How Urinary System Works In Humans

In humans urinary system comprises of a pair of kidneys, ureter, urinary bladder and urethra which empties near the vagina in females or through the penis in males. A pair of kidneys consist of millions of functional units, nephrons. The nephron have extensive blood supply via the renal arteries, which leave kidney via the renal vein. The function of kidney and blood in clearing wastes is very evident from the fact that weight of kidneys account for less than 1 percent of the total body weight while receives 20 percent of blood supplied with each cardiac beat. Following filtration of blood and further processing through the tubular system urine is collected in a central of the kidney, pelvis. Urine leaves kidney through duct ureter. The ureters of both the kidneys drain into urinary bladder through ureteral orifice. Urine leaves the body, during urination, from the bladder through the tube called urethra.  Sphincter muscles near the junction of the urethra and the bladder control the urine in bladder.

• Nephron:

The functional units, nephrons, in human kidneys are arranged along two distinct regions, an outer cortex and inner medulla. The nephron arranged along the cortex are called as cortical, however those arranged along the border of cortex and medulla with their tubular system looping deep in inner medulla are juxtamedullary nephrons.  In each nephron inner end forms a cup-shaped swelling, called Bowmans's capsule and it is around the ball of capillaries called glomerulus. Glomerulus circulates blood through capsule as it arrives through afferent arteriole and leaves the capsule by efferent arteriole. The blood vessels subdivide again into another network of capillaries, the peritubular capillaries. Bownman capsule continue as extensively convoluted proximal tubule, loop of Henle and the distal tubule, which empties into collecting tubules. The collecting tubule than opens into pelvis.

In humans, mechanism of urinary system comprises of following main phases.

• Filtration:

Blood passing through glomerulus is filtered into Bowmans capsule. It is specifically filtered here, unlike at the other parts of the vessels, because the glomerulus walls are porous, and the fraction of the blood pressure reaching here provides the filtration pressure. The filtrate appearing in glomerulus is called as glomerular filtrate, which contains numerous useful substances such as glucose, amino acids, salts etc in aqueous solution. 

• Re-absorption:

All the constituents of the glomerular filtrate are reabsorbed in proximal tubules and when filtrate leaves proximal tubules, it mostly contains nitrogenous wastes.

• Secretion:

    The tubular epithelium also secretes substances into the lumen, this secretion is very selective and is mainly of hydrogen ions to balance pH value of the filtrate passing through the tubule.

• Role of hormones:

The active uptake of sodium in the ascending limb or thick loop of Henle is promoted by the action of aldosterone, the hormone secreted from adrenal cortex. The outer site in nephron, where reabsorption of water takes place is collecting tubules. ADH released from posterior pituitary lobe acts to actively transport water from filtrate in collecting tubules back to kidney.