Organisms must exchange materials and energy with its environment and this exchange ultimately occurs at the cellular level.
Cells live in aqueous surroundings.
The materials they need must move across the plasma membrane into the cytoplasm, and metabolic wastes must move out.
CIRCULATORY SYSTEMS REFLECT PHYLOGENY
Internal transport and gas exchange are functionally related in most animals.
Diffusion is not enough for transporting substances over long distances (more than a few millimieters) in animals.
The time it takes to diffuse a substance is a function of the square of the distance.
The same amount will take 1 sec to diffuse 1μm; 100 sec for 1 mm; 3 hours for 1 cm.
INVERTEBRATES
In all animals, fluid between the cells, called interstitial fluid or tissue fluid, bathes the cells and provides a medium for diffusion of oxygen and nutrients.
Sponges, cnidarians, ctenophorans, platyhelminthes, etc. depend on diffusion for internal transport.
The gastrovascular cavity serves for both digestion and distribution of substances throughout the body.
Triploblastic animals with more complex body plan use open and closed circulatory systems.
Both systems have a circulatory fluid (blood), a set of tubes (blood vessels) and a pump
(heart).
Heart creates blood pressure that acts as the motive force to move the fluid through the set of tubes.
Arthropods and mollusks have an open circulatory system .
Blood flows into a hemocoel bathing the tissues directly.
The hemocoel is made of spaces or sinuses that surround the organs. The hemocoel is not part of the coelom.

Hemolymph : blood and interstitial fluid are indistinguishable.

When the heart contracts, hemolymph is pushed out of the tubes into the sinuses; when the heart relaxes, hemolymph is pulled into the tubes through openings called ostia .

Hemocyanin : an oxygen-transporting pigment found in some mollusks and arthropods; contains copper.

Some invertebrates (e.g. cephalopods, echinoderms, annelids) and vertebrates have a closed circulatory system.

Blood is confined to the vessels.

Blood is different from the interstitial fluid.

Chemical exchange occurs between the blood and interstitial fluid.
Nemerteans have a primitive circulatory system that is closed but does not have a pumping organ. Blood moves depending on the movements of the animal and contractions in the wall of the large blood vessels.
VERTEBRATES
Functions of the vertebrate circulatory system:
1.
Transports oxygen, metabolic wastes, nutrients and hormones.
2.
Helps maintain fluid balance.
3.
Defends the body against invading microorganisms.
4.
Distributes metabolic heat to maintain normal body temperature.
5.
Helps maintain appropriate pH.
Exchange of materials occurs through the thin wall of capillaries.
The human circulatory system is also known as the cardiovascular system .
Circulatory systems consists of
1.
Blood , a connective tissue made of cells, cell fragments and a fluid known as plasma.
2.
The heart consists of one or two atria , which receive the blood, and one or two ventricles , which pump the blood.
3.
A system of blood vessels or spaces through which the blood circulates: arteries, veins and capillaries forming networks or capillary beds .
1.
In fish there is one atrium and one ventricle and blood flows in a single circuit .
Atrium

ventricle

aorta

gill capillaries

organ capillaries

atrium

Blood must pass through two capillary beds in each circuit. Blood pressure drops substantially and oxygen-rich blood flows slowly through the system.
2.
In amphibians there are two atria and one ventricle.

Systemic and pulmonary circulation, a double circuit .
Ventricle

aorta

body capillaries

veins

right atrium

ventricle

pulmonary artery

lung and skin capillaries (pulmocutaneous)

veins

left atrium

ventricle

Oxygen-poor blood is pumped out the ventricle before the oxygen-rich blood enters it.

3.
Reptiles have a double circuit blood flow and the ventricle is partly divided .

Some mixing of blood occurs.

Ventricle sides contract at different times: oxygen-rich blood is diverted to the systemic flow, and oxygen-poor blood is diverted to the pulmocutaneous circulation.

Crocodiles have two ventricles.
4.
In crocodilians, birds and mammals the heart ventricles are separated. There are two ventricles.
Body capillaries

veins

right atrium

right ventricle

pulmonary arteries

lung capillaries

pulmonary veins

left atrium

left ventricle

aorta

body organs
 veins

right atrium...

The left side of the heart receives and pumps oxygen-rich blood; the left side of the heart receives and pumps oxygen-poor blood.

