NEET BiologyNCERT Class 11Chapter 16

Excretory Products and their Elimination: NEET notes

This chapter is about how animals get rid of the wastes their metabolism piles up, above all the nitrogen-containing ones. It compares ammonia, urea and uric acid and the excretory organs of different animal groups, then works through the human kidney: the nephron, the three steps of urine formation, what each part of the tubule does, the counter current mechanism that concentrates urine, hormonal control by ADH, renin–angiotensin and ANF, micturition, the help given by lungs, liver and skin, and kidney disorders and their treatment.

What NEET asks

NEET leans on the numbers (nearly one million nephrons per kidney, 1100–1200 mL of blood filtered per minute, GFR 125 mL/min or 180 L/day, about 99 per cent reabsorbed, 300 to 1200 mOsmol L⁻¹, urine about four times concentrated, 1–1.5 L urine and 25–30 g urea a day, pH 6.0), on which animals are ammonotelic, ureotelic or uricotelic, and on the permeability of the two limbs of Henle's loop. Marks are lost by swapping the permeabilities of the descending and ascending limbs, by mixing up what ADH, renin–angiotensin and ANF each do, and by placing the loop of Henle in the cortex.

1. Nitrogenous wastes

NCERT §16 (opening text)

Animals build up ammonia, urea, uric acid, CO₂, water and ions such as Na⁺, K⁺, Cl⁻, phosphate and sulphate, some from metabolism and some from other routes such as eating too much of something. These must be removed, wholly or partly.
The chief nitrogenous wastes are ammonia, urea and uric acid. Ammonia is the most toxic and needs a lot of water to be flushed out; uric acid is the least toxic and can be removed while losing very little water.
Ammonotelism is excretion of ammonia. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic.
Ammonia dissolves readily, so it usually leaves by diffusion as ammonium ions across the body surface, or across the gill surface in fish. The kidneys play no significant part in removing it.
Life on land made water precious, so land animals switched to less toxic wastes, urea and uric acid, which save water.
Ureotelic animals excrete mainly urea: mammals, many terrestrial amphibians and marine fishes. Their liver turns metabolic ammonia into urea, which enters the blood and is filtered out and excreted by the kidneys.
Some ureotelic animals keep a little urea in the kidney matrix to hold the osmolarity they need there.
Uricotelic animals excrete uric acid as a pellet or paste, losing hardly any water: reptiles, birds, land snails and insects.

2. Excretory structures in animals

NCERT §16 (opening text)

Across the animal kingdom excretory organs vary: most invertebrates have simple tubular structures, while vertebrates have complex tubular organs, the kidneys.
Protonephridia (flame cells) are found in Platyhelminthes such as Planaria, in rotifers, in some annelids and in the cephalochordate Amphioxus. Their main job is osmoregulation: regulating ions and fluid volume.
Nephridia are the tubular excretory organs of earthworms and other annelids. They remove nitrogenous wastes and keep the fluid and ionic balance.
Malpighian tubules serve most insects, including cockroaches. They remove nitrogenous wastes and carry out osmoregulation.
Antennal glands, also called green glands, do the excretory work in crustaceans such as prawns.
Whatever the organ, excretion is more than dumping nitrogen: these structures also help keep the ionic and acid–base balance of the body fluids.

3. Human excretory system

NCERT §16.1

Four organs make up the human excretory system: two kidneys, two ureters, one urinary bladder and one urethra.
The kidneys are reddish brown and bean shaped. They sit high against the back wall of the abdominal cavity, on its inner dorsal side, between the levels of the last thoracic and the third lumbar vertebra.
An adult kidney is 10–12 cm long, 5–7 cm wide and 2–3 cm thick, and weighs on average 120–170 g.
The hilum is a notch at the centre of the kidney's inner concave surface; the ureter, blood vessels and nerves enter through it.
Just inside the hilum is the renal pelvis, a broad funnel-shaped space with projections called calyces (singular calyx).
A tough capsule covers the kidney. Inside, the tissue forms two zones: the cortex on the outside and the medulla within.
The medulla is split into a few cone-shaped medullary pyramids that project into the calyces. The cortex dips in between the pyramids as the renal columns, the columns of Bertini.

4. The nephron

NCERT §16.1

Each kidney has nearly one million nephrons, the complex tubular structures that are its functional units. Every nephron is built of two parts, a glomerulus and a renal tubule.
The glomerulus is a capillary tuft. It is formed by the afferent arteriole, a fine branch of the renal artery, and drained by the efferent arteriole.
The renal tubule starts as Bowman's capsule, a double-walled cup around the glomerulus. Glomerulus plus Bowman's capsule is the Malpighian body, or renal corpuscle.
From the capsule the tubule runs as the highly coiled proximal convoluted tubule (PCT), then the hairpin-shaped Henle's loop with a descending and an ascending limb, then the highly coiled distal convoluted tubule (DCT).
The DCTs of many nephrons open into one straight collecting duct. Many collecting ducts join and open into the renal pelvis through the medullary pyramids in the calyces.
The Malpighian corpuscle, PCT and DCT lie in the cortex; the loop of Henle dips into the medulla.
In most nephrons the loop is very short and reaches only a little way into the medulla: cortical nephrons. In some the loop is very long and runs deep into the medulla: juxta medullary nephrons.
The efferent arteriole forms a fine capillary network around the tubule, the peritubular capillaries. One minute vessel of this network runs alongside Henle's loop as a U-shaped vasa recta, which is absent or highly reduced in cortical nephrons.

