Fascinating facts about human kidneys and urinary tract

The kidneys are a pair of been shaped organs situated in the upper part of the abdominal cavity near the back one on either side of the vertebral column.

Each kidney weighs about 150 grams with a length of about 11 centimeters, breadth of about 7 centimeters and thickness of 2.5 centimeters.

Each kidney receives its blood supply from the renal artery which directly arises from the aorta in the abdomen. As the kidneys are concerned with cleaning of the blood they have to get plenty of blood.

Approximately 20 percent of the cardiac output goes to the kidneys which is about one liter per minute and more than 1400 liters per day.

The structural and functional unit of a kidney is the nephron. It is a tube like structure with one end looking somewhat like a cup called the Bowman’s capsule. The total length of all the nephrons in the two kidneys is estimated to be about 100 kilometers (one hundred kilometers). The Bowman’s capsule contains a tuft of capillaries called glomerulus. Filtration of blood starts when it flows through the glomerulus.

The metabolic functions of the human body produce many waste products which can be harmful or even fatal if allowed to accumulate in the body. It may be surprising for many people to know that ammonia is produced in the human body due to breakdown of proteins.

The liver converts this ammonia into a compound called urea and the kidneys eliminate urea in the urine. If ammonia accumulates it can cause problems to the brain and even lead to death.

The biochemical reactions in the body produce some acids. The kidneys get rid of these acids so that the blood remains alkaline. If the ability of the kidneys to remove these acids is impaired the normal functioning of the body will go haywire and may even culminate in death.

As blood flows through the glomerulus the liquid portion of the blood (plasma) is filtered into the Bowman’s Capsule. This fluid is called glomerular filtrate. Apart from water it contains sodium, potassium, glucose, urea and other low molecular substances in the plasma.

The plasma proteins are big molecules and normally do not enter the Bowman’s capsule. Glomerular Filtration Rate (GFR) is the amount of filtrate formed per unit time. Normal value is 125 milliliter per minute and about 80 liters per day.

About 99% of filtrate is reabsorbed and the remaining portion forms urine. The pituitary gland secretes a hormone called antidiuretic hormone which increases re-absorption of water from the renal tubules.

From the Bowman’s capsule the filtrate flows through the remaining part of the nephron. The tube like structure is divided into proximal convoluted tubule, a loop (called loop of Henle) and distal convoluted tubule. Individual nephrons culminate in larger collecting tubes, which then end up in a chamber called pelvis of the kidney.

From the pelvis another tube like structure (ureter) comes down. Each ureter enters a bag like organ called urinary bladder.

Urine collects in the bladder and when it is full the nerves send impulses to the brain and the person empties the bladder through another tube called urethra.

The male urethra is longer than the female urethra. Hence urinary tract infections are more common in females than in males.

In babies and small children there is no voluntary bladder control and the bladder functions in a reflex manner. As the higher centers in the brain develop a person can exercise control and hold the urine for a longer period.

The ureters enter the bladder at an oblique angle. There is a valve like mechanism at the junction of ureter and bladder which prevents urine from going up the ureters when the bladder contracts to void urine.

Failure of this mechanism results in a condition called vesico-ureteric reflux in which urine goes up the ureter when the bladder contracts. This predisposes to recurrent urinary tract infection and enlargement of ureter (hydro-ureter ) and kidney (hydro nephrosis). If not promptly treated, the condition can result in irreversible damage to the kidneys.

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