Though excretion in human beings takes place through lungs, skin, etc., the kidney is the main excretory organ in humans. Kidney has a structural and functional unit called nephron. Each kidney consists of millions of nephron which plays a significant role in the filtration and purification of blood. The nephron is divided into two portions, namely, the glomerulus and the renal tubule. The glomerulus is constituted of capillaries of the renal artery. The Bowman’s capsule, proximal convoluted tubule (PCT), Henle’s loop, distal convoluted tubule (DCT) and collecting duct, all together form the tubules of the kidney (renal tubule). Let’s go through the functions of the renal tubules in urine formation and excretion.
Nephron: Functions of the Renal Tubules
The capillaries of glomerulus are enclosed by a cup-like structure called Bowman’s capsule. This structure extends to form highly coiled tubules called PCT. PCT continues to form the loop of Henle which ascends to DCT, which in turn opens into the collecting duct. The major function of tubules is reabsorption and the process can either be active or passive. In addition, secretions by tubules help in the urine formation without affecting the electrolyte balance of the body.
Proximal Convoluted Tubule (PCT)
The blood brought by the renal artery is filtered by the glomerulus and then passed to the PCT. Maximum reabsorption takes place in PCT of the nephron. PCT is the region of renal tubule where reabsorption of essential substances like, glucose, proteins, amino acids, a major portion of electrolytes and water takes place. The surface area for reabsorption is facilitated by the lining of simple cuboidal epithelium in them. Reabsorption takes place at the expense of energy, i.e., the process is active. PCT selectively secretes ions such as hydrogen, ammonia, and potassium into the filtrate and absorbs HCO3–from it. Thus, PCT maintains the electrolyte and acid-base balance of the body fluids.
Henle’s loop has a descending and an ascending limb. Being parts of the same loop, both the descending and ascending limbs show different permeability. The descending limb is permeable to water but impermeable to electrolyte, while the ascending limb is permeable to electrolytes but impermeable to water. Since the electrolytes get reabsorbed at the ascending loop of Henle, the filtrate gets diluted as it moves towards the ascending limb. But reabsorption is limited in this segment.
Distal Convoluted Tubule (DCT)
PCT, DCT also secretes ions such as hydrogen, potassium, and NH3 into the filtrate while reabsorbing the HCO3–from the filtrate. A conditional reabsorption of sodium ions and water takes place in DCT. Thus, it maintains the pH and sodium-potassium level in the blood cells.
Collecting duct is a long, straight tube where H+ and K+ ions are secreted to maintain the electrolyte balance of the blood. This is also the region where the maximum reabsorption of water takes place to produce a concentrated urine.
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