Blood

Blood is a body fluid that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.Two types of blood vessels carry blood throughout our bodies:


 * Arteries carry oxygenated blood (blood that has received oxygen from the lungs) from the heart to the rest of the body.
 * Blood then travels through veins back to the heart and lungs, so it can get more oxygen to send back to the body via the arteries.

As the heart beats, you can feel blood traveling through the body at pulse points — like the neck and the wrist — where large, blood-filled arteries run close to the surface of the skin.

The blood that flows through this network of veins and arteries is whole blood, which contains three types of blood cells:


 * red blood cells (RBCs)
 * white blood cells (WBCs)
 * platelets

Functions
Blood performs many important 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 (e.g., blood lipids))
 * Removal of waste such as carbon dioxide, urea, and lactic acid
 * Immunological functions, including circulation of white blood cells, and detection of foreign material by antibodies
 * Coagulation, the response to a broken blood vessel, the conversion of blood from a liquid to a semi-solid gel to stop bleeding
 * Messenger functions, including the transport of hormones and the signaling of tissue damage
 * Regulation of body pH
 * Regulation of core body temperature
 * Hydraulic functions

Blood Plasma
About 55% of blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in color. It is essentially an aqueous solution containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. Plasma circulates dissolved nutrients, such as glucose, amino acids, and fatty acids, and removes waste products, such as carbon dioxide, urea, and lactic acid.

Circulation
Blood is circulated around the body through blood vessels by the pumping action of the heart. In humans, blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, and venous blood carries carbon dioxide, a waste product of metabolism by cells, to the lungs to be exhaled. However, one exception includes pulmonary arteries, which contain the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.

Hemoglobin
Hemoglobin in the blood carries oxygen from the respiratory organs (lungs or gills) to the rest of the body (i.e. the tissues). There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells.

Red Blood Cells
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, is an important source of T lymphocytes.

Anemia
Anemia, is usually defined as a decrease in the amount of red blood cells (RBCs) or hemoglobin in the blood. It can also be defined as a lowered ability of the blood to carry oxygen. When anemia comes on slowly, the symptoms are often vague and may include: feeling tired, weakness, shortness of breath or a poor ability to exercise. Anemia that comes on quickly often has greater symptoms, which may include: confusion, feeling like one is going to pass out, loss of consciousness, or increased thirst. Anemia must be significant before a person becomes noticeably pale.