Urinary system of human beings consists of kidney, ureter, urinary bladder and urethra.1
The main functions od urinary system are:
1 Excrete waste products from the body
2. Regulate the blood volume and blood pressure
3. Control the level of electrolytes and metabolites
4. Control the pH of the body2
The basic unit of the kidney is nephron3. Mechanism of renal system is the blood enters the kidney through the renal artery and gets filtered through it and then the blood returns to the systemic circulation through renal veins.
The blood passes through the glomerulus (a structure in the nephron) and the small metabolites and waste products get filtered through it while large molecules like protein cannot cross the filtering membrane.
Useful compounds are reabsorbed from the filtrate and only waste products are left in the filtrate. Filtrate mostly contain ammonia, bilirubin, creatinine, environmental toxins and pharmacological compounds.
The filtrate is named as urine as it enters the ureter. From there, the urine is stored in the urinary bladder and gets excreted through the urethra4.
The urinary system under the influence of nervous system and endocrine system controls the blood pressure and blood volume. If the blood volume is low then the renal system retains the sodium due to which water is retained in the body and low blood volume is compensated. And if the blood volume increases, the renal system excretes the extra amount of water to bring it down to the normal level5.
Kidneys secrete aldosterone which plays an important function in maintaining the blood pressure. It reabsorbs sodium which results in retention of water and increase of blood volume and blood pressure6. Antidiuretic hormone (ADH) retains water in the body and constricts blood vessels7.
Kidney secretes hydrogen ion in the urine. At the same time, it conserves bicarbonate ions which help in buffering the pH. Charged ions are of vital importance in our body. Calcium ion is useful in contraction of the muscle. Sodium and potassium ions are useful in signal transmission. Kidney regulates the number of these ions which help in the conduction of these processes8.
Kidney secretes the hormone erythropoietin which stimulates the production of red blood cells. Kidney also synthesizes the active form of vitamin D in collaboration with skin and liver9.
There are two ureters in the body. Both the ureters pass urine from the kidney to the bladder and then prevent urine backflow from urinary bladder to the kidney. The bladder stores urine prior to elimination of urine from the body. And then expels urine into the urethra for micturition. The process of micturition is controlled by both involuntary and voluntary muscles. The urethra is used for the expulsion of urine. It is also used as a duct for ejaculation of semen10.