Excretory Products and their Elimination | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 16, Excretory Products and their Elimination: the nitrogenous wastes of animals, the human excretory system, the nephron, urine formation, the counter current mechanism, the hormonal regulation of kidney function, micturition, the role of other organs, and disorders of the excretory system. Learn the measurements and numbers in this chapter exactly.

Which nitrogenous wastes do animals excrete?

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na⁺, K⁺, Cl⁻, phosphate and sulphate, either by metabolic activities or by other means like excess ingestion. These substances have to be removed totally or partially. Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by animals.

In ammonotelic animals the kidneys do not play any significant role in the removal of ammonia. In some ureotelic animals some amount of urea may be retained in the kidney matrix to maintain a desired osmolarity.

Note: Terrestrial animals are generally ureotelic or uricotelic, not ammonotelic, because ammonia is the most toxic waste and needs a large amount of water for its elimination. Terrestrial adaptation necessitated the production of less toxic nitrogenous wastes, urea and uric acid, for the conservation of water.

What excretory structures do other animals have?

In most invertebrates the excretory structures are simple tubular forms, whereas vertebrates have complex tubular organs called kidneys.

A survey of the animal kingdom presents a variety of excretory structures. The table lists the structures, the animals that have them and the work they do. Protonephridia, nephridia and Malpighian tubules are concerned with osmoregulation, that is ionic and fluid volume regulation; nephridia and Malpighian tubules also help to remove nitrogenous wastes.

What are the parts of the human excretory system and the kidney?

In humans the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.

Two bean-shaped kidneys lie on either side of two large vertical vessels, the inferior vena cava and the dorsal aorta. An adrenal gland sits on top of each kidney. A renal artery and a renal vein connect each kidney to the large vessels. One kidney is shown cut open to show the cortex, the medulla and the pelvis. A ureter runs down from each kidney to the urinary bladder, and the urethra leads out from the bladder.

See Fig. 16.1 in your NCERT textbook

Internal structure of the kidney

• Hilum: a notch towards the centre of the inner concave surface, through which the ureter, blood vessels and nerves enter.
• Renal pelvis: a broad funnel-shaped space inner to the hilum, with projections called calyces (singular calyx).
• Capsule: the tough outer layer of the kidney.
• Cortex and medulla: the two zones inside the kidney, an outer cortex and an inner medulla.
• Medullary pyramids: the few conical masses into which the medulla is divided; they project into the calyces.
• Columns of Bertini: the renal columns formed where the cortex extends in between the medullary pyramids.

The kidney is cut lengthwise. The renal capsule forms the outer boundary and the cortex lies just inside it. The striped, cone-shaped medullary pyramids point inwards, with renal columns of cortex between them. The tip of each pyramid opens into a calyx, and the calyces join the renal pelvis, which narrows into the ureter. The renal artery and the renal vein enter and leave beside the pelvis.

See Fig. 16.2 in your NCERT textbook

What is the structure of a nephron?

Each kidney has nearly one million complex tubular structures called nephrons, which are the functional units. Each nephron has two parts: the glomerulus and the renal tubule.

The glomerulus is a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery. Blood from the glomerulus is carried away by an efferent arteriole.

The renal tubule has these parts in order. The collecting duct, the last item, is not part of one nephron: the DCTs of many nephrons open into it.

1. Bowman's capsule: a double-walled cup-like structure that encloses the glomerulus. The glomerulus along with Bowman's capsule is called the malpighian body or renal corpuscle.
2. Proximal convoluted tubule (PCT): a highly coiled network.
3. Henle's loop: a hairpin-shaped part with a descending limb and an ascending limb.
4. Distal convoluted tubule (DCT): another highly coiled tubular region, into which the ascending limb continues.
5. Collecting duct: a straight tube into which the DCTs of many nephrons open. Many collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.

The malpighian corpuscle, PCT and DCT of the nephron are situated in the cortical region of the kidney, whereas the loop of Henle dips into the medulla.

