Chapter 8
Transport in mammals
Circulatory system
The system that contains the heart and the blood vessels and moves blood
throughout the body.
What happens when carbaminohaemoglobin, hydrogen carbonate ions reach
The need for a circulatory system
the lungs?
 The cells of all living organisms need a constant supply of reactants for
 When blood reaches the lungs, the reactions described above go into reverse.
 The carbon dioxide of carbaminohaemoglobin leaves the red blood cell.
 Hydrogen carbonate ions and hydrogen ions recombine to form carbonic acid
which is later on breakdown to produce carbon dioxide and water molecules
once more.
 This leaves the haemoglobin molecules free to combine with oxygen, ready to
begin another circuit of the body.
metabolism, e.g. oxygen and glucose.
 Single-celled organisms can gain oxygen and glucose directly from their
surroundings, and the molecules can diffuse to all parts of the cell quickly due
to short diffusion distances.
 Larger organisms, however, are made up of many layers of cells, meaning that
the time taken for substances such as glucose and oxygen to diffuse to every cell
in the body would be far too long.
 To solve this problem their exchange surfaces are connected to a mass transport
system.
 For example, The digestive system is connected to the circulatory system, and the
lungs are connected to the circulatory system.
What happens when carbaminohaemoglobin,
hydrogen carbonate ions reach the lungs?
 Mass transport is the bulk movement of gases or liquids in one direction, usually
via a system of vessels and tubes.
 The circulatory system in mammals is a well-studied example of a mass transport
system; the one-way flow of blood within the blood vessels carries essential nutrients
and gases to all the cells of the body.
Types of circulatory system
 Circulatory systems are either described as being open or closed.
 In a closed circulatory system, blood is pumped around the body and is
always contained within a network of blood vessels.
 All vertebrates and many invertebrates have closed circulatory systems.
 In an open circulatory system, blood is not contained within blood vessels
but is pumped directly into body cavities.
 Organisms such as arthropods and molluscs have open circulatory systems.
Double circulation system
 Some carbon dioxide remains as carbon dioxide molecules and simply
 A mechanism in which blood passes through the heart
3. As dissolved carbon dioxide molecules in the blood plasma
 Humans have closed Double circulation system
twice on one complete circuit of the body. It is called a
double circulatory system because the blood flows
dissolves in the blood plasma.
 About 5% of the total is carried in this form.
from the heart to the lungs, then back to the heart,
then around the rest of the body, and then back to
the heart again.
 Double circulation has two parts.
2. As carbaminohaemoglobin
 Carbon dioxide that is not catalyzed by carbonic anhydrase inside RBC, combines
1. Systemic circulation: The part of the circulatory
system that carries blood from the heart to all of the
directly with the terminal amine groups (–NH2 ) of some of the haemoglobin
body except the lungs, and then back to the heart.
molecules. The compound formed is called carbaminohaemoglobin.
 In the lungs, Oxygen binding to hemoglobin reduces hemoglobin's affinity for
carbon dioxide, causing the release of carbon dioxide​ from carbaminohaemoglobin.
 Blood is pumped out of the left ventricle into the
aorta and travels from there to all parts of the body
except the lungs.
The carbon dioxide is then released into the alveoli and exhaled.
 About 10% of the carbon dioxide is carried in this way.
 It returns to the right side of the heart through the
vena cava.
2. Pulmonary circulation: The part of the
circulatory system that carries blood from the
heart to the lungs and then back to the heart.
 The blood is pumped out of the right ventricle into
the pulmonary arteries, which carry it to the lungs.
 The final part of the journey is along the
pulmonary veins, which return it to the left side of
the heart.
Carbon dioxide transport
 The blood transports carbon dioxide in three different ways.
1. As hydrogen carbonate ions in the blood plasma
 Hydrogencarbonate ions are produced in the Bohr shift process which we already know.
 These are formed in the cytoplasm of the red blood cell because this is where the enzyme
carbonic anhydrase is found.
 Most of the hydrogen carbonate ions then diffuse out of the red blood cell into the
blood plasma, where they are carried in solution.
