The renal system is vital for the excretion of metabolic products from the body. The key processes that occur in the kidney include glomerular filtration, tubular secretion, tubular reabsorption, and excretion. The kidney has millions of nephrons that comprise five main parts. These are the Bowman’s capsules where filtration occurs. The filtered fluid then goes to the proximal tubules where the principal process is the reabsorption of nutrients, especially glucose that is filtered in the glomerulus. The subsequent part of the nephron is the loop of Henle. It has a thin descending limb that is permeable to water. The other part of the loop of Henle is the thick ascending limb that is impervious to water, and this is where potassium, sodium, and chloride are exchanged (Constanzo, 2007, pp 235-247). From the loop of the henle, the fluid next goes to the distal convoluted tubule, which together with the collecting tubule is under the influence of the antidiuretic hormone (arginine vasopressin).
The kidney also plays a role in vitamin D synthesis. It hydroxylates 25-hydroxy-calciferol to convert it to 1, 25- dihydroxycholecalciferol. This is a hormone that is essential for the metabolism of calcium. It increases the assimilation of calcium from the gastrointestinal tract; it also decreases the excretion of calcium, and it also mobilizes calcium from bone. As the hormone acts to increase the levels of calcium in the body, it does vice versa for phosphate. It increases the excretion of phosphate from the body (Barrett & Ganong, 2010, pp 709-715).
The kidney is a crucial organ in the clearance of substances from the body. The kidney’s clearance rate of albumin is zero because; albumin is essentially not filtered through the glomerulus except in disease conditions. On the other hand, the kidney’s clearance of glucose is comparable to that of albumin, but this is because of a different reason. At the glomerulus, glucose is filtered, but it is reabsorbed at the proximal convoluted tubules. Glucose is reabsorbed through two processes. First of all, it is absorbed through a sodium-glucose co-transporter, which transports glucose, from the luminal side. It is also transported by facilitated transport across the peritubular membrane. This is why the urine is devoid of glucose or proteins in normal physiological states. As for a product like urea, sodium, hydrogen, and phosphates, their clearance is more than zero because, once they are filtered through the glomerulus, they are then partially reabsorbed, and hence, some get excreted in the urine. For other organic products like para-amino hippuric acid, the clearance is considerably greater than the former products because they are not only filtered; they are also secreted.
Glucose reabsorption is up to a transport maximum, beyond which it can no longer be absorbed. The rest is excreted and in such instances glucosuria is noted. This may occur in diabetic disease, or when there is a severe anomaly with the sodium-glucose co-transporter. This compromises glucose reabsorption, consequently, glucose is detected in urine.
The kidney is also involved in the regulation of pH, in the body. This is done by the excretion of hydrogen ions and reabsorption of bicarbonate ions. In cases of metabolic alkalosis, there is increased retention of hydrogen ions. In cases of metabolic acidosis, there is increased secretion of hydrogen ions while more bicarbonate ions are retained. Kidneys also take up glutamine, convert it to ammonia that binds hydrogen ions and these are excreted from the body in the form of ammonium ions.
The renal system is involved in the excretion of products of metabolism. It excretes urea, a product of amino acids metabolism. It also excretes urobilinogen, which is a product of the breakdown of bilirubin. Creatinine is yet another product of metabolism; it arises as a result of catabolism in the muscles. Subsequently creatinine level will be elevated in muscular people. Water is also a product of metabolism, and the kidneys are one of the ways through which the body excretes excess of water. The kidneys excretion of water is under the influence of antidiuretic hormone, which is, released from the posterior pituitary. For instance, if plasma osmolality is high, more arginine vasopressin is released and, subsequently, water excretion is minimized in a bid to lower the plasma osmolality.
Kidneys also play a vital role in the pharmacokinetics of drugs. They are mainly involved in the excretion of drugs. For instance, aspirin, a commonly used NSAID is excreted through the kidney. The urine has to be alkaline so that the by-product of aspirin, salicylic acid, can be excreted. If the urine is acidic, the drug accumulates to toxic levels in the body and causes salicylism (Katzung, Masters, &Trevor, 2009, pp 7-12).
The renal system is, as a result, a vital organ of the metabolic activities, in the body. It is involved in the excretion of the metabolic end products of biomolecules in the body. In addition to that, it is a major organ of excretion in the pharmacokinetics of drugs. In cases of renal dysfunction, there may be an accumulation of waste products in the body, a state called uremia. Some drugs require their dosages to be adjusted in the case of renal disease.
References
Barrett, K. E., & Ganong, W. F. (2010). Ganong’s review of medical physiology (23rd ed.). New York: McGraw-Hill Medical.
Constanzo, L. (2007). Physiology (2nd ed.). Philadelphia: Saunders.
Katzung, B. G., Masters, S. B., & Trevor, A. J. (2009). Basic & clinical pharmacology (11th ed.). New York: McGraw-Hill Medical.