Anatomy and Function of the Human Circulatory System

The Human Circulatory System

Blood Vessels: Arteries, Veins, and Capillaries

• Arteries: Blood flows from the heart to all other organs of the body. Walls are thicker and more elastic to withstand the high pressure with which blood flows, driven by the heart.
• Veins: Blood flows from the various organs back to the heart. The walls are thin and less elastic than the arteries as blood circulates under less pressure.
• Capillaries: Microscopic vessels that connect like a network, linking the endings of the arteries with the start of the veins.

Essential Components of Blood

There are three main blood types.

Plasma

The liquid, straw-colored component built of 90% water. It contains a variety of dissolved substances, including various proteins, carbohydrates (Glucids), etc.

Cellular Components of Blood

• Red Blood Cells (RBC): Small, biconcave cells that have no nucleus. They possess elastic and deformable characteristics that allow passage through capillaries.
• White Blood Cells (WBC): Cellular components that are larger than red blood cells and have a nucleus.
• Platelets: Cellular fragments without a nucleus, derived from a giant cell in the bone marrow. They are involved in blood coagulation.

Understanding the Cardiac Cycle

The cardiac cycle describes the sequence of mechanical and electrical events that occur during one heartbeat.

1. Atrial Systole: The atria, which are full of blood, contract. This contraction determines the opening of the mitral and tricuspid valves, allowing blood to flow into the ventricles.
2. Ventricular Systole: Almost immediately following atrial systole, the contraction of the ventricles occurs. This contraction determines an increase in ventricular blood pressure, which causes:
   • The mitral and tricuspid valves to close (to prevent blood return to the atria).
   • The semilunar valves to open, allowing blood to exit into the arteries (the pulmonary artery from the right ventricle and the aorta from the left ventricle).
3. Diastole: Ventricular systole ends when the ventricles relax, and the lowering ventricular blood pressure causes the semilunar valves to close. At this point, blood from the vena cava and pulmonary veins begins to fill the atria. In a later period of ventricular relaxation, atrial systole occurs.

Circulatory System Disorders

Atherosclerosis

Atherosclerosis signifies the hardening of the arteries. It occurs mainly due to the deposit of fat and cholesterol on the inside walls of the arteries, which causes the inside diameter to decrease and obstructs the passage of blood.

Myocardial Infarction (Heart Attack)

Atherosclerosis can affect any artery in the body, but it is especially important when it affects the coronary arteries, which are responsible for supplying the heart muscle (myocardium) with blood, oxygen, and nutrients.