 About 85% of the carbon dioxide transported by the blood is carried in this way.
Why chloride shift is important?
 If the chloride shift did not happen, the inside of the red blood cell would
develop an overall positive charge, because hydrogen ions (from the
dissociation of carbonic acid) would accumulate and would decrease blood pH.
The pressure in the systemic circulation is considerably higher
than in the pulmonary circulation.
Single circulatory system
A circulatory system in which the blood passes through the heart only once on a
complete circuit.
The chloride shift
 The chloride shift is the movement of chloride ions into red blood cells that
occurs when hydrogen carbonate ions are formed .
 The hydrogen carbonate ions that are produced inside red blood cells, diffuse
out of the cells and into the blood plasma.
 These ions have a negative charge and, to balance their movement, chloride
ions (which also have a negative charge) move from the blood plasma into
the red blood cells.
 This is called the chloride shift.
Blood vessels
Three main types of vessels make up the circulatory system.
1. Arteries: these are vessels with thick, strong walls that carry high-pressure
oxygenated blood away from the heart.
 Small arteries are called arterioles.
 Large arteries are called aorta.
Only exception is the pulmonary artery which carries deoxygenated blood from the
heart to the lungs.
Why haemoglobin/Bohr shift is important? Or How haemoglobin helps to
2. Veins: these are vessels with relatively thin walls that carry low-pressure
maintain pH?
deoxygenated blood back to the heart.
 Small veins are called venules.
 Large veins are called vena cava. Vena cava has two types:
 Superior vena cava: carries blood from the head, chest, and upper extremities.
 Inferior vena cava: it is the body's largest vein that carries blood from all parts
 Haemoglobin removes excess hydrogen ions from the solution. When carbon dioxide
dissolves and dissociates, a high concentration of hydrogen ions is formed. This
produces a low pH. If the hydrogen ions were left in the solution, the blood would be
very acidic. By removing the hydrogen ions from the solution, haemoglobin
helps to maintain the pH of the blood close to neutral. It is acting as a buffer.
of the body below the diaphragm.
Only exception is the pulmonary vein which carries oxygenated blood from the
Biological buffers are organic substances that maintain a constant pH
over a given range by neutralizing the effects of hydrogen ions.
lungs to the heart.
 In the Lungs, Oxygen from the alveoli binds to hemoglobin, breaking down
haemoglobinic acid and releasing hydrogen ions.
3. Capillaries: are tiny vessels that link arterioles and venules.
 It is the place where all the exchanges occur.
 Capillaries deliver nutrients, hormones, and other requirements to body
 The hydrogen ions combine with hydrogen carbonate ions to reform carbonic acid.
cells, and take away their waste products.
 Capillaries form a network throughout every tissue in the body except the
 Carbonic acid then breaks down into water and carbon dioxide which is exhaled.
brain, cornea, and cartilage.
Reaction of carbonic anhydrase
 Carbon dioxide is continually produced by respiring cells. It diffuses from the cells and
into the blood plasma, from where some of it diffuses into the red blood cells.
 In the cytoplasm of red blood cells, there is an enzyme, carbonic anhydrase which
converts carbon dioxide and water into carbonic acid.
 Then carbonic acid breaks down into hydrogen ions and hydrogen carbonate ions.
 Hydrogen ions cannot diffuse out from the RBC but hydrogen carbonate ions can.
 Hydrogen ions readily combine with the haemoglobin, forming haemoglobinic
acid, HHb. When haemoglobin does this, it releases the oxygen which it is carrying.
Endothelium: a tissue that lines the inner surface of a structure such as a blood
The Bohr shift
vessel.
 Changes in the oxygen dissociation curve as a result of carbon dioxide levels
Squamous epithelium: one or more layers of thin, flat cells forming the lining of
are known as the Bohr effect or Bohr shift.
some hollow structures, e.g. blood vessels and alveoli.
 The amount of oxygen haemoglobin carries is affected not only by the partial
Smooth muscle: a type of involuntary muscle that can contract steadily over long
pressure of oxygen but also by the partial pressure of carbon dioxide.
periods of time.
