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Essay B, Student 5, Expert Marker
Describe the mechanisms regulating blood flow to the diverse organs of vertebrates
Vertebrates all have closed circulatory systems which allow easy regulation of the
distribution of blood to different organs. and Systems vary between phyla due to
different organism’s individual needs. Means and devices that vertebrates use to
regulate blood flow to their organs vary from the different numbers of heart
chambers, and processes affecting the heart, to blood vessel structure and function.
Exercise, gravity, and diving underwater all cause organisms to regulate blood flow
to specific organs.
Mammals and birds have 4 chambered hearts in a double circulatory system. The
right side receives deoxygenated blood from the systemic circuit and pumps it to the
lungs; the left side receives oxygenated blood from the pulmonary circuit and pumps
it to the rest of the body. Most reptiles also have 3 chambered hearts with a septum
partially dividing the single ventricle. Amphibians have 3 chambered hearts with a
ridge within the ventricle directing deoxygenated blood to the pulmocutaneous
circuit, the skin and lungs, for gas exchange. The 3 chambered hearts of reptiles
and amphibians means they can divert blood away from pulmonary/pulmocutaneous
circuits when they are underwater. Crocodilians are the exception to this rule as
they have a 4 chambered heart, however arterial valves can force blood to bypass
the pulmonary circuit. Bony fish have a single circulatory system such that blood
passes through a heart consisting of two chambers, a single atrium and ventricle.
Contraction pumps blood to the gills and the systemic system before it returns to the
heart. Although blood pressure drops significantly after leaving the gills, the animal’s
motion through the water helps to push it back to the heart. (Campbell et al, 2008).
Cardiac muscle fibres are different to those in any other muscle so that they can
contract in a co-ordinated way. Action potentials, produced first by the Sinoatrial
Node (SAN) and second by the Atrioventricular Node (AVN), cause the heart muscle
to contract. Those produced by the SAN cause the atria to contract, forcing blood
into the ventricles, and those produced by the AVN cause the ventricles to contract
pumping blood to the lungs or around the body. These contractions are both
spontaneous and myogenic, meaning that they will continue for a short time after the
Essay B, Student 5, Expert Marker
heart is removed from a dead animal and that they are generated by the heart and
not by the brain. Only the autonomic nervous system can act on the heart to change
the rate of contraction. The sympathetic system inputs adrenaline or noradrenaline
to speed the heart rate up and the parasympathetic system inputs acetyl-choline to
slow the heart rate down. These systems are used in exercise or sleep respectively
where more or less blood is required to the muscles.
The 3 main types of blood vessel are differentiated by lumen and wall size, blood
content, and pressure. Arteries have thick, elastic muscular walls and a lumen which
is slightly wider than the walls are thick (Hill et al, 2008), and carry oxygenated blood
to the systemic system under high pressure. Veins have thinner less elastic walls
than arteries, carry deoxygenated blood from the systemic system under low
pressure, and they use valves to keep the blood flow unidirectional. Capillaries have
a very narrow lumen with a diameter only slightly larger to that of a single red blood
cell and their walls consist of a single layer of epithelial cells; they are the site for
gas, nutrient, waste product and hormone exchange. The smooth muscle of the
arterioles and smooth muscle sphincters at capillary junctions control blood
distribution to each capillary bed (Randall et al, 2002).
‘The smooth muscle walls of arterioles are responsible for the vasomotor control of
blood distribution. That is, by vasoconstriction and vasodilation the smooth muscle
walls of an arteriole determine the amount of blood flowing through the vessel, by
controlling the area of the lumen, and hence the rate of flow to the capillary bed’ (Hill
et al, 2008). The dilation of arterioles in working muscle, by vasodilation, causes the
blood pressure to fall and an increase of oxygenated blood to the muscles (Campbell
et al, 2008). During exercise blood flow to the working skeletal muscles is increased
by about 20% because they need a larger amount of oxygen to work harder than
normal. The proportion of the total Cardiac Output to the muscles increases by 4-5
times, and that to the viscera and skin is slightly decreased. Contractions of skeletal
muscles also play an important role in returning blood to the heart. By squeezing
and constricting veins these muscles force blood to flow towards the heart since
reverse flow is impossible due to one-way valves, so in exercise, when muscles are
moving more, blood returns through the veins faster than at rest.
Essay B, Student 5, Expert Marker
‘Gravity has a significant effect on blood pressure’ (Campbell et al, 2008). When
standing, arterial pressures are increased in the lower limbs and decreased in the
head (Hill et al, 2008), this is because it is harder to pump blood up to the head
against gravity but very easy to pump it to the legs with gravity.
Diving mammals use the same mechanisms to control blood flow distribution as
described above but temporarily slow their metabolism in certain tissues and organs
to prioritise the brain and swimming muscles. The classic “Dive Response” is
bradycardia, which is a decrease in heart rate.
Certain mechanisms involved in regulating blood flow to certain organs, such as
valves in veins, are used by all vertebrates almost all the time. In periods of exercise
methods such as the sympathetic system are used, and if some of the mammals
organs are more subject to gravity than normal, such as a Giraffes brain, cardiac
output and pressures are changed to compensate. In the case of diving mammals
that need to preserve oxygen, bradycardia is used and large parts of the body are
cut off from blood flow completely.
References
Campbell, N. et al. (2008) Biology, 8th Edition, San Francisco: Pearson Education,
Inc.
Hill, R., Wyse, F. and Anderson, M. (2008) Animal Physiology, 2nd Edition,
Sunderland, Massachusetts: Sinauer Associates, Inc.
Randall, D., Burggren, W. and French, K. (2002) Animal Physiology: Mechanisms
and Adaptations, 5th Edition, New York: W. H. Freeman and Co.