Blood, constitution

Human blood constitutes about 8% of the body s weight. It consists of cells and cell fragments in an aqueous medium, the blood plasma. The proportion of cellular elements, known as hematocrit, in the total volume is approximately 45%. 

The human body is a remarkable machine. It relies on a variety of safeguards to keep blood pH constant. Our blood constitutes a buffer system — meaning, it has components that can react with excess base or excess acid. Carbon dioxide, which is produced by the metabolism of food, dissolves in blood to produce carbonic acid, and carbonic acid can neutralize any excess base. The bicarbonate ion, also present in blood, will promptly take care of any surplus acid. The level of carbon dioxide in the blood adjusts to a body s rate of respiration. If blood pH drops — which actually means that the blood has... 

Repeated small transfusions of fresh blood constitute the most efficient form of therapy in classical hemophilia. Unfortunately, the patient often becomes refactory to transfusion, probably because of the development of neutralizing antibodies. For these reasons it is sometimes postulated that small amounts of the antihemophilic factor are present in the blood, and that the primary deficiency is in the inhibition or excessive breakdown of that factor. 

Blood is composed of a cellular portion, the formed elements, suspended in plasma. The formed elements constitute approximately 40—45% of the blood volume, ie, the hematocrit. When a test tube with blood is centrifuged, the formed elements are packed onto the bottom of the tube, leaving plasma on top... 

The formed elements consist primarily of red blood cells, ie, erythrocytes. Less than 1 /600 of the total volume of the formed elements is composed of white blood cells, ie, leukocytes, and less than 1/800 are platelets, ie, thrombocytes. Table 1 gives the typical constitution of human blood. 

Factor VII. This is a vitamin K-dependent serine protease that functions in the extrinsic coagulation pathway and catalyzes the activation of Factors IX and X. Factor VII is present constitutively in the surface membrane of pericytes and fibroblasts in the adventitia of blood vessels, vascular endothehum, and monocytes. It is a single-chain glycoprotein of approximately 50,000 daltons. 

The absorption, distribution, and accumulation of lead in the human body may be represented by a three-part model (6). The first part consists of red blood cells, which move the lead to the other two parts, soft tissue and bone. The blood cells and soft tissue, represented by the liver and kidney, constitute the mobile part of the lead body burden, which can fluctuate depending on the length of exposure to the pollutant. Lead accumulation over a long period of time occurs in the bones, which store up to 95% of the total body burden. However, the lead in soft tissue represents a potentially greater toxicological hazard and is the more important component of the lead body burden. Lead measured in the urine has been found to be a good index of the amount of mobile lead in the body. The majority of lead is eliminated from the body in the urine and feces, with smaller amounts removed by sweat, hair, and nails. 

Basophils constitute a subgroup of circulating blood cells (leucocytes). In many aspects they resemble... 

Irritable bowel syndrome (DBS) is an exceedingly common condition in all societies, characterized by abdominal discomfort or pain in association with altered bowel habit or incomplete stool evacuation, bloating and constipation or diarrhoea, easily go undetected and do not show up with common tests such as blood tests or x-rays. The estimated prevalence in the community is about 10%. Irritable bowel syndrome and its variants, collectively called functional gastrointestinal disorders, constitute 40-50% of all the patients seen by gastroenterologists in Western countries. 

The Qxo, or temperature coefficient, is the factor by which the rate of a biologic process increases for a 10 °C increase in temperature. For the temperatures over which enzymes are stable, the rates of most biologic processes typically double for a 10 °C rise in temperature (Qjo = 2). Changes in the rates of enzyme-catalyzed reactions that accompany a rise or fall in body temperature constitute a prominent survival feature for cold-blooded life forms such as lizards or fish, whose body temperatures are dictated by the external environment. However, for mammals and other homeothermic organisms, changes in enzyme reaction rates with temperature assume physiologic importance only in circumstances such as fever or hypothermia. 

Higher than normal quantities of ketone bodies present in the blood or urine constitute ketonemia (hyperke-tonemia) or ketonuria, respectively. The overall condition is called ketosis. Acetoacetic and 3-hydroxybutyric acids are both moderately strong acids and are buffered when present in blood or other tissues. However, their continual excretion in quantity progressively depletes the alkah reserve, causing ketoacidosis. This may be fatal in uncontrolled diabetes mellitus. 

Glycosphingolipids (GSLs) (neutral GSLs, gangliosides, and complex species, including the ABO blood group substances) constitute about 5-10% of the total lipid. 

The monocytes, which constitute about 5% of the total blood leucocytes. They migrate into the tissues and mature into macrophages (see below). 

The carboxyl proteases are so called because they have two catalytically essential aspartate residues. They were formerly called acid proteases because most of them are active at low pH. The best-known member of the family is pepsin, which has the distinction of being the first enzyme to be named (in 1825, by T. Schwann). Other members are chymosin (rennin) cathepsin D Rhizopus-pepsin (from Rhizopus chinensis) penicillinopepsin (from Penicillium janthinel-lum) the enzyme from Endothia parasitica and renin, which is involved in the regulation of blood pressure. These constitute a homologous family, and all have an Mr of about 35 000. The aspartyl proteases have been thrown into prominence by the discovery of a retroviral subfamily, including one from HIV that is the target of therapy for AIDS. These are homodimers of subunits of about 100 residues.156,157 All the aspartyl proteases contain the two essential aspartyl residues. Their reaction mechanism is the most obscure of all the proteases, and there are no simple chemical models for guidance. 

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