 When the partial pressure of carbon dioxide in the blood is high, the saturation
percentage of haemoglobin is reduced.
Elastic arteries
 They are relatively large arteries, which have a lot of elastic tissue and little
 Haemoglobin has a low affinity for oxygen at low concentrations of oxygen.
 Low partial pressure (at respiring cells) of oxygen means, a low concentration of
oxygen.
muscle tissue in their walls.
 The elasticity of these artery walls is important in allowing them to stretch,
which reduces the likelihood that they will burst.
 The artery walls stretch as the high-pressure blood surges into them and then
recoil inwards as the pressure drops.
1. At low partial pressure of oxygen, haemoglobin binds less to the oxygen because of
its less affinity, so the saturation percentage is low.
2. At medium pressure (at pulmonary veins and artery) of oxygen, haemoglobin binds
more easily to oxygen because of its high affinity and saturation increases quickly.
3. At high pressure (at lungs) of oxygen, haemoglobin binds easily to oxygen because
of its high affinity and saturation increases quickly.
Muscular arteries
 Arteries that take blood from an elastic artery and deliver it close to its final
destination have more smooth muscle in their walls which allows them to
constrict and dilate.
Vasoconstriction: the narrowing of a muscular artery or arteriole,
caused by the contraction of the smooth muscle in its walls in response to
The haemoglobin dissociation curve
 Partial pressure: is the pressure exerted by a single gas in a mixture of gases
which helps to measure specific concentration of a gas.
 Saturation: a sample of haemoglobin that has combined with a maximum
nerve impulses from the brain.
amount of oxygen is said to be saturated.
combined with oxygen, calculated as a percentage of the maximum amount
the relaxation of the smooth muscle in its walls in response to nerve
 Percentage saturation: the degree to which the haemoglobin in the blood is
Vasodilation: the widening of a muscular artery or arteriole, caused by
with which it can combine.
impulses from the brain.
 Dissociation curve: a graph showing the percentage saturation of a pigment
(such as haemoglobin) with oxygen, plotted against the partial pressure of
oxygen.
Heart structure
 The human heart has a mass of around 300g and is roughly the size of a closed fist.
Cardiac muscle: The muscle of the heart.
Coronary arteries: arteries that branch from the aorta and spread over the walls of the
heart, supplying the cardiac muscle with nutrients and oxygen.
Heart structure
 The heart is a hollow, muscular organ located in the chest cavity.
 It is protected in the chest cavity by the pericardium, a tough and fibrous sac.
 The heart is divided into four chambers. The two top chambers are atria and the
bottom two chambers are ventricles.
 The left and right sides of the heart are separated by a wall of muscular tissue,
called the septum which is very important for ensuring blood doesn’t mix
between the left and right sides of the heart.
Haemoglobin
 Oxygen is transported around the body inside red blood cells in combination with the
protein haemoglobin.
 Each haemoglobin molecule is made up of four polypeptides, each containing one
haem group. Each haem group can combine with one oxygen molecule.
 Overall, then, each haemoglobin molecule can combine with four oxygen molecules
(eight oxygen atoms).
 The portion of the septum that separates the left and right atria is called the
interatrial septum, while the portion of the septum that separates the left and
right ventricles is called the interventricular septum.
Structural features of a WBC
• White blood cells all have a nucleus, although the shape of this varies in
different types of white cells.
• Most white blood cells are larger than red blood cells, although one
type, lymphocytes, may be slightly smaller.
• White blood cells are either spherical or irregular in shape, not a
biconcave disc.
Valves
Valves prevent the backward flow of blood. There are two types of valves.
Lymphocytes
 Lymphocytes are smaller than phagocytes.
 They have a large nucleus that fills most of the cell.
1. B-lymphocytes and
allow blood to flow from the atria into the ventricles but prevent it from going in the
 There are two types of lymphocytes, which are produced in the bone marrow.
1. Atrioventricular valves: The valves between the atria and the ventricles. They
2. T-lymphocytes
opposite direction.