Endotherms consume more energy than ectotherms and circulatory system must deliver about 10 times more oxygen and fuel to their tissues than ectotherms of equal size.
Mammalian Circulation: The Pathway - Double Circulation In Mammals
Systemic circulation delivers blood to the tissues.

Coronary arteries feed the heart.

Carotid arteries bring blood to the brain.

Subclavian arteries to the shoulder region and arms.

Mesenteric arteries to the intestines.

Renal arteries to the kidneys.

Iliac arteries to the legs.
Four arteries deliver blood to the brain: two carotids and two vertebral arteries.
Blood returns to the heart in veins.

The superior vena cava collects blood from jugular and subclavian veins drain the brain and arms.

Renal , iliac and hepatic veins empty into the inferior vena cava .

Coronary capillaries empty in the coronary veins, which in turn join to form a large vein, the coronary sinus that empties directly into the right atrium.
The hepatic portal system delivers nutrients to the liver.

The hepatic portal system delivers blood rich in nutrients to the liver.

Blood flows from the liver to the small intestine through the superior mesenteric artery .

Blood flows through the capillaries of the intestine and collect glucose, amino acids and other nutrients.

This blood passes to the mesenteric vein and then into the hepatic portal vein , which delivers the nutrient rich blood to the liver.
Mammalian Heart

A sac of connective tissue, the pericardium , protects human heart.
The inner surface of the pericardium and outer surface of the heart are covered by a smooth layer of endothelium.
The space in between the pericardial cavity is filled with a fluid, which reduces friction during heartbeats.
The heart consists of two atria , which receive the blood, and two ventricles , which pump the blood.
On the upper surface of each atria lies a small muscular pouch called the auricle .
1.
The right atrio-ventricular valve (AV) or tricuspid valve controls the blood flow between the right atrium and right ventricle.
2.
The left AV is called the mitral valve .
3.
The cordae tendinae attach the valves to the papillary muscles of the heart.
4.
The semilunar valves guard the exits from the heart: aortic and pulmonary valves.
When the heart contracts it pumps blood, when it relaxes is fills with blood. One complete sequence of pumping and filling is called the cardiac cycle .
The contraction of the heart is called systole , and the relaxation of the heart is known as diastole .
When the semilunar valves do not close tightly during diastole, the blood flows back with a hiss known as a heart murmur .
Cardiac output is the volume of blood pumped by the left ventricle into the aorta in one minute.
The stroke volume is the volume of blood pumped into the aorta during one beat, ml/stroke.

The average stroke volume is 75ml.
Heart rate is the number of contractions per minute, strokes/min.

Cardiac output = stroke volume X heart rate, ml/min.
Pulse is the rhythmic stretching of the arteries caused by the blood pressure due to the contraction of the ventricles.
The electrical activity of the heart spreads through the body fluids to the body surface and can be recorded in a graph called the electrocardiogram (ECG or EKG) .
The oscilloscope and the electrocardiograph are the instruments used to record and monitor the heart activity.

A heart murmur is a hissing sound caused by a defect in one of the valves. A stream of blood squirts back through the valve.
Heartbeat
The heart is capable of beating independently of the nervous system. It is called a myogenic heart.
Most arthropod hearts beat under the control of the motor nerves outside of the heart; they are called neurogenic hearts .
At the end of cardiac muscle cells there are dense bands called intercalated discs , gap junctions in which two cells are connected through pores.

The sinoatrial node (SA) or pacemaker initiates the heartbeat. It is located near the point where the superior vena cava enters the right atrium.

Cardiac muscle cells branch and interconnect via intercalated discs.

Because cardiac muscle cells are coupled by the gap junctions of the intercalated discs, the electrical impulse they produce spread rapidly through the wall of the atria making them contract in unison.

Atrial muscle fibers conduct the action potential to a relay center, the atrioventricular node located in the wall between the right atrium and the right ventricle, on the lower part of the septum.

The AV bundle divides in two branches sending each branch into each ventricle through the ventricular wall.
Check the following sites about the Purkinje fibers:

http://en.wikipedia.org/wiki/Bundle_of_His http://www.med.uiuc.edu/histo/large/atlas/objects/642.htm
http://www.bioeng.auckland.ac.nz/anatml/anatml/database/cells/cells/parts/part/part_22.html
Structure of veins and arteries
1.
Arteries carry blood away from the heart.
2.
Veins carry blood to the heart.
3.
Capillari es are thinwalled vessels through which materials pass back and forth between blood and tissues.
4.
Smaller secondary branches of arteries are called arterioles , and of veins venules .
Notice that arteries and veins are distinguished by the direction in which they carry blood and not by the characteristic s of the blood.

