This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.

Why don't we have cells as big as elephants?
Surface area to volume ratio decreases with size.This limits transport of nutrients and wastes.
Multicellular organisms have a problem getting materials to and from cells.

Many invertebrates have open circulatory systems where some important organs are bathed in blood and diffusion from other tissue allows exchange of nutrients and wastes.
All vertebrates have closed cardiovascular systems that move blood in an orderly and controlled manner around the body.

Although it is called a closed system in fact it is not completely closed. Fluid continually is lost from capillaries and enters the interstitial space between cells, carrying nutrients.
This fluid is returned to the cardiovascular system by the lymph system.
Closed systems are more efficient as it allows greater exchange rates because of the constant flow of blood.

The vertebrate body is approximately 70% water.
This is distributed in the body in three major categories
Intracellular water 45%
Interstitial water 20%
Plasma 5%

Cardiovascular system is responsible (together with the lymphatic system) for moving the water and maintaining homeostasis of the interstitial fluid that baths the cells.
Cells are constantly exchanging ions, gases, nitrogenous waste, amino acids sugars etc. with the interstitial fluid in order to maintain there own internal environments.
Added to the distribution of solutes is the distribution of heat.

Each class of vertebrates has a uniform pattern of circulation but there are substantial differences between classes. A major difference in circulation pattern exists between water breathing fish and air breathers.

Fish
Fish have a two chambered heart with a single atrium and a single muscular ventricle.
Just before the atrium is an enlarged area called the sinus venosus that acts as a reservoir and receiving chamber for blood.
As blood leaves the ventricle in teleosts (bony fish) it passes through the bulbous arteriosis.
The bulbous arteriosus is an area of the aorta with thickened muscular walls which acts to absorb pressure and even blood flow from the ventricle.
In elasmobranchs blood leaving the ventricle passes through the conus arteriosus.
Valves are present on the conus arteriosus that prevent backflow of blood.
Blood flows forward from the heart towards the gills.
After passing through the gills and picking up oxygen it collects in arteries and is distributed to the body.

Amphibians
Along with air breathing amphibians have evolved a split circulation system, one systemic and one pulmocutaneous.
Amphibians have a three chambered heart.
Two atria and a single ventricle.
The left atria receives blood from the lungs and the right atria receives blood from the body.

As the blood passes from the atria to the single ventricle there is relatively little mixing of the blood.
A short conus arteriosus with a spiral valve directs blood as it leaves the ventricle.
Generally the blood from the left atria is directed toward the systemic artery (body) and the blood from the right atria is directed toward the pulmocutaneous artery.

The circulation via the pulmocutaneous artery is unique to amphibians.
It branches into the pulmonary and cutaneous arteries.
This allows the skin to also receive unoxygenated blood and facilitates diffusion across those surfaces.

At times when the lungs are not being used capillary recruitment in the skin provides increased blood to the skin and vasoconstriction decreased flow to the lungs.
With decreased flow from the lungs some of the blood returning from the skin moves to the systemic artery.

Reptiles
Lizards, snakes and turtles
Lizards, snakes and turtles have a three chambered heart with two atria and one partially divided ventricle.
There is a well developed double circulation because under normal circumstances there is little mixing of blood in the ventricle.
These reptiles do have the ability to alter their circulation.

If pulmonary resistance is increased, systemic venous return will not all flow into pulmonary artery.
Some blood will be pumped primarily into the aorta on ventricular contraction.
This is termed the right-to-left shunt. i.e. the lungs are bypassed.
Right-to-left shunt is used by diving reptiles and by others during periods of apnea. Similarly, if resistance is low in the pulmonary circulation, a left-to-right shunt takes place.

Crocodilians
Crocodilians have a four chambered heart.
There are two aortic arches. The left aortic arch originates in the right ventricle and the right aortic arch originates in the left ventricle.
The two aortic arches are joined by the foramen of Panizzae.
The pulmonary artery originates in the right ventricle also.
Under normal(?) conditions, higher pressure from the left ventricle causes pressure in the left aortic arch to close a valve at its junction with the right ventricle and there is no mixing of oxygenated and non-oxygenated blood in the aorta.