 Tricuspid valve: located between the right atrium and the right ventricle.
 Mitral valve (Bicuspid): located between the left atrium and the left ventricle.
Monocytes/macrophages
 They are made in the bone marrow and circulate in the blood as monocytes.
 When there is any attack from bacteria or viruses, they eventually leave the blood.
 When they leave the blood they become macrophages.
 Monocytes and macrophages can live for several months.
 They play a crucial role in initiating immune responses, since they do not destroy
pathogens completely, but cut them up to display antigens that can be
recognized by lymphocytes.
Semilunar valves: The valves in the entrances to the aorta and the
pulmonary artery. These valves allow blood to flow from the ventricles into
the arteries, but stop it going the other way.
 Pulmonary valve: located between the right ventricle and the
pulmonary artery.
 Aortic valve: located between the left ventricle and the aorta.
Neutrophils
 About 60-70% of the white blood cells in the blood are neutrophils.
 It can be recognized by its lobed nucleus and granular cytoplasm.
 They travel throughout the body, often leaving the blood by squeezing through the
walls of capillaries to move through the tissues engulfing any pathogens that they find.
 During an infection, neutrophils are released in large numbers from their stores, but
they are short-lived cells (5.4 days).
Phagocytes
 Phagocytes are cells that protect the body by ingesting harmful foreign particles,
bacteria, and dead or dying cells.
 Phagocytes are produced throughout life in the bone marrow. They are stored
there before being distributed around the body in the blood.
 They are scavengers, removing any dead cells as well as invasive microorganisms.
 There are two types of phagocytes:
1. Neutrophils or
2. Monocytes/macrophages
Why valves are important?
 Valves in the heart open when the pressure of blood behind them is greater
than the pressure in front of them and close when the pressure of blood in
front of them is greater than the pressure behind them.
 Valves are important for keeping blood flowing forward in the right direction
and stopping it from flowing backward. They are also important for
maintaining the correct pressure in the chambers of the heart.
The Walls of the Heart
 The muscular walls of the atria are thinner than those of the ventricles.
 When the atria contract, the thin muscular walls do not generate much pressure, but
White blood cells (Leukocytes)
 White blood cells, like red blood cells, are made in the bone marrow.
enough to force blood down into the ventricles, through the atrioventricular valves.
 In contrast, the walls of the ventricles are thicker and more muscular.
 They can be divided into two main groups:
2. lymphocytes.
inwards, increasing its pressure and pushing it out of the heart through the right and
1. phagocytes and
 Following the contraction of the atria, the ventricles contract and squeeze blood
left semilunar valves.
Left & right ventricle
 Some capillaries are even narrower than the diameter of a red blood cell but
they can still pass through these vessels by squashing. This is possible
because the cells have a specialized cytoskeleton which is made up of a mesh-
 The muscle of the left ventricle is significantly thicker than the right ventricle.
 This is because the blood leaving the right ventricle travels less distance than
blood leaving the left ventricle.
 The blood pumped out from the right ventricle travels to the lungs, whereas blood
like network of protein fibres.
 Red blood cells have no nucleus, no mitochondria, and no endoplasmic
reticulum. The lack of these organelles means that there is more room for
leaving the left ventricle has to travel to the rest of the body to deliver oxygen for
respiration.
 To reach the rest of the body, the blood leaving the left ventricle must be under high
pressure.
haemoglobin, so maximising the amount of oxygen that can be carried by each
 This is generated by the contraction of the muscular walls of the left ventricle.
red blood cell.
 The right ventricle generates less pressure from the contraction of its thinner walls, as
blood only has to reach the lungs.
Structural features of a red blood cell
The cardiac cycle
 Red blood cells are shaped like a biconcave disc.
 The sequence of events that takes place during one heartbeat. Our heart beats
 Red blood cells are very small and have a size of about 7 μm.
 This small size means that no haemoglobin molecule within the cell is very far
around 70 times a minute.
from the cell surface membrane, and the haemoglobin molecules can therefore
Diastole: is when the heart muscle relaxes.
quickly exchange oxygen with the fluid outside the cell.
