blood smear:
a - erythrocytes; b - neutrophil;
c - eosinophil; d - lymphocyte.
A scanning electron microscope (SEM) image of a normal red blood cell, a platelet, and a white blood cell.
Blood is a specialized bodily fluid (technically a tissue).
In vertebrates it is composed of blood cells suspended in a liquid called blood plasma. Plasma, which comprises 55% of blood fluid, is mostly water (90% by volume), and contains dissolved proteins, glucose, mineral ions, hormones, carbon dioxide (plasma being the main medium for excretory product transportation), platelets and blood cells themselves. The blood cells present in blood are mainly red blood cells (also called RBCs or erythrocytes) and white blood cells, including leukocytes and platelets (also called thrombocytes).
The most abundant cells in vertebrate blood are red blood cells. These contain hemoglobin, an iron-containing protein, which facilitates transportation 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. Insects and some molluscs 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 suffices 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.[1] 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.
Medical terms related to blood often begin with hemo- or hemato- (BrE: haemo- and haemato-) from the Greek word "αἷμα" for "blood." Anatomically and histologically, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen.
Blood circulation:
Red = oxygenated
Blue = deoxygenated
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Contents [hide] |
[edit] Functions
Hemoglobin
green = heme groups
red & blue = protein subunits
Heme
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)
- 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 (the normal pH of blood is in the range of 7.35 - 7.45)
- Regulation of core body temperature
- Hydraulic functions
[edit] Constituents of human blood
See also: Reference ranges for common blood tests
Two tubes of EDTA anticoagulated blood.
Left tube: after standing, the RBCs have settled at the bottom of the tube.
Right tube: contains freshly drawn blood.
Blood accounts for 7% of the human body weight,[2] with an average density of approximately 1060 kg/m³, very close to pure water's density of 1000 kg/m3.[3] The average adult has a blood volume of roughly 5 litres, composed of plasma and several kinds of cells (occasionally called corpuscles); these formed elements of the blood are erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets). By volume the red blood cells constitute about 45% of whole blood, the plasma constitutes about 55%, and white cells constitute a minute volume.
Whole blood (plasma and cells) exhibits non-Newtonian fluid dynamics; its flow properties are adapted to flow effectively through tiny capillary blood vessels with less resistance than plasma by itself. In addition, if all human hemoglobin was free in the plasma rather than being contained in RBCs, the circulatory fluid would be too viscous for the cardiovascular system to function effectively.
[edit] Cells
Further information: Complete blood count
One microliter of blood contains:
- 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes:[4] In mammals, mature red blood cells lack a nucleus and organelles. They contain the blood's hemoglobin and distribute oxygen. The red blood cells (together with endothelial vessel cells and other cells) are also marked by glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45%. The combined surface area of all the red cells in the human body would be roughly 2,000 times as great as the body's exterior surface.[5]
- 4,000-11,000 leukocytes:[6] White blood cells are part of the immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. The cancer of leukocytes is called leukemia.
- 200,000-500,000 thrombocytes:[6] Platelets are responsible for blood clotting (coagulation). They change fibrinogen into fibrin. This fibrin creates a mesh onto which red blood cells collect and clot, which then stops more blood from leaving the body and also helps to prevent bacteria from entering the body.
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Constitution of normal blood | |
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Parameter |
Value |
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45 ± 7 (38-52%) for males | |
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7.35-7.45 | |
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-3 to +3 | |
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PO2 |
10-13kPa (80-100 mmHg) |
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Akrum Hamdy [email protected] 01006376836 نشرت فى 25 يوليو 2008
بواسطة AkrumHamdy
أ.د/ أكـــرم زيـن العــابديــن محـــمود محمـــد حمــدى - جامعــة المنــيا
[email protected] [01006376836] Minia University, Egypt »
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