SS2: WEEK 4, SECOND TERM CIRCULATORY SYSTEM

Introduction to the Circulatory System in Mammals

The circulatory system in mammals is a transport system responsible for the movement of blood throughout the body. It ensures the delivery of oxygen, nutrients, hormones, and antibodies to body tissues and the removal of metabolic wastes such as carbon dioxide and urea. Mammals possess a closed, double circulatory system, which allows blood to flow efficiently under high pressure.

Components of the Mammalian Circulatory System

The circulatory system consists of:

1. The Heart – a muscular pump that drives blood circulation.

2. Blood Vessels – arteries, veins, and capillaries that transport blood.

3. Blood – the fluid medium that carries substances.

Structure of the Mammalian Heart

The mammalian heart is a four-chambered muscular organ located in the thoracic cavity between the lungs, slightly tilted to the left. It is enclosed by a protective membrane called the pericardium.

 External Structure

1. Pericardium: A double-layered membrane containing pericardial fluid that reduces friction during heartbeats.

2. Coronary arteries and veins: Supply oxygenated blood to the heart muscle itself.

Internal Structure

The heart is divided into four chambers:

1. Right Atrium—receives deoxygenated blood from the body through the superior and inferior vena cava.

2. Right Ventricle – pumps deoxygenated blood to the lungs via the pulmonary artery.

3. Left Atrium – receives oxygenated blood from the lungs through the pulmonary veins.

4. Left Ventricle – pumps oxygenated blood to the rest of the body through the aorta.

Heart Valves

Valves ensure one-way flow of blood:

1. Tricuspid valve – between right atrium and right ventricle.

2. Bicuspid (Mitral) valve – between left atrium and left ventricle.

3. Semilunar valves – pulmonary and aortic valves prevent backflow into the ventricles.

Heart Wall

The heart wall consists of three layers:

1. Epicardium – outer protective layer.

2. Myocardium – thick muscular middle layer responsible for contraction.

3. Endocardium – smooth inner lining.

The left ventricle has the thickest myocardium because it pumps blood to the entire body.

Functions of the Heart: The heart performs the following functions:

1. Pumps oxygenated blood to body tissues.

2. Pumps deoxygenated blood to the lungs for oxygenation.

3. Maintains blood pressure and continuous circulation.

4. Ensures separation of oxygenated and deoxygenated blood.

Types of Blood Vessels

1. Arteries

1. Carry blood away from the heart.

2. Have thick, elastic walls to withstand high pressure.

3. Usually carry oxygenated blood (except pulmonary artery).

2. Veins

1. Carry blood towards the heart.

2. Have thinner walls and wider lumen.

3. Possess valves to prevent backflow.

4. Usually carry deoxygenated blood (except pulmonary veins).

3. Capillaries

1. Microscopic vessels connecting arteries and veins.

2. Walls are one-cell thick to allow diffusion.

3. Site of exchange of gases, nutrients, and wastes.

Types of Circulation in Mammals: Mammals exhibit double circulation, which involves two distinct pathways.

1. Pulmonary Circulation: This involves blood flow between the heart and the lungs.

1. Deoxygenated blood flows from the right ventricle to the lungs via the pulmonary artery.

2. Blood becomes oxygenated in the lungs.

3. Oxygenated blood returns to the left atrium via pulmonary veins.

Importance:

1. Ensures efficient oxygenation of blood.

2. Systemic Circulation: This involves blood flow between the heart and the rest of the body.

1. Oxygenated blood flows from the left ventricle through the aorta to body tissues.

2. Oxygen and nutrients are delivered to cells.

3. Deoxygenated blood returns to the right atrium via the vena cava.

Importance:

1. Supplies nutrients and oxygen to all body cells.

Mechanism of Circulation (Cardiac Cycle): The cardiac cycle refers to one complete heartbeat, involving contraction and relaxation of the heart.

1. Atrial Systole

2. Atria contract.

3. Blood is forced into the ventricles.

Ventricular Systole

1. Ventricles contract.

2. Blood is pumped into the pulmonary artery and aorta.

3. Atrioventricular valves close, producing the first heart sound ("lub").

Diastole

1. Heart muscles relax.

2. Chambers fill with blood.

3. Semilunar valves close, producing the second heart sound ("dub").

8. Regulation of Heartbeat

Heartbeat is regulated by:

1. Pacemaker (Sinoatrial node) – initiates heartbeat.

2. Autonomic nervous system – sympathetic nerves increase heart rate; parasympathetic nerves reduce it.

3. Hormones – adrenaline increases heart rate.

Importance of Double Circulation in Mammals

1. Prevents mixing of oxygenated and deoxygenated blood.

2. Allows high blood pressure in systemic circulation.

3. Supports high metabolic rate and temperature regulation.