Function of Human Kidney

 

EXCRETION IN MAN

In man excretion occurs by a pair of kidneys and structures. The two kidneys are located against the dorsal abdominal wall lateral to the vertebral column. The right kidney lies a little lower than the left one. The depression in the middle of the kidney is called hilus. Hilus is the site of which the man blood vessels, lymph vessels and nerves enter the kidney. A tube called ureter also emerges from the hilus, entends downwards and opens into the urinary blander. The blander opens to the outside through another tube known as urethra.

 

INTERNAL STRUCTURE OF KIDNEY

Urinary system of man

The longitudinal section of the kidney shows that it consists of two parts:

i)                     Cortex---------- an outer darker region,

ii)                   Medulla-------- a thicker inner region.

 

The cavity in the central portion is known as renal pelvis which represents the enlarged end of the ureter within the kidney.

Each kidney is composed of about million tiny tubes known as urinary tubules or nephrons which are held together by means of connective tissue. Each nephron is a highly coiled tubule.

 

The nephron is blind at one and where it becomes cup-like known   as Bowman’s capsule.

The bowman’s capsule along with some coiled part of nephron lies in the cortex region. The Bowman’s capsule surrounds a thick cluster of capillaries called glomerulus. The glomerulus capillaries and arteries arise from the renal artery.

For some part the urinary tubule becomes quite convoluted, next portion, called Loop of Henle, consists of a fairly straight portion which extends into the medulla region but loops back towards the cortex. The last portion of the tubule is again slightly convoluted and lies close to Bowman’s capsule. It joins, together with the last portion of several other nephrons to open into larger ducts called collecting tubules which finally opens into the pelvis.

 

Internal structure of Human Kidney.

FUNCTIONS OF KIDNEY

The function of the kidney is the formation of urine which involves three processes that occur in each nephron.

1             pressure filtration through the glomerulus;

2             reabsorption of the filtration as it passes through the various portion of the tubule;

3             tubular secretion

 

UREA FORMATION

 

In man 60% to 90% of all the nitrogenous materials consists of urea which is the major component of the urine. It is produced mainly in the liver from ammonia, certain amino groups and carbon dioxide. In fact excessive carbohydrates (glycogen) by th removal of amino group from the amino acids. The formation of urea involves a very complex cyclic sequence of enzymatic reaction called urea cycle. This urea then enters the blood stream and is carried to the kidneys.

 

Human kidney showing the arrangement of urinary tubules and blood circulation 

URINE FORMATION

 

1.          Pressure filtration through the Glomerulus

 

When blood passes through the tortuous capillaries of the glomerulus, they resist the flow of blood so that a high pressure is set up. This pressure causes the fluid to filter out through the capillary walls and collect in the Bowman’s capsule. The blood cells, plasma protiens and fats cannot cross the capillary walls. Therefore glomerular filtrate contains sugars, amino acids, salts, nitrogenous wastes especially urea and other dissolved substances.

 

2.            Reabsorption of the Filtrate

 

When glomerular filtrate passes along the nephron qenal tubule, the useful substances are reabsorbed selectively. This selective reabsorption prevents the loss of useful substances from the blood stream. About 85% of the water. sodium ions, chloride ions, most of the bicarbonates and all the glucose of the glamerular filtrate are absorbed and returned to the blood. Further selective reabsorption occur, along the loop of Henle and last convoluted part of the nephron. Now the remaining liquid that enters into the collecting duct called urine. Thus urine contains only the waste products such as urea, inactive hormones, excess salts and sugars, and water.

 

3.         Tubular Secretion

 

                Severval substances are transported from the blood by the tubule cells into the filtrate and that process is known as tubular secretion. This process increases the overall efficiency of the nephron in extracting waste products from the blood.

                The energy for filtration is supplied by the heart via the pressure of blood. Then urine passes down the collecting tubule and from there enters the pelvis where it continues down the ureter to the bladder. The sphincter muscles at the base of the bladder hold the urine. Phe urine is passed out at intervals. In babies, the sphincter muscle is controlled by a reflex it is emptied almost involuntarily. After about two years or so, the muscles can be controlled voluntarily.