16.10 Exercise Problems

Q1. Define Glomerular Filtration Rate (GFR)

Answer: Glomerular filtration rate is the amount of glomerular filtrate formed in all the nephrons of both the kidneys per minute. In a healthy individual, it is about 125 \(\mathrm{mL} /\) minute. Glomerular filtrate contains glucose, amino acids, sodium, potassium, urea, uric acid, ketone bodies, and large amounts of water.

Q2. Explain the autoregulatory mechanism of GFR.

Answer: The mechanism by which the kidney regulates the glomerular filtration rate is autoregulative. It is carried out by the juxtaglomerular apparatus. Juxta glomerular apparatus is a microscopic structure located between the vascular pole of the renal corpuscle and the returning distal convoluted tubule of the same nephron. It plays a role in regulating the renal blood flow and glomerular filtration rate. When there is a fall in the glomerular filtration rate, it activates the juxtaglomerular cells to release renin. This stimulates the glomerular blood flow, thereby bringing the GFR back to normal. Renin brings the GFR back to normal by the activation of the renin angiotensin mechanism.

Q3. Indicate whether the following statements are true or false :
(a) Micturition is carried out by a reflex.
(b) \(\mathrm{ADH}\) helps in water elimination, making the urine hypotonic.
(c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule.
(d) Henle’s loop plays an important role in concentrating the urine.
(e) Glucose is actively reabsorbed in the proximal convoluted tubule.

Answer: (a) True
(b) False
(c) True
(d) True
(e) True

Q4. Give a brief account of the counter current mechanism.

Answer: The counter current mechanism operating inside the kidney is the main adaptation for the conservation of water. There are two counter current mechanisms inside the kidneys. They are Henle’s loop and vasa recta. Henle’s loop is a U-shaped part of the nephron. Blood flows in the two limbs of the tube in opposite directions and this gives rise to counter currents. The Vasa recta is an efferent arteriole, which forms a capillary network around the tubules inside the renal medulla. It runs parallel to Henley’s loop and is U-shaped. Blood flows in opposite directions in the two limbs of vasa recta. As a result, blood entering the renal medulla in the descending limb comes in close contact with the outgoing blood in the ascending limb.

The osmolarity increases from \(300 \mathrm{mOsmoIL}^{-1}\) in the cortex to \(1200 \mathrm{mOsmoIL} \mathrm{L}^{-1}\) in the inner medulla by counter current mechanism. It helps in maintaining the concentration gradient, which in turn helps in easy movement of water from collecting tubules. The gradient is a result of the movement of \(\mathrm{NaCl}\) and urea.

Q5. Describe the role of the liver, lungs, and skin in excretion.

Answer: Liver, lungs, and skin also play an important role in the process of excretion.
Role of the liver: Liver is the largest gland in vertebrates. It helps in the excretion of cholesterol, steroid hormones, vitamins, drugs, and other waste materials through bile. Urea is formed in the liver by the ornithine cycle. Ammonia – a toxic substance – is quickly changed into urea in the liver and thence eliminated from the body. Liver also changes the decomposed haemoglobin pigment into bile pigments called bilirubin and biliverdin.
Role of the lungs: Lungs help in removing waste materials such as carbon dioxide from the body.
Role of the skin: Skin has many glands which help in excreting waste products through pores. It has two types of glands – sweat and sebaceous glands. Sweat glands are highly vascular and tubular glands that separate the waste products from the blood and excrete them in the form of sweat. Sweat excretes excess salt and water from the body. Sebaceous glands are branched glands that secrete an oily secretion called sebum.

Q6. Explain micturition.

Answer: Micturition is the process by which the urine from the urinary bladder is excreted. As the urine accumulates, the muscular walls of the bladder expand. The walls stimulate the sensory nerves in the bladder, setting up a reflex action. This reflex stimulates the urge to pass out urine. To discharge urine, the urethral sphincter relaxes and the smooth muscles of the bladder contract. This forces the urine out from the bladder. An adult human excretes about 1-1.5 litres of urine per day.

