Internal Environment & Renal Physiology

Internal Environment: The body is made up of organs and tissues each composed of cells and fibers that constitute their histology. Water with its solvents needed for the health of the cells is termed as body fluid and this fluid is partly inside and partly outside the cells. Water constitutes about 70 per cent of fat free body weight. Water is the fabric of everything that lives. The body of a baby contains mostly water, whilst the old man or woman shrivels up like a wilted plant. We are completely immersed in water during the first nine months of life in the mother's womb. Water is involved in the health, disease and death. Loss of water leads to dehydration and may cause death if not corrected. Retention of water leads to edema and may cause death if remedial action is not taken. For each cell in the body the same conditions prevail as for the single-celled creatures fixed on the bed of a flowing stream which brings their food and oxygen and carries away their waste material. In our body water or the body fluid is controlled in the two major compartments: (1) intracellular compartment for intracellular fluid (2) extracellular compartment for extracellular fluid. The extracellular fluid is of two subtype (1) interstitial fluid (2) blood plasma. Intracellular fluid makes up about 40 to 50 per cent of the body weight and bulk of it being contained in muscles. Extracellular fluid represents about 20 per cent of the body weight, of which 15 per cent is interstitial fluid including lymph and 5 per cent constitute the blood plasma. The interstitial fluid constitutes the real internal environment. It is the adjustable segment in the total water content of the body. Its volume and solutes are regulated by the kidneys, lungs, endocrine glands, and are influenced by sweat glands and gastrointestinal tract. The blood plasma is in equilibrium with the interstitial fluid. Both the vascular and intracellular compartments contain a lot of protein. The normal intake of water in an adult is about 2500 ml. About 2100-2200 ml of this is taken by mouth as food and pure water and rest is the endogenous water from cellular oxidation. Renal Physiology: The word renal pertains to kidney in medical terminology. Water regulation in our body is achieved by water loss through four routes: (1) intestine (2) lungs (3) skin and (4) kidneys. Kidneys play a major role in water regulation as these excrete 50 to 70 per cent of excess water. Major functions of kidneys are: (a) excretion/elimination of excess water from body (b) excretion/elimination of waste products of metabolism e.g. urea and creatinine (c) excretion/elimination of foreign substances such as drugs (d) retention of substances necessary for normal body functions (major substances are proteins, amino acids and glucose) (e) regulation of electrolyte balance and osmotic pressure of the body fluids. Sodium ions, potassium ions, bicarbonate ions and chloride ions are major electrolytes. Urea is the main product of protein metabolism in the body. Removal of amino groups from amino-acids, from which urea is formed, takes place in the liver. Urine urea estimations are most commonly carried out as part of renal efficiency tests. A high concentration of urea in the urine shows that the kidneys possess a good concentrating power. However, in cases where there is increased blood urea due to non-renal or pre-renal factors, urine urea may be quite high. On an average concentration of urea in urine should be 2.0 per cent over the day. The total urea excretion in an adult is about 30 grams daily. At least 1500 ml of water must be excreted by kidneys daily to carry the solids which have to be eliminated. There is a pair of kidneys in our body to accomplish the above task. The kidney is the organ concerned with the regulation of the volume and composition of body fluids. Each kidney contains over 1000,000 functional units called nephrons. Each nephron consists of (a) glomerulus with its afferent and efferent arteriole (b) proximal convoluted tubule (c) loop of Henle (d) distal convoluted tubule and (e) collecting tubule. The structure of the glomerulus is that of a filtration mechanism. The afferent arteriole divides into 3 or 4 branches, which gives the lobulated appearance to the glomerular tuft. Each branch gives rise to 40 to 50 capillary loops, which probably do not anastomose with one another. The diameter of efferent arteriole is only half of that of afferent arteriole and the efferent arteriole splits up into a huge network of capillaries containing blood that is highly viscous by reason of the preceding loss of water. The viscous blood moves slowly, so raises the pressure in the glomerular tuft and thus facilitate filtration. The glomerular tuft consists of four main components: (i) the endothelium lining the capillaries (ii) the basement membrane which separates the endothelium from (iii) the epithelium and (iv) the mesangium. Mechanism of Renal Function: Every minute about 1000 ml of blood containing about 500 ml of plasma flows through the glomeruli of kidneys and about 100 ml of it is filtered out as raw-urine called glomerular filtrate. The plasma containing all the salts, glucose and other small substances is filtered in the glomerular filtrate. The cells and plasma proteins are too big to pass through the pores of the filter and stay behind in the blood stream. The glomerular filtrate then passes through the real tubules and 85% of it is absorbed automatically by the proximal tubules, where essential substances are reabsorbed. The fate of remaining 15% depends upon the degree of further reabsorption of water in the distal tubules. The reabsorption is controlled by antidiuretic hormone (ADH) released from the posterior pituitary gland (an endocrine gland). Loss of ADH results in defective reabsorption of water in the distal tubules and causes diabetes insipidus. An increase in the electrolyte osmotic pressure (osmolality) of the extracellular fluids results in an increased release of ADH with an increased water reabsorption in distal tubules. Conversely any decrease in the osmolality will lead to opposite effect. This complex controlling mechanism is termed as neurohypophysial-renal axis. Just as the electrolytic osmotic pressure of the extracellular fluid is controlled by the ADH, the volume of that fluid is controlled by aldosterone from the adrenal gland. The ADH regulates the retention or excretion of water and the aldosterone regulates the reabsorption of sodium and thus the retention of water. Thus the secretion of urine is accomplished in three steps: (a) glomerular filtration. (b) tubular reabsorption and (c) tubular secretion. By comparing the amount filtered by the glomeruli per day with the amount usually excreted in the urine we can see how selective is renal function? Daily about 150 liters of water is filtered and about 1500 ml is excreted; about 750 grams of salts are filtered and 15 grams are excreted; about 150 grams of glucose are filtered and no amount is excreted and about 50 grams of urea are filtered and about 30 grams excreted.