BLOOD VESSELS
[Photo credit: Kelvin Song]
The blood vessels are the tubes that carry blood within the body. In other words, blood vessels are the tubes within the body, through which blood moves "to and fro" the heart. i.e from the heart and to the heart. They are majorly three:
- arteries
- veins
- capillaries
1. Arteries: These are the blood vessels, through which blood moves away from the heart to the body. They carry blood away from the heart to the body. They have thick muscular walls with small lumen or canal. The walls of the arteries are elastic in order to withstand the high pressure of the blood, coming from the heart. They divide and become smaller as they live the heart to form arterioles.
2. Veins: These are blood vessels, through which blood moves from the body to the heart. They carry blood to the heart. They are less elastic than arteries with large lumen or canal. Valves are present in veins to prevent back flow of blood due to reduced pressure. The smaller veins from capillaries which join them to form bigger veins are called venules
3. Capillaries: These are blood vessels which join the arteries to the veins. They are tiny. They are located in the junction between arteries and veins around tissues and organs of the body. They have thin walls. They are tiny so as to penetrate to all parts of the internal organs. They aid the diffusion of oxygen, nutrients and waste products.
Differences Between Artery and Vein
The following are some of the differences between artery and vein.
Artery
|
Vein
|
It carries
blood away from the heart
|
It carries blood
to the heart
|
Its wall is
elastic
|
Its wall is
not elastic
|
It has thick
wall
|
It has thin
wall
|
Pressure is
high
|
Pressure is
low
|
It has small
lumen
|
It has large
lumen
|
THE HEART
The human heart is muscular in shape. It is responsible for the pumping of blood round the body. If it stops functioning, living organisms die. It works continuously throughout the life of a person.
The heart is located in the thoracic cavity, within the chest, behind the sternum, and between the two lobes of lungs. Each pumping action of the heart is referred to as heartbeat. The average heart beat per minute in man is about 72, while in rats is about 200. The average heartbeat in large animals like elephant is around 12. Rate of heartbeat can increase during activities.
The heart is reddish in color. It is made up of special muscles called cardiac muscles. It has two-layered membrane called the pericardium. The space between the two layers is filled with fluid called pericardial fluid. This fluid reduces the friction caused by the pumping movement of the heart between the heart walls and the surrounding tissue.
The Structure of the Heart
The mammalian heart is divided into four chambers:
- the right auricles
- the left auricles
- the right ventricles
- the left ventricles
The walls of the ventricles are normally thicker than those of the auricles. Also, the wall of the left ventricle is thicker and more muscular than that of the right ventricle. This is because it is throughout the left ventricle that blood is pumped or forced out of the heart to all other parts of the body.
An aperture guarded by a valve known as biscupid or mitral valve is between the left auricle and left ventricle. The valve is made of two cusps which moves only in one direction, i.e into the ventricle, thereby enabling valve to channel the blood in one direction which is from the auricle into the ventricle. When the ventricle is filled with blood, the cusps close up the aperture between the left auricle and left ventricle. This makes sure that the blood does not flow back into the auricle.
Also, a triscupid valve consisting of three cusps is located at the aperture between the right auricle and right ventricle. It functions in the same manner as the biscupid valve, i.e it allows the flow of blood from the right auricle to the right ventricle, and not vise versa.
Heartbeat
Heartbeat is the each pumping action of the heart. It is caused by contraction and relaxation of the four muscular chambers of the heart. It occurs in two stages:
- diastole
- systole
1. Diastole: This is the first stage of heartbeat. During this stage, the two auricles contract, creating a high blood pressure in the blood contained in them. The pressure causes the biscupid and triscupid valves to fold downwards into the ventricles, allowing blood to flow from auricles into ventricles, hence deoxygenated blood enters the right ventricle from the right auricle while oxygenated blood enter from the left auricle. When the ventricles are full, the cuspid valves close up the apertures between the upper and lower chambers of the heart. This happens by means of the blood forcing the cusps upwards so that they meet one another at the center of the apertures.
2. Systole: This is the second stage of heartbeat. This stage involves the contraction of the two ventricles. This leads to blood being forced into two trunks of the main arteries and out of the heart. Deoxygenated blood from the right ventricle passes into the pulmonary artery while oxygenated blood from the left ventricle passes into the aorta. During this occurrence, blood from the body fills up the auricles again.
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