Q7. Match the items of column I with those of column II :

\(
\begin{array}{|l|l|l|l|}
\hline & {\text { Column I }} & & {\text { Column II }} \\
\hline \text { (a) } & \text { Ammonotelism } & \text { (i) } & \text { Birds } \\
\hline \text { (b) } & \text { Bowman’s capsule } & \text { (ii) } & \text { Water reabsorption } \\
\hline \text { (c) } & \text { Micturition } & \text { (iii) } & \text { Bony fish } \\
\hline \text { (d) } & \text { Uricotelism } & \text { (iv) } & \text { Urinary bladder } \\
\hline \text { (d) } & \text { ADH } & \text { (v) } & \text { Renal tubule } \\
\hline
\end{array}
\)

Answer:

\(
\begin{array}{|l|l|l|l|}
\hline & {\text { Column I }} & & {\text { Column II }} \\
\hline \text { (a) } & \text { Ammonotelism } & \text { (iii) } & \text { Bony fish } \\
\hline \text { (b) } & \text { Bowman’s capsule } & \text { (v) } & \text { Renal tubule } \\
\hline \text { (c) } & \text { Micturition } & \text { (iv) } & \text { Urinary bladder } \\
\hline \text { (d) } & \text { Uricotelism } & \text { (i) } & \text { Birds } \\
\hline \text { (d) } & \text { ADH } & \text { (ii) } & \text { Water reabsorption } \\
\hline
\end{array}
\)

Q8. What is meant by the term osmoregulation?

Answer: Osmoregulation is a homeostatic mechanism that regulates the optimum temperature of water and salts in the tissues and body fluids. It maintains the internal environment of the body by water and ionic concentration.

Q9. Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?

Answer: Terrestrial animals are either ureotelic or uricotelic, and not ammonotelic. This is because of the following two main reasons:
(a) Ammonia is highly toxic in nature. Therefore, it needs to be converted into a less toxic form such as urea or uric acid.
(b) Terrestrial animals need to conserve water. Since ammonia is soluble in water, it cannot be eliminated continuously. Hence, it is converted into urea or uric acid. These forms are less toxic and also insoluble in water. This helps terrestrial animals conserve water.

Q10. What is the significance of juxta glomerular apparatus (JGA) in kidney function?

Answer: Juxta glomerular apparatus (JGA) is a complex structure made up of a few cells of glomerulus, distal tubule, and afferent and efferent arterioles. It is located in a specialised region of a nephron, wherein the afferent arteriole and the distal convoluted tubule (DLT) come in direct contact with each other. The juxtaglomerular apparatus contains specialised cells of the afferent arteriole known as juxtaglomerular cells. These cells contain the enzyme renin that can sense blood pressure. When glomerular blood flow (or glomerular blood pressure or glomerular filtration rate) decreases, it activates juxtaglomerular cells to release renin. Renin converts the angiotensinogen in blood into angiotensin I and further into angiotensin II. Angiotensin II is a powerful vasoconstrictor that increases the glomerular blood pressure and filtration rate. Angiotensin II also stimulates the adrenal cortex of the adrenal gland to produce aldosterone. Aldosterone increases the rate of absorption of sodium ions and water from the distal convoluted tubule and the collecting duct. This also leads to an increase in blood pressure and glomerular filtration rate. This mechanism, known as renin-angiotensin mechanism, ultimately leads to an increased blood pressure.

Q11. Name the following:
(a) A chordate animal having flame cells as excretory structures
(b) Cortical portions projecting between the medullary pyramids in the human kidney
(c) A loop of capillary running parallel to the Henle’s loop.

Answer: (a) Amphioxus is an example of a chordate that has flame cells as excretory structures. Flame cell is a type of excretory and osmoregulatory system.
(b) The cortical portions projecting between the medullary pyramids in the human kidney are the columns of Bertini. They represent the cortical tissues present within the medulla.
(c) A loop of capillary that runs parallel to Henle’s loop is known as vasa recta. Vasa recta, along with Henle’s loop, helps in maintaining a concentration gradient in the medullary interstitium.

Q12. Fill in the gaps:
(a) The ascending limb of Henle’s loop is ____ to water whereas the descending limb is ______ to it.
(b) Reabsorption of water from distal parts of the tubules is facilitated by hormone ______.
(c) Dialysis fluid contains all the constituents as in plasma except ________.
(d) A healthy adult human excretes (on an average) ______ gm of urea/day.

