61. Homeostatic role of the liver. Hepatocytes. Glycemia. Lipid metabolism.bile.

The liver is a vital organ present in vertebrates and some other animals. The liver is necessary for survival; there is currently no way to compensate for the absence of liver function long term, although liver dialysis can be used short term.This organ plays a major role in metabolism and has a number of functions in the body: a) glycogen storage, b) decomposition of red blood cells, c) plasma protein synthesis, d) hormone production, e) detoxification.

A hepatocyte is a cell of the main tissue of the liver. Hepatocytes make up 70-80% of the liver's cytoplasmic mass. These cells are involved in: a) Protein storage, b) Transformation of carbohydrates, c) Synthesis of cholesterol, bile salts and phospholipids.

The enzyme systems of liver are capable of catalyzing the majority of lipid metabolism reactions. Processes like synthesis of higher fatty acids, triglycerides, phospofipids, cholesterol and its esters, the production o keton bodies. Liver is the major side for production of blood plasma VLDL and HDL.

Bile – is a golden yellowish fluid to grinish yellow in colour, secreted by the liver and delivered to small intestine via bille duct. The daily production o bile is about 500 – 700ml (10 ml per kg of body mass).

Bile is a composition of the following materials: water (85%), bile salts (10%), mucus and pigments (3%), fats (1%), inorganic salts (0.7%) and cholesterin (0.3%).

62. detoxyfication function of liver - The liver detoxifies the blood of substances originating from the gut or elsewhere in the body. Part of this function is physical in nature-bacteria and other particulates are trapped in and broken down by the strategically-located Kupffer cells. The remaining reactions are biochemical, and mediated in their first stages by the large number of cytochrome P450 enzymes expressed in hepatocytes. These convert xenobiotics and other toxins to inactive, less lipophilic metabolites. Detoxification reactions are divided into phase I (oxidation, hydroxylation, and other reactions mediated by cytochrome P450s) and phase II (esterification). Ultimately, metabolites are secreted into the bile for elimination via the gastrointestinal tract. In this regard, in addition to disposing of drugs, the liver is responsible for metabolism of essentially all steroid hormones. Liver disease can therefore result in the apparent overactivity of the relevant hormone systems.

Cytochrome P-450 - Human CYPs are primarily membrane-associated proteins, located either in the inner membrane of mitochondria or in the endoplasmic reticulum of cells. CYPs metabolize thousands of endogenousand exogenous chemicals. Cytochrome P450 enzymes are present in most tissues of the body, and play important roles in hormone synthesis and breakdown (including estrogenand testosterone synthesis and metabolism), cholesterol synthesis, and vitamin D metabolism. Cytochrome P450 enzymes also function to metabolize potentially toxic compounds, including drugsand products of endogenous metabolism such as bilirubin, principally in the liver.

Microsomal oxidation – is the result of enzyme systems mostly within microsomes of liver. Oxygen acts as an electron acceptor to produce water in final stage; activated oxygen becomes directly incorporated into substrate liable to oxidation. Enzymes in microsomes are divided into: dioxygenases and monooxygenases. Monooxygenases catalyze addition of only one oxygen atom of molecular oxygen to the substrate molecule. The hydrogen atoms which are engaged in reduction of the other oxygen atom to water are supplied by NADPH2. The microsomal enzyme chain contains: cytochrome P-450, flavoprotein with FAD as a coenzyme, protein containing nonheme iron.

63. Conjugation reactions in hepatocytes. The role of conjugating enzymes is best understood by looking at the interaction between phase I (mostly cytochromes P-450) and phase II (conjugation) enzymes of drug metabolism. A balance between phase I and II enzymes of detoxication largely determines the disposition to drug toxicity. Reactive electrophilic metabolites, generated by phase I enzymes, are often controlled by GSH-tansferases, whereas nucleophilic metabolites such as phenols are controlled by UDP-glucuronosyltransferases (GT) and sulfotransferases. It is more and more recognized that the control of the more stable and more abundant nucleophiles is as important as the control of electrophiles, since the former can be readily converted to electrophiles. For example, phenols and quinols can undergo quinone/quinol redox-cycles with the generation of reactive oxygen species. In the case of benzo(a)pyrene-3,6-quinol toxicity can be prevented by glucuronidation.

