Blood

How can we describe blood?
Blood is made up of a mixture of red blood cells, white blood cells, platelets lying in a fluid called plasma. The cells present in the blood are specialized cells and perform certain unique functions. The blood derives its color from the red blood cells, which are the largest in quantity. The number of red cells (RBCs) in the blood is so large that blood appears red in color.

Components of Blood

The total amount of blood in the circulation of an average person is about 5 liters. Blood is made up of

• Red blood cells
  These number about 5 million in each cubic milliliter of blood. They transport oxygen to the tissues of the body. RBCs have a large surface area to absorb oxygen from the lungs. They are also very flexible and can squeeze through the smallest of capillaries with ease. They contain a substance called as hemoglobin. This is a combination of a protein (globin) and an iron particle. Hemoglobin is responsible for carrying oxygen. Hence lack of iron in the body causes improper oxygenation of our cells and thus causes disease.
• White blood cells
  These number about 7,000 to 11,000 in each cubic milliliter of blood. These act as the defense mechanism of the body. There are five major types of white blood cells. These are neutrophils, eosinophils, basophils, lymphocytes and monocytes. All these cells have a particular role to play. Some act as killer cells or phagocytes and engulf foreign bodies or bacteria present in the blood.
• Platelets
  These are cells responsible for sealing wounds and thus preventing further bleeding. There are about 400,000 platelets in every ml of blood. These are the smallest cells in the blood and always converge at the site of a wound. These cells secrete certain clotting factors that assist in clotting of blood.
• Plasma
  The remaining part of blood is made up of plasma, a thick fluid containing all the cells mentioned above. It is made of water, salts, hormones, proteins and nutrients. The protein, fibrinogen which causes clotting is also present in plasma.

What is blood?
The trillions of cells in the human body derive nutrition every second from some source. Cells of the body need oxygen, glucose and other nutrients to perform their daily activities. This source of nutrition is called as blood. It is a fluid media containing specialized cells and a fluid component called as plasma. The nutrition to the cells is provided by the blood in the circulation. Blood being a fluid medium is able to reach all the cells in the body. The blood circulates about once every minute at rest throughout the whole body. This gives lots of scope for adequate nutrition of all the living tissues of the body.

How are blood cells formed?
All the red cells, platelets and most of the white cells are formed in the bone marrow. The bone marrow is the soft tissue inside the cavity of bones. The bones which contain marrow that actively forms blood cells are flat bones like the shoulder blades, ribs, pelvic bones and breastbone.

All cells are formed from a single parent cell called as the stem cell. Production of these cells is under the control of hormones like:

• Erythropoietin from the kidney
• Thyroid hormones
• Steroid hormones from the adrenal gland
• Growth hormone from the pituitary gland in the skull

Functions of Blood

The blood carries out major functions essential for survival of the body. These are

• Transport of Oxygen
  Blood reaches the lungs where it collects the oxygen present in the lungs. This oxygen combines with hemoglobin in the red blood cells to form oxyhemoglobin. When it reaches a site in the body that needs oxygen, it releases the oxygen molecule and reverts back to hemoglobin. Therefore, more the hemoglobin. More is the oxygen carrying capacity of the blood.
• Blood Clotting
  Blood helps in the formation of a clot during an injury. The mechanism of clotting is as follows. As soon as a blood vessel is damaged, it narrows down to decrease the blood supply passing through it. Platelets are attracted to the damaged lining of the vessel and start clumping at the site. This leads to more release of chemicals by the platelets. Finally the fibrinogen in the plasma forms a mesh of sticky strands. This traps the other blood cells of the body forming a clot. Clot formation prevents further bleeding and promotes healing.