6.2 The Blood System Flashcards
Functions of blood
- transport: red blood cells (erythrocytes) transport oxygen from lungs to cells, blood plasma transports nutrients, carbon dioxide, antibodies, urea. Heat is transported from parts of the body that produce it, to the skin, where it is lost.
- defense against infectious diseases: leukocytes (white blood cells), blood clotting
Double circulation
humans, mammals and birds have a double circulation system with a four chambered heart
fish have a single circuit and repties have double circuit but only three chambers
The human heart is a four chambered organ, consisting of two atria and two ventricles
The atria act as reservoirs, by which blood returning to the heart is collected via veins (and passed on to ventricles)
The ventricles act as pumps, expelling the blood from the heart at high pressure via arteries
The reason why there are two sets of atria and ventricles is because there are two distinct locations for blood transport
The left side of the heart pumps oxygenated blood around the body (systemic circulation)
The right side of the heart pumps deoxygenated blood to the lungs (pulmonary circulation)
There is therefore a separate circulation for the lungs (right side of heart) and for the rest of the body (left side of heart)
The left side of the heart will have a much thicker muscular wall (myocardium) as it must pump blood much further
Discoveries of William Harvey
Prior to Harvey’s findings, scientists held to the antiquated views of the Greek philosopher Galen, who believed that:
Arteries and veins were separate blood networks (except where they connected via invisible pores)
Veins were thought to pump natural blood (which was believed to be produced by the liver)
Arteries were thought to pump heat (produced by the heart) via the lungs (for cooling – like bellows)
Based on some simple experiments and observations, Harvey instead proposed that:
Arteries and veins were part of a single connected blood network (he did not predict the existence of capillaries however)
Arteries pumped blood from the heart (to the lungs and body tissues)
Veins returned blood to the heart (from the lungs and body tissues)
Arteries structure and function
The function of arteries is to convey blood at high pressure from the heart ventricles to the tissues of the body and lungs
To this end, arteries have a specialised structure in order to accomplish this task:
They have a narrow lumen (relative to wall thickness) to maintain a high blood pressure (~ 80 – 120 mmHg)
They have a thick wall containing an outer layer of collagen to prevent the artery from rupturing under the high pressure
The arterial wall also contains an inner layer of muscle and elastic fibres to help maintain pulse flow (it can contract and stretch)
Flow of blood - arteries
Blood is expelled from the heart upon ventricular contraction and flows through the arteries in repeated surges called pulses
This blood flows at a high pressure and the muscle and elastic fibres assist in maintaining this pressure between pumps
The muscle fibres help to form a rigid arterial wall that is capable of withstanding the high blood pressure without rupturing
Muscle fibres can also contract to narrow the lumen, which increases the pressure between pumps and helps to maintain blood pressure throughout the cardiac cycle
The elastic fibres allow the arterial wall to stretch and expand upon the flow of a pulse through the lumen
The pressure exerted on the arterial wall is returned to the blood when the artery returns to its normal size (elastic recoil)
The elastic recoil helps to push the blood forward through the artery as well as maintain arterial pressure between pump cycles
Veins structure and function
The function of veins is to collect the blood from the tissues and convey it at low pressure to the atria of the heart
To this end, veins have a specialised structure in order to accomplish this task:
They have a very wide lumen (relative to wall thickness) to maximise blood flow for more effective return
They have a thin wall containing less muscle and elastic fibres as blood is flowing at a very low pressure (~ 5 – 10 mmHg)
Because the pressure is low, veins possess valves to prevent backflow and stop the blood from pooling at the lowest extremities
Flow of blood - veins
Blood is at very low pressure in the veins which can make it difficult for the blood to move against the downward force of gravity
The veins contain numerous one-way valves in order to maintain the circulation of blood by preventing backflow
Veins typically pass between skeletal muscle groups, which facilitate venous blood flow via periodic contractions
When the skeletal muscles contract, they squeeze the vein and cause the blood to flow from the site of compression
