Physiology of Excretion

BiologyExcretory Products and their EliminationFor NEET aspirants

The physiology of excretion explains how the kidneys form urine and how the body controls this work. This page covers glomerular filtration and GFR, tubular reabsorption and secretion along the nephron, the counter current mechanism that concentrates urine and its regulation by ADH, the renin-angiotensin mechanism and ANF. It also covers micturition, other excretory organs and kidney disorders. It follows the NCERT Class 11 chapter Excretory Products and their Elimination. In the physiology of excretion, NEET often asks GFR values, the function of each tubule segment and the hormones that regulate the kidney.

On this page1Urine formation2Functions of the tubules3Counter current mechanism4Regulation of kidney function5Micturition6Other excretory organs7Disorders8Exam essentials9Quick revision10Solved examples11Practice

Key Points at a Glance

★ Must learn Urine formation has three steps: glomerular filtration, reabsorption and secretion.
★ Must learn GFR is about 125 ml per minute (180 L per day); urine is about 1.5 L per day, so nearly 99% of the filtrate is reabsorbed.
Filtration (ultrafiltration) passes through the endothelium, the basement membrane and the epithelium of Bowman's capsule (podocytes with slit pores); proteins stay back.
★ Must learn The PCT reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water.
Descending limb: permeable to water, almost impermeable to electrolytes. Ascending limb: impermeable to water.
★ Must learn Counter current flow in Henle's loop and vasa recta keeps medullary osmolarity rising from 300 to 1200 $mOsmol L^{- 1}$ , mainly through NaCl and urea.
ADH (vasopressin) from the neurohypophysis increases water reabsorption and prevents diuresis.
★ Must learn Renin → angiotensin I → angiotensin II → aldosterone: raises blood pressure and GFR; ANF checks it by vasodilation.
Urine: 1-1.5 L per day, light yellow, slightly acidic (pH 6.0); 25-30 g of urea per day.
Glucose (glycosuria) and ketone bodies (ketonuria) in urine indicate diabetes mellitus.
Lungs remove about 200 mL of $C O_{2}$ per minute; the liver, skin and saliva also help in excretion.
Uremia is treated by haemodialysis (heparin before, anti-heparin after) or by kidney transplantation.

1. Urine Formation

1.1 Three processes

Urine formation involves three main processes, which take place in different parts of the nephron:

Glomerular filtration: blood is filtered by the glomerulus.
Reabsorption: useful substances and most of the water return from the filtrate to the blood.
Secretion: tubular cells add substances such as $H^{+}$ , $K^{+}$ and ammonia to the filtrate.

Memory Trick

Fill, Recover, Send: Filtration fills Bowman's capsule, Reabsorption recovers useful substances, Secretion sends extra ions into the tubule.

1.2 Glomerular filtration

The first step is the filtration of blood by the glomerulus, called glomerular filtration.
★ Exam imp On average, 1100-1200 ml of blood is filtered by the kidneys per minute.
This is roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute.
Filtration is caused by the glomerular capillary blood pressure.
Filtration is a non-selective process.

Three layers the blood is filtered through (from blood to capsule)

Endothelium of the glomerular blood vessels
Basement membrane, lying between the two cell layers
Epithelium of Bowman's capsule, made of podocytes

Podocytes: epithelial cells of Bowman's capsule, arranged in an intricate manner.
They leave minute spaces called filtration slits or slit pores.
★ Exam imp Almost all constituents of the plasma except the proteins pass into the lumen of Bowman's capsule.
Because blood is filtered so finely, glomerular filtration is called ultrafiltration.

Memory Trick

Three layers, from blood outwards: Every Blood Exit = Endothelium, Basement membrane, Epithelium (podocytes).

1.3 Glomerular filtration rate (GFR)

★ Very important Glomerular filtration rate (GFR): the amount of filtrate formed by the kidneys per minute. In a healthy individual it is about 125 ml per minute, that is, 180 litres per day.

125 ml min^{- 1} \times 1440 min = 180000 ml = 180 L per day

About 1200 ml of blood filtered per minute forms about 125 ml of filtrate in Bowman's capsule per minute.

1.4 Regulation of GFR by the JGA

The kidneys have built-in mechanisms to regulate the GFR.
★ Exam imp Juxta glomerular apparatus (JGA): a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole, where they touch.
A fall in GFR can activate the JG cells to release renin.
Renin can stimulate the glomerular blood flow and so bring the GFR back to normal.

NEET Focus

The JGA is formed by the DCT and the afferent arteriole, not by the PCT or the efferent arteriole.
180 litres per day is the filtrate (GFR), not the urine; urine is only about 1.5 litres per day.
Proteins are the plasma constituents that do not pass into the filtrate.

