Human Urinary System

 Urinary System

The main organ that carries out nitrogenous excretion is the kidney.

A pair of kidneys and other organs are organized to form urinary system.

The main parts of the urinary system are as follows

• Pair of kidneys
• Pair of ureters
• Urinary bladder
• Urethra

The waste materials in blood enter through renal arteries are filltered inside the kidney. This filtrate is known as urine and it is transported through ureters and stored temporary in urinary bladder. Next it is released out of the body through urethra.

Process of urine formation 

Urine formation in kidney follows three main processes, 

• Ultra filtration
• Selective reabsorption 
• Secretion 

 Ultrafiltration

Each afferent arteriole enters into each Bowman's capsule, where they further divide forming a dense network of capillaries. It is known as glomerulus. The blood flow  through the glomerulus is having a high blood pressure because the diameter of efferent arteriole is smaller than diameter of afferent arteriole. So blood gets filtered through the wall of glomerulus and the inner wall of the bowman’s capsule and collected into the cavity of Bowman’s capsule. This process is known as ultrafiltration. This filtered fluid is referred to as glomerular filtrate. Large molecules like plasma proteins and blood cells are not filtered into the glomerular filtrate. Glomerular filterate is as same as blood plasma. The constituents of glomerulur filtrate are water, glucose, amino acids, vitamins, drugs, various ions, hormones and urea. 

Selective reabsorption

When glomerular filterate moves along the nephron most of the constituents absorb  again into the blood capillaries associated with nephron. This is called Selective reabsorption.  90% of the water, all glucose, amino acids and vitamins part of salts, small amount of  urea, uric acid and durgs reabsorb into blood. The composition of glomerular filtrate change with selecteve reabsorption. Then the glomerular filtrate is released into collecting ducts and then to the pelvis. The volume of glomerular filterate formed during one minute in a healthy adult is about 120 cm3. But 95% of the glomerular filtrate reabsorb when it moves along the nephron.

100% of glucose is reabsorbed in a healthy adult. But in diabetes patients glucose is not totally reabsorbed. The remaining glucose is  released with urine. 

Secretion 

Some of the materials in the blood capillaries associated with nephron enter the tubules of  nephron. This process is known as secretion. 

E.g (¡ Hydrogen ions (H+), Potasium ions (K+), Ammonium ions (NH+ 4), Creatinine, drugs,      Vitamin B

Removal of urine from the body 

Urine released into the pelvis is transported along ureters into bladder and is temporally stored in bladder. Release of urine takes place according to the need of urination.