3.4.1 Circulation

Question 1

what causes atrial systole

Answer 1

• higher pressure in atria than ventricles
• blood moves into ventricles
• increase ventricular volume, decreased atrial volume
• AV valve opens, semi-lunar valves close

Question 2

what causes ventricular systole

Answer 2

• higher pressure in ventricles than atria
• blood moves into the arteries
• increase in arterial volume, decrease in ventricular volume

Question 3

characteristics of arteries

Answer 3

• transport blood under highest pressure from heart organs
• accommodate variations in pressure
• have thick elastic tissue to accommodate high pressure
• narrow lumen to resist flow and maintain pressure
• thick muscle wall to contract and control blood flow

Question 4

characteristics of arterioles

Answer 4

• transport blood under high pressure
• change diameter to control pressure changes
• have thick elastic tissue to accommodate high pressure
• narrow lumen to resist flow and maintain pressure
• thick muscle wall to contract and control blood flow

Question 5

characteristics of veins

Answer 5

• Transport blood under lowest pressures, from organs to heart
• Maintain pressure so that blood is returned to the heart.
• Achieved by:
- Thin elastic and muscle tissue layer: some ability to contract to maintain pressure
- Wide lumen to reduce resistance to flow
- Pocket valves: close to prevent the backwards flow of blood

Question 6

what is the structure of capillaries

Answer 6

• Single layer of squamous (flattened) endothelial cell
• basement membrane of connective tissue
• Narrow (approximately the same diameter as one red blood cell)

Question 7

how does the capillary structure relate to its function

Answer 7

• Reduces diffusion distance for rapid diffusion;
• Many capillaries in a network, branch between cells to further minimise diffusion distance
• Reduced velocity of blood flow due to narrow vessels - more time for exchange

Question 8

how is tissue fluid formed

Answer 8

• In capillaries the hydrostatic pressure is higher at the arterial end.
• This pushes fluid out of the capillary.
• It is opposed by the water potential of the blood, which is lower than that of the tissue fluid & the hydrostatic pressure of the tissue fluid.
• Overall fluid is forced OUT from the capillary by ultrafiltration

Question 9

how is tissue fluid returned

Answer 9

• Further along the capillary, at the venous end, the hydrostatic pressure falls.
• Formation of the tissue fluid reduces the water potential of the blood slightly, because the plasma proteins cannot cross the basement membrane, but the volume of water has decreased
• So fluid moves INTO the capillary, but some tissue fluid (around 10%) drains into the lymphatic system.