1.5 Reabsorption

Filtrate formed per day: 180 litres. Urine released per day: 1.5 litres.
★ Exam imp So nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This process is reabsorption.

Reabsorbed = \frac{180 - 1.5}{180} \times 100 \approx 99%

The tubular epithelial cells in different segments reabsorb by active or passive mechanisms.
Water is also reabsorbed passively in the initial segments of the nephron.

Mechanism	Substances reabsorbed
Active transport	Glucose, amino acids, $N a^{+}$ , and similar substances
Passive transport	Nitrogenous wastes; water in the initial segments of the nephron

1.6 Tubular secretion

During urine formation, the tubular cells secrete substances such as $H^{+}$ , $K^{+}$ and ammonia into the filtrate.
★ Exam imp Tubular secretion helps maintain the ionic and acid-base balance of body fluids.

Reabsorption

Filtrate to blood
About 99% of the filtrate
Active: glucose, amino acids, $N a^{+}$ ; passive: nitrogenous wastes, water

Secretion

Tubular cells to filtrate
$H^{+}$ , $K^{+}$ and ammonia
Keeps ionic and acid-base balance

Quick Recall: tap to check

What is the GFR of a healthy person?

About 125 ml per minute, that is, 180 litres per day.

Name the epithelial cells of Bowman's capsule that leave filtration slits.

Podocytes. The minute spaces between them are the filtration slits or slit pores.

Which constituent of plasma does not pass into the filtrate?

Proteins. So glomerular filtration is called ultrafiltration.

Which two structures form the JGA?

The distal convoluted tubule and the afferent arteriole, at their point of contact.

Key idea

Filtration makes 180 L of protein-free filtrate a day; reabsorption returns about 99% of it, and secretion fine-tunes ions and pH.

2. Functions of the Tubules

Test yourself: hide labels

Figure 1: Reabsorption and secretion of major substances at different parts of the nephron. Arrows show the direction of movement: the PCT reabsorbs the most, while the loop of Henle and the collecting duct handle mainly water, NaCl and urea.

2.1 Proximal convoluted tubule (PCT)

The PCT is lined by simple cuboidal brush border epithelium, which increases the surface area for reabsorption.
★ Exam imp Nearly all the essential nutrients and 70-80 per cent of electrolytes and water are reabsorbed here.
The PCT maintains the pH and ionic balance of body fluids.
It does so by selective secretion of hydrogen ions and ammonia into the filtrate, and by absorption of $HC O_{3}^{-}$ from it.
The PCT is the major site of reabsorption and selective secretion.

2.2 Henle's loop

Reabsorption is minimum in its ascending limb.
This region plays a significant role in maintaining the high osmolarity of the medullary interstitial fluid.
★ Exam imp Descending limb: permeable to water but almost impermeable to electrolytes. This concentrates the filtrate as it moves down.
★ Exam imp Ascending limb: impermeable to water but allows transport of electrolytes, actively or passively.
So, as the concentrated filtrate moves up, it gets diluted as electrolytes pass into the medullary fluid.

Descending limb

Permeable to water
Almost impermeable to electrolytes
Filtrate becomes concentrated

Ascending limb

Impermeable to water
Electrolytes move out, actively or passively
Filtrate becomes diluted

Memory Trick

Down the loop, water leaves; up the loop, salt leaves. Water leaving concentrates the filtrate, and salt leaving dilutes it.

2.3 Distal convoluted tubule (DCT)

★ Exam imp Conditional reabsorption of $N a^{+}$ and water takes place here.
The DCT can also reabsorb $HC O_{3}^{-}$ .
It selectively secretes hydrogen and potassium ions and $N H_{3}$ .
This maintains the pH and the sodium-potassium balance in blood.

2.4 Collecting duct

A long duct that extends from the cortex to the inner parts of the medulla.
★ Exam imp Large amounts of water can be reabsorbed here to produce a concentrated urine.
It allows small amounts of urea to pass into the medullary interstitium, to keep up the osmolarity.
It maintains the pH and ionic balance of blood by selective secretion of $H^{+}$ and $K^{+}$ .
The DCT and collecting duct together allow extensive reabsorption of water and certain electrolytes, which helps in osmoregulation.

