Name: Excretion in Human Beings
Uploaded: 2019-07-19
Duration: 2 min 52 s
Description: Kidneys eliminate the toxic nitrogenous wastes generated in the body as a result of metabolism. The liver, too, plays a crucial role in this. Take a sneak peek.

Question

Mention the physiological mechanism by which urine is formed in the kidney.

Accepted Answer

Kidney plays an essential role in the elimination of harmful and toxic substances from the body of an individual. Urine formation is a complex process and every part of the nephron actively participates in this process. The process involves three different functions of the nephrons. glomerular filtration, tubular reabsorption, and tubular secretion which are described as follows :

Glomerular filtration: The malpighian corpuscle of the nephron acts as an ultra-filtering organ. The process which serves to separate colloid solutions from crystalloids and to separate particles of different sizes in the colloid mixture and takes place under pressure is known as ultrafiltration. The visceral layer of Bowman's capsule and the walls of the glomerular capillaries form a special filtering membrane by which the ultrafiltration of blood takes place. The pores of the filtering membrane only allow the water and the non-colloidal materials dissolved in the water of plasma to pass across the membrane. The pores of the membrane allow to pass those plasma components whose molecular weight is not more than 68000 microns. Thus, the blood cells and the plasma proteins are separated out from the remaining components of the plasma of the glomerular blood. The water and non-colloidal materials are filtered into the cavity of the Bowman s capsule. Thus, ultrafiltration helps in the first stage of urine formation which is referred to as glomerular filtration.

Tubular reabsorption: Tubular reabsorption is the process by which the important constituents of the blood i.e. water, glucose. amino acid, K. Na,CI. vitamins, etc. are absorbed from the glomerular filtrate by the epithelial cells of renal tubules and returned into the bloodstream of the surrounding blood capillaries.
The epithelial cells of the renal tubules reabsorb the substances selectively from the glomerular filtrate through active and passive reabsorption processes. Active reabsorption involves the expenditure of energy derived from ATP which is synthesised in the mitochondria present in the epithelial cells. Active reabsorption generally occurs against the concentration gradient. Whereas, passive reabsorption is driven by the concentration gradient or electrical potential without requiring energy. As the glomerular filtrate from the cavity of the Bowman's capsule passes through the renal tubule and flows towards the collecting duct, some of the constituents of the filtrate returned to the blood by active or passive reabsorption. Reabsorption is the phenomenon of returning of the materials from the glomerular filtrate to the blood after the active or passive absorption of that materials by the cells of the renal tubule. As a result, the filtrate changed into the urine gradually. This phenomenon is also called urine formation.

Tubular secretion: The process by which some substances are actively transported from the plasma and then actively secreted these substances by the tubular epithelial cells into the fluid of the renal tubular lumen is called tubular secretion. Tubular secretion is the reverse process of reabsorption by which the different substances are entered or released into the luminal fluid by the epithelial cells of the renal tubule. Creatinine, K, hippuric acid, ammonia, pigments, penicillin, etc secreted into the luminal filtrate by the epithelial cells of the proximal convoluted tubule. Amongst these. K. H⁺ creatinine, hippuric acid, penicillin are secreted actively but ammonia passively released into the filtrate. In the ascending limb of Henle's loop urea enters the filtrate by simple diffusion.
Thus, through those above-mentioned reabsorption and secretion processes, the glomerular filtrate convened into the urine. But normally the urine is hypertonic to plasma or filtrate and also acidic, which is only due to the reabsorption of the huge amount of water from the filtrate and the addition of water to the filtrate by the tubular secretion.

Mention the physiological mechanism by which urine is formed in the kidney.

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