Veins typically run parallel to arteries, and a similar effect can be caused by the rhythmic arterial bulge created by a pulse
Capillaries structure and function
The function of capillaries is to exchange materials between the cells in tissues and blood travelling at low pressure (<10mmHg)
Arteries split into arterioles which in turn split into capillaries, decreasing arterial pressure as total vessel volume is increased
The branching of arteries into capillaries therefore ensures blood is moving slowly and all cells are located near a blood supply
After material exchange has occurred, capillaries will pool into venules which will in turn collate into larger veins
Capillaries have specialised structures in order to accomplish their task of material exchange:
They have a very small diameter (~ 5 µm wide) which allows passage of only a single red blood cell at a time (optimal exchange)
The capillary wall is made of a single layer of cells to minimise the diffusion distance for permeable materials
They are surrounded by a basement membrane which is permeable to necessary materials
They may contain pores to further aid in the transport of materials between tissue fluid and blood
Capillaries structure may vary depending on its location in the body and specific role:
The capillary wall may be continuous with endothelial cells held together by tight junctions to limit permeability of large molecules
In tissues specialised for absorption (e.g. intestines, kidneys), the capillary wall may be fenestrated (contains pores)
Some capillaries are sinusoidal and have open spaces between cells and be permeable to large molecules and cells (e.g. in liver)
Flow of blood - capillaries
Blood flows through the capillaries very slowly and at a very low pressure in order to allow for maximal material exchange
The high blood pressure in arteries is dissapated by extensive branching of the vessels and the narrowing of the lumen
The higher hydrostatic pressure at the arteriole end of the capillary forces material from the bloodstream into the tissue fluid
Material that exits the capillaries at body tissues include oxygen and nutrients (needed by the cells for respiration)
The lower hydrostatic pressure at the venule end of the capillary allows materials from the tissues to enter the bloodstream
Materials that enters the capillaries at body tissues include carbon dioxide and urea (wastes produced by the cells)
Valves
ensure that the blood always flows in the correct direction
Action of the heart
- the walls of the atria contract, pushing blood from the atria into the ventricles through the atrioventricular valves, which are open. The semilunar valves are closed, so the ventricles fill with blood
- the walls of the ventricles contract powerfully and the blood pressure rapidly rises inside them. This rise in pressure first causes the atrioventricular valves to close, preventing back-flow of blood to the atria and causes the semilunar valves to open, allowing blood to be pumped into the arteries. At the same time the atria starts to refill as they collect blood from the veins.
- The ventricles stop contracting and as the pressure falls inside them the semilunar valves close, preventing back-flow of blood from the arteries to the ventricles. When the ventricular pressure drops below the arterial pressure, the atrioventricular valves open. Blood entering the atrium of the veins then flows on to start filling the ventricles. The next heartbeat begins when the walls of the atria contract again.
Systole (contraction)
pressure is high
blood returning to the heart will flow into the atria. The atria will contract (atrial systole), increasing pressure in the atria and forcing blood into ventricles. The ventricles contract (ventricular systole) and the atrioventricular valves close to prevent back flow. The aortic valve then opens and blood is released into the aorta and away to the body
Diastole (relaxation)
The blood from the veins fills the atria and the ventricles. Pressure is low as the heart is not pushing blood into the arteries. As blood leaves the heart via the aorta, the ventricular pressure falls. The aortic valve closes to prevent back flow. The atrioventricular valves open and blood can flow from the atria to ventricle.
Aortic valve (semilunar)
The aortic valve is one of four heart valves and is the final one encountered by oxygenated blood as it leaves the heart. It is between the left ventricle and the aorta to ensure that oxygen-rich blood does not flow back into the left ventricle.
Atrioventricular valve
The bicuspid (left) and the tricuspid (right) valves, also known as the atrioventricular valves, are located between the top chambers of the heart, the atria, and the lower chambers of the heart, the ventricles.
