Thursday, 16 August 2018



All vertebrates have a closed circulatory system in which the walls of the heart and blood vessels are continuously contracted, and blood never leaves the blood vessels. Blood moves from the heart, through arteries, arterioles, capillaries, venules, veins, and back to the heart. Exchange between the blood and extracellular fluid only occurs at the capillary level.


Vertebrate blood transports oxygen, carbon dioxide, and nutrients; defends against harmful microorganisms, cells, and viruses; prevents blood loss through coagulation (clotting); and helps regulate body temperature and pH. Because it is a liquid, vertebrate blood is classified as a specialized type of connective tissue. Like other connective tissues, blood contains a fluid matrix called plasma and cellular elements called formed elements.


Plasma is the straw-colored, liquid part of blood. In mammals, plasma is about 90% water and provides the solvent for dissolving and transporting nutrients.
 A group of proteins (albumin, fibrinogen, and globulins) comprises another 7% of the plasma. The concentration of these plasma proteins influences the distribution of water between the blood and extracellular fluid.  Albumin about 60% of the total plasma proteins, it plays important roles to water movement. Fibrinogen is necessary for blood coagulation (clotting), and the globulins include the immunoglobulins and various metal-binding proteins.
Serum is plasma from which the proteins involved in blood clotting have been removed. The gamma globulin portion functions in the immune response because it consists mostly of antibodies. But 3% portion of plasma is consist of hormones, metabolic wastes, traces of many inorganic and organic molecules.
amino acids, glucose and other nutrients,  and various enzymes,

Formed Elements
The formed-element fraction (cellular component) of vertebrate blood consists of erythrocytes (red blood cells; RBCs), leukocytes (white blood cells; WBCs), and platelets (thrombocytes) (White blood cells are present in lower number than are red blood cells, generally being 1 to 2% of the blood by volume.

White blood cells are divided into agranulocytes (without granules in the cytoplasm) and granulocytes (have granules in the cytoplasm). The two types of agranulocytes are lymphocytes and monocytes. The three types of granulocytes are eosinophils, basophils, and neutrophils. Fragmented cells are called platelets
(thrombocytes). Each of these cell types is now discussed in more

Red Blood Cells

 Red blood cells vary dramatically in size, shape, and number in the different vertebrates. The RBCs of some vertebrates are nucleated, and mammalian RBCs are enucleated (without a nucleus). Some fishes and amphibians have enucleated RBCs. Some vertebrates such as the salamander has the largest RBC.
Avian RBCs are oval-shaped, nucleated, and larger than mammalian RBCs. Among birds, the ostrich has the largest RBC. Most mammalian RBCs are biconcave however, the camel and llama have elliptical RBCs. The shape of a biconcave disk provides a larger surface area for gas diffusion than a flat disk or sphere.  
Almost the entire mass of a RBC consists of hemoglobin, an iron-containing protein. The major function of an erythrocyte is to pick up oxygen from the environment, bind it to hemoglobin to form oxyhemoglobin, and transport it to body tissues. Blood rich in oxyhemoglobin is bright red.
 As oxygen diffuses into the tissues, blood becomes darker and appears blue when observed through the blood vessel walls. However, when this less oxygenated blood is exposed to oxygen, it instantaneously turns bright red. Hemoglobin also carries waste carbon  dioxide from the tissues to the lungs (or gills) for removal from the body.

White Blood Cells

 White blood cells are those blood cell that destroy microorganisms at infection sites, remove foreign chemicals, and remove debris that results from dead or injured cell.
Among the granulocytes, eosinophils are phagocytic, and ingest foreign proteins and immune complexes rather than bacteria. In mammals, eosinophils also release chemicals that counteract the effects of certain inflammatory chemicals released during allergic reactions. Basophils are the least numerous WBC. When they react with a foreign substance, their granules release histamine and heparin. Histamine causes blood vessels to dilate and leak fluid at a site of inflammation, and heparin prevents blood clotting. Neutrophils are the most numerous of the white blood cells.
They are chemically attracted to sites of inflammation and are active phagocytes. The two types of agranulocytes are the monocytes and lymphocytes.  
Two distinct types of lymphocytes are B cell and T cells, both of which are central to the immune response.
B cells originate in the bone marrow and colonize the lymphoid tissue, where they mature.
  T cells are associated with and influenced by the thymus gland before they colonize lymphoid tissue and play their role in the immune response. When B cells are activated, they divide and differentiate to produce plasma cells.


Platelets or thrombocytes are disk-shaped cell fragments that initiate blood clotting. When a blood vessel is injured, platelets immediately move to the site and clump, attaching themselves to the damaged area, and thereby beginning the process of blood coagulation.


Arteries are elastic blood vessels that carry blood away from the heart to the organs and tissues of the body. The central canal of an artery (and of all blood vessels) is a lumen. Surrounding the lumen of an artery is a thick wall composed of three layers, or tunicae.
Most veins are relatively inelastic, large vessels that carry blood from the body tissues to the heart. The wall of a vein contains the same three layers (tunicae) as arterial walls, but the middle layer is much thinner, and one or more valves are
present. The valves permit blood flow in only one direction, which is important in returning the blood to the heart.
Arteries lead to terminal
arterioles (those closest to a capillary). The arterioles branch to form capillaries (L. capillus, hair), which connect to venules and then to veins. Capillaries are generally composed of a single layer of endothelial cells and are the most numerous blood vessels in an animal’s body.
An abundance of capillaries makes an enormous surface area available for the exchange of gases, fluids, nutrients, and wastes between the blood and nearby cells.


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