64. Role in liver in turnover of bile pigments. Hemoglobin catabolism - When red cells reach the end of their life due to aging or defects, they are broken down, the hemoglobin molecule is broken up and the iron gets recycled. When the porphyrin ring is broken up, the fragments are normally secreted in the bile by the liver. This process also produces one molecule of carbon monoxide for every molecule of heme degraded. This is one of the few natural sources of carbon monoxide production in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air. The other major final product of heme degradation is bilirubin. Increased levels of this chemical are detected in the blood if red cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage. Transformation of bilirubin: 1) The initial stage in hemoglobin break-down is the scission of one methane bridge to produce verdoglobin. An iron ion and globin are split off from the verdoglobin molecule. This results in formation of biliverdin, which is structurally built of a chain of 4 pyrrole rings linked to one another via methane bridges. Biliverdin when subjected to reduction, converts to bilirubin- indirect (uncojugated). It is insoluble in water and gives an indirect reaction with diazo reagent. 2) In the liver, bilirubin becomes coupled to glucoronic acid. This reaction is catalyzed by the enzyme UDP-glucoronyltransferase. Glucoronic acid enters into the reactrion in an active form, that is, us UDP-glucuronate. Bilirubin glucuronide thus formed is referred to as conjugated bilirubin (b.diglucuronide). t is soluble in water and gives a direct color reaction with diazo reagent.

65) pathobiochemistry of jaundices. Hereditary (enzymatic) jaundices. Jaundice is a yellowish pigmentation of the skin, the conjunctival membranes over the sclerae and other mucous membranes caused by hyperbilirubinemia. This hyperbilirubinemia subsequently causes increased levels of bilirubin in the extracellular fluid. Typically, the concentration of bilirubin in plasma must exceed 1.5 mg/dL^[2](>26µmol/L), three times the usual value of approximately 0.5 mg/dL^[2], for the coloration to be easily visible. Hemolytic j.: catabolytes is intensified in the macrophage system cells. The liver thus rendered incapable of producing bilirubin glucuronides in adequate amount and the unconjugated bilirubin becomes accumulated in blood. Hepatocellular j.: the liver cells undergo degradation, the excretion of conjugated bilirubin int the bile capillaries is disturbed and this bilirubin is excreted directly in the bloodstream, which leads to its increased level in blood. Obstructive j.: the bile excretion is impaired which leads to an increased level of conjugated bilirubin in blood.

66) Porphyrins are a group of organic compounds, many naturally-occuring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is acofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH-). Porphyrins are aromatic. Biosynthesis: The "committed step" for porphyrin biosynthesis is the formation of D-aminolevulinic acid (dALA) by the reaction of the amino acid glycine with succinyl-CoA from the citric acid cycle. Two molecules of dALA combine to give porphobilinogen (PBG), which contains a pyrrole ring. Four PBGs are then combined through deamination into hydroxymethyl bilane (HMB), which is hydrolysed to form the circular tetrapyrrole uroporphyrinogen III. This molecule undergoes a number of further modifications. Intermediates are used in different species to form particular substances, but, in humans, the main end-product protoporphyrin IX is combined with iron to form heme. Bile pigments are the breakdown products of heme. Heme synthesis: The enzymatic process that produces heme is properly called porphyrin synthesis, as all the intermediates are tetrapyrroles that are chemically classified as porphyrins. The process is highly conserved across biology. In humans, this pathway serves almost exclusively to form heme. In other species, it also produces similar substances such as cobalamin (vitamin B₁₂).

The pathway is initiated by the synthesis of D-Aminolevulinic acid (dALA or δALA) from the amino acid glycine and succinyl-CoA from the citric acid cycle (Krebs cycle). The rate-limiting enzyme responsible for this reaction, ALA synthase, is strictly regulated by intracellular iron levels and heme concentration. A low-iron level, e.g., in iron deficiency, leads to decreased porphyrin synthesis, which prevents accumulation of the toxic intermediates. This mechanism is of therapeutic importance: infusion of heme arginate or hematin can abort attacks of porphyria in patients with an inborn error of metabolism of this process, by reducing transcription of ALA synthase.

67) water: composes produced from nutrient oxidation. 2,5L is excreted by kidneys (1-1,5L), skin vapolarization (0,5-1L), lungs (400mL), and feces (50-200mL). Water of blood composes ~5%. It is the circulating blood volume. 93% is pure water, rest in blood cellular elements. Intacelullar water 35-45% is constantly regulated and must be not changed. Present in: a) bounded to hydrophilic cytoplasm structures c) in cytoplasmic lacunes extracellular 15% the interstitial fluid is close to blood plasma. Trnscellular 1-3% forms digestive juice, cerebrospinal fluid, kidney tubule fluid. Water balance regulation: a) oncotic pressure b) osmotic pressure c) interstitial tissue pH d) hydrodynamic pressure difference between blood and extracellular fluid. E) parcipitation of aldosterone of adrenal cortex, which regulates Na+ reabsorption from primary urine into the blood. *** Excess of sodium cell shrinkage, dry skin, arterial hypertension, thirst, oliguria or anuria. Deficit of sodium cell swelling, edema, polyuria, absence of thirst, a.hypotension, vomiting. Excess of potassium diarrhea, metabolic acidosis. Deficit of potassium vomiting, thirst, met.alkalosis, inability to concentrate urine.

