THE BLOOD VESSELS AND THE HEART

THE BLOOD VESSELS

There are three main kinds of blood vessels:

• The arteries

• The veins

• The blood capillaries

The following figure shows interconnection between these three blood vessels:

The Arteries

• Function: carry blood away from the heart

• Structure of wall: thick, strong containing muscles and elastic fibres

• Width of lumen: smaller than that of veins

• Reasons for structure: thick wall needed to withstand high pressure as blood is pumped out of the heart. Being elastic enables the arteries to stretch and recoil with the force of the blood as the ventricles contract and relax. This helps to make the flow of blood smoother

The Veins

• Function: carry blood to the heart

• Structure of the wall: quite thin, less muscular and less elastic compared to arteries

• Width of lumen: wider than that in the arteries. Valves are also present along the length of the veins

• Reason for structure: strong wall no required in veins as most of the pressure of the blood has been lost. Wide lumen offers less resistance to blood flow and the presence of valves prevent backflow of blood

• Movement of blood along the veins is assisted by the action of the skeletal muscles on the veins

The Blood Capillaries

• One cell thick

• Function: supply all cells with their requirements (food and oxygen) and take away waste products (Carbon dioxide and urea)

• The lumen is very narrow, just wide enough for a red blood cell to squeeze through

• Reason for structure: strong wall not required as most of the pressure of the blood has been lost. Thin walls and narrow lumen bring blood into close contact with tissues.

Differences Between Arteries and Veins

Arteries	Veins
Carry blood away from the heart	Carry blood towards the heart
Blood is under high pressure hence it flows fast, in spurts, reflecting the rhythmic pumping action of the heart	Blood is not under high pressure and hence it flows more slowly and smoothly
Have thick and elastic muscular walls	Have relatively thin, slightly muscular walls
Have no semi lunar valves	Have semi lunar valves along their length to prevent back flow of blood
Carry red oxygenated blood (except: pulmonary arteries which carry deoxygenated blood from the heart to the lungs and umbilical arteries which carry deoxygenated blood from the foetus to the mother's blood)	Carry bluish-red deoxygenated blood (except: pulmonary veins which carry oxygenated blood from the lung to the heart and umbilical vein which carries oxygenated blood from the mother's blood to the foetus)

Double Circulation in Mammals

In mammals, there is a double circulation because of the lungs. Blood passes through the heart twice in one complete circuit. The double circulation in mammals consists of two parts:

• Pulmonary circulation: From the heart, the pulmonary arteries carry the deoxygenated blood to the lungs. In the lungs the blood gains oxygen and at the same time releases carbon dioxide. The oxygenated blood is then returned to the heart by the pulmonary veins

• Systemic circulation: From the left side of the heart, the oxygenated blood is then distributed by arteries to all parts of the body (except the lungs). In the body parts the blood releases the oxygen to be used for tissue respiration and at the same time gains carbon dioxide. From the body parts, the now deoxygenated blood is then carried back to the right side of the heart by veins.

Advantages of Double Circulation:

• Blood entering the lungs is at a low pressure. This ensures that the blood well oxygenated before it is returned to the heart

• Blood leaving the heart for the systemic circulation is at a high pressure. This ensures that the oxygenated blood is distributed to the body tissues at a faster rate, thus maintain the high metabolic rate in mammals)

THE HEART

• Lies in the thorax behind the chest-bone and between the two lungs

• Function: to pump blood all over the body

• The pumping action of the heart is driven by cardiac muscle in the walls

• Surrounded by two-layered bag known as pericardium. The inner membrane is in contact with the heart. Between the two layers of pericardial membranes is the pericardial fluid which helps to reduce friction when the heart is beating.

• Has four chambers: the two upper chambers are the auricles or atria (singular: atrium) whilst the two lower chambers are the ventricles.

• The right side is completely separated from the left side by means of a muscular wall called the median septum. Hence the deoxygenated blood in the right side is unable to mix with the oxygenated blood in the left side.

• Tricuspid valve: lies between the right atrium and the right ventricle. Consists of three flaps (hence the name). The flaps are attached to the walls of the right ventricles by tendons. The flaps point downwards to permit easy flow of blood from the atrium into the ventricle.

• Bicuspid valve (mitral valve): lies between the left atrium and the left ventricle. This is similar in structure and function to the tricuspid valve except that it has two flaps instead of three flaps.

• Tricuspid and bicuspid valves prevent backflow of blood from the ventricles to the atria.

• Right ventricle has thinner walls than the left ventricle? The right ventricle pumps blood to the lungs which are a short distance from the heart. Therefore, the blood in the pulmonary arteries is at a lower pressure than the blood in the aorta. This gives sufficient time for gaseous exchange to occur in the lungs

• The atria have thinner walls than the ventricles? The atria only have to force blood into the ventricles and this does not require much power. On the other hand the ventricles have to force blood out of the heart. Hence they have relatively thick walls especially the left ventricle which has to pump blood round the whole body.

• Coronary arteries supply the heart with oxygen and food substances.

Mode of Action of the Heart

• When the two atria contract, blood is forced into the relaxed ventricles. After a slight pause, the two ventricles contract forcing the blood into the aortic arch and pulmonary arch respectively.

• The backflow of blood into the atria is prevented by the sudden closing of the tricuspid and the bicuspid valves.