Systole: is when the heart muscle contracts.
 Because of their size, they can easily squeeze through blood capillaries and
bring oxygen as close as possible to cells that require it.
Volume and pressure changes
 Contraction of the heart muscle causes a decrease in volume in the
corresponding chamber of the heart, which then increases again when the muscle
relaxes.
 Volume changes lead to corresponding pressure changes.
1. When volume decreases, pressure increases.
2. When volume increases, pressure decreases.
Atrial systole
 The stage of the cardiac cycle is when the atria are filled with blood, and muscles in
the walls of the atria contract.
Red blood cells (Erythrocytes)
 The red colour of red blood cells is caused by the pigment haemoglobin, a
globular protein.
 When the walls of the atria contract, atrial volume decreases and pressure increases.
 The pressure in the atria rises above that in the ventricles, forcing the
atrioventricular (AV) valves open.
 Then, blood is forced into the ventricles.
 The main function of haemoglobin is to transport oxygen.
 Red blood cells do not live very long. Old ones are broken down in the liver,
and new ones are constantly made in the bone marrow.
 They do not have nucleus.
Ventricular systole
 The stage of the cardiac cycle is when the ventricles are filled with blood, and the muscles
in the walls of the ventricles contract.
 When the walls of the ventricles contract, ventricular volume decreases and pressure
increases.
 The pressure in the ventricles rises above that in the atria, forcing the AV valves to close,
and preventing backflow.
 The pressure in the ventricles rises above that in the aorta and pulmonary artery.
 This forces the semilunar(SL) valves to open so blood is forced into the arteries and out of
the heart.
Diastole/ cardiac diastole
 The ventricles and atria are both relaxed.
 The pressure in the ventricles drops below that in the aorta and pulmonary artery,
forcing the SL valves to close, and preventing backflow.
 The atria continue to fill with blood via the vena cava and pulmonary vein.
The volume of fluid that leaves the capillary to form tissue fluid is the
result of two opposing forces.
1. Hydrostatic pressure/ Water potential
2. Solute concentration
 Pressure in the atria rises above that in the ventricles, forcing the AV valves open.
 Blood flows passively into the ventricles without the need of atrial systole.
Oedema: is a build-up of fluid in the body
 The cycle then begins again with atrial systole.
Tissue fluid
 The almost colorless fluid that fills the spaces between body cells.
 It forms from the fluid (plasma) that leaks from blood capillaries.
 Tissue fluid is almost identical in composition to blood plasma.
 However, it contains far fewer protein molecules than blood plasma, because
these are too large to escape easily through the capillary endothelium.
 Red blood cells are much too large to pass through, so tissue fluid does not
contain these, but some white blood cells can squeeze through and move
around freely in tissue fluid.
Plasma: a pale yellow liquid blood component, in which the blood cells
float. It carries a very large range of different substances in solution.
Plasma proteins: a range of several different proteins dissolved in the
blood plasma.
Tissue fluid
Control of heartbeat
 Blood comprises about 55% blood plasma and about 45% different types of
Myogenic: a word used to describe muscle tissue that controls heartbeat without
blood cells.
 Over 90% (95%) of blood plasma is water, while less than 10% consists of
dissolved substances, mostly proteins.
any external stimulus.
Sinoatrial node (SAN) or pacemaker: a patch of cardiac muscle in the right atrium
of the heart that contracts and relaxes in a rhythm, sets the pattern for the rest of the
 Over 99% of the solid particles in blood are red blood cells (erythrocytes).
heart muscle.
 The rest are pale or colorless white blood cells (leukocytes) and platelets
Atrioventricular node (AVN): a patch of tissue in the septum of the heart that
(thrombocytes).
transmits the wave of excitation from the walls of the atria and transmits it to the
Purkyne tissue.
Purkyne tissue: a network of Purkyne fibers that carry the cardiac impulse from the
AVN to the ventricles of the heart and cause them to contract.
SAN Atrium AVN Purkyne tissue Ventricle