The blood

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The Blood

      Blood is a liquid connective tissue that travels through tubular vessels. Its primary function is to transport cells and dissolved matter from place to place.In addition,blood is a viscous (thick) fluid that varies in color from bright to dark red, depending on how much oxygen it is carrying. Its quantity differs with the size of the person; for example the average adult male, weighing 70 kg has about 5-6 litres of blood. This volume accounts for about 8% of the total body weight. It is carried through a closed system of vessels pumped by the heart. The circulating blood is of fundamental importance in maintaining the internal environ Human.

      Blood is classified as a connective tissue, since nearly half of it is made up of cells. However, it differ from other connective tissues in that its cells are not fixed in position, instead they move freely in the liquid portion of the blood, the plasma.


 In general the blood has distinctive physical characteristics as follows.

1.Amount a person has 4 to 6 liters of blood, depending on his or her size. Of the total blood volume in the human body, 38% to 48% is composed of the various blood cells, which are also known formed elements. The remaining 52% to 62% of the blood volume is plasma, the liquid portion of blood.

2.Color. The blood also poses the colored characteristics. For example  you’re probably saying to yourself, “Of course, it’s red!” Mention is made of this obvious fact, however, because the color does vary. Arterial blood is bright red because it contains high levels of oxygen. Venous blood has given up much of its oxygen in tissues, and has a darker, dull red color. This may be important in the assessment of the source of bleeding. If blood is bright red, it is probably from a severed artery, and dark red blood is probably venous blood.

  1. PH. The normal pH range of blood is 7.35 to 7.45, which is slightly alkaline. Venous blood normally has a lower pH than does arterial blood because of the presence of more carbon dioxide.
  2. Viscosity. This means thickness or resistance to flow. Blood is about three to five times thicker than water. Viscosity is increased by the presence of blood cells and the plasma proteins, and this thickness contributes to normal blood pressure.

Functions of the Blood.

  1. Transportation.The blood transports oxygen- from inhaled air diffuses into the blood through the thin lung membranes and is carried to all the tissue of the body. Carbon dioxide, a waste product of cell metabolism, is carried from the tissues to the lungs, where it is breathed out. Also the blood transports foods and other needed substances such as minerals and vitamins, to the cells. These materials may inter the blood from the digestive system or may be released into the blood from body stores. In addition, the blood transports waste products from the cells to the sites from which they are released. The kidney removes excess water, minerals, and urea from protein metabolism and maintains the acid-base balance of the blood. The liver removes bile pigments and drugs. Finally, the blood carries hormones from their sites of origin to the organs they affect.
  1. Regulation.The blood regulates body temperature in several ways. Plasma absorbs and distributes heat throughout the body. If the body needs to be cooled, the blood vessels in the dermis dilate and dissipate the excess heat through the integument. Conversely, when the body needs to conserve heat, the dermal blood vessels constrict, and the warm blood is shunted to deeper blood vessels in the body .Blood also helps regulate pH levels in the body’s tissues the  blood help keep the PH of body fluids at about 7.4. The term pH is a measure of how acidic or alkaline a fluid is. Finally the blood serves to regulate the amount of fluid in the tissues by means of substances (mainly proteins) that maintain the proper osmotic pressure
  1. Defense/Protection. Blood defends the body against invasion by pathogens (microscopic infectious agents, such as bacteria and viruses) in several ways. Certain blood cells are capable of engulfing and destroying pathogens, and others produce and secrete antibodies into the blood. Antibodies incapacitate pathogens, making them subject to destruction, sometimes by white blood cells. When an injury occurs, blood forms a clot, and this prevents blood loss. Blood clotting involves platelets and the plasma protein fibrinogen. Without blood clotting, we could bleed to death even from a small cut.

Composition of Blood.

        The blood is composed of two prime elements: as already mentioned, the liquid element is called plasma; the cells and fragments of cells are called formed elements or corpuscles .The formed elements are classified as follows:

  1. Red Blood Cells (Erythrocytes), from erythro, meaning “red,” are the red blood cells, which transport oxygen.
  2. Leukocytes, from leuko, meaning “white,” are the several types of white blood cells, which protect against infection.
  3. Platelets, also called thrombocytes, are cell fragments that participate in blood clotting.

Figure 1.Components of blood and the relationship of blood to other body tissues