A summary of A level OCR Biology topic 3.2, covering transport in animals.
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In animals, a transport system is required to supply oxygen/nutrients
and to remove waste products.
The following factors influence the need for a transport system:
Size:
In a large organism, the diffusion pathway is long, reducing the diffusion
rate.
Surface area to volume ratio:
In smaller animals, this ratio is larger. For larger animals, each tissue has
a smaller surface area for exchange.
Metabolic activity:
Animals release energy from food by aerobic respiration, which requires
oxygen.
What makes a good, efficient transport system?
Pump:
- Creates pressure to push the fluid around the body. Eg; heart.
Exchange surfaces:
- Substances can enter/leave the blood where needed. Eg;
capillaries.
Fluid/medium:
- To carry oxygen, nutrients, and waste around the body. Eg; blood.
Tubes/vessels:
- To carry blood by mass flow.
2 circuits
- One to pick up oxygen, and another to deliver oxygen to the
tissues.
Fish have single circulatory systems. The blood flows through the heart
once for each circuit of the body. The blood has a low pressure and
travels slowly. This circuit delivers sufficient and nutrients for the fish’s
needs, since there are not as metabolically active and mammals.
Mammals have a double circulatory system. The system has 2 separate
circuits, the pulmonary circuit (to pick up oxygen from the lungs) and
the systematic circuit (to carry oxygen and nutrients around the body).
In the pulmonary circuit, the pressure is low, so no damage is done to
the lungs.
, In the systematic circuit, the pressure is high, and blood flow is faster.
3.2.2 Blood Vessels
Insects have an open circulatory system. The blood is not always held
within vessels, the fluid circulates through the body cavity. In some
animals, bodily movements help to circulate the blood. In some animals,
there is a muscular pump, lying under the upper surface of the body.
Blood enters the heart through pores called ostia. The heart pumps this
by peristalsis.
There are disadvantages of this system, the blood pressure is low and
blood flow is slow.
Larger animals have a closed circulatory system, where the blood stays
within vessels. A separate fluid, tissue fluid, bathes the cells and
tissues.
There are advantages of this system, higher pressure and quicker blood
flow, and the transport is independent of body movements.
Arteries carry blood away from the heart. The blood pressure is high so
artery walls must be thick. The lumen is quite small, to maintain this
pressure.
Capillaries allow exchange of materials between the blood and tissue
fluid. The lumen is narrow to decrease the diffusion distance, and the
walls are said to be leaky.
Veins carry blood back to the heart. The blood pressure is low, and the
lumen is relatively large. The walls are thin, and the blood vessel
contains valves to prevent the backflow of blood.
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