Human Circulatory System: Blood, Heart, and Vessels Explained

Blood: Composition and Vital Functions

Blood Composition

• Blood Plasma: The liquid component of blood.
• Blood Cells:
  • Red Blood Cells (Erythrocytes): Primarily responsible for oxygen transport.
  • White Blood Cells (Leukocytes): Key components of the body's immune and defense system.
  • Platelets: Essential for blood clotting and preventing excessive bleeding.

Key Functions of Blood

Blood serves as an effective transport system for various substances throughout the body:

• Oxygen Transport: Carries oxygen gas, primarily bound to hemoglobin. When hemoglobin is highly saturated with oxygen, it is called oxyhemoglobin.
• Nutrient and Waste Transport: Moves nutrients to cells and carries metabolic waste products away for excretion.
• Heat Distribution: Helps regulate and distribute body heat, maintaining a stable internal temperature.
• Defense: Protects the body against infections and foreign invaders through white blood cells.
• Bleeding Control: Initiates clotting mechanisms to prevent excessive blood loss from injuries.

Common Blood-Related Diseases

Anemia

The most common type of anemia is caused by iron deficiency. Symptoms often include:

• Fatigue
• Loss of vitality and energy

Leukemia

Leukemia is a form of cancer that affects the bone marrow cells, which are responsible for producing blood cells. This leads to an overproduction of abnormal white blood cells.

Hemophilia

Hemophilia is a hereditary condition characterized by problems with blood clotting, leading to prolonged bleeding after injury or surgery.

Blood Vessels: The Circulatory Network

Blood vessels are the tubes through which blood circulates throughout the body. There are three main types:

• Arteries: These strong, elastic-walled vessels carry blood from the heart to the organs. They withstand high pressure from the heart's pumping action.
• Veins: These vessels carry blood from the organs back to the heart. They have thinner walls and often contain valves to prevent the backflow of blood.
• Capillaries: These are vessels of extremely small diameter that connect the arterial and venous systems. They are the primary sites for the exchange of oxygen, nutrients, and waste products between blood and tissues.

The Heart: Structure and Function

The heart's walls are composed of a specialized muscle called the myocardium.

Heart Chambers

The human heart has four cavities, or chambers:

• Two upper chambers called atria (singular: atrium)
• Two lower chambers called ventricles

Blood enters the atria of the heart, propelled by the veins, and is then pumped out of the ventricles into the arteries.

The Cardiac Cycle

The rhythmic beating of the heart involves a sequence of contractions and relaxations, known as the cardiac cycle:

• Atrial Systole: Contraction of the atria, pushing blood into the ventricles.
• Ventricular Systole: Contraction of the ventricles, pumping blood out of the heart.
• Diastole: Relaxation of the entire heart, allowing it to fill with blood.

The Double Circulatory System

The human circulatory system is a double circuit, meaning blood passes through the heart twice for each complete circuit of the body.

Pulmonary Circuit (Lung Circuit)

Blood leaves the right ventricle, travels through the pulmonary arteries to the lungs. In the lungs, specifically within the alveoli and their surrounding capillaries, gas exchange occurs: the blood releases carbon dioxide and picks up oxygen. This oxygenated blood then returns to the heart via the pulmonary veins, entering the left atrium.

Systemic Circuit (General Circuit)

Oxygenated blood leaves the left ventricle through the large artery called the aorta and is distributed to all organs of the body. In the capillaries of these organs, oxygen and essential nutrients are delivered to the cells, while waste products are collected. The capillaries from various organs then converge into veins, which return the deoxygenated blood to the heart, specifically entering the right atrium.

Atherosclerosis: Hardening of Arteries

Atherosclerosis is a condition characterized by the hardening and narrowing of arteries. It is produced by the deposition of fatty plaques, including cholesterol, on the inner walls of the arteries, which can impede blood flow.