Segment	Main reabsorption	Added to the filtrate
PCT	Nearly all nutrients; 70-80% of electrolytes and water; $HC O_{3}^{-}$	$H^{+}$ , ammonia
Descending limb of Henle's loop	Water	None
Ascending limb of Henle's loop	Electrolytes (minimum reabsorption overall)	Urea enters the thin segment
DCT	Conditional $N a^{+}$ and water; $HC O_{3}^{-}$	$H^{+}$ , $K^{+}$ , $N H_{3}$
Collecting duct	Large amounts of water; urea passes into the interstitium	$H^{+}$ , $K^{+}$

Memory Trick

Track the secreted ions: $H^{+}$ is secreted at all three (PCT, DCT, collecting duct); $K^{+}$ only at the two distal parts (DCT, collecting duct); ammonia only at the two convoluted tubules (PCT, DCT).

Tips and Tricks

To decide whether the filtrate becomes more concentrated or more dilute, ask what leaves it. Water leaving concentrates it (descending limb, collecting duct); salt leaving dilutes it (ascending limb).

In Figure 1, arrows pointing away from the tubule mean reabsorption. Arrows pointing into the tubule mean secretion, or entry of urea into the thin ascending limb.

Quick Recall: tap to check

Which segment of the nephron is lined by brush border epithelium?

The proximal convoluted tubule (PCT).

Which limb of Henle's loop is impermeable to water?

The ascending limb.

Which segment shows conditional reabsorption of sodium ions and water?

The distal convoluted tubule (DCT).

In Figure 1, which part receives urea from the collecting duct region?

The thin segment of the ascending limb of Henle's loop.

Key idea

The PCT does most of the reabsorption, Henle's loop builds the medullary gradient, and the DCT and collecting duct fine-tune water, ions and pH.

3. Mechanism of Concentration of the Filtrate

Mammals can produce a concentrated urine.
Henle's loop and the vasa recta play a significant role in this.
★ Exam imp The filtrate flows in opposite directions in the two limbs of Henle's loop. This forms a counter current.
Blood also flows through the two limbs of the vasa recta in a counter current pattern.
The closeness of Henle's loop and the vasa recta, and the counter current in them, maintain an increasing osmolarity towards the inner medullary interstitium.
★ Exam imp Osmolarity rises from 300 $mOsmol L^{- 1}$ in the cortex to about 1200 $mOsmol L^{- 1}$ in the inner medulla.
This gradient is caused mainly by NaCl and urea.

How NaCl builds the gradient

The ascending limb of Henle's loop transports NaCl into the interstitium.
This NaCl is exchanged with the descending limb of the vasa recta.
The ascending portion of the vasa recta returns NaCl to the interstitium.

How urea builds the gradient

Small amounts of urea enter the thin segment of the ascending limb of Henle's loop.
The collecting tubule transports the urea back to the interstitium.

Test yourself: hide labels

Figure 2: Counter current mechanism in the nephron and vasa recta. Opposite flows in the two limbs keep the medullary interstitium rising from 300

mOsmol L^{- 1}

near the cortex to 1200

mOsmol L^{- 1}

in the inner medulla.

★ Very important Counter current mechanism: the transport of NaCl and urea made possible by the special arrangement of Henle's loop and the vasa recta. It maintains a concentration gradient in the medullary interstitium.

The interstitial gradient allows easy passage of water from the collecting tubule.
This concentrates the filtrate, which becomes urine.
Electrolytes and urea are retained in the interstitium by this arrangement.
★ Exam imp Human kidneys can produce urine nearly four times as concentrated as the initial filtrate.
The DCT and collecting duct concentrate the filtrate from 300 $mOsmol L^{- 1}$ to 1200 $mOsmol L^{- 1}$ , an excellent mechanism for conserving water.

\frac{1200 mOsmol L ^{- 1}}{300 mOsmol L ^{- 1}} = 4

Memory Trick

Count the levels in multiples of 300: 300, 600, 900, 1200 from the cortex to the inner medulla. The inner medulla (1200) is four times the cortex (300), so urine can become four times as concentrated as the filtrate.

NEET Focus

NaCl from the ascending limb of Henle's loop is exchanged with the descending limb of the vasa recta, not with the descending limb of Henle's loop.
Urea enters the thin ascending limb and is returned to the interstitium by the collecting tubule.
The gradient is caused mainly by NaCl and urea, and osmolarity increases towards the inner medulla.
In both Henle's loop and the vasa recta, the two limbs carry fluid in opposite directions.

Key idea

Opposite flows in Henle's loop and the vasa recta trap NaCl and urea in the medulla. Water then leaves the collecting duct, and urine becomes up to four times concentrated.

4. Regulation of Kidney Function

Kidney function is monitored and regulated by hormonal feedback mechanisms.
These involve the hypothalamus, the JGA and, to a certain extent, the heart.

