Introduction

• The components of the circulatory system are the heart, blood vessels and blood.

• Blood passes through the heart twice in a complete circuit. This is termed double circulation.

• Double circulation consists of:

  (a) Systemic circulation – Carries oxygenated blood (oxygen-rich) from the heart to all body organs and returns oxygen-poor blood to the heart

  (b) Pulmonary circulation–Carries deoxygenated blood(oxygen-poor)from the heart to the lungs for gaseous exchange before returning blood to the heart for transport to the body organs via systemic circulation

• The three main types of blood vessels are: (a) Arteries – Vessels that carry blood away from the heart to body organs. Arteries branch into arterioles and then into capillaries. (b) Capillaries – Microscopic vessels that connect between the arteries and veins. They converge into venules which converge into veins. They form networks called capillary beds that are present in most body tissues. (c) Veins – Vessels that return blood to the heart

• The main vessels of the human circulatory system are: (a) Pulmonary arteries that supply oxygen-poor blood from the heart to the lungs (b) Pulmonary veins that bring oxygen-rich blood from the lungs to the heart (c) Aorta that supplies oxygen-rich blood from the heart to the rest of the body. The aorta branches into: coronary arteries which supply cardiac tissue, an anterior branch leading to the head and arms and a posterior branch (dorsal aorta) leading to abdominal organs and legs. (d) Branches of the dorsal aorta include:

  (i) Hepatic artery from the heart to the liver

  (ii) Arteries to the alimentary canal

  (iii) Renal arteries from the heart to the kidneys

  (e) Vena cava consists of an anterior branch which returns blood from the head and arms to the heart and a posterior branch. (f) Posterior vena cava collects blood from the posterior parts of the body, such as from:

  (i) Hepatic veins from the liver to the heart

  (ii) Renal veins from the kidneys to the heart

  (g) Hepatic portal vein transports blood from the alimentary canal to the liver. Blood from the liver is returned to the heart via the hepatic vein.

Role of blood in transport

• Blood plasma transports: (a) Simple sugars, amino acids, fatty acids and glycerol from the capillaries in the small intestine (b) Waste products of metabolism from tissues:

  (i) Carbon dioxide in the form of bicarbonate ions. Carbon dioxide enters the blood from body tissues by diffusion into red blood cells, which contain the enzyme carbonic anhydrase to convert it to hydrogen carbonate. The hydrogen carbonate then diffuses out of red blood cells to be carried in plasma. In the lungs, the reverse occurs.

  (ii) Nitrogenous waste products such as urea, uric acid and creatinine to the kidneys to be removed

  (c) Hormones from the glands to target tissues (d) Heat from muscles and liver throughout the body

• Red blood cells transport: (a) Oxygen as oxyhaemoglobin (b) A small amount of carbon dioxide bound to haemoglobin

Transport of oxygen by red blood cells

• As air enters the lungs, oxygen dissolves in the fluid covering the moist epithelium of the alveoli.

• The oxygen diffuses into the capillaries of the lungs where they bind reversibly with haemoglobin in red blood cells to form oxyhaemoglobin.

• When blood is transported to oxygen-poor respiring tissues, oxyhaemoglobin releases its oxygen which then diffuses into tissue cells.

Function of white blood cells

• Phagocytosis refers to the ingestion of harmful foreign particles, bacteria and dead or dying cells by certain types of white blood cells called phagocytes.

• When phagocytes detect a foreign particle, it engulfs it by stretching itself around the particle and enclosing it. It then digests the particle and kills it.

• After phagocytosis, these cells die and form pus.

• Antibodies are special proteins found in blood and other bodily fluids that help phagocytes identify and neutralise foreign particles. Antibodies also activate other immune responses.

• When pathogens enter the blood, they stimulate lymphocytes to produce antibodies.

• Antibodies may be present in the blood long after infection has been cured, conferring immunity to that particular infection.

Blood Clotting

• The blood clotting process begins at the site of injury when blood vessels are damaged.