Veins and arteries have three layers of tissues.

Tunica intima consists of squamous epithelium ( endothelium ).

Tunica media is made of connective tissue and smooth muscle .

Tunica adventitia consists of connective tissue rich in elastic and collagen fibers.
The smooth muscle in the wall of arteries can constrict ( vasoconstriction ) or dilate
( vasodilation ).
The thick wall of the arteries and veins prevent gases from passing through.
Capillaries form a network between arterioles and venules.
Precapillary sphincters are located whenever a capillary branches off a metarteriole. These sphincters open and close continuously to direct blood to needed sectors of the tissues.
Vasoconstriction and vasodilation help maintain the appropriate blood pressure and control the volume of blood passing to a particular tissue.
Changes in blood flow are regulated by the autonomic nervous system in response to metabolic needs of tissues.
Blood flow
Blood flow follows the law of continuity :

If a pipe's diameter changes over its length, a fluid will flow through narrower segments of the pipe faster than it flows through wider segments.

The volume of flow per second must be constant through the entire pipe.
Blood flows faster in the aorta than in a capillary.
The total cross-sectional area of the capillaries determines the rate of flow.
The number of capillaries is so great that the total cross-sectional area is much greater in capillary beds than in any other part of the circulatory system.
Capillaries are the only vessels with walls thin enough to permit the transfer of substances between the blood and interstitial fluid. The slow blood flow enhances the exchange.
Blood pressure
Fluids flow from the area of greater pressure to the area of lower pressure.
Blood pressure is the hydrostatic pressure the blood exerts against the wall of the vessel and that propels the blood.
Blood pressure is greater in arteries than in veins.
When the heart contracts, the blood pressure is the highest in the arteries: systolic pressure .
Resistance is the opposition to flow. It depends on...


Blood viscosity: the greater the viscosity, the greater the resistance.

Total blood vessel length: the longer the vessel, the longer the resistance.

Blood vessel diameter: the smaller the vessel, the greater the resistance.
Peripheral resistance refers to the friction the blood encounters when it flows through the circulatory system.
As the elastic arteries return to their more relaxed condition during diastole, the pressure in the circulatory system is maintained. This is called the diastolic pressure .
120 diastolic pressure (diastole: relaxation of the heart muscles)
80 systolic pressure (systole: contraction of the heart muscles)
Nerve impulses and hormones control the arteriole muscles and vasodilation or vasoconstriction.
Cardiac output is adjusted in concert with changes in peripheral resistance. This coordination maintains a constant blood flow.
Blood flow through capillaries
Capillaries in the brain, heart, liver and kidneys are usually filled to capacity.
Nerve impulses and hormones control the contraction mechanism of the smooth muscles of arterioles and precapillary sphincters , and the distribution of blood in the capillary beds.

After a meal blood is diverted to the digestive track.

During strenuous exercise, blood flow to the muscles increases.
Capillary exchange
All exchange of substances takes place across the thin endothelial wall of the capillary.
Some material may be transported across the wall cell by endocytosis and then released at the other end by exocytosis.
Small molecules like oxygen and carbon dioxide diffuse down the concentration gradient.
Diffusion also occurs through intercellular spaces.
Capillary pressure pushes out fluid with sugars, salts, oxygen and urea into the interstitial fluid.
Blood cells and blood proteins are too large to pass through the endothelium causing an increase in solute concentration (osmolarity).
Water is recuperated downstream near the venule end of the capillary. About 85% of the fluid is recuperated that way. The remaining 15% is returned to the blood stream by the lymphatic system.
LYMPHATIC SYSTEM

The lymphatic system is an accessory circulatory system, which...
1.
Collects and returns interstitial fluid to the blood.
2.
Defends against disease-causing organisms.
3.
Absorb lipids from the small intestine.
The lymphatic system consists of...

Lymphatic vessels that conduct lymph .

Lymphatic tissue organized into lymph nodes and nodules .

Tonsils, thymus gland and spleen.
Interstitial fluid enters the lymph capillaries and is called lymph.
Lymph capillaries are dead-end and extend into almost all tissues of the body.
Lymph capillaries join to form large lymphatics (lymph veins).

Thoracic duct empties the lymph into the left subclavian vein.

