Blood

What is Blood?

Blood is a living tissue present in our body in form of liquids and solids. Its main function is to transport essential substances to the body cells. This include nutrients and oxygen and also it transports waste products away from the body cells.

In human, it is composed of blood cells suspended in a liquid form called blood plasma. Blood plasma covers 55% of the blood components. Blood plasma is mostly water(90%), on the other hand it contains dissolved proteins, glucose, mineral ions, carbon dioxide, hormones, platelets and blood cells. The blood cells present in blood are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells (leukocytes) and platelets (also called thrombocytes).

The main blood cells in blood are red blood cells which contains hemoglobin, an iron-containing protein, which provides the trasnportation of oxygen by reversibly binding to this respiratory gas and greatly increasing its solubility in blood. In contrast, carbon dioxide is almost entirely transported extracellularly dissolved in plasma as bicarbonate ion.

Vertebrate blood is bright red when its hemoglobin is oxygenated. Some animals, such as crustaceans and mollusks, use hemocyanin to carry oxygen, instead of hemoglobin. Some insects use a fluid called hemolymph instead of blood, the difference being that hemolymph is not contained in a closed circulatory system. In most insects, this "blood" does not contain oxygen-carrying molecules such as hemoglobin because their bodies are small enough that their tracheal system manages for supplying oxygen.

Jawed vertebrates have an adaptive immune system, based largely on white blood cells. White blood cells help to resist infections and parasites. Platelets are important in the clotting of blood. Arthropods, using hemolymph, have hemocytes as part of their immune system.

Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals having lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.

Blood Functions

Blood performs many vital functions within the body including:

• Supply of oxygen to tissues (bound to hemoglobin which is carried in red cells)
• Supply of nutrients such as glucose, amino acids and fatty acids (dissolved in the blood or bound to plasma proteins (eg blood lipids)
• Removal of waste such as carbon dioxide, urea and lactic acid
• Immunological functions, including circulation of white cells, and detection of foreign material by antibodies
• Coagulation, which is one part of the body's self-repair mechanism
• Messenger functions, including the transport of hormones and the signaling of tissue damage
• Regulation of body pH
• Regulation of core body temperature
• Hydraulic functions

Production and degradation of blood cells

In vertebrates, the various cells of blood are made in the bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red blood cells; and myelopoiesis, the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the thymus gland, found in the mediastinum, is an important source of lymphocytes. The proteinaceous component of blood (including clotting proteins) is produced predominantly by the liver, while hormones are produced by the endocrine glands and the watery fraction is regulated by the hypothalamus and maintained by the kidney.

Healthy erythrocytes have a plasma life of about 120 days before they are degraded by the spleen, and the Kupffer cells in the liver. The liver also clears some proteins, lipids and amino acids. The kidney actively secretes waste products into the urine.

Transport of hydrogen ions

Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin binds most of the hydrogen ions as it has a much greater affinity for more hydrogen than does oxyhemoglobin.

Lymphatic system

In mammals, blood is in equilibrium with lymph, which is continuously formed in tissues from blood by capillary ultrafiltration. Lymph is collected by a system of small lymphatic vessels and directed to the thoracic duct, which drains into the left subclavian vein where lymph rejoins the systemic blood circulation.

Thermoregulation

Blood circulation transports heat throughout the body, and adjustments to this flow are an important part of thermoregulation. Increasing blood flow to the surface (e.g. during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss, while decreasing surface blood flow conserves heat.

Hydraulic functions

The restriction of blood flow can also be used in specialized tissues to cause engorgement resulting in an erection of that tissue; examples are the erectile tissue in the penis, nipples or clitoris.

Another example of a hydraulic function is the jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump, without the need for bulky muscular legs.

Color

Hemoglobin

(A bleeding finger)

Hemoglobin is the principal determinant of the color of blood in vertebrates. Each molecule has four heme groups, and their interaction with various molecules alters the exact color. In vertebrates and other hemoglobin-using creatures, arterial blood and capillary blood are bright red as oxygen imparts a strong red color to the heme group. Deoxygenated blood is a darker shade of red; this is present in veins, and can be seen during blood donation and when venous blood samples are taken. Blood in carbon monoxide poisoning is bright red, because carbon monoxide causes the formation of carboxyhemoglobin. In cyanide poisoning, the body cannot utilize oxygen, so the venous blood remains oxygenated, increasing the redness. While hemoglobin containing blood is never blue, there are several conditions and diseases where the color of the heme groups make the skin appear blue.

Vacuum

If human blood was exposed to vacuum, it would lose all oxygen (bound to the hemoglobin molecules in its red cells) as it does in part when it supplies oxygen to various organs as it passes through the body. This state of deoxyhemoglobin has a purplish colour.