Question

Reabsorption in tubules of nephrons occurs by
(a)Osmosis
(b)Diffusion
(c)Active transport
(d)Both B and C

Answer 1

Hint: Reabsorption also known as tubular reabsorption is the process of absorbing water and electrolytes from the filtrate and returning it back to the blood. This is done to concentrate the urine formed and also reduce the amount of loss of water and electrolytes from the body.

Complete Answer:
Every day 180 liters of filtrate are formed in the kidneys but only 1.5 liters of urine are released out from the body. This is because our body has mechanisms to reduce the amount of water loss through urine. This is important because if 180 liters of water were to be lost from our body each day, then we would have to constantly keep drinking water to make up for the water loss. Urine is concentrated by tubular reabsorption in which the renal tubules absorb up to 99% of the filtrate formed. Substances like glucose, amino acids, sodium ions are actively absorbed whereas nitrogenous wastes like urea are passively diffused back.

Additional information:
The process of urine formation involves the following steps:
Glomerular filtration: It is the first step of urine formation and it is carried out in the glomerulus and hence it is known as glomerular filtration. Blood coming from the afferent arteriole is filtered through 3 layers which are the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule, and a base membrane that is present between them. The filtered out substances from the glomerular filtrate.
Reabsorption: The amount of urine formed is less than the amount of filtrate formed because the most amount of the filtrate (99%) is reabsorbed by the renal tubules through active and passive transport.
Secretion: Some substances like hydrogen ions, potassium ions, and ammonia are selectively secreted into the filtrate to maintain the ionic and acid-base balance of body fluids.

So, the correct option is ‘Both B and C’.

Note:
-70-80% of reabsorption of all electrolytes and water occurs in the proximal convoluted tubule (PCT) of the nephron.
-The process of glomerular filtration is known as ultrafiltration because blood is filtered very finely through all the membranes such that all the components of the blood plasma are passed on except proteins.
-Juxtaglomerular apparatus (JGA) is a mechanism used to regulate blood pressure and water loss from the body.