Human Circulatory and Lymphatic Systems Explained

The Human Circulatory and Lymphatic Systems

The human body contains two primary transport circuits: the cardiovascular system, which circulates blood, and the lymphatic system, which circulates lymph.

The Cardiovascular System

The cardiovascular system consists of the heart, which acts as a pump, and blood vessels, which serve as the pathways for blood flow. These vessels are categorized into three types:

• Arteries
• Veins
• Capillaries

Blood Composition

Blood is a vital red liquid composed of two main parts: plasma and solid components.

Plasma (55%–60% of blood)

Plasma consists of 90% water and various dissolved substances, including:

• Gases: Oxygen, CO₂, and nitrogen.
• Minerals: Calcium, sodium, potassium, and chloride.
• Nutrients: Glucose (0.08%–0.12%), amino acids, lipids (cholesterol), triglycerides, and vitamins.
• Proteins: Albumin (for transport), gamma globulins (immunoglobulins), and enzymes.
• Hormones and waste substances.

Solid Components (40%–45% of blood)

The solid portion of blood includes red blood cells, platelets, and white blood cells.

1. Red Blood Cells (Erythrocytes)

These disc-shaped cells lack a nucleus and contain hemoglobin. Men typically have 5 million per cubic milliliter, while women have about 4.5 million. Produced in the red bone marrow and destroyed in the liver and spleen, a deficiency in iron, hemoglobin, or cell count leads to anemia.

2. Platelets

Platelets are not complete cells but cytoplasmic fragments derived from large cells in the red bone marrow. With a count of 250,000 to 300,000 per cubic milliliter, they are essential for blood clotting and vessel wall repair.

3. White Blood Cells (Leukocytes)

These pale, nucleated cells (5,000–10,000 per cubic milliliter) are produced in the bone marrow, spleen, lymphatic system, and thymus. They provide immune defense and can cross vessel walls. A deficiency is called leukopenia, while an excess may indicate leukemia. They are categorized as:

• Agranulocytes: Cells without visible grains in the nucleus.
  • Monocytes: Horseshoe-shaped phagocytes (macrophages) representing 4% of white blood cells.
  • Lymphocytes: The smallest white blood cells, produced in the lymphatic system, representing 23% of the total count.