sexual characteristics such as broad
shoulders, growth in muscle mass and pubic hair.
Maintains sex drive in adults.
Causes pre-natal development of female reproductive organs, if testosterone is not present.
Duri g pu ert , raised le els auses’ de elop e t of se o dar se ual hara teristi s like
growth of breasts, pubic hair and enlargement of hips.
Produced by the developing follicles in ovaries and the corpus luteum.
Promotes the thickening of the endometrium and the growth of blood vessels, in
preparation of egg implantation.
Inhibits FSH and LH when the estrogen levels are high.
Progesterone:


Produced by the ovaries and the corpus luteum.
Prepares the uterus during the menstrual cycle for embryo implantation.
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
LH:

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

Secreted by the pituitary gland.
Triggers the release of the egg during ovulation.
Stimulates the growth of the corpus luteum
Promotes secretion of estrogen and progesterone.
FSH:



Secreted by the pituitary gland.
Stimulates the growth of the follicles in the ovaries.
Promotes the thickening of the follicle wall.
In vitro fertilization (IVF)


IVF has been used extensively to overcome fertility problems in either male or female parent.
The following steps are usually involved;
1. Down-regulation:


IVF treatment begins by taking drugs each day to halt the regular secretion of the hormones
FSH and LH. The secretion of progesterone and estrogen therefore also stops.
This suspends the normal menstrual cycle and effectively allows the doctor to take control
of the timing and egg production of the wo a ’s o aries.
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2. Artificial hormone doses:



The woman is then injected with large amounts of FSH for about 10 days to induce the
production of many follicles.
LH is also injected to promote the release of many eggs.
This process is called superovulation, which can produce around 20 eggs.
3. Egg retrieval & fertilization:





Eggs are then stimulated to mature by an injections of HCG (Human Chorionic
Gonadotrophin), a hormone usually secreted by the developing embryo.
These eggs are then surgically removed from the ovary of the woman.
Sperms are collected from a male individual.
Each egg is mixed with around 50,000-100,000 sperms in a petri dish.
The sperm and eggs in the petri dish are incubated at 37ºC in sterile conditions.
4. Embryo transfer:




The eggs are analyzed for successful fertilization (two nuclei inside the egg).
Healthy embryos are selected and are placed into the female uterus for implantation.
To increase chance of implantation, more than one embryo is transferred into the uterus.
The woman is usually given extra progesterone so that the uterus lining is maintained.
Harvey & discovery of sexual reproduction
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

William Harvey who was known for his discovery of the circulation of the blood in the 17th
century, was also interested in sexual reproduction and how life is formed.
Aristotle’s theor as alled the seed a d soil theor , hi h stated that the male produces a
seed which then forms an egg when it mixes with menstrual blood of the female. The egg then
develops into a fetus inside the mother.
Harvey tested Aristotle’s theor usi g a atural e peri e t i ol i g deer.
He studied the uterus of the deer during mating season by slaughtering them and then
dissecting them expecting to find eggs immediately after mating.
Harvey only found signs of fetal development 2 to 3 months after the start of mating season.
He concluded with proof that Aristotle’s theor of seed and soil was incorrect.
But he also incorrectly stated that the offspri g does ’t result from mating.
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