Biology · Excretory Products and Their Elimination · NEET
They are opposite directions. Reabsorption moves useful things (water, glucose, ions) OUT of the filtrate and BACK into the blood. Tubular secretion moves waste and extra ions (H+, K+, NH3) FROM the blood INTO the filtrate. So reabsorption saves substances, secretion removes them. Both happen in the tubule at the same time.
Learn these three from NCERT: H+ (hydrogen ions), K+ (potassium ions), and NH3 (ammonia). NCERT lists exactly these. In humans NH3 is later handled as ammonium, but for NEET write H+, K+ and NH3 as the secreted items.
It can happen along the tubules, but the PCT (proximal convoluted tubule) is the major site of selective secretion, as NCERT states. The DCT and collecting duct also secrete H+ and K+ to help control pH and ion balance.
Yes, most of it is ACTIVE transport, so it needs ATP. The cells pump ions against their gradient from blood into the filtrate. This is why it is called selective secretion.
When blood becomes too acidic, the tubule secretes extra H+ into the filtrate to be removed in urine, and secretes NH3 which carries away more acid. This is why NCERT says secretion helps 'maintain the ionic balance and pH of body fluids.'
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the active movement of H+, K+ and NH3 from the peritubular blood into the tubular filtrate, the third step of urine formation.
It maintains the ionic balance and the pH (acid-base balance) of body fluids by removing extra H+, K+ and ammonia.
It is mainly active transport and uses ATP, because ions are moved against their concentration gradient.
The proximal convoluted tubule (PCT) is the major site of selective secretion, according to NCERT.
Filtration is passive bulk filtering of plasma at the glomerulus; secretion is the selective active addition of specific ions into the tubule.
