There are 3 types of blood cells known as erythrocytes, leucocytes and thrombocytes.
Erythrocytes transport oxygen around the body. It is carried as oxyhemoglobin. Hemoglobin is the protein that absorbs the oxygen. Erythrocytes are produced in the bone marrow, they last around 120 days and they are broken down in the spleen and liver. (Tucker 2000:55)
Leucocytes protect the body. They increase very quickly when the body has an infection. They defend the system against viruses and bacteria, produce antibodies, eat bacteria and other micro organisms. (Tucker 2000:55)
Thrombocytes also called platelets are the cells responsible for blood clotting. They are formed in the red bone marrow. (Tucker 2000:55) When damaged they release an enzyme called thromboplastin, this converts a plasma protein called prothrombin into an active enzyme called thrombin. This process needs calcium. Thrombin then changes the plasma protein fibrinogen into fibrin. It’s the fibrin that blocks the damaged blood vessel by forming a net like cover. This net catches the blood cells which then form into a clot. (Tucker 2000:65)
3. The Heart
The heart is a muscle that is around the size of the person’s fist. Its purpose is to pump blood around the body. It beats around 70 times per minute. (NHS Direct, Undated). It is positioned in the thorax. (Tucker, 2000:56)
3.1 External Structure of the heart
The heart (heart wall) is made up of 3 layers of tissue: Pericardium, myocardium and endocardium.
The pericardium is made up of 2 parts. The outer part is made of fibrous tissue and it prevents over distension of the heart. The inner part is made up of serous membrane which has a pericardial space inside it containing pericardial fluid. This fluid allows movement between the layers when the heart beats.
The myocardium is made up of cardiac muscle. The endocardium forms the lining of the myocardium; it is a smooth membrane which allows the flow of blood through the heart. (Waugh & Grant, 2001:83-84)
3.2 Internal structure of the heart
The heart is divided into 4 chambers. The left and right atriums are at the top of the heart and the left and right ventricles at the bottom of the heart. The atriums and ventricles are separated by atrioventricular valves. The right and left side of the heart is separated by a muscular wall called the septum. This prevents the mixing of venous and arterial blood. (Tucker, 2000:56)
3.3 Blood flow through the heart
Deoxygenated blood enters the heart via the superior and inferior vena cava. It is pumped from the right atrium through the tricuspid valve into the right ventricle. It leaves the right ventricle via the pulmonary valve into the pulmonary artery. This is when the blood leaves the heart and goes through the lungs where gas exchange takes place. It leaves the lungs oxygenated and re-enters the heart through the pulmonary veins which lead into the left atrium. Then the blood is pumped through the mitral valve into the left ventricle. The left ventricle pushes the blood up through the aortic valve along the aorta which splits into the descending and ascending aorta and then the blood is taken around the body. (Figure 1. The normal heart)
Fig 1 (UAB Health Systems /2007)
4. The Blood vessels
The heart pumps blood around the body via various blood vessels.
Arteries and arterioles are blood vessels that carry blood away from the heart. There are many different sizes but they are all made up of 3 layers of tissue, the tunica intima inside layer, the tunica media and the tunica adventitia the outside layer. The inside of the vessel is called the lumen. Artery walls are thick because they need to be strong for the high pressure of arterial blood.
Veins and venules return blood to the heart at a lower pressure. They have the same 3 layers as arteries and arterioles but the walls are thinner because there is less muscle and elastic tissue. Some veins have valves which prevent the back flow of blood. These are found mainly in the lower limbs of the body. (Waugh & Grant, 2001:78-79)
5. Conducting mechanism of the heart
The heart has its own conductive system which stimulates the heart to beat automatically. The myocardium contains specialized neuromuscular cells which initiate and conduct impulses which synchronize the contraction of the heart. The Sinoatrial node (SA node) is found in the wall of the right atrium near the opening of the superior vena cava and is known as the pacemaker. The atrioventricular node (AV node) is situated in the wall of the atrial septum. The atrioventricular bundle is a group of specialized fibres which branch into finer fibres called the purkinje fibres. These convey electrical impulses from the AV node to the apex of the myocardium, causing the wave of contractions which pump blood into the pulmonary artery and the aorta. (Waugh & Grant, 2001:87)
6. Circulation
6.1 Systematic circulation
Systematic circulation is the flow of blood from the heart to the body as mentioned in detail earlier in paragraph 3.3 (blood flow through the heart).
