Blood

Blood contains mainly four substances, namely:

• Plasma

It is a yellowish liquid. About 90% of plasma is water in which a complex mixture of various substances are dissolved. Examples of such substances are soluble proteins such as serum albumin, serum globulin, fibrinogen and prothrombin. Fibrinogen and prothrombin play an important part in the clotting of blood. Antibodies are also present which fight diseases. Other substances found in plasma are dissolved mineral salts, food substances such as glucose, amino acids, fats and vitamins, excretory products and finally, hormones.

Red blood cells (erythrocytes)

Red blood cells are produced by the bone marrow. They are circular, flattened and biconcave in shape as they have no nucleus. A red pigment haemoglobin in the red blood cell is essential in transporting oxygen as it contains the protein iron. Each red blood cell is less that 0.01 mm large and is elastic, aiding it in squeezing through capillaries smaller than itself. Its lifespan is 3 to 4 months and is worn out and destroyed after the lifespan.

• White blood cells (Leucocytes)

White blood cells are colourless and do not contain haemoglobin. They, however, contain a nucleus. Red blood cells outnumber them by the ratio of   700:1. It is irregular in shape and contains a nucleus. It can move and change its shape and squeeze through walls of fine blood vessels into the spaces among the tissue cells. The two main types of white blood cells are the lymphocytes and the phagocytes. Lymphocytes are produced by the lymph glands or lymph nodes. They tend to be nearly round and show little signs of movement. Phagocytes are produced by the bone marrow. They can ingest foreign particles such as bacteria, thus, these cells play a vital role in keeping our body healthy.

Blood platelets (Thrombocytes)
These platelets help in the clotting of the blood. When there is a wound, the platelets gather at the wound and seal it up to stop the bleeding. Blood also transports digested food substances, excretory products, hormones, heat, and oxygen.

Heart

The Structure of the Heart

The heart of a mammal is a complicated organ. Its size varies with the animal and in man, it is about the size of a clenched fist. It lies in the thorax between the chest bone and between the two lungs. It is conical in shape and slants with its apex directed slightly towards the left side of the body. The whole heart is surrounded by a two-layered bag known as the pericardium, in which the inner membrane being in contact with the heart. Between the two-pericardial membranes is the perifluid, which helps to reduce friction when the heart is beating.

The mammalian heart has four chambers: two upper chambers called the auricles or the atria, below that are two large chambers known as ventricles. The right side of the heart is completely separated from the left side by means of a muscular wall (the median septum) which runs down the middle of the heart. In this way deoxygenated blood in the right side is unable to mix with the oxygenated blood in the left side. Blood from various parts of the body is returned to the right atrium. Blood from the head, neck and arms is returned to the right atrium by a large vein called the common anterior or superior vena cava. Blood from the other parts of the body is brought back by the posterior or interior vena cava. Thus the right atrium receives deoxygenated blood from the two venae cavae. The union of left and right anterior venae cavae forms the superior vena cava.

When the right atrium contracts the blood flows into the right ventricle. Between the right atrium and the right ventricle is the tricuspid valve, which consists of three flaps. These flaps are attached to muscular projections (papillary muscles) in the walls of the right ventricle by cord-like tendons called chordae tendineae. They point downwards to permit easy flow of blood from the atrium into the ventricle. When the right ventricle contracts, the blood pressure forces the flaps to close the opening into the atrium. This prevents back flow of blood into the atrium. The chordae tendineae prevents the flaps from being turned back into the atrium. The blood leaves the right ventricle by only one route, namely the pulmonary arch. The latter leaves the heart and divides into two pulmonary arteries, one to each lung. Return of blood to the ventricle is prevented semi-lunar valves in the pulmonary arch.

Oxygenated blood from the lungs is brought back to the heart by way of the pulmonary veins that open into the left atrium. Blood from the left atrium enters the left ventricle. Between the left atrium and the left ventricle is the bicuspid valve or mitral valve. This is similar in structure and function of the tricuspid valve except that it has two flaps instead of three. When the left ventricle contracts blood leaves by way of a large artery, the aortic arch. From the aortic arch, blood is distributed to all parts of the body.

The aortic arch curves upward from the left ventricle as a U-shaped tube and it also possesses semi-lunar valves to prevent the back flow of blood into the left ventricle. The blood entering the aorta is at a high pressure.

The right ventricle has thinner walls than the left ventricle. It pumps blood into the lungs that 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 also means a slower blood flow, allowing time for gaseous exchange in the lungs. Two small coronary arteries originate from the aortic arch. They give off branches that supply oxygen and food substances to the heart muscles.

The ventricles have thicker walls when compared to the atria, as they have to force blood out of the heart while the atria only have to force blood into the ventricles.

Mode of Action of the Heart

The two atria of the heart work simultaneously. They relax at the same time to receive the blood from the veins. The right atrium receives blood from the two venae cavae while the left atrium receives blood from the pulmonary veins. The two atria then contract, forcing the blood into the ventricles. After a pause, the two ventricles contract. Blood from the right ventricle enters the pulmonary arch and the blood from the left ventricle enters the aortic arch. After the ventricles have fully contracted, they relax. As they relax a systole and a diastole make up one heartbeat. There is a short pause between two heartbeats. The rate of heartbeat varies with age and size of the individual.

The Main Arteries and Veins

Arteries

The arteries leaving the heart are the pulmonary arteries from the right ventricle and the aortic arch from the left ventricle. From the aortic arch the following arteries arise:

• The carotid arteries which supply blood to the head and neck
• The subclavian arteries which supply blood to the arms or forelimbs, one subclavian artery to each arm
• The dorsal aorta. The aortic arch curls backward to the left side of the heart and continues downwards or posteriorly as the dorsal aorta. The dorsal aorta lies on the left side of the backbone and runs backwards, parallel to the backbone. From the dorsal aorta the following main arteries are given off: - The hepatic artery to the liver, the mesenteric arteries to the intestines, the renal arteries, one to each kidney, the iliac arteries, one to each hindlimb or leg.

Veins

Blood is returned to the heart by the main veins as follows:

• The pulmonary veins bring blood from the lungs to the left atrium of the heart.
• The jugular veins collect blood from the head and neck.
• The subclavian veins bring blood from the forelimbs. The jugular and the subclavian veins unite to form the superior vena cava, which opens into the right atrium of the head.
• The inferior vena cava which runs forward along the mammal's back, parallel to the dorsal aorta and close to the backbone.

As the inferior vena cava travels forward through the abdominal cavity it is joint by whole parts of our body. Amongst these veins are: