Medical brain notes 30

  DISORDERS OF WATER, ELECTROLYTES AND URATEBALANCES INTRODUCTION Homeostasis is  the maintenance of a stable internal environment within the body. This stability is necessary for optimum functioning of proteins, particularly enzymes, cells, tissues, organs and systems. Many substances have to be maintained at appropriate concentrations, including water, electrolytes, such as Na + , K + , Ca 2+ , Mg 2+  and P i , and the acid–base components H +  and HCO 3 – . In addition, waste products, such as urea and urate, must be kept below toxic levels. Normally biochemical and physiological mechanisms regulate and control the concentrations of all these components and, in general, homeostatic controls involve negative feedback mechanisms. A receptor detects unacceptable levels of a particular substance under homeostatic control and sends a signal to a regulatory center that initiates a response that corrects the imbalance and returns conditions to a physiologically acce ptab...

  KIDNEYS The functions of the kidney include maintaining the volume of the plasma and its concentrations of electrolytes, such as Na +  and K + , and keeping its pH within normal physiological ranges. This is largely achieved by varying the amounts of water and salts excreted, the removal of excess H +  and the regeneration of HCO 3 – . Kidneys also excrete waste products, such as urea, urate and creatinine, and produce the enzyme rennin and the hormones erythropoietin and calcitriol (also called 1  α ,25-dihydroxycholecalciferol, 1  α ,25DHCC). These control blood pressure, stimulate the production of erythrocytes by the bone marrow and regulate the absorption of Ca 2+  by the gastrointestinal tract (GIT) respectively. Kidneys also synthesize prostaglandins and degrade hormones, such as insulin. Each kidney is composed of an outer fibrous capsule, a cortex, a middle medulla and an inner pelvis region ( Figure 8.2A ). The tough capsule surrounding each kid...

  RENAL FUNCTION TESTS Renal function tests are used to detect the presence of renal diseases and assess their progress. They are, however, of little use in determining the causes of renal disease. The most widely used test is to measure the glomerular filtration rate (GFR), that is, the rate of filtrate formation by the kidneys. The value of the GFR depends on the net pressure across the glomerular membrane, the physical nature of the membrane and the surface area of the membrane that reflects the number of functioning glomeruli. All three factors can change as a result of disease and this will be reflected in the value of the GFR. In adults, the GFR is about 120 cm 3  per minute although it is related to body size, being higher in men than women. The GFR is also affected by age and declines in the elderly.   Measuring the GFR The GFR is determined by measuring the concentration of a substance in the urine and plasma that is known to be completely filtered from the plasm...

  RENAL FAILURE Renal failure is the cessation  of renal function and it can be acute or chronic. In acute renal failure there is rapid loss of renal function within hours or days, although the condition is potentially reversible and normal renal function can be regained. The deterioration is sudden, with increases in the concentrations of urea, creatinine and H +  in serum. Patients with acute renal failure often, but not always, present with  oliguria , where there is less than 400 cm 3  of urine passed per day. Indeed, patients are sometimes  anuric  and do not pass any urine at all. Chronic renal failure is the gradual, progressive deterioration of kidney function. As kidney function declines, there is accumulation of waste products that eventually reach toxic levels in the blood and may a ffect other organs. Acute renal failure Acute renal fa ilure can be categorized as prerenal, where the loss in renal function is due to a decrease in renal blood...

  DISORDERS OF WATER HOMEOSTASIS Water  is necessary to maintain the volumes of body compartments, for excretion of waste products and as a medium in which biochemical reactions occur. Water intake is variable and can depend, to some extent, on social habits but is supplied in the diet, from food as well as water and as a product of oxidative metabolism. Its loss is variable although an almost fixed amount, called the insensible loss, occurs from the GIT, skin and lungs. An average 70 kg man has 42 dm 3  of water distributed between various body compartments ( Figure 8.4 ). Water accounts for 60% of body weight in men but only 55% in women given they have a higher proportion of fat. In disease, patients can be  dehydrated , where water loss caused by vomiting and diarrhea exceeds gain,or  overhydrated , with an accumulation of water in body compartments. The clinical features of dehydration and over hydration are listed in  Table 8.1 . A reduced extracellul...

  DISORDERS OF Na +   HOMEOSTASIS Sodium ions are significant constituents of tissues, including bone, they control the volume of ECF and are required for normal neuromuscular functions. The intake of Na +  is variable, from less than 100 mmol to more than 300 mmol day –1 . Losses are also variable, but renal loss is normally matched to intake. Small amounts of Na +  are lost via skin and in feces and, under some circumstances, the GIT can be a major route of Na +  loss, as in diarrhea. The average 70 kg man contains 3700 mmol of Na +  ( Figure 8.5 ), of which 75% is found in the ECF. Hyponatremia and hypernatremia refer to serum concentrations of Na +  below and above the reference range of 135–145 mmol dm –3 . Hyponatremia is caused by an excessive retention of water or the loss of Na + , these two conditions resulting in different clinical features. The retention of water produces behavioral disturbances, headaches, confusion, convulsions and eventu...

  DISORDERS OF K +   HOMEOSTASIS Potassium ions are necessary to maintain cell volume, for the optimal activities of a number of enzymes, and to maintain the resting potential of cell membranes and therefore neuromuscular functions, especially in the heart . The intake of K +  varies between 30 and 100 mmol day  –1  and losses are equally variable. The kidneys excrete most ingested K +  with a smaller amount being eliminated by the GIT. A high concentration of plasma K +  stimulates the release of aldosterone that, in turn, increases the renal excretion of K + . Gastrointestinal losses can be significant during vomiting and diarrhea. Only very small amounts of K +  are lost in sweat. The average 70 kg human contains about 3600 mmol of K + ( Figure 8.8 ), almost all being found in the ICF. Values for the concentration of K +  in the serum below and above the reference range of 3.4 to 4.9 mmol dm –3  are called hypokalemia and hyperkalemia...

  DISORDERS OF Ca 2+   HOMEOSTASIS Calcium is required for bone and teeth structure, the release of neuro-transmitters and initiation of muscle contraction, as a cofactor for coagulation factors , some enzyme activities and it also acts as an intracellular second messenger for a number of hormones . The normal dietary intake of Ca 2+ of about 25 mmol day –1  is supplemented by the reabsorption of Ca 2+  from gastrointestinal secretions. Approximately 19 mmol of Ca 2+  is lost in the feces daily. The kidneys normally filter about 240 mmol of Ca 2+  daily but, as most of this is reabsorbed by the tubules, normal renal loss of Ca 2+  is only about 6 mmol per day ( Figure 8.9 ). Calcium is the most abundant mineral in the body and the average adult contains approximately 1 kg or 25 000 mmol of Ca 2+ . Approximately 99% of Ca 2+  is present in the bone. About 500 mmol of Ca 2+  is exchanged daily between bone and the ECF. The ECF contains about 22...