Right lymphatic duct empties into the right subclavian vein
Valves within the lymph veins prevent the lymph from flowing backwards.
When blood enters the capillaries under pressure some plasma and proteins filters out into the tissues forming the interstitial fluid .
Only about one fourth of the blood proteins pass into the tissues.
Lymph capillaries are made of overlapping cells that separate under pressure allowing excess interstitial fluid and proteins in it to enter and drain the tissue.
Obstruction of the lymph vessels causes edema , the swelling that occurs due to the accumulation of interstitial fluid.
BLOOD
In invertebrates with an open circulatory system, blood is no different from interstitial fluid, the hemolymph.
Animals with a closed circulatory system have blood, which is different from the interstitial fluid.
Plasma
Blood is a type of connective tissue containing different kinds of cells suspended in a liquid matrix, the plasma .
Plasma makes about 55% of the blood. The remaining 45% are made up of blood cells and platelets.

Plasma is about 92% water, 7% proteins and the rest consists of nutrients, organic wastes and electrolytes (ions).
Blood makes up about 8% of the body weight.
Humans have 4 to 6 liters of blood.
The plasma contains ions, nutrients, wastes, hormones and respiratory gases.
The plasma and interstitial fluid are similar in composition except that the plasma contains a higher protein concentration than the interstitial fluid.
When proteins involved in blood clotting have been removed from the blood; the remaining liquid is called serum .
Ions in the plasma help maintain the osmotic balance on the blood, and contribute to buffer the blood that usually has a pH of 7.4 in humans.
Plasma proteins act as buffers in order to maintain a constant pH of 7.4
.
Globulins are of three kinds:

HDL, high-density lipoproteins, transport fats and cholesterol.

Some globulins are lipoproteins that bind to minerals, vitamins, lipids and cholesterol to dissolve and transport.

Other globulins are antibodies that provide immunity against certain diseases.

Globulins make up 33% of the plasma proteins.
Albumins help to regulate the amount of fluid in the plasma and interstitial fluid and help maintain osmotic pressure and proper blood volume. They constitute 60% of plasma proteins.
Fibrinogen and prothrombin function in the clotting reaction.

When proteins involved in blood clotting have been removed from the blood; the remaining liquid is called serum .
Blood cells
Red blood or erythrocytes cells (RBC) transport oxygen and carbon dioxide.

Made in the bone marrow ribs, long bones, vertebrae and skull bones.

5.4 million/
 l (mm 3 ) in men and 5.0 million/
 l (mm 3 ) in women.

Lack nucleus and mitochondria, and live for about 120 days.

Liver and spleen remove old RBC from circulation.

Hemoglobin is the oxygen transporting protein; contains Fe.

Hemoglobin binds to O
2
and NO (nitric oxide); NO relaxes the walls of the capillaries and helps in the diffusion of oxygen.
White blood cells or leukocytes (WBC) defend the body against disease-causing microorganisms.


About 7,000 cells/
 l (mm 3 ) in human blood. 5,000 - 10,000 cells on the average. The number increases temporarily during infections.

Made in the bone marrow.

Travel in the blood stream for a short time and can migrate across the endothelial lining of the capillaries.

Their collective function is to fight infections.
Platelets or thrombocytes function in blood clotting.
They are pinched off from very large cells called megakaryocytes in the red bone marrow.
Cell fragments containing enzymes; they are 2 to 3 μm.
Lack nucleus.
About 300,000 platelets/
 l.
Stem cells and the replacement of cellular elements
Cellular elements: erythrocytes, leukocytes and platelets.
Cellular elements wear out and are replaced constantly throughout the person's life.
Phagocytes in the spleen and liver destroy red blood cells. Their chemicals are recycled into new cellular material.
Erythrocytes, leukocytes and platelets develop from a population of cells called pluripotent stem cells found in the red bone marrow of long bones, ribs, vertebrae, breastbone and pelvis.
Pluripotent cells have the potential to differentiate into any kind of cells or into cells that produce platelets.
The population of stem cells renews itself while replenishing the cellular elements.
RBCs are produced via a negative feedback mechanism. When the oxygen reaching the tissues is low, the kidney converts a plasma protein to a hormone called erythropoietin , which stimulates production of erythrocytes.
Blood clotting
Coagulation:
Fibrinogen and prothrombin are proteins found in the plasma.
Platelets release several factors that combine with Ca 2+ in order to convert prothrombin to the active enzyme thrombin .
Thrombin then converts the soluble protein fibrinogen into the insoluble fibrin.
Fibrin polymerizes and sticks to the damaged surface forming a web. RBC and platelets get trapped in the web and form the clot.
There are more than 30 factors interacting during the clotting process.

