ABC of Vascular Disease

The Vascular System

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The vascular system is the set of structures in the body that are concerned with circulating the blood.  The vascular system has four hollow components: the heart, the arteries, the capillaries and the veins which contain blood and which form a loop that allows the blood to circulate over and over again.  In the body there are two circulations that are linked to each other.  The pulmonary circulation starts at the heart and only goes to the lungs and then returns to the heart.  The systemic circulation starts at the heart and goes to all the other parts of the body except the lungs before returning to the heart

The heart
The heart is a muscular pump which is actually two pumps in one.  One half pumps blood around the pulmonary circulation (right side of the heart), the other half pumps blood around the systemic circulation (left side of the heart).  Each half of the pump has two parts: a priming chamber called the atrium and the main chamber called the ventricle.  This means that the heart has four chambers: right atrium (RA), right ventricle (RV), left atrium (LA) and left ventricle LV). 

The arteries
The arteries are the vessels which carry blood away from the heart.  Just as there are two halves to the heart, there are two sets of arteries: the pulmonary arteries carry blood from the right side of the heart to the lungs and the systemic arteries carry blood from the left side of the heart to the rest of the body.  The arteries are like a tree, with a single large trunk that gives off smaller branches, that themselves divide in turn into smaller branches, on and on until the branches become too small to see with the naked eye.  These terminal branches of the arteries connect with a network of microscopic vessels called capillaries.

The capillaries
The capillaries are a network of microscopic vessels that permeate all the tissues of the body.  The capillaries are very delicate vessels because their walls are only one cell thick! As blood passes through the capillaries molecules can pass through the walls of the capillaries to the surrounding cells.  These cells use these molecules to provide energy and materials for growth and repair.  The waste products of these cellular processes are also molecules that can pass back through the wall of the capillary into the blood and be carried away for disposal.

The veins
The veins are more like a river than a tree.  The smallest veins are microscopic and connect with the network of capillaries to collect the blood.  These smallest veins joining with others to form larger veins that also join with each other, on and on until they form large veins that run beside the arteries and eventually return all the blood back to the heart.

The circulation
Blood circulates around this system of vessels, passing first from the heart through the lungs, back to the heart then around the body then back to the heart and then to the lungs, and so on.  As the blood passes through the lungs it picks up oxygen and turns from a dark blue-red colour to a bright scarlet red colour.  When the blood passes through the body it releases the oxygen which is used to generate energy and heat.  As blood loses it's oxygen it turns back to a dark blue-red colour. The waste product from the body is carbon dioxide which passes back into the blood but, unlike oxygen, it doesn't change the colour of the blood.  The carbon dioxide is transported in the blood to the lungs where it is released as a gas and breathed out.

Blood only moves around this vascular system because of the pumping action of the heart.  If the heart stops, the blood does not circulate and the body does not get enough blood to deliver the necessary oxygen.  This lack of blood circulation is called ischaemia.  When the cells are starved of oxygen they cannot carry out their normal work.  This is usually a reversible process if the circulation is restored quickly.  If the ischaemia is too severe or lasts too long the cells will die.  The death of a whole region of cells is called infarction and is irreversible.  Different parts of the body can tolerate oxygen starvation better than others.  The brain is the most sensitive: after only  a few seconds of oxygen starvation the brain cells stop working properly and the person becomes unconscious and may have a fit.  If the oxygen starvation continues for several minutes the brain cells start to die, resulting in permanent mental disability and possibly death.

The most important function of the vascular system is to supply blood continuously to the brain and other vital organs.  To achieve this the body has a number of sophisticated regulatory mechanisms which ensure the heart beats at the correct rate and with the correct force, and that blood flows to each part of the body at the correct rate for its needs.

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S.R.Dodds 2001