1.6 Understand why many animals have a heart and circulation (mass transport to overcome the limitations of diffusion in meeting the requirements of organisms)
1.7 Understand how the structures of blood vessels (capillaries, arteries and veins) relate to their functions
Transport in large organisms
- Have a smaller surface area to volume ratio (SA/V) compared to small organisms
- Diffusion alone is not suffice to satisfy the needs
- Need special organs (e.g. lungs) to increase surface area for exchange and a mass transport system (e.g. heart and circulation) to move food and waste around the body
Structure and function of different blood vessels
- Relatively thick wall --> withstand high blood pressure from the pumping action of the heart
- Smooth muscles --> change diameter of lumen to control blood flow
- Elastic fibers --> allow walls to stretch when blood is pumped into the artery and then recoil to ensure smooth blood flow
- Lined with smooth layer of endothelial cells --> low friction surface to enable smooth blood flow
- Narrow Lumen
- Very thin wall (one cell thick) --> allow for more rapid diffusion and substance exchange between the blood and surrounding tissues
- Relatively thin wall --> thick wall not necessary as blood flowing under low pressure
- Very little smooth muscle or elastic fibers --> no pulsation of blood so no stretching and recoiling
- Wide lumen (i.e. large volume) --> allow the veins to act as a blood reservoir
- Have valves to prevent backflow of blood
Common misconception in the IAL Biology exam:
Smooth muscles in arteries do not squeeze blood but only controls the amount of blood flowing into different tissues. A stable blood pressure during diastole is due to the recoil of elastic fibers.
Time for a break!
1. Edexcel AS Biology Revision Guide. Edexcel.