In the urinary system, the two kidneys form and secretes urine by peristalsis. Urine is conveyed from each kidneys into the urinary bladder. This provides temporary storage for about 300 ml urine which is eventually voided to the exterior through the urethra.
The kidneys are situated at the posterior of the abdomen, one on either side of the vertebral column with the right kidney a little lower than the left due to the space occupied by the liver.
Each kidney has a bean shaped and is enclosed in a capsule of fibrous tissue that can easily stripped off. Underneath this capsule lies the cortex followed by the medulla (made up of renal pyramid) then the hilum (this is where the renal arteries and nerves enter and the renal vein and ureter leaves the kidney).
Microscopically, the kidney is made up of collecting tubules and nephrons. The nephron is the functional unit of the kidney and each kidney has approximately one million nephrons. It consist of the Bowman’s capsule or glomerular, enter site loop of Henle, enter site proximal convoluted tubule and the distal convoluted tubule.
Functions Of The Kidney
I.Maintenance of water and electrolyte balance of the body
When there is a fluctuation in the amount of water and/or electrolyte in the body, their excretion is regulated by the kidney with the help of aldosterone and antidiuretic hormone which restore the body fluids to it normal volume and composition.
The osmolality of the plasma varies only slightly under normal condition, despite wide variations of fluid and electrolyte intake of the body. When excess amount of water is taken, it tend to dilute the plasma and reduces its osmotic pressure. This produces a renal response which results in the excretion of an increased volume of urine with an osmolality less than that of the plasma.
Therefore, the kidney maintains the water balance of the body by excreting water in excess of the solutes. On the other hand, on restricted fluids an increase in plasma osmolality is solved by the excretion of concentrated urine with an osmolality higher than that of the plasma which shows that more solutes are being excreted than water.
II. Blood pH Maintenance
Blood normal pH is slightly alkaline (7.36-7.42).The kidneys are involved in the removal of substances responsible for the acidity or more alkalinity of blood. Waste product of protein metabolism such as Uric acid, Urea, and Creatinine which are constantly formed and must be excreted are responsible for the acidity of urine.
Salts such as Sodium, Potassium, Calcium, and Magnesium and phosphorus increases urine alkalinity and must be excreted especially when their concentration is too high in the blood.
III.Excretion of drugs and toxins
Drugs after completing their action in the body leave waste products which are excreted by the kidneys. If these drugs are not metabolized they themselves are excreted. Toxins in the body are rendered harmless by detoxication in the liver, and the conjugated compounds so formed excreted in the urine.
Formation of urine
The formation of urine takes place in the nephrons by a combination of two processes, simple filtration and selective reabsorption.
- SIMPLE FILTRATION
It takes place between the glomerulus and glomerular capsule. The blood supply to the kidneys averages 1200 ml per min. A higher pressure in the glomerulus meets negligible pressure in the Bowman’s capsule, this result in the passage of substances in solution through the semi-permeable membrane at a rate of about 120 ml/min (glomerular filtrate rate). The filtrate contains glucose, salts, uric acid, urea and other substances of small molecular size. Plasma proteins and Cells with a large molecular size do not pass through the semi-permeable membrane.
- SELECTIVE REABSORPTION
When the filtrate enters the tubule, the tubular epithelium reabsorbs water and selected essential substances into the peritubular capillaries. By passive and active reabsorption the cells adjust the composition of urine to meet the body requirements. Approximately 80% of the water and sodium chloride content, together with phosphate, glucose and amino acids, are absorbed in the proximal tubule. About 20% of the tubular fluid enters the loop of Henle where water is passively absorbed, 6 ml per min of concentrated tubular fluid now enters the distal tubule, where there is an active reabsorption of sodium. The fluid leaves the distal tubule at the rate of approximately 1 ml/min, passing into the collecting ducts in the form of urine. Over a period of 24 h this will give a urine volume of 1-1.5 litres.
The constituents of the glomerular filtrate have either a high, medium, low or no-threshold value. This means that the threshold of a given substance in the plasma is the highest level at which the constituent is present in the blood before it appears in the urine.
Glucose is a high threshold substance which is completely reabsorbed from the filtrate and only appears in the urine when the blood level is 175-180 mg/dl, this being the normal threshold value. Urea has a threshold value of zero, because only small amounts of it are reabsorbed in the tubules, and it is always present in the urine no matter what the blood level happens to be.
Creatinine is a no-threshold substance because it is not reabsorbed and is always present in the urine. (Tubular cells can also secrete some creatinine into the filtrate.) The threshold of any substance can be altered by impaired renal function.
Source: Introduction to Medical Laboratory Technology (baker and silverton)
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