4.1 ADH and the hypothalamus

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 activates these receptors.
They stimulate the hypothalamus to release antidiuretic hormone (ADH), or vasopressin, from the neurohypophysis.
ADH facilitates water reabsorption from the latter parts of the tubule, preventing diuresis (passing out of a large volume of dilute urine).
An increase in body fluid volume switches off the osmoreceptors and suppresses ADH release. This completes the feedback.

ADH also affects kidney function through its constrictory effect on blood vessels.
This raises blood pressure, which increases the glomerular blood flow and so the GFR.

4.2 Renin-angiotensin mechanism (JGA)

A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells.
The JG cells release renin.
Renin converts angiotensinogen in blood to angiotensin I, and further to angiotensin II.
Angiotensin II, a powerful vasoconstrictor, increases the glomerular blood pressure and so the GFR.
Angiotensin II also activates the adrenal cortex to release aldosterone.
Aldosterone causes reabsorption of $N a^{+}$ and water from the distal parts of the tubule, raising blood pressure and GFR.

★ Exam imp This complex mechanism is generally known as the renin-angiotensin mechanism.

Memory Trick

Order of the chain: Rain Always Arrives After April = Renin, Angiotensinogen, Angiotensin I, Angiotensin II, Aldosterone.

Extra Depth: In the body, the conversion of angiotensin I to angiotensin II is carried out by angiotensin converting enzyme (ACE), mainly in the lungs. For the exam, remember the chain as renin acting on angiotensinogen to give angiotensin I and then angiotensin II.

4.3 Atrial natriuretic factor (heart)

An increase in blood flow to the atria of the heart can cause the release of atrial natriuretic factor (ANF).
ANF causes vasodilation (dilation of blood vessels) and so decreases blood pressure.
★ Exam imp The ANF mechanism therefore acts as a check on the renin-angiotensin mechanism.

Hormone or factor	Released from	Trigger	Main action
ADH (vasopressin)	Neurohypophysis, on a signal from the hypothalamus	Excessive loss of body fluid (osmoreceptors)	Water reabsorption from the latter parts of the tubule; constricts blood vessels
Renin	JG cells of the JGA	Fall in glomerular blood flow, blood pressure or GFR	Angiotensinogen to angiotensin I and then angiotensin II
Angiotensin II	Formed in blood	Renin	Powerful vasoconstrictor; stimulates aldosterone release
Aldosterone	Adrenal cortex	Angiotensin II	$N a^{+}$ and water reabsorption from distal parts of the tubule
ANF	Atria of the heart	Increased blood flow to the atria	Vasodilation; lowers blood pressure

Tips and Tricks

Sort by effect on blood pressure: ADH, angiotensin II and aldosterone raise it; ANF lowers it. Only ANF causes vasodilation, which is why it checks the renin-angiotensin mechanism.

Quick Recall: tap to check

Which hormone is also called vasopressin?

Antidiuretic hormone (ADH), released from the neurohypophysis.

Which gland releases aldosterone in the renin-angiotensin mechanism?

The adrenal cortex, when activated by angiotensin II.

What does ANF do to blood vessels?

It causes vasodilation and so decreases blood pressure.

Key idea

ADH saves water, the renin-angiotensin-aldosterone chain raises blood pressure and GFR, and ANF from the heart opposes it.

5. Micturition

Urine formed by the nephrons is carried to the urinary bladder.
It is stored there until a voluntary signal is given by the central nervous system (CNS).
As the bladder fills, it stretches; the stretch receptors on its walls send signals to the CNS.
The CNS passes on motor messages.
The smooth muscles of the bladder contract while the urethral sphincter relaxes at the same time, releasing urine.

★ Exam imp Micturition: the process of release of urine.
Micturition reflex: the neural mechanism that causes micturition.

Features of human urine

Volume: an adult excretes, on average, 1 to 1.5 litres of urine per day.
Appearance: a light yellow, watery fluid with a characteristic odour.
Reaction: slightly acidic, pH 6.0.
Urea: on average, 25-30 g of urea is excreted per day.

Various conditions can change the characteristics of urine.
Analysis of urine helps in the clinical diagnosis of many metabolic disorders and of kidney malfunction.
★ Exam imp Glucose in urine (glycosuria) and ketone bodies in urine (ketonuria) indicate diabetes mellitus.

Key idea

Bladder stretch sends signals to the CNS, which contracts the bladder and relaxes the urethral sphincter: the micturition reflex.

6. Role of Other Organs in Excretion

Besides the kidneys, the lungs, liver and skin also help eliminate excretory wastes.

