Essay B, Student 6, Expert Marker
Describe the Mechanisms Regulating Blood Flow to the
Diverse Organs of Vertebrates
Blood is an important liquid tissue in vertebrates and it makes up the
cardiovascular system alongside the heart and blood vessels. The main
components of blood are blood cells; both red and white, platelets, hormones and
plasma containing nutrients and ions such as K+ and Na+. Blood plays an important
role in the transport of substances to all body organs. The main transport material is
oxygen. Red blood cells contain haemoglobins and each can carry 4 molecules of
oxygen. To make sure that substances reach the desired organ destinations there
are a few mechanisms that regulate the blood flow which involves the pump, blood
vessels and muscles.
Vertebrates have a closed circulatory system. This has several advantages
over open circulations in arthropods and molluscs; closed systems can control the
direction of blood to specific tissues by changing the resistance in the vessels, rapid
transport of nutrients and wastes to and from cells and also molecules and
specialised cells that help in the transport of materials are contained in the vessels
(K.PURVES et al., 2004).
The heart’s function is to pump blood. The hearts of vertebrates consist of two
or more chambers. The main chambers are the atria and ventricles, which is further
divided in vertebrates with more than two chambers, to make up the left and right
atria and ventricles. The walls of the heart are made up of cardiac muscles. These
muscles contract spontaneously making up the heartbeat. The ventricles have
thicker muscles as they pump blood out of the heart to the rest of the body. The atria
on the other hand, receive blood from other parts of the body. For instance, the
unusual feature in a reptilian heart, they have the ability to control the amount of
blood going to the lungs and other parts of the body. Reptiles can bypass their blood
flow to the lungs and direct all their blood to the rest of their body when not breathing
(K.PURVES et al., 2004). This is possible with an additional aorta. This is of course
logical since there’s no need for blood to go through the pulmonary circuit if there’s
no breathing hence saving energy.
Essay B, Student 6, Expert Marker
Blood vessels also play a part in regulating blood flow. The major vessels are
the arteries, veins and capillaries. The arteries have the thickest walls to withstand
the high pressure of rapid flowing blood pumped out of the heart. The walls contain a
lot of elastic fibres and collagen for that specific function. Arteries and arterioles are
also known as resistance vessels as their resistance can vary (K.PURVES et al.,
2004). Smooth muscles in the arterial walls allows the vessels to be dilated or
constricted which then changes their resistance to blood flow and hence changes the
amount of blood that passes through. They also posses precapillary sphincters
(smooth muscle “cuffs”) which can block blood supply to the capillary bed. These
changes are controlled by neuronal and hormonal mechanisms.
“Most arteries and arterioles are innervated by the autonomic nervous system,
especially the sympathetic division” (K.PURVES et al., 2004). Norepinephrine are
released by most sympathetic neurone, which causes smooth muscle cells to
contract, constricting the vessels and hence reducing blood flow. Some specialized
sypathetic neurones in skeletal muscle however releases acetylcholine which does
the exact opposite to what norepinephrine does.Other hormones can also affect the
vessels such as epinephrine which is released during the fight-or-flight response,
angiotensin which is produced when blood pressure in the kidney falls and
vasopressin which is released by the posterior pituitary when blood pressure falls.
Hormone secretions can also be caused by changes in emotions. These hormones
affect the arterioles in the extremities or in tissues that do not need continuous
maintenance. They increase the central blood pressure and blood flow to important
organs , the heart, brain and kidneys.
The blood then goes back to the heart via the veins. Veins have a larger
lumen but a thinner wall compared to arteries. The large lumen allows larger
volumes of blood to flow back towards the heart. Furthermore, veins are the only
blood vessel that possesses valves. These valves are there to prevent the backflow
of blood and make sure that blood flow in one direction. Furthermore, veins have
thinner walls so they do not have adequate muscles, they then depend on
neighbouring muscles which helps to push blood upwards while doing movements
and even by breathing. As observed in the venules of the bat wing, peristaltic
contractions of the smooth muscle of venules (the vessel connecting capillaries and
Essay B, Student 6, Expert Marker
veins) can also help to promote venous flow towards the heart (RANDAL et al.,
2002).
To sum up, there are a lot of mechanisms at work to keep and control the
blood flow towards all the organs in a vertebrates’ body. A healthy body system
could sense if there was something in error (autoregulation) and produce a
correcting mechanism such as the negative feedback mechanism or by secreting
hormones to the desired area of the body and keep the blood pressure in check. The
cardiovascular system is an effective transport system for blood to reach every part
of the body. The highly branched vessels played a major role in regulating blood flow
as well. The heart keeps the motion going so without the heart there’s no point in
having all these mechanisms as they will not work. The flow of blood toward organs
below the heart is also helped by the gravity force. On the other hand, the blood
flowing upwards is made possible with valves and movements of muscles around the
body. And lastly, these regulations are important to supply enough nutrients and
oxygen and remove waste products to and from cells to keep a vertebrate alive.
Bibliography
Essay B, Student 6, Expert Marker
K.PURVES, William, David SADAVA, Gordon H. ORIANS, and H. Craig HELLER. 2004.
Circulatory systems. In: Sinauer ASSOCIATES, (ed). Life: The Science of Biology, U.S.A: W.
H. Freeman and Company, pp.940-960.
RANDAL, David, Warren BURGGREN, and Kathleen FRENCH. 2002. Circulation. In: Jason
NOE, Morgan RYAN, and Jane O'NEILL, (eds). Eckert Animal Physiology Mechanisms and
Adaptations, New York: W. H. Freeman and Company, pp.473-522.