Test: Urine Formation (NCERT) - NEET MCQ

Potassium, sodium, glucose, amino adds, hormones, vitamins etc. are reabsorbed in proximal convoluted tubule (PCT). Potassium is reabsorbed by active transport.

Water reabsorption in descending limb of loop of Henle and collecting duct occurs under different conditions. In descending limb, water is reabsorbed due to increasing osmolarity of interstitial fluid. Collecting duct is entirely permeable to water. Water is reabsorbed here, under the influence of anti-diuretic hormone (ADH), which is released into the blood from the posterior lobe of pituitary gland.

The kidneys filter a large volume of blood, producing a correspondingly large volume of filtrate. However, most of this filtrate is reabsorbed into the bloodstream through the walls of the nephron tubules and collecting ducts. Essential substances such as water, glucose, and ions are reabsorbed, while waste products are concentrated in the remaining fluid, which is excreted as urine.

About 180 litres (100%) of fluid is filtered out of blood every day by nephrons but an adult human excretes, on an average, 1 to 1.5 litres (1 %) of urine per day. The remaining 99% of glomerular filtrate is absorbed into the blood by renal tubules to maintain constant homeostasis in the body. Hence nearly 99% of glomerular filtrate is reabsorbed by renal tubules.

Most reabsorption of the major substances from the glomerular filtrate takes place in proximal convoluted tubule. Na⁺ ions, Cl^- ions and water are reabsorbed in distal convoluted tubules. Afferent arteriole carries the blood towards the glomerulus.

Malpighian corpuscle (A) is the site of ultrafitration. A considerable amount of water is reabsorbed in the collecting duct (E) under the influence of ADH. Proximal convoluted tubule (C) is the main site for the reabsorption of glucose and amino acids. In distal convoluted tubule (D), both hydrogen ions and ammonium ions are secreted, thus it maintains blood pH.

The diameter of afferent arteriole is larger than the efferent arteriole. This increases the volume of blood in glomerulus and increases the filtration rate. This causes the effective filtration pressure which is the total pressure that promotes filtration.

About 65% of the glomerular filtrate is reabsorbed in the proximal convoluted tubule (PCT). Glucose, amino acids, vitamins, hormones, sodium, potassium, chlorides, phosphates, bicarbonates, much of water and some urea are reabsorbed form the filtrate.

A fall in GFR can stimulate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR comes back to normal.

Juxtaglomerular cells are the smooth muscle cells of both the afferent and efferent arterioles that are swollen and contain dark granules. These cells secrete renin. Macula densa are the epithelial cells of the distal convoluted tubule that come in contact with the afferent and efferent arterioles. These cells are more dense than the other tubular cells. They act as chemoreceptors. Lacis cells are located in the interval between the macula densa and the afferent and efferent arterioles. The function of lacis cells is unknown. All these cells make up juxtaglomerular apparatus.

DCT maintains the pH and ionic balance in the blood by reabsorption of Na⁺, water, HCO₃^- and selective secretion of H⁺, K⁺ and NH₃. Collecting duct plays a role in the maintenance of pH and ionic balance of blood by reabsorption of water, and selective secretion of H⁺ and K⁺ ions.

Along the entire length of the nephron the formation of urine takes place by ultrafiltration in the glomerulus, reabsorbtion in the tubules and the elimination by the collecting duct.
The collecting duct is a long duct extending from the cortex of the kidney to the inner parts of the medulla. Large amounts of water could be reabsorbed from this region to produce a concentrated urine. This segment allows passage of small amounts of urea into the medullary interstitium to keep up the osmolarity. 
Hence, both the statements are true and statement II is the reason for statement I.

Descending limb of loop of Henle is impermeable to salts.

NH3​ is secreted while NaCl is reabsorbed at PCT.

In the proximal convoluted tubule, maximum reabsorption of ions takes place e.g. H+, Na+, K+, Cl−, NH3​, etc. 

The movement of Na+, Cl−, and NH3​ are incorrect and shown inverted. Sodium and Chlorine ions gets reabsorbed from the PCT into the capillaries, and Ammonia gets secreted from the capillaries into the PCT.

Juxtaglomerular nephrons form about 15% of total nephrons. Their glomeruli are found in the inner margin of the cortex. They control plasma volume when water supply is short.

Net pressure gradient = Glomerular hydrostatic pressure - (Blood colloidal osmotic pressure + Capsular hydrostatic pressure)
= 60 mm Hg - (30 mm Hg + 20 mm Hg)
= 60 - 50 = 10 mm Hg

In descending limb of loop of Henle, water is reabsorbed due to increasing osmolarity of interstitial fluid.

The maximum absorption of glucose, amino acids, and inorganic ions like Na⁺, K⁺, and Cl⁻ take place in the proximal convoluted tubule or PCT, and the maximum absorption of urea takes place in the ascending loop of Henle.
Thus, the correct answer is '(i) PCT (ii) PCT (iii) PCT (iv) Ascending limb of the loop of Henle.'

In the kidney, urine formation occurs in three steps, Glomerular filtration, Reabsorption, and tubular secretion. Glomerular filtration is the process by which nitrogenous waste likes Hippuric acid, creatinine and ketone bodies from glomerular blood are filtered through minute pores called filtration slits present in the wall of Bowman's capsule, tubular secretion is the transfer of nitrogenous waste materials like Hippuric acid, creatinine, and ketone materials from the peritubular capillaries to the renal tubular lumen. Hence Hippuric acid, creatinine, and ketones are added to urine through Glomerular filtration and Tubular secretion.

Three proceesses involved in urine formation are glomerular filtration, reabsorption and secretion. Filtration of blood by the glomerulus is called glomerular filtration, absorption of selected materials from the filtrate into the blood of the peritubular capillaries or vasa recta is called reabsorption and exxretion of additional wastes from the blood stream into the filtrate is called secretion.