About our Blood

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About our Blood

Post by Cursakandine on Thu Nov 26, 2009 7:08 pm

Blood is a specialized bodily fluid (technically a tissue) that is composed of a liquid called blood plasma and blood cells suspended within the plasma. The blood cells present in blood are red blood cells (also called RBCs or erythrocytes), white blood cells (including both leukocytes and lymphocytes) and platelets (also called thrombocytes). Plasma is predominantly water containing dissolved proteins, salts and many other substances; and makes up about 55% of blood by volume. Mammals have red blood, which is bright red when oxygenated, due to hemoglobin. Some animals, such as the horseshoe crab use hemocyanin to carry oxygen, instead of hemoglobin.

The most abundant cells in 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. White blood cells help to resist infections and parasites, and platelets are important in the clotting of blood.

Blood is circulated around the body through blood vessels by the pumping action of the heart. 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.

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.

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)
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 signalling 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

Constituents of human blood

Blood accounts for 7% of the human body weight, with an average density of approximately 1060 kg/m³, very close to pure water's density of 1000 kg/m3. 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 effectvely.

Cells

One microliter of blood contains:

4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes: 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.
4,000-11,000 leukocytes: 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: 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.

Plasma
About 55% of whole blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in color. The blood plasma volume totals of 2.7-3.0 litres in an average human. 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 (dissolved in the blood or bound to plasma proteins), and removes waste products, such as, carbon dioxide, urea and lactic acid.

Other important components include:

Serum albumin
Blood clotting factors (to facilitate coagulation)
Immunoglobulins (antibodies)
Various other proteins
Various electrolytes (mainly sodium and chloride)
The term serum refers to plasma from which the clotting proteins have been removed. Most of the proteins remaining are albumin and immunoglobulins.

The normal pH of human arterial blood is approximately 7.40 (normal range is 7.35-7.45), a weak alkaline solution. Blood that has a pH below 7.35 is too acidic, while blood pH above 7.45 is too alkaline. Blood pH, arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2) and HCO3 are carefully regulated by complex systems of homeostasis, which influence the respiratory system and the urinary system in the control the acid-base balance and respiration. Plasma also circulates hormones transmitting their messages to various tissues.

Oxygen transport

About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the Hgb. About 1.5% is physically dissolved in the other blood liquids and not connected to Hgb. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species (for exceptions, see below).

With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules and veins carry deoxygenated blood back to the heart.

Under normal conditions in humans at rest, hemoglobin in blood leaving the lungs is about 98-99% saturated with oxygen. In a healthy adult at rest, deoxygenated blood returning to the lungs is still approximately 75% saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions. Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia. Sustained hypoxia,(oxygenation of less than 90%) is dangerous to health, and severe hypoxia (saturations of less than 30%) may be rapidly fatal.

A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% of the level found in an adult's lungs) and so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) in order to function under these conditions.

Carbon dioxide transport
When blood flows through capillaries, carbon dioxide diffuses from the tissues into the blood. Some carbon dioxide is dissolved in the blood. Some carbon dioxide reacts with hemoglobin and other proteins to form carbamino compounds. The remaining carbon dioxide is converted to bicarbonate and hydrogen ions through the action of RBC carbonic anhydrase. Most carbon dioxide is transported through the blood in the form of bicarbonate ions.

Carbon dioxide (CO2), the main cellular waste product is carried in blood mainly dissolved in plasma, in equilibrium with bicarbonate (HCO3-) and carbonic acid (H2CO3). 86%-90% of CO2 in the body is converted into carbonic acid, which can quickly turn into bicarbonate, the chemical equilibrium being important in the pH buffering of plasma.Blood pH is kept in a narrow range (pH between 7.35-7.45).
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