Directions: In the following questions, a statement of assertion is followed by a statement of reason. Mark the correct choice as :
(a) If both assertion and reason are true and reason is the correct explanation of assertion.
(b) If both assertion and reason are true but the reason is not the correct explanation of assertion.
(c) If the assertion is true but the reason is false.
(d) If the assertion is false but the reason is true.
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Assertion: Sharks are said to be ammonotelic animals.
Reason: Sharks minimise water loss from their body to adjust the concentrated saline water of sea.
(d) Sharks are not ammonotelic, but they are ureotelic animals. Ureotelic animals excrete urea instead of ammonia as the major nitrogenous waste product. Sharks need to avoid water loss from body, thus they cannot excrete ammonia, as it requires enough water to be eliminated. Sharks can retain so much urea in their blood that their blood osmotic pressure approaches that of sea water. This minimises water loss from their body to adjust to the concentrated saline water of the sea.
Assertion: Nephrons are of two types : cortical and juxtamedullary according to their relative position in the cortex.
Reason: Juxtamedullary nephrons have short loop of Henle while cortical nephrons have long loop of Henle.
(c) On the basis of location, the nephrons are of two types. In the majority of nephrons, the loop of Henle is too short and extends only a little into the medulla. These nephrons are called cortical nephrons and form \(85 \%\) of the total nephrons. In some of the nephrons, the loop of Henle is very long and runs deep into the medulla. These nephrons are called juxtamedullary nephrons and form \(15 \%\) of the total nephrons.
Assertion: Vasa recta is absent or highly reduced in cortical nephrons.
Reason: Cortical nephrons are mainly concerned with the concentration of urine.
(c) Cortical nephrons lie in the renal cortex. They have short loop of Henle and no vasa recta. Hence, they are not involved in the concentration of urine. They control plasma volume when water supply is normal.
Assertion: Glomerular filtration requires the expenditure of energy by kidney.
Reason: Glomerular filtration occurs because the pressure in the glomerular capillaries is higher than the pressure in Bowman’s capsule.
(d) Glomerular filtration occurs because the pressure of the blood flowing in the glomerular capillaries is higher than the pressure of the filtrate in Bowman’s capsule. In other words, blood pressure drives glomerular filtration, and because the process takes advantage of a pressure gradient, glomerular filtration does not require the expenditure of energy by kidney cells.
Assertion: Tubular secretion removes potassium, hydrogen ions and ammonia from the body.
Reason: As much as 99 percent of the material in the filtrate is reabsorbed from the body because of tubular secretion.
(c) Certain chemicals in the blood that are not removed by filtration from the glomerular capillaries are removed by a third process of urine formation called tubular secretion. These chemicals are removed from the blood in the peritubular capillaries to the nephron tubule by both passive and active transport. The chemicals removed by tubular secretion include potassium, hydrogen ions and ammonia, etc.
Assertion: In the descending limb of loop of Henle the urine is hypotonic, while in ascending limb of loop of Henle, the urine is hypertonic.
Reason: Descending limb is permeable to water while ascending limb is permeable to \(\mathrm{Na}^{+}\).
(d) The descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down. The ascending limb is impermeable to water but allows the transport of electrolytes actively or passively. Therefore, as the concentrated filtrate pass upward, it gets diluted due to the passage of electrolytes to the medullary fluid.
Assertion: DCT and collecting duct maintain the \(\mathrm{pH}\) and ionic balance of blood.
Reason: DCTs of many nephrons open into a collecting duct.
Correct answer is (b) If both assertion and reason are true but reason is not the correct explanation of assertion
Assertion: The Henle’s loop and vasa recta play a significant role in producing concentrated urine.
Reason: The counter-current arrangement of Henle’s loop and vasa recta helps in this.
(a) Mammals have the ability to produce concentrated urine. The Henle’s loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of vasa recta is also in a counter current pattern. The proximity between the Henle’s loop and vas recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium. Presence of such interstitial gradient helps in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed
Assertion: Antidiuretic hormone (ADH) controls the amount of water in the urine.
Reason: ADH determines the permeability of the collecting duct to water.
(a) Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid from the body can activate these receptors which stimulate the hypothalamus to release antidiuretic hormone ADH or vasopressin from the neurohypophysis. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release to complete the feedback.
Assertion: Angiotensin II increases the glomerular blood pressure thereby GFR.
Reason: Angiotensin II activates the JG cells to release renin.
(c) Angiotensin II activates the adrenal cortex to release aldosterone.
Assertion: Stimulation of renin secretion will increase the volume of the extracellular fluid (ECF).
Reason: The increased ECF occurs due to decreased active reabsorption of \(\mathrm{Na}^{+}\).
(c) Renin is a proteolytic enzyme, secreted into the blood by juxtaglomerular cells of the kidney under the control of the sympathetic nervous system. The only known physiological effect of renin is to cause the formation of angiotensin II from its plasma substrate, angiotensinogen. Angiotensin II, stimulates the release of aldosterone from the zona glomerulosa of the adrenal cortex. Aldosterone promotes reabsorption of \(\mathrm{Na}^{+}\)ions by epithelial cells of the collecting duct. Retaining \(\mathrm{Na}^{+}\)raises the osmotic pressure of blood and reduces water loss from the body. Hence, causing an increase in extracellular fluid (ECF) volume.
Assertion: The kidneys have built in mechanisms for the regulation of glomerular filtration rate (GFR).
Reason: ANF can cause vasoconstriction and thereby increasing blood pressure.
(c) The kidneys have built-in mechanisms for the regulation of glomerular filtration rate. One such efficient mechanism, known as renin-angiotensin mechanism, is carried out by juxtaglomerular apparatus (JGA). A fall in GFR can activate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR back to normal. An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure. ANF mechanism, therefore, acts as a check on the renin-angiotensin mechanism.
Assertion: During micturition, urine is prevented from flowing back into the ureters.
Reason: Urethral sphincters contract during micturition.
(c) The micturition is the process of evacuation of the formed urine from the bladder time to time. Urine is prevented from flowing back into the ureters because the terminal part of each ureter passes obliquely through the bladder wall and is consequently closed due to compression by the contracting bladder muscles. When enough urine has accumulated in the bladder to distend the bladder and raise its pressure sufficiently, a spontaneous nervous activity (reflex) is initiated; this causes the smooth muscles on the bladder wall to contract and the urethral sphincters, which guard the urethra, to relax. Urine consequently flows from the bladder through the urethra to the exterior.
Assertion: Renal threshold of glucose is said to be \(180 \mathrm{mg} / 100 \mathrm{~mL}\).
Reason: Glucose starts appearing in the urine when its blood level exceeds \(180 \mathrm{mg} / 100 \mathrm{~mL}\) of blood.
(a) Renal threshold of a substance is its highest concentration in the blood up to which it is totally reabsorbed from the glomerular filtrate. Renal threshold of glucose is about \(180 \mathrm{mg} / 100 \mathrm{~mL}\). Glucose is reabsorbed and does not appear in the urine as long as its blood level does not exceed \(180 \mathrm{mg} / 100 \mathrm{~mL}\).
Assertion: Liver is referred to as the primary excretory organ in vertebrates.
Reason: Liver helps in excretion of bile pigments and many drugs.
(d) In vertebrates, the lungs, liver and skin are referred as accessory excretory organs because besides the urinary system, these organs also participate in the removal of waste products from the body. The liver helps in the excretion of cholesterol, bile pigments (bilirubin and biliverdin), inactivated products of steroid hormones, some vitamins and many drugs. These are carried by the bile to the intestine and eliminated with the faeces. It has no role in secretion of urine.
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