At the top the afferent arteriole enters the glomerulus, which sits in the cup of Bowman's capsule, and the efferent arteriole leaves it. From Bowman's capsule the tubule runs as the coiled proximal convoluted tubule, then down as the descending limb of the loop of Henle, turns, and returns as the ascending limb. It continues as the coiled distal convoluted tubule, which joins the straight collecting duct on the right. Blood vessels follow the tubule, and the vasa recta runs alongside Henle's loop.

See Fig. 16.3 in your NCERT textbook

The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule, called the peritubular capillaries. A minute vessel of this network runs parallel to Henle's loop, forming a U-shaped vasa recta.

How is urine formed?

Urine formation involves three main processes, which take place in different parts of the nephron: glomerular filtration, reabsorption and secretion.

Glomerular filtration

The first step is the filtration of blood, carried out by the glomerulus. On an average, 1100 to 1200 ml of blood is filtered by the kidneys per minute, which is roughly one-fifth of the blood pumped out by each ventricle of the heart in a minute.

The glomerular capillary blood pressure causes filtration of blood through three layers:

• the endothelium of the glomerular blood vessels,
• the epithelium of Bowman's capsule, and
• a basement membrane between these two layers.

The epithelial cells of Bowman's capsule, called podocytes, are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores. Blood is filtered so finely through these membranes that almost all the constituents of the plasma, except the proteins, pass into the lumen of Bowman's capsule. It is therefore considered a process of ultra filtration.

Definition: The glomerular filtration rate (GFR) is the amount of the filtrate formed by the kidneys per minute. In a healthy individual it is approximately 125 ml per minute, that is 180 litres per day.

Worked example: from GFR to the volume reabsorbed

1. GFR = 125 ml per minute. In one hour the filtrate is 125 × 60 = 7,500 ml.
2. In one day the filtrate is 7,500 × 24 = 1,80,000 ml, that is 180 litres.
3. The urine released is about 1.5 litres per day, so the volume reabsorbed is 180 − 1.5 = 178.5 litres.
4. Fraction reabsorbed = 178.5 ÷ 180 ≈ 0.99, that is nearly 99 per cent of the filtrate.

The juxta glomerular apparatus

The kidneys have built-in mechanisms for the regulation of the glomerular filtration rate. One efficient mechanism is carried out by the juxta glomerular apparatus (JGA). The JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. A fall in GFR can activate the JG cells to release renin, which can stimulate the glomerular blood flow and thereby bring the GFR back to normal.

Reabsorption and secretion

What does each part of the tubule do?

The PCT is lined by simple cuboidal brush border epithelium, which increases the surface area for reabsorption. Henle's loop plays a significant role in the maintenance of the high osmolarity of the medullary interstitial fluid. The collecting duct is a long duct that extends from the cortex of the kidney to the inner parts of the medulla.

The nephron is drawn across two zones, the cortex above and the medulla below. Arrows pointing out of the tubule show reabsorption and arrows pointing in show secretion. At the proximal convoluted tubule HCO₃⁻, NaCl, H₂O, nutrients and K⁺ leave, and H⁺ and NH₃ enter. The descending limb of the loop of Henle loses H₂O. The ascending limb, in its thin and thick segments, loses NaCl. At the distal convoluted tubule NaCl, H₂O and HCO₃⁻ leave, and K⁺ and H⁺ enter. The collecting duct loses H₂O, and urea passes from it towards the thin segment of the ascending limb.

See Fig. 16.5 in your NCERT textbook

Note: The two limbs of Henle's loop are opposites. The descending limb is permeable to water and almost impermeable to electrolytes. The ascending limb is impermeable to water and lets electrolytes out. So the filtrate becomes concentrated going down and dilute coming up.

How does the kidney concentrate the filtrate?

Mammals have the ability to produce a concentrated urine. Henle's loop and the vasa recta play a significant role in this.

Definition: The counter current mechanism is the transport of substances facilitated by the special arrangement of Henle's loop and the vasa recta, which maintains a concentration gradient in the medullary interstitium.