During diving events however, muscular constriction of the pulmonary artery causes increased pressure in the right ventricle and blood passes from the right ventricle into the aorta and systemic circulation.
i.e. the lungs are bypassed - right to left shunt.
Shunting of blood has been reported to be as high as 100% in some diving reptiles.
Oxygen in lungs is used as a reservoir and intermittent changes in shunting has been observed. It has also been suggested also as a mechanism for thermoregulation and to prevent oxygen loss through the skin.

Three chambered heart not imperfect version of mammal heart but organ that meets demands for ectotherms.

Birds and mammals
Birds and mammals have a four chambered heart with separate pulmonary and systemic circulations.
Complete separation allows differences in blood pressure in these circulations.

Peripheral resistance is much lower in the pulmonary system.
Mean blood pressure in aorta of humans is approximately 100 mm Hg.
Mean blood pressure in pulmonary artery of humans is approximately 20 mm Hg.

So how does the heart work?
Most study has been done on the mammal heart and we will use that as a model.
The mammalian heart is basically two pumps with two inlets and two outlets.
Blood is expelled when cardiac muscle contracts and the volume inside the chambers decreases.
Blood flow is directed by pressure variances and a series of valves which prevent backflow.

A heartbeat consists of a rhythmical contraction (systole) of the cardiac muscle followed by relaxation (diastole).
Contraction is initiated at the sinoatrial node (sinus venosus in fish).
The sinoatrial (SA) node is the pacemaker. It consists of specialized muscle fibers that have 'leaky' membranes and undergo regular depolarization, i.e. after a refractory period Na⁺ enters the cell depolarizing it toward threshold (spontaneous depolarization).

Action potentials in the SA node spread to other cells via gap junctions. The AP spreads across both atria at a rate of 0.8-1.0 m/s.
Atria are insulated from the ventricles and the AP does not travel unimpeded across the entire heart.
The atrial AP stimulates cells in the atrioventricular node. These are specialized muscle cells that conduct the AP slowly (0.03-0.1m/s) to the ventricle.

This slow down of conduction is important in allowing blood time to fill the ventricles.
The atrioventricular (AV) node cells conduct the AP to other faster conducting cells, the bundle of His, right and left bundle branches and finally to the Purkinje fibers (5m/s).
Ventricular contraction begins about 0.1-0.2 seconds after contraction of the atria.
Purkinje fibers stimulate cardiac muscle and a wave of contraction sweeps across the ventricle from the apex.

An action potential in a cardiac fiber is slightly different from skeletal muscle. Rapid inflow of Na⁺ is followed by a more sustained inflow of Ca⁺⁺, which sustains depolarization and extends the refractory period. Eventually K⁺ channels open and the cell is repolarized. A cardiac muscle twitch will last in the order of 0.3 s.

Electrical activity can be recorded on an electrocardiogram (ECG or EKG). The water in our body make it a good conductor of electricity and the depolarization events of the heart can be measured on the skin surface.
A cardiac cycle produces 3 distinct ECG waves, P, QRS and T.

The P wave represents the spread of atrial depolarization.
QRS wave represents ventricular depolarization.
T wave represents ventricular repolarization

The base heart rate is determined by the pacemaker cells in the SA node.
The SA node is influenced by the autonomic nervous system.
Parasympathetic stimulation via the vagus nerve (ACh) decreases heart rate. ACh increases K⁺ leakage in the pacemaker cells and it therefore takes longer to depolarize to the threshold level.
Norepinephrine from the sympathetic nervous system increases Na⁺ and Ca⁺⁺ conductance and possibly reduces K⁺ outflow during repolarization and increases heart rate.
Epinephrine from the adrenal gland will also have this effect.

Cardiac output is a combination of heart rate and stroke volume.
Heart rate is controlled by autonomic input.
Stroke volume is controlled by 3 factors
1. end diastolic volume - determined by pressure in the veins and atrial contraction.
2. total peripheral resistance
3. force of contraction - autonomic control - norepinephrine increases force of contraction

The Frank-Starling law of the heart describes the intrinsic property of the heart that as end diastolic volume increases, the cardiac muscle is stretched and within normal limits becomes more efficient (remember length of muscle and force of contraction) and force increases to expel contents.