The absence of one of these factors due to genetic mutation is the cause of hemophilia .
Cardiovascular diseases
Diseases of the heart and blood vessels usually result in a heart attack or a stroke.
Heart attack is the death of heart muscle due to oxygen deprivation.
Stroke is the death of brain tissue, usually caused by the rupture or blockage of an artery.
Heart attacks and strokes are frequently caused by a clump of platelets and fibrin (blood clot or thrombus) that is formed within the blood vessels.
The traveling clot is called an embolus , and can get trapped in an artery that is too narrow for it to pass. The artery gets clogged and the blood flow stops to the tissues downstream, which become deprived of oxygen and begin to die.
Atherosclerosis is often the cause of thrombus formation. Plaques form when fibrous connective tissue and lipids partially close the lumen of the artery.
If the plaque becomes hardened with calcium deposits, the disease is called arteriosclerosis .
The rough walls formed by the plaques facilitate the clustering of platelets and the formation of blood clots.
Hypertension or high blood pressure promotes atherosclerosis and increases the risk of heart attack and stroke.

Low-density lipoproteins or LDL tend to deposit cholesterol and build plaques in the arteries.

High-density lipoproteins or HDL reduce cholesterol deposition.
Desirable — Less than 200 mg/dL
Borderline high risk — 200–239 mg/dL
High risk — 240 mg/dL and over
Your LDL cholesterol level
Your LDL cholesterol level greatly affects your risk of heart attack and stroke. The lower your LDL cholesterol, the lower your risk. In fact, it’s a better gauge of risk than total blood cholesterol. Your LDL cholesterol will fall into one of these categories:
LDL Cholesterol Levels
Less than 100 mg/dL Optimal
100 to 129 mg/dL
130 to 159 mg/dL
160 to 189 mg/dL
Near Optimal/ Above Optimal
Borderline High
High
190 mg/dL and above Very High

http://www.americanheart.org/presenter.jhtml?identifier=183
GAS EXCHANGE IN ANIMALS
The exchange of gases between an organism and its environment is called respiration .

Organismic respiration brings oxygen from the environment to the cells.

Aerobic respiration occurs within the cell in the mitochondria.

Oxygen is taken in by the organism and carbon dioxide is released into the environment.
The respiratory medium is the source of oxygen: air or water depending on the habitat of the organism.
Ventilation is the movement of air or water over the respiratory surfaces.
The tissue where the gas exchange takes place is called the respiratory surface .
Gas exchange occurs entirely by diffusion .
The rate of diffusion is proportional to the area of the respiratory surface and inversely proportional to the square of the distance (thickness) through which the molecules must move.
Respiratory surfaces are thin and with large areas.
In order for oxygen and carbon dioxide to diffuse across a cell membrane, they must dissolve in water.
Respiratory surfaces must be maintained moist and air has to pass through a long series of tubes to reach these surfaces.
Living cells must be bathed is water in order to maintain their plasma membrane.
TYPES OF RESPIRATORY SURFACES
1.
Body surface. Used by protists and small animals with low metabolic rate, e. g. sponges, cnidarians and flatworms.
2.
Gills containing capillaries.

Outfoldings of the body surface that are suspended in the water.

Specialized body appendages drive water over the gill surface: increase ventilation.

Water is more viscous and dense than air and the aquatic animal must spend a lot of its energy moving water over the gills.

Aquatic animals spend 20% of its energy while terrestrial animals spend 1 -2 % of its total energy.

Echinoderms have dermal gills.

Chordates usually have internal gills.

In bony fish the gills are protected by a bony plate, the operculum


Counter current system is an efficient method of obtaining oxygen; more than 80% of the oxygen in the water can be removed by this method.
3.
Tracheal system is an adaptation to terrestrial living that delivers oxygen to all parts of the body.

Air contains much larger concentration of oxygen than water.

Terrestrial animals have to compensate for water loss during breathing. By having the respiratory surface inside the body, evaporation is decreased.

It consists of a network of tracheal tubes or tracheae that deliver air directly to the body cells through the tiny final branches of the tracheoles .

All body cells lie within a very short distance (μm) of a tracheole.

The tracheae open on the body surface through up to 20 tiny openings called spiracles .