• The closing of these valves produces a loud “lub” sound which we can hear in a heartbeat

• After the ventricles have fully contracted, they start to relax. As they relax, the blood in the arteries tends to flow back into the ventricles. This is prevented by the sudden closing of the semi-lunar valves which produces a soft “dub” sound.

• Ventricular contraction (systole) makes a “lub” sound

• Ventricular relaxation (diastole) makes a “dub” sound

• A systole and a diastole make up one heartbeat

• There is a short pause between two heartbeats

• The average normal heart beat of an adult is about 72 times per minute

Blood Pressure

• Blood pressure is the force of the blood exerted on the walls of the blood vessels

• Blood pressure is the highest during systole and the lowest during diastole

• Blood pressure varies, being the highest near the aortic arch and becoming weaker the further away the arteries are from the heart

• Blood pressure is very low in veins, being the lowest in the vena cava

• Blood pressure varies with the individual person. An average person has a systolic pressure ranging from 120 to 140mm of mercury and a diastolic pressure ranging from 75 to 90mm of mercury.

• Blood pressure can be measured using sphygmomanometer

The Pulse

• Every time the ventricles contract, blood is pumped into the aortic arch and into the arteries which are already filled with blood

• The sudden increase in pressure causes the arteries to dilate

• After each dilation, the walls of the arteries recoil and force the blood along in a series of waves

• Each wave is called the pulse

• The pulse rate is the same for all arteries though the pulse is weaker in parts of the artery furthest from the heart

• A pulse is produced after every ventricular contraction

• By counting the number of pulse beats per minute we actually get the number of heartbeats per minute

CORONARY HEART DISEASE (CHD)

• The function of the coronary arteries of the heart is to supply the heart with blood containing the necessary nutrients and oxygen

• If these arteries become blocked or narrowed by the build up of fatty deposits, the blood supply to the heart muscles can be greatly reduced

• This can cause angina (chest pain) and heart attack

• Angina: the blood flow to the heart muscles is sufficient at rest. During exercise, when the heart rate is much higher, the coronary arteries are unable to deliver the extra blood to meet the demand imposed. This results in chest pain.

• Obstructions in the coronary arteries can be caused by atherosclerosis.

• Atherosclerosis: this is a condition in which fatty materials are deposited in the lining of the arteries forming atheroma. This results in the narrowing of the diameter of the coronary arteries. Consequently, blood flow to the heart is limited

• The rough inner surface of such affected coronary arteries increases the risk of a blood clot being trapped in it. The formation of a local blood clot (thrombus) in an artery is called a thrombosis. If it occurs in the coronary arteries, the supply of blood may be completely cut off, resulting in a heart attack. In this case, the muscles in the affected region of the heart die due to shortage of oxygen and nutrients.

Strokes

• Generally similar to heart disease, but in strokes, it is the arteries supplying blood containing nutrients and oxygen to the brain are blocked.

• The results of a stroke depend on the area of the brain affected

• Muscles may be paralysed, and speech or memory affected.

• Death occurs if the brain damage is extensive.

Causes of Heart Disease

• Smoking: carbon monoxide and other chemicals in cigarette smoke may damage the inner lining of the arteries, resulting in atheroma formation.

• Fatty diet: cholesterol (coming from fatty food, particularly from animal fats) may be deposited in the inner lining of the arteries forming atheroma.

• Stress: this often leads to a raised blood pressure. High blood pressure may increase the rate at which atheroma is formed in the arteries.

• Lack of exercise: sedentary life style and lack of exercise may slow down blood flow. This may lead to atheroma formation in the arteries. In addition, people who do not exercise enough, may not burn as much fats as those who do regular exercise. Fats are easily deposited in the arteries of those people who do not exercise enough.

THE LYMPHATIC SYSTEM

Lymph and Tissue Fluid

• Capillaries leak! The cells in the walls of the blood capillaries do not fit together exactly. So there are small gaps between them. Plasma can therefore leak out from the blood.

• White blood cells can also get through these gaps in the walls of the blood capillaries. They are able to move and can squeeze through out of the capillaries

• Red blood cells however are too large and cannot pass through these gaps

• So plasma and white cells are continually leaking out of the blood capillaries

• The fluid formed in this way is called tissue fluid. It surrounds all the cells in the body.

Function of Tissue Fluid

• It supplies cells with oxygen and food nutrients diffuse from the blood

• It also removes waste products such as carbon dioxide from the body cells back into the blood through the walls of the capillaries by diffusion

Lymph

• The plasma and white blood cells which leak out of the blood capillaries must eventually be returned to the blood.

• In the tissues, together with the blood capillaries are the lymphatic capillaries

• The tissue fluid slowly drains into the lymphatic capillaries. The fluid is now called lymph.

• The lymphatic capillaries gradually join up to form larger lymphatic vessels.

• The lymphatic vessels carry the lymph to the sub-clavian veins where the lymph enters the blood again.

• The lymphatic system has no pump to make the lymph flow.

• The lymph vessels do have valves to ensure that movement is only in one direction.

• Lymph flows much more slowly than blood

• On its way from the tissues to the sub-clavian vein, lymph flows through several lymph nodes.

• Lymph nodes contain large numbers of white cells. Most bacteria or toxins in the lymph can be destroyed by these cells.