1. 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 the vasa recta is also in a counter current pattern.
2. The proximity between Henle's loop and the vasa recta, and the counter current in them, help in maintaining an increasing osmolarity towards the inner medullary interstitium: from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.
3. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle's loop, and is exchanged with the descending limb of the vasa recta.
4. NaCl is returned to the interstitium by the ascending portion of the vasa recta.
5. Small amounts of urea enter the thin segment of the ascending limb of Henle's loop, and are transported back to the interstitium by the collecting tubule.
6. The 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 as concentrated as the initial filtrate formed. The numbers agree: 1200 ÷ 300 = 4.

The vasa recta is drawn as a U-shaped vessel on the left and the nephron on the right. Dashed horizontal lines divide the figure into cortex, outer medulla and inner medulla. A scale on the left gives the osmolarity of the interstitium: 300 mOsmol L⁻¹ at the top, then 600 and 900, and 1200 mOsmol L⁻¹ at the bottom of the loop. Numbers inside the limbs rise as they go down and fall as they come up. Arrows show NaCl moving out of the ascending limb of the loop and H₂O moving out of the descending limb and the collecting duct, with urea leaving the collecting duct in the inner medulla.

See Fig. 16.6 in your NCERT textbook

How is kidney function regulated?

The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, the JGA and, to a certain extent, the heart.

ADH

1. Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration.
2. 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.
3. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis.
4. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release, to complete the feedback.

ADH can also affect kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure, which can increase the glomerular blood flow and thereby the GFR.

The renin-angiotensin mechanism

1. A fall in glomerular blood flow, glomerular blood pressure or GFR can activate the JG cells to release renin.
2. Renin converts angiotensinogen in blood to angiotensin I, and further to angiotensin II.
3. Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby the GFR.
4. Angiotensin II also activates the adrenal cortex to release aldosterone.
5. Aldosterone causes reabsorption of Na⁺ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR.

Atrial natriuretic factor

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. The ANF mechanism therefore acts as a check on the renin-angiotensin mechanism.

What is micturition, and what is normal urine like?

Definition: Micturition is the process of release of urine. The neural mechanism causing it is called the micturition reflex.

1. Urine formed by the nephrons is carried to the urinary bladder, where it is stored till a voluntary signal is given by the central nervous system (CNS).
2. The signal is initiated by the stretching of the urinary bladder as it gets filled with urine.
3. In response, the stretch receptors on the walls of the bladder send signals to the CNS.
4. The CNS passes on motor messages to initiate the contraction of the smooth muscles of the bladder and the simultaneous relaxation of the urethral sphincter, causing the release of urine.

Analysis of urine helps in the clinical diagnosis of many metabolic disorders, as well as of malfunctioning of the kidney. For example, the presence of glucose (glycosuria) and ketone bodies (ketonuria) in urine is indicative of diabetes mellitus.

What role do other organs play in excretion?

Other than the kidneys, the lungs, liver and skin also help in the elimination of excretory wastes.

What are the disorders of the excretory system?

Hemodialysis, step by step

1. Blood is drained from a convenient artery and pumped into a dialysing unit, called the artificial kidney, after adding an anticoagulant like heparin.
2. The unit contains a coiled cellophane tube surrounded by a fluid (the dialysing fluid) that has the same composition as plasma, except for the nitrogenous wastes.
3. The porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient.
4. As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood.
5. The cleared blood is pumped back to the body through a vein, after adding anti-heparin to it.

In kidney transplantation, a functioning kidney is used from a donor, preferably a close relative, to minimise its chances of rejection by the immune system of the host.

How do you answer the NCERT exercise questions?

Osmoregulation is the regulation of the ionic concentration and fluid volume of the body. It is the primary work of the protonephridia, and the Malpighian tubules also help in it.

