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Intro to CVS

Organisation of the Body

QuestionAnswer
What is circulation needed for? The circulatory system is an evolutionary response to the limits of diffusion
What would happen without circulation? In the context of our activities and size solute concentrations could vary substantially in time and space This would be difficult to interpret, maintain and change Circulation is a form of convection that mixes the milieu and prevents this
Diffusion Most transport occurs by diffusion Diffusion time is distance travelled squared divided by 2x the diffusion constant (a property of the solute in its solvent) Thus a 2 x increase in distance takes 4x the time to cover Difficult to control
Why is diffusion scope limited Our body is made of 40 trillion cells These arise from 45 cell divisions If cells had the same size thats a 2^45 fold increase in volume Thats a 2^30 fold increase in diffusion time
When is diffusion inadequate At distances about 100um - a typical cell is 10um Infarction occurs when the diffusion distance is above 100um even if there is a rich supply of oxygen available
What does division of labour predispose us to Non-uniformity We have an oxygen sink in the brain and a rich oxygen source in the lungs This would result in a large concentration gradients Mixing by circulation prevents this occurring
Convection - Bulk flow Transit time = distance travelled/velocity Distance travelled is not squared so increases in distance have less effect Velocity is a property of blood, not the solute - usually 2m/min Regulated
Role of diffusion at exchange surfaces Short distances at exchange surfaces are supplied by diffusion - distances less that 100um can be supplied by diffusion e.g. each myocardial cell is supplied by a blood vessel
What is distributed by bulk flow Substrates Wastes Signals Heat Pressure
Components of a cardiovascular system A medium to carry substances by bulk flow - blood Tubes to connect the medium - vessels A pump to drive the flow of medium - heart
What is the energy source of bulk flow This usually takes the form of pressure energy as a result of heart contraction
Primitive circulations Flies - open circulation with many small hearts to generate a larger pressure in a smaller area Earthworms - closed circulation containing blood in vessels - still with many small hearts These are both hard to coordinate and regulate
Single pump circulation One ventricle which forces blood around the circulation with primitive atria to collect blood This leads to downstream tissues being perfused at low pressures as blood must pass through two capillary beds
Two pump circulation Heart 1 - pumps blood under low pressure around the lungs Heart 2 - pumps blood under high pressure around the tissues Allows for different pressures to be maintained in each circulation Two pumps are collected in one organ which acts as a syncytium
How is blood distributed around the body Blood leaves the ventricles in a large vessel, which splits into smaller capillaries These than fuse back into a larger vessel which supplied the atrium
Krogh cylinder Tissue surrounding a blood vessel is no more that 100um away This allows diffusion to occur in order to supply blood to the tissues
Divisions of the cardiovascular tissue Arteries - 25mm diameter Arterioles - highest resistance to blood flow Capillaries - site of exchange - diameter of 6um Venules - Veins - 30mm diameter
Effects of changes in cross sectional area Capillaries have a high cross sectional area to slow the flow of blood - this allows time for exchange to occur and diffusion to reach equlibrium Larger vessels have a low cross sectional area to allow rapid flow of blood
Distribution of blood 10% - heart 10% - pulmonary circuit 80% - systemic circuit Most of the blood is in systemic veins - provide lowest resistance to flow
Initial views on circulation - Ancient Greeks Were aware that the heart is connected to the rest of the body via arteries Believed that arteries were filled with air as in death blood moves into veins (lowest resistance) leaving arteries full of air
Galen's 2nd century model Gut produces substrate for blood Liver produces blood - growth and energy Lung delivers air to the left heart Heart mixes air with blood across porous septum One way traffic - no circulation of blood
Incorrect assertations of Galen's model Two chambered heart - no atria Heart septum is porous Venous and arterial systems are separate Artery wall generates pulse
Concept of pulmonary circulation Septum is not porous Blood flow from right heart flows to the lungs via pulmonary artery Blood returns from lungs to left side of the heart via pulmonary vein Predicted the existence of capillaries - connect arteries with veins
De motu cordis - Harvey Blood is ejected from cut arteries - they carry blood not air under pressure Volume of blood expelled by heart per day is much greater than food intake - argues for circulation of blood A rod cannot be pushed down a vein - only unidirectional flow
Modern view of circulation The heart is a muscular pump that circulates the blood Air turns blood red and is needed for the heartbeat Blood could be transfused from an artery of one dog to a vein of another - shows blood is a molecule not a spirit
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