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The processes by which the nephrons remove the nitrogenous wastes as urine from the blood without losing at the same time valuable small molecules, water and ions, are as follows:
1. Glomerular filtration:
When the blood in the afferent arteriole enters the glomerulus a part of the water and some dissolved constituents of the blood of low molecular weight like nitrogenous wastes, glucose and mineral salts filter out through the capillary walls into the surrounding Bowman’s capsule, by a process called glomerular filtration.
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This phenomenon was established by Richards and his associates in the year of 1924. According to Ludwig glomerular filtration is caused due to changes in blood pressure and, thus, the dynamics of the renal corpuscle (nephron) were established in physical terms of blood pressure, osmotic pressure and pressures of the capsular fluid.
This shows that the process of glomerular filtration in the renal corpuscles or nephrons is made possible by the high blood pressure in the glomerular capillaries and the osmotic pressure exerted by the contents on either side of the membrane. A sharp drop in blood pressure leads to decrease urinary excretion.
The glomerular filtrate resembles the blood plasma in its chemical composition, except for the absence of large molecules.
About 180 liters (approximately 45 gallons) of fluid are filtered from the plasma into Bowman’s capsule from the glomerular vessels every 24 hours.
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(In other words, the entire plasma volume is filtered about 60 times a day). However, only about 1 to liters of urine are produced every day.
2. Tubular reabsorption:
This is the second step in the urine formation. The glomerular filtrate in Bowman’s capsule flows on through the convoluted tubule into the collecting tubule and thence into the pelvis of the kidney and down the ureter to the bladder.
As the glomerular filtrate flows through the proxmial convoluted tubule some water, physiologically important solutes like glucose, amino acids and inorganic salts like sodium chloride and sodium bicarbonate are reabsorbed into the blood in the capillaries around this portion of the tubule, according to Forster, 1961.
The remaining constituents in the filtrate are waste products to be excreted.
3. Active secretion:
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It is the third step in the urine formation. As the tubular fluid, or filtrate flows through the distal convoluted tubule the unwanted substances which could not filtered out in the glomerulus, are actively secreted by the tubular wall into the filtrate from the blood.
As a result of this entire process, homeostasis of the blood is maintained and all the waste products remained in the tubular fluid constitute urine which is ready for excretion from the body.
Mammals produce hypertonic urine. Urine remains isotonic in the proximal convoluted tubule but becomes progressively more hypertonic as it slowly descends the loop of Henle.
Within the ascending limb of this loop it becomes gradually less hypertonic. Within the distal tubule it is either hypotonic or once more isotonic to the surrounding tissue fluids.
As the urine passes through the collecting tubule in the medulla it again becomes hypertonic.
It is suggested that the medullary tubules constitute a countercurrent multiplier (Gottschalk 1960, Wirz 1961) and that the active mechanism is a cellular transport of sodium from the urine in the thick portion of the ascending limb of Henle’s loop.
Recent evidence suggests chloride ions, rather than sodium ions are actively pumped out and that sodium ions follow passively.
The basic features of the mechanism by which hypertonic urine is formed are as follows : The glomerular filtrate that enters Bowman’s capsule and the contents of the proximal tubule are isosmotic with the plasma of the blood.
About 75% of the original filtrate volume is reabsorbed by the time the tubular fluid enters the Henle’s loop.
This reabsorption of water occurs osmotically, as a result of dilution of the filtrate due to active uptake of sodium by the cells lining-the walls of tubule.
The descending tubules of Henle’s loops are permeable to diffusion of water, sodium and chloride ions.
As the tubular fluid passes through Henle’s loop, it enters regions of increasing osmotic, sodium and chloride ion concentrations.
Water then is osmotically withdrawn and sodium and chloride ions are actively pumped into the tubule, keeping tubular fluid concentrations close to those existing outside the tubules.
The cells of the ascending limb of Henle’s loop are impermeable to diffusion of water and chloride ions.
However, these actively transport chloride ions outward. As a result, the tubular fluid remains reduced in volume (about 50% of its volume was removed osmotically in the descending limb), but it becomes progressively more dilute as it reaches the distal tubule.
This tubular fluid of low osmotic concentration enters the distal tubule. The cells of this region are permeable to water, so additional osmotic removal of water occurs and filterate becomes isotonic.
The final concentration of the urine takes place in the collecting ducts whose cells can vary greatly in their permeability to water, this variation being under the control of at least one and possibly two chemicals.
These chemicals are the neurohypophyseal hormone vasopressin (also called the antidiuretic hormone, ADH) and the enzyme hyaluronidase.
The action of ADH is well understood while that of hyaluronidase is still uncertain. Thus, the tubular fluid in the distal tubule and later in the collecting tubule becomes hypertonic and is known as urine.
Hypertonic urine, thus, produced in the nephrons passes into the collecting tubules and thence into the calyces which lead to the renal pelvis.
The urine passes from the pelvis through a ureter into the urinary bladder from where it is removed outside the body time to time.
Blood leaves the kidney through the renal vein and returns to the general circulation by way of the inferior vena-cava.