Circulatory system

Question 1

Explain the common features of transport systems (4)

Answer 1

1. Suitable Medium to transport materials
2. Form of mass transport where medium moved in bulk over large distances (more rapid than diffusion)
3. Closed system of tubular vessels that branch out distributing medium
4. Mechanism for moving transport medium such as pressure difference from one part of system to another

Question 2

Explain why large organisms have transport systems (3)

Answer 2

1. The surface area to volume ratio decreases for large organisms so diffusion over body surface is insufficient
2. So large organisms need a specialised transport system to deliver nutrients and remove wastes from tissues
3. More active organisms also have a greater need for specialised transport systems

Question 3

Explain diastole of the cardiac cycle (3)

Answer 3

1. blood enters atria via Vena cava & pulmonary vein. Atria relax and fill with blood.
2. When pressure in atria exceeds pressure in ventricle, the atrioventricular valves open letting blood through, aided by gravity
3. Ventricles relax reducing pressure so that pressure is lower than in the aorta/pulmonary artery and so semi-lunar valves close

Question 4

Explain systole of the cardiac cycle (3)

Answer 4

1. (Atrial systole) Contraction of atrial walls forces remaining blood into ventricle from atria
2. (Ventricular systole) Ventricles contract increasing pressure which forces atrioventricular valves shut preventing back flow of blood
3. When pressure in ventricles exceeds aorta/pulmonary artery, the semi-lunar valves open and blood is pumped through.

Question 5

Explain what is cardiac output (2)

Answer 5

1. Cardiac output is the volume of blood pumped by one ventricle in one minute (dm³min⁻¹)
2. Cardiac output = heart rate x stroke volume (volume of blood pumped at each beat)

Question 6

Explain the electrical impulses involved in initiating the cardiac cycle (4)

Answer 6

1. Electrical impulse begins at the Sinoatrial node (SA node)
2. Wave of depolarisation passes through wall of atria reaching atrioventricular node (AV node) (atria contracts)
3. The AV node transmits impulses through the ‘bundle of his’
4. Wave of depolarisation spreads through perkinje fibres and into ventricle walls (ventricle contracts) cycle repeats

Question 7

Explain the structure of the artery and relate it to its function (3)

Answer 7

1. Thick muscle layer - smaller arteries can be constricted and dilate to control volume of blood passing through
2. Thick elastic layer - Stretch and recoil action maintains high pressure
3. Overall thickness of wall is great - resists vessel bursting under pressure

Question 8

Explain the structure of the arteriole and relate it to its function (2)

Answer 8

1. Muscle layer relatively thicker than arteries - contraction of muscle layer allows constriction of blood into capillaries
2. Elastic layer thin - as pressure is lower

Question 9

Explain the structure of the vein and relate it to its function (4)

Answer 9

1. Muscle layer thin - carries blood away from tissues no need to be controlled
2. Elastic layer thin - low blood pressure
3. Overall thickness of wall is small - low pressure, no risk of bursting, allows them to be flattened aiding in flow of blood by body muscle contraction.
4. Pocket valves - prevent back flow of blood

Question 10

Explain the structure of the capillary and relate it to its function (4)

Answer 10

1. One cell thick walls - short diffusion distance, increased rate of diffusion
2. Highly branched - increased surface area, increased diffusion
3. Narrow lumen - squeezes red blood cells against lining, reducing diffusion distance
4. Spaces between lining - so white blood cells can escape to deal with infections

Question 11

Explain the formation of tissue fluid (3)

Answer 11

1. At arterial end of capillaries hydrostatic pressure (created by heart pumping) causes tissue fluid to move out of blood plasma
2. Hydrostatic pressure of tissue fluid outside capillaries and lower water potential of blood, which causes water to move back into blood, both oppose this force
3. The combined effect is that tissue fluid is moved out of the capillaries however, only small molecules can pass the membrane of the capillaries (ultrafiltration)

Question 12

Explain how tissue fluid returns to circulatory system (6)

Answer 12

1. After tissue fluid has exchanged metabolic materials with cells, it returns to the circulatory system (reabsorption)
2. Most tissue fluid returns to the blood plasma through the capillaries as the loss of tissue fluid from capillaries reduces the hydrostatic pressure inside
3. Therefore, as blood reaches the venous end of the capillary network its hydrostatic pressure is lower than the tissue fluid outside
4. So tissue fluid is forced back into capillaries by higher hydrostatic pressure outside
5. In addition, the plasma has lost water and so has a lower water potential than the tissue fluid
6. So, water enters capillary by osmosis down a water potential gradient

Question 13

Explain the role of the lymph with tissue fluid (4)

Answer 13

1. Not all tissue fluid returns to capillaries, remainder drains through lymphatic capillary and is transported in the lymphatic system
2. Movement of contents through lymphatic system is provided by hydrostatic pressure of tissue fluid that has left capillary
3. And contraction of body muscles that squeeze lymph vessels (which have valves to prevent back flow)
4. Lymph is later returned to bloodstream