Question

Q. After active removal of small molecules (glucose, amino acids, vitamins electrolytes, etc.), urine inside tubules in the renal cortex does not become hypotonic relative to plasma.  Why is this?

a. Urea concentration is high inside the urine.

b. Small molecules are removed by antiport mechanisms and are therefore replaced by other osmotically active particles.

c. Removal of small molecules results in a drop in osmotic pressure, water leaves the urine.

d. Urine does not tarry in the cortex, it enters the tubules in the medulla as soon as small molecules are removed.

e. ATP is consumed in the transport of small molecules; the breakdown of one molecule of ATP produces two molecules (ADP + P_i) of osmotically active molecules.

Q. What would happen if the loop of Henle does not exist, and the proximal convoluted tubule leads directly to the collecting duct?

a. The glomeruli will decrease in size.

b. The body can secrete nitrogenous waste only in the form of ammonia.

c. The concentration of Na⁺ and urea in urine will be the same.

d. Urine will become hypotonic to plasma.

e. Urine will have the same osmolarity as plasma.

Q. What would happen if the loop of Henle does not exist, and the proximal convoluted tubule leads directly to the collecting duct?

a. Volume of excreted urine would increase.

b. Metabolic activity in the kidney will decrease.

c. Urine osmolarity would decrease.

d. Urine would become isotonic with blood plasma.

e. All of the above.

Q. The role of antidiuretic hormone is to:

a. reduce water permeability in the loop of Henle.

b. reduce water permeability of the collecting ducts.

c. reduce total blood volume.

d. increase urine volume.

e. increase urine osmolarity.

Answer 1

To answer these questions, we need to have detailed knowledge of a nephron structure. Nephron is a single unit of structure and function in kidneys, consists mainly of four parts- Bowman’s capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), all leading to the collecting duct. Urine formation occurs in three steps- glomerular filtration, tubular reabsorption and tubular secretion.

1. Glomerular filtration – Wall of the glomerulus is semi-permeable, it allows only water and small molecules to pass through it. Glomerular filtrate contains glucose, amino acids, inorganic ions, urea, creatinine, hormones, vitamins and almost equals the osmotic pressure and composition of the surrounding plasma.

2. Tubular reabsorption - Now, this concentrated glomerular filtrate is obviously not excreted out of the body, rather it undergoes a transition and forms concentrated urine which is excreted out. Glomerular filtrate passes onto the PCT. The cells lining the PCT has increased mitochondria content which supports active transport of the molecules back into the blood from the filtrate, and excess microvilli for absorption. Almost all of the glucose, vitamins, amino acids, most of the inorganic ions, water and some urea is absorbed back into the plasma from the initial filtrate.

PCT is followed by Loop of Henle, the first broad descending part of the loop is impermeable to water and ions. The second narrower part allows removal of water from the filtrate to the plasma, which has higher osmotic pressure owing to the presence of sodium chloride and urea. The filtrate becomes hypertonic as it reaches the turn of the loop, ascending loop is impermeable to water and permeable to ions. Inorganic ions are secreted out of the filtrate, whereas urea is secreted within the filtrate. Now, the filtrate becomes hypotonic with respect to the plasma and is passed into the DCT.

When the water content in plasma is low, antidiuretic hormone (also known as vasopressin) is secreted from posterior pituitary lobe which results in excess water absorption from filtrate and decreased amount of urine. When the water content in plasma is high, antidiuretic hormone (also known as vasopressin) secretion is restrained from posterior pituitary lobe which results in less water absorption from filtrate and increased amount of urine. As the filtrate passes from DCT to collecting tubule, Na+ ions are absorbed back into the plasma by the effect of aldosterone which facilitates more water reabsorption back into the body, making the urine concentrated.

3. Tubular secretion – Creatinine, hippuric acid, foreign substances, hydrogen ions and ammonia are secreted into the filtrate in the PCT region. Urea diffuses into the filtrate in the ascending loop of henle. Potassium ions, hydrogen ions, ammonia and bicarbonate ions are secreted into the filtrate in the DCT region.

The final filtrate which is excreted out of the body is urine.

Answer 1

Small molecules are removed from the filtrate back to the plasma in the PCT region, on the other hand, excreted materials like creatinine, hippuric acid, urea, hydrogen ions are secreted back into the filtrate. This is why the filtrate does not become hypotonic. Hypotonic solution is referred to as a solution which has low solute concentration and low osmotic pressure in comparison to the solution outside the cell. The most appropriate option is c.

Option a is not correct as urea is not the only component present inside the urine. Option b is not appropriate as antiport is not always involved. PCT and DCT are present in the renal cortex, where active transport of molecules occurs, therefore, option d is also incorrect. Active transport does involve ATP, but ADP and Pi are not used to maintain osmotic pressure, rather they are recycled, option e is also incorrect.

Answer 2

If loop of Henle does not exist, the body can secrete nitrogenous waste only in the form of ammonia, which is excreted within the filtrate in the DCT region. Urea is secreted into the filtrate in the loop of Henle, moreover, water reabsorption also occurs in loop of henle. The most appropriate answer is option b.

Loop of Henle has nothing to do with the glomerulus size, reduction of the pore size will perhaps decrease the load of the secreted molecules into the glomerular filtrate, but most of the molecules are absorbed back into the body in the PCT region, option a is incorrect. Sodium absorption also occurs in the collecting duct, so option c is not the appropriate option. Solute concentration is dereased from the filtrate in the PCT region, thereby lowering the osmotic pressure within the filtrate, option d incorrect. Urine will not have the same osmotic concentration because PCT has a huge role in molecular absorption, option e is incorrect.

Answer 3

The correct option is e. Loop of henle permits absorption of water from the filtrate back into the plasma, which maintains osmolarity and tonicity of the urine. Urine has lower osmotic pressure and hypertonic to plasma solution. Loop of henle also allows tubular secretion of urea into the filtrate, therefore, absence of the loop will lead to all of the above phenomenons.

Answer 4

Antidiuretic hormone or vasopressin increases water permeability in the DCT region, where it allows water reabsorption back into the plasma when the concentration of water is low in the plasma. The correct option is e.

Vasopressin stimulates water absorption back into the body, so option a and b is incorrect. It has nothing to do with blood volume, option c is incorrect. Vasopressin reduces urine volume by increasing water uptake from the filtrate, option d is also incorrect.