Describe the mechanism of transportation of materials in the human body (the mammalian heart, blood vessels, blood, blood circulation)

Location and External Structure

The human heart is located in the center of the chest cavity between the two lungs, slightly pointing to the left side. It is enclosed in a double layer of tough inelastic membrane called the pericardium, which protects the heart and secretes lubricating fluid to reduce friction during beating.

The heart wall has three layers:

• Epicardium (outer layer)
• Myocardium (middle layer containing cardiac muscle)
• Endocardium (inner layer)

Internal Structure

The heart has four chambers:

• Right atrium (formerly right auricle)
• Right ventricle
• Left atrium (formerly left auricle)
• Left ventricle

The walls of the ventricles are thicker than those of the atria because ventricles pump blood to longer distances. The left ventricle has the thickest walls because it pumps oxygenated blood to the entire body, requiring high pressure.

Heart Valves

The heart contains valves that ensure one-way blood flow:

• Tricuspid valve – between right atrium and right ventricle
• Bicuspid (mitral) valve – between left atrium and left ventricle
• Semilunar valves – at the bases of the pulmonary artery and aorta

The septum is a muscular wall that separates the left and right sides of the heart, preventing mixing of oxygenated and deoxygenated blood.

Blood Flow Through the Heart

1. Deoxygenated blood enters the right atrium through the vena cava (superior and inferior)
2. Right atrium contracts, pushing blood through the tricuspid valve into the right ventricle
3. Right ventricle contracts, pumping blood through the pulmonary artery to the lungs
4. In the lungs, blood picks up oxygen and releases carbon dioxide (gas exchange)
5. Oxygenated blood returns to the left atrium through the pulmonary vein
6. Left atrium contracts, pushing blood through the bicuspid valve into the left ventricle
7. Left ventricle contracts strongly, pumping oxygenated blood through the aorta to the body

The heart beats approximately 60-80 times per minute in adults. The sinoatrial node (SAN) acts as a natural pacemaker, setting the rate of contraction.

Arteries

Arteries are thick-walled, muscular, and elastic vessels that carry blood away from the heart. They have:

• Narrow lumens
• Thick muscular walls
• No valves (except where connected to the heart)
• High blood pressure

All arteries carry oxygenated blood except the pulmonary artery, which carries deoxygenated blood to the lungs.

The rhythmic contraction and relaxation of arteries creates a pulse that can be felt at various points in the body.

Veins

Veins are blood vessels that carry blood toward the heart. They have:

• Wide lumens
• Thin muscular walls
• Valves at regular intervals to prevent backflow
• Low blood pressure

All veins carry deoxygenated blood except the pulmonary vein, which carries oxygenated blood from the lungs to the heart.

Capillaries

Capillaries are the smallest blood vessels. They:

• Connect arterioles and venules
• Have walls only one cell thick
• Are permeable, allowing exchange of materials between blood and tissues
• Form networks called capillary beds in tissues

Blood is a specialized fluid tissue consisting of four main components:

1. Plasma (55%)

A pale-yellow fluid containing:

• Water (92%)
• Dissolved proteins (albumins, globulins, fibrinogen)
• Dissolved salts, hormones, and enzymes
• Nutrients, oxygen, and waste products

Functions: Transports nutrients, wastes, hormones, and blood cells; distributes heat; maintains pH balance.

2. Red Blood Cells (Erythrocytes)

• Red, biconcave, and lack a nucleus when mature
• Contain haemoglobin – a red pigment that binds oxygen
• Transport oxygen from lungs to tissues and carbon dioxide from tissues to lungs
• Formed in red bone marrow; destroyed in liver and spleen after about 120 days

Oxygen transport: Haemoglobin + oxygen ⇌ oxyhaemoglobin

3. White Blood Cells (Leucocytes)

• Irregular shaped cells with nuclei
• Protect the body against infections
• Perform phagocytosis (engulfing pathogens)
• Produced in bone marrow and lymph nodes

4. Platelets (Thrombocytes)

• Small fragments of cells without nuclei
• Essential for blood clotting
• Help form clots to stop bleeding and prevent entry of bacteria

Human beings have double circulation – blood passes through the heart twice for each complete circuit.

Pulmonary Circulation

This is the movement of blood between the heart and lungs:

• Deoxygenated blood → right ventricle → pulmonary artery → lungs
• Oxygenated blood → pulmonary vein → left atrium

Systemic Circulation

This is the movement of blood between the heart and all body parts:

• Oxygenated blood → left ventricle → aorta → body tissues
• Deoxygenated blood → vena cava → right atrium

Formation of Tissue Fluid

At the arterial end of capillaries, high blood pressure forces water, oxygen, and nutrients out through capillary pores to form tissue fluid. This bathes the cells, allowing exchange of materials. At the venous end, tissue fluid returns to capillaries. Excess fluid enters the lymphatic system.

ABO Blood Groups

Rhesus Factor

• Rh⁺ – Rhesus factor present on red blood cells
• Rh⁻ – Rhesus factor absent

Blood Compatibility

• Group O – Universal donor (can give to all groups)
• Group AB – Universal recipient (can receive from all groups)

Successful blood transfusion requires compatible blood groups to prevent agglutination (clumping of red blood cells), which can be fatal.

• Hypertension – High blood pressure (above 140/90 mmHg); controlled by exercise, balanced diet, reducing salt intake
• Arteriosclerosis – Hardening and narrowing of arteries due to fat deposits
• Coronary Thrombosis – Blood clot in coronary arteries supplying the heart
• Stroke – Blockage or rupture of arteries supplying the brain
• Sickle-cell Anaemia – Genetic disorder producing abnormal haemoglobin and sickle-shaped red blood cells

Understanding the circulatory system is important for everyday life in Tanzania. For example, when you visit a health facility for a medical check-up, knowing about blood pressure helps you understand why the nurse measures it – normal blood pressure is about 120/80 mmHg. If someone needs a blood transfusion due to surgery or an accident, knowledge of blood groups ensures the correct type (O, A, B, or AB) is used, preventing dangerous reactions. Additionally, avoiding excessive fatty foods, exercising regularly, and reducing salt intake can prevent hypertension and heart diseases, which are increasingly common in Tanzania.