HEART AND CIRCULATORY SYSTEM (Part-I)

Diastole and systole are two phases of the cardiac cycle. They occur as the heart beats, pumping blood through a system of blood vessels that carry blood to every part of the body. Systole occurs when the heart contracts to pump blood out, and diastole occurs when the heart relaxes after contraction. The SA node generates electrical impulses and conducts them throughout the muscle of the heart, stimulating the heart to contract and pump blood.

Thyroxine hormone can increase the heart beat; its principal function is to stimulate the consumption of oxygen and thus the metabolism of all cells and tissues in the body.

Beta-blockers, also known as beta-adrenergic blocking agents, are medications that reduce blood pressure. Beta-blockers work by blocking the effects of the hormone epinephrine, also known as adrenaline. Beta-blockers cause the heart to beat more slowly and with less force, which lowers blood pressure. Beta-blockers also help widen veins and arteries to improve blood flow.

A heart attack occurs when an artery that sends blood and oxygen to the heart is blocked. Fatty, cholesterol-containing deposits build up over time, forming plaques in the heart’s arteries. If a plaque ruptures, a blood clot can form. The clot can block arteries, causing a heart attack.

In mammals, as in birds, the right and left ventricles of the heart are completely separated so that pulmonary (lung) and systemic (body) circulations are completely independent. Oxygenated blood arrives in the left atrium from the lungs and passes to the left ventricle, forcing it through the aorta to the systemic circulation.

• The opening between the right auricle or atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve. It is also called as an atrioventricular valve. It prevents the backflow of blood into the right atrium. Likewise, a bicuspid valve guards the opening between the left atrium and the left ventricle.

The rigid vessels are called arteries which originate from the heart and supply blood to various organs of the body.
(i) Arteries start from heart and supply oxygenated blood to various organs on the body. The biggest artery is the aorta.
(ii) Pulmonary artery carries blood to lungs.
(iii) Coronary artery supply blood to muscles of heart.

Blood plasma is the liquid component of whole blood, and makes up approximately 54% of the total blood volume. It is composed primarily of water with small amounts of minerals, salts, ions, nutrients, and proteins in solution. In whole blood, red blood cells, leukocytes, and platelets are suspended within the plasma.

Heart rate is controlled by the two branches of the autonomic (involuntary) nervous system i.e. The sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The sympathetic nervous system (SNS) releases the hormones (catecholamine epinephrine) to accelerate the heart rate. The parasympathetic nervous system (PNS) releases the hormone acetylcholine to slow the heart rate.

The sino-atrial (SA) node is a specialised bundle of neurons located in the upper part of the right atrium of the heart. The cardiac impulse originating from the SA node triggers a sequence of electrical events in the heart, thereby controlling the sequence of muscle contraction that pumps blood out of the heart. Since the SA node initiates and maintains the rhythmicity of the heart, it is known as the natural pacemaker of the human body.