Organ	What it removes	Note
Lungs	Large amounts of $C O_{2}$ (about 200 mL per minute) and significant quantities of water every day	-
Liver (largest gland of the body)	Bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs	Most of these pass out with the digestive wastes
Skin: sweat glands	Sweat: a watery fluid with NaCl, small amounts of urea, lactic acid, etc.	Main function of sweat is cooling the body surface
Skin: sebaceous glands	Sebum, carrying sterols, hydrocarbons and waxes	Sebum forms a protective oily covering for the skin
Saliva	Small amounts of nitrogenous wastes	-

Memory Trick

Substances in bile: Brave Boys Can Do Very Difficult tasks = Bilirubin, Biliverdin, Cholesterol, Degraded steroid hormones, Vitamins, Drugs.

Sebum: Sebum Holds Wax = Sterols, Hydrocarbons, Waxes.

Key idea

Lungs remove carbon dioxide and water, and the liver removes bile pigments and other substances. The skin removes some wastes in sweat and sebum, and saliva carries a little nitrogenous waste.

7. Disorders of the Excretory System

★ Exam imp Uremia: accumulation of urea in the blood due to malfunctioning kidneys. It is highly harmful and may lead to kidney failure.
In such patients, urea can be removed by haemodialysis (also spelt hemodialysis).

Steps of haemodialysis

Blood is drained from a convenient artery.
An anticoagulant such as heparin is added.
The blood is pumped into a dialysing unit called the artificial kidney.
The unit has a coiled cellophane tube surrounded by dialysing fluid, which has the same composition as plasma except the nitrogenous wastes.
The porous cellophane membrane lets molecules pass according to the concentration gradient; nitrogenous wastes move out freely, clearing the blood.
The cleared blood is pumped back into the body through a vein, after adding anti-heparin.

Haemodialysis is a boon for thousands of uremic patients all over the world.
★ Exam imp Kidney transplantation is the ultimate method of correcting acute renal failure (kidney failure).
A functioning kidney from a donor, preferably a close relative, is used to minimise the chance of rejection by the host's immune system.
Modern clinical procedures have increased the success rate of this complicated technique.
Renal calculi: stones, or insoluble masses of crystallised salts (oxalates, etc.), formed within the kidney.
Glomerulonephritis: inflammation of the glomeruli of the kidney.

Memory Trick

A comes before V, heparin before anti-heparin: blood leaves through an Artery with heparin added, and returns through a Vein with anti-heparin added.

Key idea

Uremia is treated by haemodialysis or, ultimately, by kidney transplantation; renal calculi are kidney stones and glomerulonephritis is inflammation of the glomeruli.

8. Exam Essentials

Pairs to Match

Structure, hormone or condition	Feature or role
Podocytes	Leave filtration slits (slit pores)
PCT	Brush border epithelium; reabsorbs 70-80% of electrolytes and water
Descending limb of Henle's loop	Permeable to water, almost impermeable to electrolytes
Ascending limb of Henle's loop	Impermeable to water
DCT	Conditional reabsorption of $N a^{+}$ and water
Collecting duct	Large amounts of water reabsorbed; urea into the interstitium
JG cells	Release renin
ADH	Released from the neurohypophysis; prevents diuresis
Angiotensin II	Powerful vasoconstrictor
Aldosterone	Adrenal cortex; $N a^{+}$ and water reabsorption
ANF	Atria of the heart; vasodilation
Glycosuria	Glucose in urine
Uremia	Urea accumulated in blood
Renal calculi	Crystallised salts (oxalates) in the kidney
Glomerulonephritis	Inflammation of glomeruli

Exceptions

Almost all plasma constituents enter the filtrate except the proteins.
Reabsorption is minimum in the ascending limb of Henle's loop.
The ascending limb is impermeable to water, unlike the descending limb.
Nitrogenous wastes are reabsorbed passively, unlike glucose, amino acids and $N a^{+}$ , which are reabsorbed actively.
ANF lowers blood pressure, unlike ADH, angiotensin II and aldosterone, which raise it.
Urine is slightly acidic (pH 6.0), not neutral or alkaline.
The primary function of sweat is cooling; excretion of wastes is only a secondary role.
Dialysing fluid matches plasma except for the nitrogenous wastes.

