The amount of filtrate formed in Bowman’s capsule every minute is called the glomerular filtration rate.
About ninety nine percent of the filtrate is absorbed during the process of reabsorption. Tubular secretion maintains the ionic and acid base balance of body fluids by secreting ions and ammonia back into the distal convoluted tubule and passing it to the collecting duct.
The kidney is the major excretory organ in humans and one of its primary functions is urine formation. Urine formation involves three major processes, namely glomerular filtration or ultrafiltration, reabsorption and secretion.
The first step in urine formation is the filtration of blood, which takes place in the glomerulus and hence the name glomerular filtration.
The amount of filtrate formed in the Bowman’s capsule of a nephron due to glomerular filtration in the kidneys every minute is called the glomerular filtration rate or GFR, which in a healthy individual is about 125 ml per minute or 180 liters per day.
The juxtaglomerular apparatus or JGA is one such microscopic structure that regulates the GFR.
Reabsorption: Did you know that of the 180 litres of glomerular filtrate formed per day, the amount of urine released is just one point five litres? This is because about ninety nine percent of the filtrate is re-absorbed in the renal tubules during the process of reabsorption.
Reabsorption is performed by the tubular epithelial cells present in the different parts of the renal tubule through active or passive mechanisms.
Apart from absorption, the cells in the renal tubule also selectively secrete substances such as potassium and hydrogen ions and ammonia into the filtrate to maintain the pH and ionic balance in body fluids. The proximal convoluted tubule is the section of the nephron situated between Bowman's capsule and Henle’s loop.
The glomerular filtrate from Bowman’s capsule enters this tubule, which is lined by cuboidal, brush border epithelial cells that help to increase the surface area for reabsorption.
The proximal convoluted tubule reabsorbs about 70-80% of the electrolytes and water and almost all the essential nutrients and vitamins.
It also selectively secretes ammonia, hydrogen ions and potassium ions into the filtrate and absorbs bicarbonate to maintain the pH and ionic balance of body fluids.
The proximal convoluted tubule is followed by Henle’s loop, where minimal reabsorption takes place. However, the region plays a vital role in maintaining the high osmolarity of medullary interstitial fluid. The descending limb of Henle’s loop is permeable to water and almost impermeable to electrolytes, which helps concentrate the filtrate as it moves down.
The ascending limb is permeable to electrolytes, actively or passively, and is impermeable to water. Therefore, electrolytes pass into the medullary fluid and dilute the concentrated filtrate as it passes upwards.
Henle’s loop is followed by the distal convoluted tubule, where conditional reabsorption of water and sodium occurs.
The distal convoluted tubules of several nephrons open into a straight tube called the collecting duct, which extends from the cortex of the kidney to the inner parts of the medulla.
The duct helps to reabsorb water, thereby increasing the concentration of urine according to the body’s state of hydration.
Therefore, the nephron filters blood by reabsorbing substances that are needed and excreting the rest as urine.
The urine produced by our body is around four times more concentrated than the initial filtrate, which reflects the conservation of water in the nephrons. Urine thus helps to eliminate the numerous waste compounds generated by our body. Urine formation is vital for human health and it is difficult to survive without producing and eliminating it.