68) Kidneys. pH and acid-base balance: *acidogenesis is secretion oh H+ ions into renal tubules. *ammoniogenesis is formation and secretion of ammonia ion (NH3) into the renal tubules Then, NH3 reacts with H+ to form NH4+. Afterwards it is accompanied by anion Cl-. Neutral salt NH4Cl is formed and excreted in urine. *reabsorption of bicarbonate (NaHCO3) in renal tubules. Consequently, urine examination reflects the acid-base state.

Filtration: A filtrate derived from plasma in the glomerulus must pass through a basement membrane of the glomerular

capillaries and through slits in the processes of the podocytes—the cells that compose the inner layer of the glomerular (Bowman’s) capsule.A. The glomerular ultrafiltrate,formed under the force of blood pressure, has a low protein concentration. Reabsorbtion: I Approximately 65% of the filtered salt and water is reabsorbed across the proximal convoluted tubules. A. Sodium is actively transported, chloride follows passively by electrical attraction, and water

follows the salt out of the proximal tubule. B. Salt transport in the proximal tubules is not under hormonal

regulation.The reabsorption of most of the remaining water occurs as a result of the action of the countercurrent

multiplier system .II A. Sodium is actively extruded from the ascending limb, followed passively by chloride. B. Since the ascending limb is impermeable to water, the remaining filtrate becomes hypotonic. C. Because of this salt transport and because of countercurrent exchange in the vasa recta, the interstitial fluid of the medulla becomes hypertonic. D. The hypertonicity of the medulla is multiplied by a positive feedback mechanism involving the descending limb, which is passively permeable to water and perhaps to salt. III. The collecting duct is permeable to water but not to salt. A. As the collecting ducts pass through the hypertonic renal medulla, water leaves by osmosis and is carried away in surrounding

capillaries. B. The permeability of the collecting ducts to water is stimulated by antidiuretic hormone (ADH).

Renal Plasma Clearance I. Inulin is filtered but neither reabsorbed nor secreted. Its clearance is thus equal

to the glomerular filtration rate. II. Some of the filtered urea is reabsorbed. Its clearance is therefore less than the glomerular filtration rate. III. Since almost all the PAH in blood going through the kidneys is cleared by filtration and secretion, the PAH clearance is a measure of the total renal blood flow. IV. Normally all of the filtered glucose is reabsorbed. Glycosuria occurs when the transport carriers for glucose become saturated as a result of hyperglycemia.

69) Urine: Urine is a transparent solution that can range from colorless to amber but is usually a pale yellow. In the urine of a healthy individual, the color comes primarily from the presence of urobilin. The smell of urine can be affected by the consumption of food. Eating asparagus is known to cause a strong odor in human urine. This is because of the body's breakdown of asparagusic acid. The pH of urine is close to neutral (7) but can normally vary between 4.4 and 8. In persons with hyperuricosuria, acidic urine can contribute to the formation of stones of uric acid in the kidneys, ureters, or bladder. The amount of urine produced depends on numerous factors including level of hydration, activities, environmental factors, weight of individual, and the individual's health. In adult humans the average production is about 1 – 2 L per day. Producing too much or too little urine needs medical attention: Polyuria is a condition of excessive production of urine (> 2.5 L/day), in contrast to oliguria where < 400 mL are produced per day, or anuria with a production of < 100 mL per day. ammoniogenesis is formation and secretion of ammonia ion (NH3) into the renal tubules Then, NH3 reacts with H+ to form NH4+. Afterwards it is accompanied by anion Cl-. Neutral salt NH4Cl is formed and excreted in urine.

70) Composition of physiological urine: Urine is an aqueous solution of greater than 95% water, with the remaining constituents, in order of decreasing concentration urea 9.3 g/L, chloride 1.87 g/L, sodium 1.17 g/L, potassium 0.750 g/L, creatinine 0.670 g/L, calcium 2,5-8,0mmol/day, phosphate 0-38mmol/day, and other dissolved ions, inorganic and organic compounds. Urobilinogen: 0,2-1,0mg/Dl.