Numbers to Remember

Blood filtered by the kidneys: 1100-1200 ml per minute, about 1/5th of the output of each ventricle.
GFR: about 125 ml per minute = 180 litres per day.
Urine: 1-1.5 litres per day; nearly 99% of the filtrate is reabsorbed.
PCT reabsorbs 70-80% of electrolytes and water.
Medullary osmolarity: 300 $mOsmol L^{- 1}$ (cortex) to about 1200 $mOsmol L^{- 1}$ (inner medulla); urine up to 4 times as concentrated as the filtrate.
Urine pH: 6.0; urea excreted: 25-30 g per day.
$C O_{2}$ removed by the lungs: about 200 mL per minute.
Layers of the filtration membrane: 3; processes of urine formation: 3.

9. Quick Revision

Urine formation: glomerular filtration, reabsorption and secretion.
1100-1200 ml of blood is filtered per minute (1/5th of each ventricle's output), driven by glomerular capillary blood pressure.
Three filtration layers: endothelium, basement membrane, epithelium of Bowman's capsule (podocytes, slit pores); proteins stay back, so it is ultrafiltration.
GFR is about 125 ml per minute (180 L per day); the JGA (DCT + afferent arteriole) releases renin when GFR falls.
About 99% of the filtrate is reabsorbed; glucose, amino acids and $N a^{+}$ actively, nitrogenous wastes passively.
Secretion of $H^{+}$ , $K^{+}$ and ammonia keeps the ionic and acid-base balance.
PCT: brush border; nearly all nutrients and 70-80% of electrolytes and water reabsorbed; secretes $H^{+}$ and ammonia; absorbs $HC O_{3}^{-}$ .
Descending limb: permeable to water, concentrates filtrate. Ascending limb: impermeable to water, dilutes filtrate.
DCT: conditional $N a^{+}$ and water reabsorption; secretes $H^{+}$ , $K^{+}$ , $N H_{3}$ . Collecting duct: water out, urea into the interstitium.
Counter current in Henle's loop and vasa recta: 300 to 1200 $mOsmol L^{- 1}$ , built mainly by NaCl and urea; urine up to 4 times concentrated.
ADH (vasopressin) from the neurohypophysis increases water reabsorption and prevents diuresis.
Renin, angiotensin I, angiotensin II, aldosterone: raise blood pressure and GFR; ANF causes vasodilation and checks this.
Micturition reflex: stretch receptors, CNS, bladder muscle contraction and urethral sphincter relaxation.
Urine: 1-1.5 L per day, light yellow, pH 6.0, 25-30 g urea per day; glycosuria and ketonuria indicate diabetes mellitus.
Lungs, liver, skin and saliva help excretion; uremia is treated by haemodialysis or transplantation.

10. Solved Examples

Solved Example 1

Match List I with List II.
List I: A. PCT, B. Descending limb of Henle's loop, C. Ascending limb of Henle's loop, D. Collecting duct
List II: I. Impermeable to water, II. Reabsorbs 70-80% of electrolytes and water, III. Permeable to water but almost impermeable to electrolytes, IV. Allows small amounts of urea into the medullary interstitium
Choose the correct answer:
(A) A-II, B-I, C-III, D-IV
(B) A-IV, B-III, C-I, D-II
(C) A-II, B-III, C-I, D-IV
(D) A-II, B-III, C-IV, D-I

Solution:

Answer: (C). The PCT reabsorbs 70-80% of electrolytes and water (II); the descending limb is permeable to water (III); the ascending limb is impermeable to water (I); the collecting duct passes urea into the interstitium (IV).

Solved Example 2

Read the statements about glomerular filtration.
A. The GFR of a healthy person is about 125 ml per minute.
B. About 180 litres of urine are formed per day.
C. Plasma proteins pass freely into Bowman's capsule.
D. A fall in GFR can make the JG cells release renin.
E. Podocytes leave minute spaces called filtration slits.
Choose the correct answer:
(A) A, B and D only
(B) A, D and E only
(C) B, C and E only
(D) A, C, D and E only

Solution:

Answer: (B). B is wrong: 180 litres per day is the filtrate; urine is about 1.5 litres. C is wrong: proteins do not pass into the filtrate, which is why the process is ultrafiltration.

Solved Example 3

Arrange the events of the renin-angiotensin mechanism in the correct order.
A. Aldosterone is released by the adrenal cortex
B. JG cells release renin
C. Angiotensinogen is converted to angiotensin I
D. Glomerular blood pressure falls
E. Angiotensin II is formed
Choose the correct answer:
(A) D, B, C, E, A
(B) B, D, C, E, A
(C) D, B, E, C, A
(D) D, C, B, E, A

Solution:

Answer: (A). A fall in glomerular blood pressure activates the JG cells to release renin, which converts angiotensinogen to angiotensin I and then angiotensin II; angiotensin II makes the adrenal cortex release aldosterone.

