1.1 Circulation

Question 1

Why do we have a heart and circulatory system?

Answer 1

• to move substances around the body
• large organisms have a small SA:vol ratio so cannot rely on diffusion
• this is because the diffusion distance is too great, and so diffusion is not efficient enough to meet all the metabolic requirements of all cells
• therefore we rely on mass transport systems that move substances efficiently by mass flow

Question 2

Why can single celled organisms rely on diffusion?

Answer 2

• the SA:vol ratio is big
• the diffusion distance is short, so the rate of diffusion is quick
• therefore diffusion is efficient enough to meet all the needs of the cell

Question 3

What needs to be delivered to cells and why?

Answer 3

glucose and oxygen for aerobic respiration, which releases energy

Question 4

What is mass transport?

Answer 4

• the bulk movement of gases or liquids in one direction, usually via a system of vessels and tubes
• there is movement in one direction only because of the difference in pressure in the blood vessels

Question 5

What are mass transport systems used for?

Answer 5

• to supply body cells with substances and to remove metabolic waste - this ensures effective cell activity
• to transport substances quickly
• to maintain diffusion gradients at exchange sites

Question 6

What are open circulatory systems? How do they work?

Answer 6

• blood circulates in large open spaces
• the heart pumps blood into the cavities around organs and diffusion of substances occurs
• blood is drawn back into the heart as it relaxes

Question 7

What are closed circulatory systems? How do they work?

Answer 7

• blood is enclosed in tubes
• blood leaves the heart at high pressures
• blood travels along the arteries, then the arterioles, then the capillaries
• diffusion occurs
• blood travels along the venules, then the veins back to the heart
• blood returns to heart at low pressures

Question 8

What are single and double circulatory systems?

Answer 8

• single: blood flows through the heart once for each complete circuit
• double: blood flows through the heart twice for each complete circuit

Question 9

What are the advantages of double circulatory systems?

Answer 9

• the heart gives the blood returning to the lungs an extra boost
• so it takes less time to circulate around the body
• this increases the rate of gas exchange in cells, to meet the demands of organisms with a high metabolic rate (e.g. mammals and birds)