6.2 Pulmonary circulation
As shown in figure 2 (circulation), pulmonary circulation is the circulation of blood from the heart to the lungs and back. Deoxygenated blood leaves the heart via the pulmonary artery; in the lungs the gas exchange takes place. The carbon dioxide is replaced with oxygen. The blood returns to the heart via the pulmonary veins. (Tucker, 2000:58)
Fig 2 Circulation (Waugh & Grant /2001)
6.3 Portal circulation
Veins from the stomach, spleen, pancreas and intestines join up and form the hepatic portal vein. The hepatic portal vein carries blood into the liver. (Tucker, 2000: 59)
6.4 Coronary circulation
The heart has its own blood supply. It is supplied with blood by the left and right coronary arteries. These branch off from the aorta. The venous blood is collected and flows through to the coronary sinus into the right atrium.(Waugh & Grant , 2000: 87)
7. Coronary heart disease
When the blood supply to the heart is blocked or interrupted this is called coronary heart disease. This is mostly caused by the build up of fatty substances in the coronary arteries. The substance is called atheroma and the narrowing of the arteries is called atherosclerosis. The vessels can become hard and loose their elasticity. If the arteries become completely blocked a heart attack (myocardial infarction) can occur. (NHS Direct / Undated)
Atheroma is an accumulation of cholesterol and other lipid compounds, excess smooth muscle and fat filled monocytes. This fat spreads along the artery wall causing swelling and or obstruction. The arteries can become partially or completely blocked. (Waugh & Grant, 2001: 112)
There are 2 types of cholesterol in the body, LDL known as bad cholesterol is what builds up on the walls of the coronary arteries, increasing the risk of heart disease. HDL or good cholesterol carries the cholesterol away from the cells back to the liver to be broken down and excreted. Normally the body has 70% LDL but if it is higher than this it can not be removed by the HDL. (NHS Direct, undated: 4)
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When the coronary arteries become partially blocked the patient can suffer with pain in the chest known as angina. 1 in 50 people suffer with angina in the UK. (NHS Direct, undated:4) When the arteries are narrowed or occluded by the atheroma ischemic heart disease occurs.
Many things contribute to the build up of this plaque like substance. The most common attributes are: Heredity factors, age, hypertension, diabetes mellitus, smoking, stress, poor diet, obesity, lack of exercise and too much alcohol. (Waugh & Grant, 2001:113)
Coronary thrombosis can occur if there is a blood clot in one of the coronary arteries. These clots stop the blood supply to the heart muscle. (NHS direc, undated: 4)
When calcium salts are deposited in the plaque, the arteries can become brittle and break, this can lead to hemorrhage. (Waugh & Grant, 2001:113)
A number of different tests can be used to detect heart problems. A radionuclide test can be used to show how strongly the heart pumps and can help diagnose coronary heart disease. Magnetic resonance imaging and electrocardiograms are used commonly to detect heart problems and measure the flow of blood through the heart and its arteries. (NHS Direct, undated: 6)
Coronary heart disease can not be cured but it can be maintained and its symptoms can be reduced and controlled. Blood clotting is the main cause of heart attack and a low dose aspirin or clot busting medicine will prevent he blood from clotting. (NHS Direct, undated: 8). Statins are medicines that lower the cholesterol in the body. They prevent atheroma from forming or increasing in the blood vessels. Beta blockers prevent angina while reducing high blood pressure. The effects of stress are reduced and the flow of blood through the heart is improved. (NHS Direct, undated: 9) There are many different types of prescribed medicines that can improve heart health but sometime surgery is needed. Angioplasty can be used to open up a blocked artery. In simple terms a mesh like tube is inserted into the artery and inflated by a small balloon. If this cant be done then a coronary bypass can be performed so that blood is diverted and bypass the restricted artery. In severe cases a heart transplant may be performed. (NHS Direct, undated: 11)
The best thing is prevention, and the best way to prevent heart disease is to limit the amount of bad cholesterol in the diet, and eat a healthy diet which is low in saturated fat, sugar and salt. Exercise increases the amount of good cholesterol and obviously can prevent obesity. Smoking encourages the hardening of the arteries and is the main cause of coronary thrombosis in people under the age of 50.
8. References
NHS Direct. (undated) Health Encyclopedia, Coronary Heart Disease. [Online] Available from:
[Accessed: 9 November 2007].
Seeley, R. & Stephens,T. & Tate, P. (2003) Anatomy & Physiology. 6th Ed, New York: McGraw Hill.
Tucker, L. (2000) An Introductory Guide to Anatomy & Physiology. Cambridge: Ruben Publishing Ltd.
UAB Health System (2007). Normal heart [Online] Alabama. available from:
[Accessed 19 November 2007]
Waugh, A. & Grant, A. (2001) Ross and Wilson Anatomy & Physiology in Health & Illness. 9th Ed. London: Churchill Livingston an imprint of Elsevier Science Ltd.