After the blood leaves the heart it enters arteries. The elastic walls of arteries absorb some of the pressure from ventricular contraction and keep the blood flowing during diastole.
Small arteries or arterioles are the site of greatest resistance to blood flow.
Contraction of circular smooth muscle in arterioles is a major control mechanism for blood flow control. There are also circular sphincters in some capillaries.

Capillaries are exchange sites for blood vessels. Combined cross-sectional area of all capillaries is much larger than other blood vessels and blood velocity is correspondingly low.
Capillary diameter is only just big enough to allow passage of red blood cells.
Endothelial cells are very thin to facilitate diffusion. Capillaries often have pores or fenestra to increase allow secretion of plasma.

Solutes with small molecular weight diffuse from the blood.
Most proteins are held in capillaries.
Some fluid forced through endothelium (semipermeable membrane) by blood hydraulic pressure.
Colloidal osmotic pressure caused by proteins in blood increases as fluid is lost until equilibrium achieved.
Plasma proteins essential to retain fluid in blood.

Capillaries not always filled with blood.
Blood flow to skin very variable and used for thermoregulation.
Blood flow to muscle can increase by 30x during exercise in human athlete.

Secretion of plasma with its accompanying dissolved nutrients is important for our cells.There is 4 times the amount of fluid in the interstitial fluid than in out blood vessels.
Each of our cells needs the homeostatic balance of the interstitial fluid to provide a healthy environment.
The interstitial fluid is drained by the lymphatic system. Everywhere there are capillaries there are lymph vessels.

Lymph nodes are expanded areas of lymph vessels the contain many phagocytes and lymphocytes.
Lymph vessels drain back into the subclavian veins.

Blood leaving the capillary beds enter venules and then veins.
Blood pressure is very low in veins and blood tends to gather here. About 60-70% of blood is in the veins and it is somewhat of a reservoir that can be mobilized by contraction of these blood vessels
Valves are present to prevent backflow and skeletal muscle contraction is important to move the blood.

In general (as will be discussed later) smaller animals require more oxygen per unit body mass than larger animals.

How is this achieved?

Body size and heart size.
Heart size is proportional to body size in all classes of vertebrates except birds where it is proportionally smaller in larger animals.
Mammals are the most studied group and the heart appears to be about 0.59% of body mass.
Reptiles - 0.51% of body mass
Amphibians - 0.46% of body mass
Fish - 0.2% of body mass
1 kg bird - 0.82% of body mass

Body size and heart rate
This phenomenon is best studied in mammals.
Resting heart rate is inversely proportional to body mass.
f_h = 241M_b^-0.25
Proportional decrease is the same as oxygen demand at rest.
3000 kg elephant has an at rest heart rate of 25 bpm
3 g shrew has an at rest heart rate of over 600 bpm

Increased oxygen demand
During activity the increased oxygen demand is met in part by increased cardiac output. This can be achieved by increased stroke volume and/or increased heart rate.
Cardiac output does not increase in proportion to oxygen consumption.
Balance of oxygen demand supplied by greater extraction of oxygen from blood.

During a 10x increase in oxygen consumption a pigeon increases heart rate by 6x to achieve 6x increase in cardiac output. Stroke volume declines slightly.
During an 8x increase in oxygen consumption a trout increases heart rate by 1.4x to achieve 3x increase in cardiac output. Stroke volume increases by 2.2x. Oxygen extraction increases by 3x.

Average human increase oxygen consumption by about 12x during hard exercise. Heart rate increases 2.5x, stroke volume increases about 1.5x, oxygen extraction increases by 3x.

Return to Index

This page was created by Peter King. Please contact the author at pking@fmarion.edu with comments.
http://people.fmarion.edu/pking/vertphys/circulation.html
Last edit January 10, 2011.
Copyright Peter King