Large insects and flying insects enhance ventilation by using muscles like bellows to pump air into the tracheal tubes.
4.
Lungs formed by in-growth of the body surface or from the wall of the body cavity.

Lungs are in one location.

The circulatory system must transport gases from the lungs to other parts of the body.
Spiders have "book lungs".

Located in an inpocketing of the abdominal wall.

Open to the outside by a spiracle.

A series of plates rich in hemolymph separated by air spaces.
Osteichthyes have a swim bladder.

It is used to control buoyancy.

Lungfishes use it breath air at certain times in their life cycle.
Amphibians and reptiles have simple lungs.

The lungs of toads and frogs are simple sacs with ridges that increase the respiratory surface.

Some amphibians do not have lungs and exchange gases through the skin.

Reptiles have sacs with folding of the wall to increase the respiratory surface.
HUMAN RESPIRATORY SYSTEM
The human respiratory system is typical of air-breathing vertebrates.
Nostrils are the opening of the nose.
Nasal cavities moisten, warm and filter the air.
Pharynx or throat is used also by the digestive system.
Larynx also called "voice box" contains the vocal cords and is supported by a cartilage.

Epiglottis is a small flap of tissue that closes the larynx during swallowing.
Trachea or windpipe is supported by rings of cartilage.
Bronchi are branches of the trachea that lead to each lung.

Both trachea and bronchi are lined with a mucous membrane containing ciliated cells.

Mucus traps dust, pollen, bacteria and other particles.
Alveoli are tiny air sacs at the end of the bronchioles and are lined with a very thin epithelium.
Capillaries surround the alveoli.
Gas exchange occurs in the alveoli of the lungs.
The lungs as such consist mostly of air tubes and elastic tissue with a very large internal surface.
Bronchioles and alveoli make most the lungs .
Each lung is covered with a pleural membrane , which also lines the thoracic cavity.
Pleural cavity is the space in between the pleural membranes and it is filled with a fluid.

The pleural fluid provides lubrication between the lungs and the body wall.
Passage of air:
Nostrils → nasal cavities → pharynx → larynx → trachea → bronchi→ bronchioles → alveoli.
BREATHING
Ventilation is accomplished by breathing.
Breathing is the mechanical processes of moving air from his environment into the lungs
( inspiration ) and expelling the air from the lungs ( expiration ).
Amphibians like frogs ventilate their lungs by positive pressure .

The lowering of the buccal cavity draws air through the nostrils into the mouth. With the nostrils and mouth closed, the frog raises the floor of the oral cavity and air is forced down the trachea. Compression of the body wall and elastic recoil of the lungs force air back out of the lungs during exhalation.
Mammals ventilate by negative pressure .
During inspiration, the volume of the thoracic cavity is increased by contraction of the diaphragm .


Contraction moves the diaphragm downward increasing the volume of the thoracic cavity.

The pressure of the air in the lungs decreases by 2 or 3 mm Hg below the atmospheric pressure.

With the increase in volume in the thoracic cavity, the pressure drops and air is forced in by the atmospheric pressure.
Expiration occurs when the diaphragm relaxes and moves up .
Diaphragm and rib muscles account for shallow breathing. During vigorous exercise, other muscles of the neck, back and chest further increase ventilation volume by raising the rib cage even more.
The normal amount of air inhaled at rest is called tidal volume : ~ 500 ml.
The vital capacity is the maximum amount of air a person can exhale after filling the lungs to the maximum extent. It brings about ~3.4 liters for college females and ~4.8 liters for males.
After forcefully exhaling, the alveoli remain inflated with the residual volume of air that cannot be expelled.
Birds have the most efficient respiratory system of any living vertebrate.
Their lungs have air sac extensions that reach into many parts of the bird's body.
BREATHING CONTROL CENTERS
Breathing is regulated by respiratory centers in the pons, medulla oblongata and in the walls of the carotid arteries and aorta .
Neurons originating in the medulla send messages to the diaphragm and external intercostal muscles causing them to contract and inspiration occurs.
Negative feedback mechanism prevents our lungs from overexpanding; stretch sensors in the lung tissue send nerve impulses back to the medulla, inhibiting its breathing control center.
After several seconds the neurons become inactive, the muscles relax, and expiration occurs.
Chemoreceptors sensitive to increases in CO
2
and H
+
and to low O
2
concentrations regulate the respiratory centers.
The medulla control center maintains homeostasis by monitoring the amount of CO
2
in the blood.
Slight drop in the pH of the blood and cerebrospinal fluid means an increase in CO
2
in the tissues and blood.
The medulla registers these changes, and increases the depth and rates of breathing so the excess of CO
2
is eliminated in exhaled air.
Oxygen concentration generally does not play an important role in breathing regulation.