Answer: (a) The Ascending limb of Henle’s loop is impermeable to water, whereas the descending limb is permeable to it.
(b) Reabsorption of water from distal parts of the tubules is facilitated by the Hormone vasopressin.
(c) Dialysis fluid contains all the constituents as in plasma, except the nitrogenous waste.
(d) A healthy adult human excretes (on an average) 25 – 30 gm of urea/day.

Exemplar Section

VERY SHORT ANSWER TYPE QUESTIONS

Q1. Where does the selective reabsorption of Glomerular filtrate take place?

Answer: DCT

Q2. What is the excretory product from kidneys of reptiles?

Answer: Uric acid

Q3. What is the composition of sweat produced by sweat glands?

Answer: Water, minerals, lactic acid and urea.

Q4. Identify the glands that perform the excretory function in prawns.

Answer: Antennal glands or green glands

Q5. What is the excretory structure in amoeba?

Answer: Contractile vacuole

Q6. The following abbreviations are used in the context of excretory functions, what do they stand for?
a. ANF
b. ADH
c. GFR
d. DCT

Answer:  a. ANF – Atrial Natriuretic factor
b. ADH – Antidiuretic hormone
c. GFR – Glomerular Filtration Rate
d. DCT – Distal Convoluted Tubule

Q7. Differentiate Glycosuria from Ketonuria.

Answer: Glycosuria – Presence of glucose in urine.
Ketonuria – Presence of ketone bodies in urine.

Q8. What is the role of sebaceous glands?

Answer: Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes through sebum. This secretion provides a protective oily covering for the skin.

Q9. Name two actively transported substances in Glomerular filtrate.

Answer: Glucose and amino acids

Q10. Mention any two metabolic disorders, which can be diagnosed by analysis of urine.

Answer: Glycosuria and ketonuria

Q11. What are the main processes of urine formation?

Answer: The main processes are filtration, reabsorption, secretion and concentration/ dilution

Q12. Sort the following into actively or passively transported substances during reabsorption of GFR.
glucose, aminoacids, nitrogenous wastes, \(\mathrm{Na}^{+}\), water

Answer: Actively transported-Glucose, aminoacids and \(\mathrm{Na}^{+}\) Passively transported-Nitrogenous wastes and water.

Q13. Complete the following:
a. urinary excretion = tubular reabsorption + tubular secretion –
b. Dialysis fluid = Plasma – 

Answer: a. Urinary excretion = tubular reabsorption + tubular secretion – Glomerular filtration.
b. Dialysis fluid = Plasma – nitrogenous wastes

Q14. Mention the substances that exit from the tubules in order to maintain a concentration gradient in the medullary interstitium.

Answer: NaCl and Urea.

Q15. Fill in the blanks appropriately
   Organ             Excretory wastes
a. Kidneys ______________________
b. Lungs ______________________
c. Liver ______________________
d. Skin ______________________ 

Answer: a. Kidneys – Urea
b. Lungs \(-\mathrm{CO}_2\) and \(\mathrm{H}_2 \mathrm{O}\)
c. Liver – Bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs
d. Skin – Sweat ( NaCl , urea, lactic acid) and sebum (sterols, hydrocarbons and waxes).

SHORT ANSWER TYPE QUESTIONS

Q1. Show the structure of a renal corpuscle with the help of a diagram.

Answer: 

Q2. What is the role played by Renin-Angiotensin in the regulation of kidney function?

Answer: Renin is released from JGA on activation due to fall in the glomerular blood pressure/flow. Renin converts angiotensinogen in blood to angiotensin-I and further to angiotensin-II. Angiotensin-II being a powerful vasoconstrictor, increase the glomerular blood pressure and thereby GFR. Angiotensin-II also activates the adrenal cortex to release aldosterone. Aldosterone causes reabsorption of Na⁺ and water from the distal parts of the tubule. This also leads to.an increase in blood pressure and thereby GFR. This is generally known as the Renin-Angiotensin mechanism.

Q3. Aquatic animals generally are ammonotelic in nature where as terrestual forms are not. Comment.

Answer: Ammonia is the most toxic form and requires large amount of water for its elimination, terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys.

Q4. The composition of glomerular filtrate and urine is not same. Comment.

Answer: A comparison of the volume of the filtrate formed per day ( 180 litres per day) with that of the urine released ( 1.5 litres), suggest that nearly \(99 \%\) of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption. For example, substances like glucose, amino acids, \(\mathrm{Na}^{+}\), etc., in the filtrate are reabsorbed actively so, these substances are not present in urine.

