Download Explain how arteries,veins,and capillaries are adapted

Explain how arteries,veins,and capillaries are adapted for their
functions. Describe what happens in coronary heart disease and
explain how lifestyle may contribute to the development of this
disease.
The human circulatory system is an essential network of tubes designed
to transport a continuous flow of blood throughout the body delivering
nutrients to and removing waste from every cell. Contemporary society is
experiencing an increasing number of people suffering from an often
preventable disease that occurs in this circulatory network named
coronary
heart
disease.
(Boyle,M.,Senior,K.
2002).
(www.nhlbi.nih.gov/health/public/heart/other/chdfacts.htm).
This essay will explain how the circulatory systems‘ arteries,capillaries and
veins are adapted for their functions,what happens in coronary heart
disease,and how some contemporary lifestyles may contribute to the
condition.
The main organ of the circulatory system is the heart,situated in the chest
between the lungs. It is a hollow muscula r structure consisting of four
main chambers.
The lower left and right ventricles have thick muscular walls to pump
blood,and the upper left and right atria that have thinner walls and receive
blood. This is effectively a double circulatory system as b oth ventricles
pump blood and both atria receive blood simultaneously.
The pulmonary system starts in the right ventricle where deoxygenated
blood is pumped via the pulmonary artery to the lungs. Here it is
reoxygenated and returned to the left atrium v ia the pulmonary vein.
The systemic system pumps the oxygenated blood from the left
ventricle via the aorta. The aorta branches into the carotid artery
that serves the head and forelimbs,the hepatic artery the liver,the
mesenteric artery the gut,and the renal artery the kidneys and
lower limbs. £rom the gut deoxygenated blood travels to the liver
via the heptic portal vein. £rom the kidneys and lower
limbs,deoxygenated blood leaves via the renal vein and from the liver
via the hepatic vein. These ve ins join the inferior vena cava which in
turn joins the superior vena cava carrying deoxygenated blood
from the head and forelimbs and re -enters the heart through the
right atria. (Roberts,M.B.V. 1976).
Arteries transport blood away from the heart at high pressure to
supply the organs and body tissue with oxygen and nutrients
required to maintain life. Their walls have a thick muscular
structure,which adapts them for high pressure and any change in
pressure due to increased activity.
The outer walls,tunica adventitia are connective tissue and the
inner walls tunica media,are elastic tissue forming smooth muscle. A
single layer of flattened endothelial cells form a smooth muscular
tube within these walls. This structure enables diastole and systole
with each pulse of blood pumped by the heart. Semilunar valves
prevent the blood from returning to the heart and ensure a one way
flow. Because of the elastic recoil of the arteries,blood pressure
does not drop much therefore maintaining high pressure. This
elasticity tends to decrease with age so diastole is less,producing
higher blood pressure. ( www.medem.com/The Vascular System).
(Boyle,M.,Senior,K. 2002).
£rom the arteries,blood is pumped into smaller arterioles which in
addition to distributing oxygenated blood,act as pressure reducing
valves between the arteries and the capillaries. They are also
important in determining blood pressure. They buffer the delicate
capillaries from
the high pressure of blood in the arteries. (www.medem.com/The
Vascular System).
Capillaries are the only blood vessels in the circulatory system that
have the unique function of permitting the exchange of substances.
They are composed of a single layer of flattened endothelial cells
that together form a smooth continuous tube. At the arterial end
of the capillaries,the blood pressure is high enough to cause tissue
fluid containing oxygen and other nutrients to filter out through
the thin walls to the surrounding cells. Branching of the capillaries
is so extensive that no cell in the body is more than a few
micrometers away from one and its life sustaining contents.
The venous end of the capillaries receive some of the tissue fluid b ack
by osmosis,this time containing carbon dioxide and other waste
materials.
(www.medem.com/The
Vascular
System).
(Boyle,M.,Senior,K. 2002).
£rom the capillary beds,the blood drains into the venous section and
into the veins,which transport the blood at a lower pressure back
to the heart. Although veins have the same composition as
arteries,they have a much thinner layer of tunica adventitia and tunic
media,as blood pressure is much lower. This structure has a wide
lumen to reduce resistance in blood flow allowing for easier
expansion and adapting them for their function as a reservoir for
blood.
To prevent the downward pull of gravity resulting in an accumulation
of blood in the lower part of the body,the veins contain one-way
pocket valves. In addition to this,the contracting of the skeletal
muscles press against the veins narrowing them so creating pressure
which forces the blood towards the heart. During inspiration,reduced
pressure in the thorax encourages blood to flow towards the heart. The
pocket valves prevent back flow during expiration when thoracic
pressure is high. (www.medem.com/The Vascular System).
(Boyle,M.,Senior,K. 2002).