Solved Example 4

The GFR of a person is 125 ml per minute and the urine output is 1.5 litres per day. About what percentage of the filtrate is reabsorbed?
(A) 80%
(B) 90%
(C) 95%
(D) 99%

Solution:

Answer: (D). Filtrate per day $= 125 \times 1440 = 180000$ ml, that is, 180 L. Reabsorbed $= \frac{180 - 1.5}{180} \times 100 = 99.2%$ , that is, nearly 99%.

Solved Example 5

Which of the following is NOT correct about the counter current mechanism?
(A) The medullary gradient is caused mainly by NaCl and urea
(B) NaCl from the ascending limb of Henle's loop is exchanged with the descending limb of the vasa recta
(C) Osmolarity of the interstitium falls from the cortex to the inner medulla
(D) It helps the kidneys produce a concentrated urine

Solution:

Answer: (C). Osmolarity rises from the cortex (300) towards the inner medulla (about 1200 milliosmoles per litre).

Solved Example 6

Statement I: ANF acts as a check on the renin-angiotensin mechanism.
Statement II: ANF causes vasoconstriction and raises blood pressure.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect

Solution:

Answer: (B). ANF causes vasodilation and lowers blood pressure; that is exactly why it checks the renin-angiotensin mechanism.

11. Practice Questions

Practice Questions

Match List I with List II.
List I: A. Ammonotelism, B. Bowman's capsule, C. Micturition, D. Uricotelism, E. ADH
List II: I. Birds, II. Water reabsorption, III. Bony fish, IV. Urinary bladder, V. Renal tubule
Choose the correct answer:
(A) A-III, B-V, C-IV, D-I, E-II
(B) A-I, B-V, C-IV, D-III, E-II
(C) A-III, B-IV, C-V, D-I, E-II
(D) A-III, B-V, C-II, D-I, E-IVAnswer: (A). Bony fishes are ammonotelic, Bowman's capsule begins the renal tubule, the bladder releases urine in micturition, birds are uricotelic and ADH aids water reabsorption.
Read the statements.
A. Micturition is carried out by a reflex.
B. ADH helps in water elimination, making the urine hypotonic.
C. Protein-free fluid is filtered from blood plasma into 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.
Choose the correct answer:
(A) A, B and C only
(B) A, C, D and E only
(C) B, D and E only
(D) A, B, C, D and EAnswer: (B). B is false: ADH increases water reabsorption and prevents diuresis.
Arrange the steps of haemodialysis in order.
A. Anti-heparin is added
B. Blood is drained from an artery and heparin is added
C. Blood is pumped into the dialysing unit
D. Nitrogenous wastes move out into the dialysing fluid
E. Cleared blood returns through a vein
Choose the correct answer:
(A) B, C, D, A, E
(B) B, D, C, A, E
(C) C, B, D, A, E
(D) B, C, A, D, EAnswer: (A). Drain and add heparin, pump into the unit, clear the wastes, add anti-heparin, return through a vein.
Which of the following is NOT eliminated through sebum?
(A) Sterols
(B) Hydrocarbons
(C) Waxes
(D) Lactic acidAnswer: (D). Lactic acid is present in sweat, not in sebum.
Statement I: Glycosuria and ketonuria indicate diabetes mellitus.
Statement II: Normal human urine is slightly alkaline.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrectAnswer: (B). Urine is slightly acidic, with a pH of about 6.0.
The juxta glomerular apparatus is formed by cellular modifications in the:
(A) PCT and efferent arteriole
(B) DCT and afferent arteriole
(C) Collecting duct and vasa recta
(D) Henle's loop and peritubular capillariesAnswer: (B). The JGA forms where the DCT touches the afferent arteriole.
Define glomerular filtration rate (GFR).Answer: GFR is the amount of filtrate formed by the kidneys per minute. In a healthy person it is about 125 ml per minute, that is, 180 litres per day.
Explain the autoregulatory mechanism of GFR. What is the significance of the juxta glomerular apparatus (JGA) in kidney function?Answer: The JGA, formed by the DCT and the afferent arteriole, senses a fall in glomerular blood flow, blood pressure or GFR. Its JG cells release renin, which converts angiotensinogen to angiotensin I and then angiotensin II. Angiotensin II constricts vessels and raises glomerular blood pressure. It also activates the adrenal cortex to release aldosterone, which causes reabsorption of sodium ions and water from the distal parts of the tubule. Blood pressure and GFR return to normal.
Give a brief account of the counter current mechanism.Answer: Filtrate flows in opposite directions in the two limbs of Henle's loop, and blood does the same in the vasa recta. NaCl from the ascending limb passes to the descending vasa recta and returns to the interstitium through the ascending vasa recta. Urea enters the thin ascending limb and returns via the collecting tubule. This keeps the interstitium at 300 to 1200 milliosmoles per litre, so water leaves the collecting duct and urine becomes concentrated.
Describe the role of the liver, lungs and skin in excretion.Answer: The lungs remove about 200 mL of carbon dioxide per minute and much water daily. The liver secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, which leave with digestive wastes. The skin removes NaCl, some urea and lactic acid in sweat, and sterols, hydrocarbons and waxes in sebum.
Explain micturition.Answer: Urine is stored in the bladder. As it fills, stretch receptors in its wall signal the CNS, which sends motor messages that contract the bladder's smooth muscles and relax the urethral sphincter, releasing urine. This neural mechanism is the micturition reflex.
Fill in the blanks: (a) The ascending limb of Henle's loop is ____ to water, whereas the descending limb is ____ to it. (b) Reabsorption of water from the distal parts of the tubules is facilitated by the hormone ____. (c) Dialysing fluid contains all the constituents of plasma except ____. (d) A healthy adult excretes, on average, ____ g of urea per day.Answer: (a) impermeable, permeable; (b) ADH (vasopressin); (c) the nitrogenous wastes; (d) 25-30.

