Blood is a specialized body fluid that is essential for life. It circulates through the entire body, delivering vital substances and removing waste, making it one of the most important components of the human body.

1. Mechanism and Working of Blood

Blood flows through a network of blood vessels—arteries, veins, and capillaries—powered by the heart. This process is known as circulation.

Oxygenated blood (rich in oxygen) is pumped from the heart to all body parts.
Deoxygenated blood (carrying carbon dioxide and waste) returns to the heart and is sent to the lungs to release CO₂ and absorb fresh oxygen.
This cycle keeps repeating, ensuring all body cells get what they need.

2. Requirements of Blood in the Body

Oxygen and nutrient transport to organs and tissues
Waste removal, including carbon dioxide and toxins
Immune defense against infections and foreign substances
Clotting to stop bleeding after injury
Regulation of body temperature and pH balance

3. Components of Blood

1. Plasma
Plasma is the liquid part of the blood, making up about 55% of total blood volume. It is a yellowish fluid that consists mostly of water (about 90–92%), along with proteins, glucose, minerals, hormones, enzymes, and waste products.
Function: Plasma transports nutrients, hormones, and proteins to the parts of the body that need them. It also helps carry waste products to the liver and kidneys for removal from the body.

2. Red Blood Cells (RBCs)
Also known as erythrocytes, red blood cells make up roughly 44–45% of the blood. They are disc-shaped cells that contain hemoglobin, a red pigment that binds with oxygen.
Function: RBCs carry oxygen from the lungs to every cell in the body and return carbon dioxide from the body back to the lungs to be exhaled.

3. White Blood Cells (WBCs)
Also called leukocytes, white blood cells are a very small portion of blood (less than 1%) but play a critical role. There are several types of WBCs, such as lymphocytes, neutrophils, monocytes, eosinophils, and basophils.
Function: WBCs protect the body from infections. They identify and destroy bacteria, viruses, fungi, and foreign substances. They are key players in the immune system.

4. Platelets
Platelets, or thrombocytes, are tiny cell fragments found in blood, also making up less than 1% of the total volume.
Function: Their main job is blood clotting. When there is a cut or injury, platelets gather at the site, stick together, and form clots to stop bleeding and help in healing.

4. Importance and Usefulness in the Body

Keeps organs functioning by supplying oxygen and nutrients.
Supports immunity, protecting from infections.
Maintains homeostasis—stable internal conditions (like temperature and pH).
Helps in healing by forming clots and stopping blood loss.
Transports chemical messengers (hormones) for body coordination.

5. Why Blood is Vital and Lethal if Lost

Blood cannot be replaced easily by artificial means. Severe loss leads to organ failure and death.
Even a loss of 30-40% of blood volume without quick medical help can be fatal.
Blood is vital because every single cell in the body depends on it for survival.

Introduction to Human Blood