The coronary arteries are an extensive network of vessels that supply the
myocardium or heart muscle itself. Coronary heart disease (CHD) or
ischaemic heart disease is the most common and the most preventable of
all heart disease and it occurs in this particular network. The main risk
factors are high blood cholesterol,hypertension,smoking,obesity,diabetes
mellitus,physical inactivity or the genetic predisposition familial
hyperlipidaemia. Those most likely to develop CHD will always have one
or more of these factors present and with the exception of the latter,the
majority are preventable. (Boyle,M.,Senior,K. 2002).
It is important to understand what happens during CHD and how certain
lifestyles contribute to or cause the condition before any improvement s
can be made.
CHD begins when the walls of the coronary arteries become narrowed by
a gradual build-up of fatty deposits. This condition is known as
atherosclerosis and the fatty substance an atheroma. Gradually the
muscle cells and fibres grow over the affected areas called fibrous
plaque. This protrudes into the lumen and starts to obstruct the blood
flow. During increased physical activity,a narrowed artery restricts the
oxygen supply to the myocardium resulting in angina pectoris. This is
normally felt as pain in the chest or left arm or shoulder and is usually the
first indication of CHD. If the plaque bursts into a vessel it damages the
endothelium and the cells underneath are exposed to the blood,triggering
the blood clotting mechanism. A re sulting blood clot or thrombus that
completely blocks any artery is called thrombosis. When this occurs in a
coronary artery it causes a heart attack and in the carotid artery,a stroke.
When a blood clot becomes mobile,it is called an embolus resulting in a
embolism when it blocks a vessel. A pulmonary embolism is usually fatal.
An aneurysm is a burst artery that,if occurring close to the heart is also
usually fatal. (Boyle,M.,Senior,K. 2002).
A major contributor to the fatty deposits that cause CHD,are actually
essential to the formation of cell membranes and energy stores. These
are triglycerides and cholesterol lipids that combine with protein to form
lipoproteins before the blood can transport them to required sites around
the body. Most are produced by the liver but some are present in the
foods we eat. Lipoproteins consist mainly of low density lipoproteins,LDL
that carry cholesterol from the liver to the tissues and high density
lipoproteins,HDL that return extra cholesterol not required to the liver.
Ideally,there should be a higher ratio of HDL to LDL to ensure any excess is
returned. When this ratio is unbalanced and the blood contains surplus
LDL rich lipoproteins,they are engulfed by phagocytes within the artery
walls just beneath the endothelium. Unable to break down the
cholesterol contained within them,an accumulation gradually occurs inside
the artery wall. This is the first sign of atherosclerosis. (Boyle,M.,Senior,K.
2002).
Saturated fats in the diet lower HDL and increase LDL cholesterol
therefore increasing the risk of an atheroma developing. Unsaturated
fats lower the LDL cholesterol levels and are therefore encouraged in the
diet. A high intake of saturated fats puts people at risk of
developing hypertension,obesity and diabetes mellitus. Too much
salt in the diet also increases the risk of hypertension by reducing
the water potential of the blood,increasing the volume and in turn
the pressure. (Eades,M.R.,Eades,M.D. 2000). (British Heart
£oundation,Eating for your Heart,1999).
Regular physical activity raises HDL but does not affect LDL and can help
to maintain a healthy weight and reduce obesity,hypertension and
medication required for diabetes mellitus. It can also help to
prevent hypertension and thrombosis by ensuring the artery walls are
kept supple and less prone to the thickening that reduces the
lumen,elevates pressure and increases the risk of damage to the
endothelial cells. (British Heart £oundation,Physical Activity and your
Heart,1999). (Boyle,M.,Senior,K. 2002).
Cigarette smoke contains a vasoconstrictor nicotine that narrows
the blood vessels increasing blood pressure. It also increases
lipoproteins in the blood and the amount of fibrinogen,which causes
the blood to clot. (Boyle,M.,Senior,K. 2002).
By reducing saturated fats,stopping smoking,increasing physical
activity and generally eating a healthy balanced diet will reduce
the risks of developing CHD. Unfortunately,the disease is still on
the increase in developed countries and is a major killer.
Luckily,there are many safe drugs available that can control
symptoms or reduce them but,as an escalating problem,lifestyle
changes are the safest,cheapest and healthiest ways of reducing or
eliminating the risks.
References
Boyle,M.,Senior,K. (2002). Uuman Biology. (2nd ed.). London: Uarper
Collins Publishers Ltd.
Eades,M.R.,Eades,M.D. (2000). Nhe Protein Power Lifeplan. hew York:
Warner Books Inc.
Roberts,M.B.V. (1976). Biology Æ Iunctional Æpproach . (2nd ed.).
Sunbury- on-Nhames: Nhomas helson and Sons Ltd.
British Ueart Ioundation. (1999). Eating for your Ueart.
British Ueart Ioundation. (1999). Physical Æctivity and your Ueart.
http://www.medem.com/Nhe Vascular System.htm [Æccessed:
2003,ebruary 1].
http://www.nhlbi.nih.gov/health/public/heart/other/chdfacts.h
m [Æccessed: 2003,Iebruary 1].