Common Mistakes to Avoid

Watch out

Saying 180 litres of urine form per day. That is the filtrate; urine is only about 1.5 litres.
Saying plasma proteins pass into Bowman's capsule. Almost everything passes except the proteins.
Writing that the ascending limb is permeable to water. It is impermeable; the descending limb is permeable.
Saying ADH makes urine dilute. ADH increases water reabsorption and prevents diuresis.
Saying the JGA is formed by the PCT and the efferent arteriole. It is the DCT and the afferent arteriole.
Saying ANF raises blood pressure. It causes vasodilation and lowers it.
Calling normal urine alkaline. It is slightly acidic, pH 6.0.
Reversing haemodialysis: heparin is added before dialysis, anti-heparin after it.

Frequently Asked Questions

What are the three steps of urine formation?

Urine formation involves glomerular filtration, reabsorption and secretion. The glomerulus filters blood into Bowman's capsule, and the renal tubules reabsorb nearly 99 per cent of this filtrate back into the blood. The tubular cells also secrete hydrogen ions, potassium ions and ammonia into the filtrate to maintain ionic and acid-base balance.

What is glomerular filtration rate (GFR)?

GFR is the amount of filtrate formed by the kidneys per minute. In a healthy person it is about 125 ml per minute, or 180 litres per day. Since only about 1.5 litres of urine is released daily, nearly 99 per cent of the filtrate must be reabsorbed by the renal tubules.

How does the juxta glomerular apparatus regulate GFR?

The JGA is a sensitive region formed by the distal convoluted tubule and the afferent arteriole where they touch. When glomerular blood flow, pressure or GFR falls, its JG cells release renin. This starts the renin-angiotensin mechanism, which raises glomerular blood pressure and brings the GFR back to normal.

What is the counter current mechanism in the kidney?

Filtrate flows in opposite directions in the two limbs of Henle's loop, and blood does the same in the vasa recta. Their close arrangement traps sodium chloride and urea in the medulla, keeping its osmolarity rising from 300 to about 1200 milliosmoles per litre. Water then leaves the collecting duct, producing concentrated urine.

How does ADH regulate kidney function?

An excessive loss of body fluid activates osmoreceptors, which stimulate the hypothalamus to release ADH, or vasopressin, from the neurohypophysis. ADH increases water reabsorption from the latter parts of the tubule and prevents diuresis. When body fluid volume rises again, the osmoreceptors switch off and ADH release is suppressed.

What is micturition?

Micturition is the release of urine. As the urinary bladder fills, stretch receptors in its wall send signals to the central nervous system. The CNS sends motor messages that contract the smooth muscles of the bladder and relax the urethral sphincter at the same time. This neural mechanism is called the micturition reflex.

How do the lungs, liver and skin help in excretion?

The lungs remove about 200 mL of carbon dioxide per minute and a lot of water. The liver secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. The skin removes sodium chloride, some urea and lactic acid in sweat, and sterols, hydrocarbons and waxes in sebum.

What is haemodialysis?

Haemodialysis removes urea from the blood of patients with uremia. Blood from an artery, mixed with heparin, is pumped through a coiled cellophane tube in dialysing fluid that matches plasma except for nitrogenous wastes. These wastes diffuse out, and the cleared blood returns through a vein after anti-heparin is added.

Previous year questions on Physiology of Excretion

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