Only if the partial pressure of oxygen drops markedly, the aortic and carotid centers become stimulated to send messages to the respiratory centers in the brain, and breathing increases.
PRESSURE GRADIENT AND THE DIFFUSION OF GASES
Oxygen transport
The difference in partial pressure of oxygen between the inhaled air and the blood allows the oxygen to diffuse.
Oxygen makes 21% (by volume) of the atmosphere.
The atmosphere exerts a pressure of 760 mm Hg at sea level.
The partial pressure of oxygen is 760 mm Hg x 0.21 = 160 mm Hg.

P ox
= 160 mm in air and P ox
= 40 mm in venous blood.

Therefore oxygen diffuses into the blood.

P ox
= 100 mm in arterial blood; P ox
= 0 - 40 mm in tissues.

Therefore oxygen diffuses into the tissues.

Fick's law : the greater the partial pressure difference and the larger the surface area, the faster the gas will diffuse.
Respiratory pigments, gas transport and blood buffering.
Most animals would not be able to support their oxygen demands without the help of respiratory pigments.
Hemoglobin increases the capacity to transport oxygen by about 75 times.
There are several types of hemoglobin.

Consists of four subunits that work cooperatively: when one subunit binds to oxygen or releases it, the other three follow quickly.

All contain iron as part of a heme group.

Heme group is bound to a protein called globin.

Protein portion varies in size and AA in different species.

It is present in some invertebrates like annelids, nematodes, mollusks and arthropods.

Hemocyanins are copper containing proteins found in arthropods and some mollusks.

Lack heme group.

Copper-containing proteins.

Dissolved in the blood rather than contained in cell.

Blue when combined with oxygen; without oxygen is colorless.

It is found many species of mollusks and arthropods.
About 97% of the oxygen is transported as oxyhemoglobin and 3% dissolves in the plasma.

About 200 ml of O
2
per liter of blood in mammals
The maximum amount of oxygen that can be transported by hemoglobin is called the oxygen carrying capacity .
The actual amount of oxygen bound to hemoglobin is the oxygen content .
The ratio of oxygen content to oxygen capacity is the percent oxygen saturation .
The conformation of the hemoglobin protein is sensitive to pH.

The affinity of hemoglobin for oxygen decreases.

Oxyhemoglobin dissociates faster in an acid medium.

Bohr Effect (Bohr Shift) : Changing the blood pH affects the % of O
2
saturation of blood.

Carbon dioxide reacts with water to form carbonic acid.

An active tissue will lower the pH of its surroundings and cause hemoglobin to release more oxygen.
Carbon dioxide transport.
Carbon dioxide is transported mainly as bicarbonate ions.

About 70% of the CO
2
dissolves in the plasma and forms HCO
3
-
and H
+
lowering the pH.


About 7% - 10% dissolves in the plasma.

About 20% - 23% enter the red blood cells and combines with hemoglobin forming carbaminohemoglobin.
This reaction occurs in the RBC catalyzed by carbonic anhydrase.
Carbonic
anhydrase
CO
2
+ H
2
O -------→ H
2
CO
3
→ H + → HCO
3
-
Most of the H
+
released from carbonic acid combine with hemoglobin and do not change the pH of the blood.
Many of the bicarbonate ions leave the RBCs and diffuse into the plasma.
As CO
2
diffuses out of the alveolar capillaries, the resulting lower CO
2
concentration reverses the previous reaction.
Deep diving
Diving mammals have high concentration of myoglobin, an oxygen binding pigment found in muscles.
Myoglobin stores oxygen in diving mammals up to ten times more than in terrestrial mammals.

Weddell seal store about 25% of its oxygen in muscle, compared to only 13% in humans.

Dive down to 500 meters and remain underwater for about 20 minutes; sometimes for more than one hour.

After 20 minutes the supply of oxygen in myoglobin is used up, and ATP is derived from fermentation.

Elephant seal dive to depth of 1,500 m (almost 1 mile) and remain submerged for two hours.
Diving reflex reduces the heart rate, blood is redistributed and other physiological changes occur that allow the diving mammal to conserve oxygen.