Human Excretory System : definition structure functions and diagram

How does the excretory system work?

the main function of the Human Excretory System or any other animal excretory system is to remove waste from our body. and this function carries out through our kidneys.

The kidneys filter blood by the renal artery and are removed by renal veins. Filtration occurs in the renal cortex. Wastes products collect in the renal pelvis and drain through the ureters to the bladder.

Kidneys dispose of waste as urine. They also help maintain a healthy salt and water balance. From the outside, kidneys resemble a pair of purplish-brown beans the size of a man’s fist. Each weighs about 140 grams. They lie on either side of the backbone.

Cut open, a kidney reveals two major areas; a pale outer layer called the cortex, and the dark inner mass, the medulla. The cortex is made mostly of blood filtration units, renal corpuscles, and tubules. The medulla contains tubules (Loops of Henle) that collect the dilute filtrate and reabsorb most of the water and the collecting ducts for the final concentrated filtrate that becomes urine.

The main function of the kidney is urine formation, which takes place in three steps. The first steps are pressure filtration. When blood enters the kidney via the renal artery, it goes to many arterioles, and then to the glomerulus.

The pressure of the blood is very high and so most of the water, salts, glucose, and urea of blood is forced out of glomerulus capillaries. This material passes into the Bowman’s capsule and is now called glomerular filtrate.

The second step is selective reabsorption. In this step, about 99% of the glomerular filtrate is reabsorbed into the blood capillaries surrounding renal tubule. It occurs through osmosis, diffusion, and active transport. Some water and most of the glucose are reabsorbed from the proximal convoluted tubule.

Here, salts are reabsorbed by active transport and then water follows by osmosis. The descending limb of Henle allows the reabsorption of the water while the ascending loop of Henle allows the reabsorption of the salts. The distal convoluted tubule again allows the reabsorption of water into the blood

The third step is tabular secretion. Different ions,  creatinine, urea, etc. are secreted from the blood into the filtrate in the renal tubule. This is done to maintain the blood at a normal pH (7.35 to 7.45).

After the above-mentioned steps, the filtrate present in the renal tubules is known as urine. It moves into collecting ducts and then into the pelvis.

Blood Filtration

About a million blood filtration units, or nephrons, are packed into each kidney. Each nephron is a coiled tubule with a loop measuring about 2-6 centimeters. Each nephron’s outer and (in the cortex) forms a double-walled cup known as Bowman’s Capsule. This envelops a bulging knot of capillaries called a glomerulus. Blood driven through the glomerulus forces small molecules out through the capillary walls into the Bowman’s Capsule.

The resulting filtrate includes water and dissolved sugar, salts, and urea. In this way, the nephron removes impurities from the blood. Water and other valuable substances are returned to the blood as they filtrate through the nephron’s convoluted tubule (the Loop of Henle). By the time the filtrate reaches the collecting tube it has been converted into concentrated urea solution (urine).

Osmoregulatory Function of Kidney

Osmoregulation is defined as the regulation of the concentration of water and salts in blood and other body fluids. Kidneys play important role in osmoregulation by regulating the water contents of blood.

It is an important process an excessive loss of water concentrates the body fluids whereas excess intake of water dilutes them. When there is excess water in body fluids, kidneys form dilute (hypotonic) urine. For this purpose, kidneys filter more water from the glomerular capillaries into Bowman’s capsule.

Similarly, less water is reabsorbed and abundant dilute urine is produced. It brings down the volume of body fluids to normal. When there is a shortage of water in body fluids, kidneys filter less water from glomerular capillaries and the rate of reabsorption of water is increased.

Less filtration and more reabsorption produce a small amount of concentrated (hypertonic) urine. It increases the volume of the body fluids to normal. The whole process is under hormonal control.