Glossary

• Ammonotelism — The process of excreting ammonia, as in many bony fishes, aquatic amphibians and aquatic insects.
• Ureotelic — Describes animals that mainly excrete urea, such as mammals, many terrestrial amphibians and marine fishes.
• Uricotelic — Describes animals that excrete uric acid as a pellet or paste, such as reptiles, birds, land snails and insects.
• Nephron — The functional unit of the kidney, made of a glomerulus and a renal tubule; each kidney has nearly one million.
• Glomerulus — A tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery.
• Malpighian body — The glomerulus along with Bowman's capsule; also called the renal corpuscle.
• Podocytes — The epithelial cells of Bowman's capsule, arranged so as to leave minute filtration slits.
• Glomerular filtration rate — The amount of filtrate formed by the kidneys per minute, about 125 ml in a healthy individual.
• Vasa recta — A minute U-shaped vessel of the peritubular capillary network that runs parallel to Henle's loop.
• Juxta glomerular apparatus — A sensitive region formed by cellular modifications in the DCT and the afferent arteriole where they touch.
• Counter current mechanism — The transport of substances between Henle's loop and the vasa recta that maintains the medullary concentration gradient.
• Micturition — The process of release of urine from the urinary bladder through the urethra.
• Uremia — The accumulation of urea in blood caused by malfunctioning of the kidneys.
• Renal calculi — Stones or insoluble masses of crystallised salts, such as oxalates, formed within the kidney.

Common errors and misconceptions

• Misconception: Uric acid is the most toxic nitrogenous waste. Correct: Ammonia is the most toxic. Uric acid is the least toxic and is removed with a minimum loss of water.
• Misconception: All fishes are ammonotelic. Correct: Many bony fishes are ammonotelic, but marine fishes are listed among the ureotelic animals, which mainly excrete urea.
• Misconception: The glomerular filtrate has the same composition as blood. Correct: Almost all the constituents of plasma pass into Bowman's capsule except the proteins. Blood cells and proteins stay behind.
• Misconception: GFR is 125 ml per day. Correct: GFR is approximately 125 ml per minute, which is 180 litres per day.
• Misconception: The ascending limb of Henle's loop is permeable to water. Correct: The ascending limb is impermeable to water. The descending limb is the one permeable to water.
• Misconception: ADH increases the volume of urine. Correct: ADH facilitates water reabsorption from the latter parts of the tubule and prevents diuresis, so less urine is formed.
• Misconception: ANF and the renin-angiotensin mechanism both raise blood pressure. Correct: Angiotensin II raises blood pressure. ANF causes vasodilation and lowers it, acting as a check on the renin-angiotensin mechanism.
• Misconception: In hemodialysis the dialysing fluid contains urea so that it can mix with blood. Correct: The dialysing fluid has the same composition as plasma except the nitrogenous wastes, so these wastes move out of the blood into it.

Exam-style questions with model answers

Q1. Define glomerular filtration rate and state its value in a healthy individual. [1 mark]

1. The glomerular filtration rate (GFR) is the amount of filtrate formed by the kidneys per minute. In a healthy individual it is approximately 125 ml per minute, that is 180 litres per day.

Q2. Why are terrestrial animals generally ureotelic or uricotelic and not ammonotelic? [2 marks]

1. Ammonia is the most toxic nitrogenous waste and requires a large amount of water for its elimination, which land animals cannot spare.
2. Terrestrial adaptation necessitated the production of less toxic nitrogenous wastes, urea and uric acid, for the conservation of water.

Q3. Distinguish between cortical and juxta medullary nephrons. [2 marks]

1. In cortical nephrons, which are the majority, the loop of Henle is too short and extends only very little into the medulla; the vasa recta is absent or highly reduced.
2. In juxta medullary nephrons the loop of Henle is very long and runs deep into the medulla.

Q4. What is the significance of the juxta glomerular apparatus in kidney function? [3 marks]

1. The JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact.
2. A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release renin.
3. Renin converts angiotensinogen to angiotensin I and then angiotensin II, which raises the glomerular blood pressure and the GFR and also activates the adrenal cortex to release aldosterone. So the JGA brings the GFR back to normal.

Q5. Describe the role of the liver, lungs and skin in excretion. [3 marks]

1. Lungs remove large amounts of CO₂ (approximately 200 mL per minute) and significant quantities of water every day.
2. The liver secretes bile containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, most of which pass out with digestive wastes.
3. In the skin, sweat glands remove NaCl, small amounts of urea and lactic acid in sweat, and sebaceous glands eliminate sterols, hydrocarbons and waxes through sebum.