• Platelets are activated, and the damaged tissue and activated platelets release thrombokinase.

• Thrombokinase converts plasma protein, prothrombin, into thrombin in the presence of calcium and vitamin K.

• Thrombin converts fibrinogen, a soluble plasma protein, to fibrin, an insoluble protein that forms long threads.

• Fibrin forms a mesh across the damaged surface and traps red blood cells, forming a clot.

• The clot prevents further blood loss, and also restricts the entry of pathogens into the blood.

Types of Blood Vessels

Exchange of substances in capillaries

• Capillaries are found between tissue cells.

• As blood enters the capillaries, the narrow lumen of the capillaries forces red blood cells to travel in a single line.

• Rate of blood flow decreases, allowing more time for the exchange of materials between tissue cells and red blood cells.

• At the arterial end of capillaries, the blood pressure is high, forcing plasma through capillary walls into tissues. Plasma proteins are unable to pass through capillary walls.

• The solution bathing tissue cells becomes known as tissue fluid, or interstitial fluid.

• There is a higher concentration of nutrients and oxygen in blood than in the interstitial fluid. They diffuse across the endothelium of the capillary into the interstitial fluid, and from there, across the plasma membranes of tissue cells.

• Waste materials from the tissue cells diffuse into the interstitial fluid, where they are present in higher concentrations than within the blood. They diffuse across the endothelium of the capillary into blood and are transported to excretory organs for removal.

Structure of the heart

• A diagram of the heart and its associated blood vessels is shown below:

  

• The heart is mainly made up of cardiac muscle tissue surrounded by a double- walled sac called a pericardium. The inner membrane of the pericardium is connected to the outer layer of the cardiac muscle. Between the two layers is the pericardial fluid, which reduces friction when the heart is beating.

• The four chambers of the heart are the right and left atria and ventricles.

• The atria are the upper chambers of the heart, with relatively thin walls. They collect blood returning to the heart and pump it into the ventricles.

• The ventricles have thick, muscular walls. The left ventricle has thicker walls than the right ventricle, as it has to pump blood to the rest of the body.

• The right side of the heart pumps deoxygenated blood and the left side pumps oxygenated blood. The septum separating the right and left sides prevent the blood from mixing, so that the maximum amount of oxygen can be carried to the tissues.

• Between the right atrium and ventricle is a valve called the tricuspid valve.

• Between the left atrium and left ventricle is a bicuspid valve (mitral valve).

• The bicuspid and tricuspid valves are collectively known as atrioventricular valves.

• Vessels associated with the heart are the anterior and posterior venae cavae, pulmonary veins and artery, aorta and coronary arteries. The coronary arteries are found on the heart surface itself, and supply blood to the heart muscles.

• Located at the start of the aorta and pulmonary arteries are semi-lunar valves.

Coronary heart disease

• Coronary heart disease occurs when the coronary arteries become blocked (occluded) or narrowed.

• The heart muscles will no longer be able to receive sufficient oxygen and nutrients.

• This can cause a heart attack. During a heart attack, blood supply to part of the heart muscle is completely cut off due to blockage in the coronary arteries. The affected part dies, which can affect the heart’s ability to pump and lead to heart failure.

• A cause of coronary heart disease is atherosclerosis, in which an artery wall thickens and hardens due to the deposition of plaque, which causes the lumen of the artery to become narrower.

• The narrowing of the lumen of the arteries causes an increase in blood pressure. This causes arteries to develop rough linings, which increases the likelihood of formation of blood clots inside the arteries. This is known as thrombosis.

• This obstructs blood flow in the afflicted artery. If it occurs in a coronary artery, a heart attack takes place.

• Factors that contribute to atherosclerosis include:

  (a) High intake of cholesterol and saturated fats

  (b) Stress

  (c) Smoking

• Preventive measures include: (a) Healthy diet – low in cholesterol and saturated fats

  (b) Not smoking – nicotine increases blood pressure

  (c) Exercising – lowers stress and strengthens the heart