Q5. What is the procedure advised for the correction of extreme renal failure? Give a brief account of it.

Answer: Kidney transplantation is the ultimate method in the correction of acute renal failures (kidney failure). A functioning kidney is used in transplantation from a donor, preferably a close relative, to minimise its chances of rejection by the immune system of the host. Modem clinical procedures have increased the success rate of such a complicated technique.

Q6. How have the terrestrial organisms adapted themselves for conservation of water?

Answer: Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys. Some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity. Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid ‘ in the form of pellet or paste with a minimum loss of water and are called uricotelic animals.

Q7. Label the parts in the following diagram.

Answer: 

Q8. Explain, why a haemodialysing unit called artificial kidney?

Answer: Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called uremia, which is highly harmful and may lead to kidney failure. In such patients, urea can be removed by a process called hemodialysis. Blood drained from a convenient artery is pumped into a dialysing unit (also called artificial kidney) after adding an anticoagulant like heparin. The unit contains a coiled cellophane tube surrounded by a fluid (dialysing fluid) having the same composition as that of plasma except the nitrogenous wastes. The porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient. As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood. The cleared blood is pumped back to the body through a vein after adding anti-heparin to it.

Q9. Comment upon the hormonal regulation of selective reabsorption.

Answer: 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 latter parts of the tubule, thereby preventing diuresis.

LONG ANSWER TYPE QUESTIONS

Q1. Explain the mechanism of formation of concentrated urine in mammals.

Answer: Mechanism of Concentration of the Filtrate:
Mammals have the ability to produce a 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 vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from \(300~ \mathrm{mOsmolL}^{-1}\) in the cortex to about \(1200~ \mathrm{mOsmolL}^{-1}\) the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s ioop which is transported back to the interstitium by the collecting tubule. The above described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism. This mechanism helps to maintain a concentration gradient in the 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.

Q2. Draw a labelled diagram showing reabsorption and secretion of major substances at different parts of the nephron

Answer: 

Q3. Explain briefly, micturition and disorders of the excretory system.

Answer:

• Micturition: Urine formed by the nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal is given by the central nervous system (CNS). This signal is initiated by the stretching of the urinary bladder as it gets filled with urine. In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine. The process of release of urine is called micturition and the neural mechanisms causing it is called the micturition reflex.
• Disorders of the Excretory System: Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called uremia, which is highly harmful and may lead to kidney failure. In such patients, urea can be removed by a process called hemodialysis. Kidney transplantation is the ultimate method in the correction of acute renal failures (kidney failure).
• Renal calculi: Stone or insoluble mass of crystallised salts (oxalates, etc.) formed within the kidney.
• Glomerulonephritis: Inflammation of glomeruli of kidney.

Q4. How does tubular secretion help in maintaining ionic and acid-base balance in body fluids?

Answer: During urine formation, the tubular cells secrete substances like \(\mathrm{H}^{+}, \mathrm{K}^{+}\)and ammonia into the filtrate. Tubular secretion is also an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids.

• PCT helps to maintain the pH and ionic balance of the body fluids by selective secretion of hydrogen ions, ammonia and potassium ions into the filtrate
• DCT is also capable of selective secretion of hydrogen and potassium ions and \(\mathrm{NH}_3\) to maintain the pH and sodium-potassium balance in blood.
• Collecting duct also plays a role in the maintenance of pH and ionic balance of blood by the selective secretion of \(\mathrm{H}^{+}\)and \(\mathrm{K}^{+}\)

Q5. The glomerular filtrate in the loop of Henle gets concentrated in the descending and then gets diluted in the ascending limbs. Explain.

Answer: A hairpin shaped Henle’s loop has a descending and an ascending limb. Reabsorption is minimum in its ascending limb. However, this region plays a significant role in the maintenance of high osmolarity of medullary interstitial fluid. 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 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.

Q6. Describe the structure of a human kidney with the help of a labelled diagram.

Answer: In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra. Kidneys are reddish brown, bean shaped structures situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 g. Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter. Inner to the hilum is a broad funnel shaped space called the renal pelvis with projections called calyces. The outer layer of kidney is a tough capsule. Inside the kidney, there are two zones, an outer cortex and an inner meditlla. The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces (sing.: calyx). The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini. Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units.