Q6. Explain micturition. [3 marks]

1. Urine is stored in the urinary bladder till a voluntary signal is given by the CNS. The signal is initiated by the stretching of the bladder as it fills with urine.
2. Stretch receptors on the walls of the bladder send signals to the CNS, which passes on motor messages.
3. These cause contraction of the smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, releasing the urine. The process is micturition and the neural mechanism is the micturition reflex.

Q7. Give an account of the counter current mechanism. [5 marks]

1. The flow of filtrate in the two limbs of Henle's loop is in opposite directions, forming a counter current. The flow of blood in the two limbs of the vasa recta is also counter current.
2. Their proximity and the counter current maintain an increasing osmolarity towards the inner medullary interstitium, from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.
3. The gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle's loop, exchanged with the descending limb of the vasa recta, and returned to the interstitium by the ascending portion of the vasa recta.
4. Small amounts of urea enter the thin segment of the ascending limb and are transported back to the interstitium by the collecting tubule.
5. The interstitial gradient allows an easy passage of water out of the collecting tubule, concentrating the urine. Human kidneys can produce urine nearly four times as concentrated as the initial filtrate.

Q8. Describe the process of glomerular filtration and the reabsorption that follows it. [5 marks]

1. Glomerular filtration is the filtration of blood by the glomerulus. On an average 1100 to 1200 ml of blood is filtered by the kidneys per minute.
2. The glomerular capillary blood pressure filters the blood through three layers: the endothelium of the glomerular blood vessels, the epithelium of Bowman's capsule and the basement membrane between them. The podocytes of Bowman's capsule leave filtration slits.
3. Almost all the constituents of plasma except the proteins pass into the lumen of Bowman's capsule, so it is called ultra filtration. The GFR is about 125 ml per minute, or 180 litres per day.
4. Since only about 1.5 litres of urine is released per day, nearly 99 per cent of the filtrate is reabsorbed by the renal tubules.
5. Glucose, amino acids and Na⁺ are reabsorbed actively, while nitrogenous wastes and, in the initial segments, water are reabsorbed passively.

Key takeaways

• Ammonia is the most toxic nitrogenous waste and needs most water; uric acid is the least toxic and needs least; urea lies between them.
• Each kidney has nearly one million nephrons, each made of a glomerulus and a renal tubule of Bowman's capsule, PCT, Henle's loop and DCT.
• Urine is formed by glomerular filtration, reabsorption and secretion; the GFR is about 125 ml per minute, or 180 litres per day.
• Nearly 99 per cent of the filtrate is reabsorbed; the PCT reabsorbs nearly all essential nutrients and 70 to 80 per cent of electrolytes and water.
• The descending limb of Henle's loop is permeable to water, and the ascending limb is impermeable to water but lets electrolytes pass.
• The counter current mechanism of Henle's loop and the vasa recta builds a gradient from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.
• ADH and the renin-angiotensin mechanism raise water reabsorption and blood pressure, and ANF acts as a check by causing vasodilation.
• Lungs, liver and skin also excrete wastes; uremia is treated by hemodialysis, and kidney transplantation corrects kidney failure.

Test yourself

Which nitrogenous waste is the most toxic?

Ammonia is the most toxic form of nitrogenous waste and requires a large amount of water for its elimination.

What are the excretory structures of insects?

Malpighian tubules are the excretory structures of most of the insects, including cockroaches.

What is the average weight of an adult human kidney?

An adult human kidney has an average weight of 120 to 170 g, and is 10 to 12 cm long.

What are the Columns of Bertini?

They are the renal columns formed where the cortex extends in between the medullary pyramids of the kidney.

Through which three layers is blood filtered in the glomerulus?

Blood is filtered through the endothelium of the glomerular blood vessels, the basement membrane and the epithelium of Bowman's capsule.

Which part of the tubule reabsorbs 70 to 80 per cent of electrolytes and water?

The proximal convoluted tubule reabsorbs nearly all of the essential nutrients and 70 to 80 per cent of electrolytes and water.

What is the osmolarity of the inner medullary interstitium?

The osmolarity rises from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.

Which substance does the dialysing fluid lack?

The dialysing fluid has the same composition as plasma except that it lacks the nitrogenous wastes.