The Blood

Blood composition

Blood is a fluid tissue consisting of cells (red blood cells, white blood cells, and platelets) suspended in plasma. An average adult human has 4 to 6 liters of blood, with a pH of 7.4.

Plasma

Plasma is a pale-yellow fluid making up about 55% of blood. It is primarily composed of water (92%) but also contains dissolved substances such as nutrients, oxygen, metabolic waste products, proteins, and mineral ions (8%).

Plasma's major functions include:

• Transporting nutrients, oxygen, and hormones
• Carrying waste products to excretory organs
• Assisting in immune function by carrying white blood cells
• Distributing heat and maintaining pH balance

Red blood cells

Also known as erythrocytes, red blood cells are biconcave, disc-shaped cells with no nucleus. They are responsible for transporting oxygen and carbon dioxide. The presence of hemoglobin in red blood cells facilitates oxygen transport and helps them perform their role efficiently.

Red blood cell

In the lungs, hemoglobin binds to oxygen to form oxyhemoglobin, which releases oxygen when it reaches tissues with low oxygen concentration.

White blood cells

Leukocytes are responsible for defending the body against infections. They have irregular shapes and are produced in the bone marrow and lymph nodes. White blood cells:

• Engulf pathogens through phagocytosis
• Produce substances to neutralize toxins
• Aid in fighting infections by causing foreign materials to clump together

White blood cells of different shapes

The Human Immunodeficiency Virus (HIV) targets helper T-cells, a type of white blood cell, which compromises the immune system.

HIV attacking T-helper cell

Platelets

Platelets are fragments of cells involved in blood clotting, playing a key role in wound healing and preventing excessive blood loss.

Blood platelets

The clotting process

1. Platelet Activation: Platelets at the site of an injury produce thromboplastin, which triggers the clotting process.
2. Thromboplastin Action: Thromboplastin, with the help of vitamin K and calcium, neutralizes heparin, an anticoagulant in the blood.
3. Conversion of Prothrombin: Heparin converts prothrombin (an inactive plasma protein) into thrombin (an active plasma protein).
4. Formation of Fibrin: Thrombin catalyzes the conversion of soluble fibrinogen to insoluble fibrin. Fibrin forms a network of fibers that traps debris and blood cells, resulting in the formation of a blood clot at the site of the wound.