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, Transport in animals
Animals need specialised transport systems because:
Metabolic demand is high
As multicellular animals get bigger SA:V gets
smaller
Molecules, e.g. hormones, may be made in 1 place
but needed in another
Food will be digested in 1 organ system, but needs
to be transported to every cell for use in
respiration
Waste products need to be removed from cells &
transported to excretory organs.
Most large multicellular animals have specialised
circulatory systems which carry gasses, nutrients,
waste products & hormones around the body.
, Circulatory systems
Open circulatory system- One in which the transport medium is not held in
vessels. It is instead pumped straight from the heart, which lies just under
the upper surface of the body, towards the head by peristalsis. The blood
then drains to the rest of the body. The open body cavity is called the
haemocoel. Blood then returns to the heart through an open-ended vessel.
Organisms with open circulatory systems have low blood pressure so slow
flow rate. The blood circulates the body but steep concentration gradients
cannot be maintained & the circulation may be affected by body movements
or lack of body movements. The amount of blood flowing to a particular
tissue cannot be varied to meet changing demand.
Open circulatory systems are found in insects. Insect blood is called
haemolymph & does not transport O₂ or CO₂.
Closed circulatory system- One in which blood is held in vessels & does not
come directly in contact with the cells of the body. The heart pumps blood
around the body at a high pressure so blood flows quickly allowing a more
rapid removal & delivery of substances. Substances leave & enter the blood
by diffusion through the blood vessel walls. The amount of blood flowing to a
particular tissue can be adjusted by widening or narrowing the blood vessels.
Most closed circulatory systems contain a blood pigment that carries the
respiratory gases.
, Single closed circulatory system The blood flows through the heart once for each
circuit of the body.
Blood passes through 2 sets of tiny capillaries composed of squamous epithelium tissue
& so the blood has a low pressure, which means a slow flow rate & so a slow rate of O₂
delivery & CO₂ removal. This limits the efficiency of the exchange process so the
activity levels of animals with single closed circulations tends to be relatively low. They
also tend to be endotherms which are organisms that cannot regulate body temp.
Fish are an exception & are very active with a single closed circulatory system. This is
because fish have an efficient countercurrent gaseous exchange mechanism that
allows them to take a lot of O₂ from the water. In addition the body weight of with is
supported by the water.
Double closed circulatory system The blood flows through the heart twice for each
circuit of the body. There is low blood pressure in
pulmonary circulation
(heartlungsheart) as the blood travels through thin
capillaries composed of squamous epithelial tissue. High
blood pressure may damage the delicate capillaries.
After it has passed through the capillary network in the
lungs the blood travels at high pressure in systematic
circulation (heartbodyheart). This increases the flow
rate meaning there is fast O₂ delivery & CO₂ removal.
This system is found in ecotherms such as mammals which
use energy to thermoregulate. Mammals need lots of energy
to maintain body temp & provide energy for activity.
Therefore a double circulatory system is more efficient.
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