5. Glomerular filtration

NCERT §16.2

Urine is made in three main steps, each in its own parts of the nephron: glomerular filtration, reabsorption and secretion.
Filtration of blood by the glomerulus comes first. On average the kidneys filter 1100–1200 mL of blood a minute, roughly one-fifth of what each ventricle pumps out per minute.
The push comes from the blood pressure in the glomerular capillaries. Blood is filtered through three layers: the lining endothelium of the glomerular vessels, the basement membrane, and the epithelium of Bowman's capsule.
The epithelial cells of Bowman's capsule, the podocytes, are arranged intricately so that tiny gaps, filtration slits or slit pores, are left between them.
The filter is so fine that nearly everything in plasma except the proteins passes into the lumen of Bowman's capsule, so the process is called ultrafiltration.
The volume of filtrate formed by the kidneys each minute is the glomerular filtration rate (GFR). In a healthy person GFR is about 125 mL/minute, which is 180 litres a day.
The kidneys regulate GFR themselves. One efficient route is the juxta glomerular apparatus (JGA), a sensitive region formed by modified cells of the DCT and the afferent arteriole where the two touch.
A fall in GFR can make the JG cells release renin, which stimulates glomerular blood flow and so brings GFR back to normal.

6. Reabsorption and secretion

NCERT §16.2

About 180 litres of filtrate are formed a day but only about 1.5 litres of urine are passed, so nearly 99 per cent of the filtrate must be taken back by the renal tubules. This is reabsorption.
The epithelial cells of the different tubule segments reabsorb by active or passive mechanisms.
Glucose, amino acids, Na⁺ and similar substances are reabsorbed actively; nitrogenous wastes are absorbed passively.
In the initial segments of the nephron, water is also reabsorbed passively.
In secretion, tubular cells add substances such as H⁺, K⁺ and ammonia to the filtrate.
Secretion is an important step too, because it keeps the ionic and acid–base balance of the body fluids.

7. Function of the tubules

NCERT §16.3

PCT: lined by simple cuboidal brush border epithelium, which gives a larger surface for reabsorption. Nearly all essential nutrients and 70–80 per cent of the electrolytes and water are reabsorbed here.
The PCT also helps keep the pH and ionic balance of body fluids: it selectively secretes H⁺ and ammonia into the filtrate and absorbs HCO₃⁻ from it.
Henle's loop: its ascending limb takes back very little, yet the loop matters greatly, because it keeps the interstitial fluid of the medulla at high osmolarity.
The descending limb is permeable to water but almost impermeable to electrolytes, so the filtrate becomes more concentrated as it goes down.
The ascending limb is impermeable to water but lets electrolytes pass out, actively or passively, so the concentrated filtrate becomes diluted as it goes up.
DCT: conditional reabsorption of Na⁺ and water happens here. The DCT can also reabsorb HCO₃⁻ and selectively secrete H⁺, K⁺ and NH₃, keeping the pH and the sodium–potassium balance of blood.
Collecting duct: a long duct running from the cortex to the inner medulla. Large amounts of water can be reabsorbed here to make concentrated urine.
The collecting duct lets a little urea leak into the medullary interstitium, which helps hold the osmolarity there. It also secretes H⁺ and K⁺ selectively, helping the pH and ionic balance of blood.

8. Counter current mechanism

NCERT §16.4

Mammals can produce concentrated urine, and Henle's loop and the vasa recta are central to this.
Filtrate flows in opposite directions in the two limbs of Henle's loop, forming a counter current. Blood in the two limbs of the vasa recta also flows in a counter current pattern.
Because the loop and vasa recta lie close together and both carry counter currents, the osmolarity of the interstitium rises steadily towards the inner medulla: from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.
The gradient is made mainly by NaCl and urea.
NaCl is transported out by the ascending limb of Henle's loop and exchanged with the descending limb of the vasa recta; the ascending part of the vasa recta returns it to the interstitium.
A little urea enters the thin part of the ascending limb of Henle's loop; the collecting tubule later hands urea back to the interstitium.
This transport, made possible by the special arrangement of Henle's loop and the vasa recta, is the counter current mechanism. It keeps the concentration gradient in the medullary interstitium.
The gradient lets water pass easily out of the collecting tubule, concentrating the filtrate. Human kidneys can make urine nearly four times as concentrated as the initial filtrate (300 to 1200 mOsmol L⁻¹).

9. Regulation of kidney function

NCERT §16.5

Kidney function is monitored and regulated by hormonal feedback involving the hypothalamus, the JGA and, to some extent, the heart.
Osmoreceptors respond when blood volume, body fluid volume or ionic concentration changes. Losing too much fluid activates them; they act on the hypothalamus, and antidiuretic hormone (ADH), also called vasopressin, is released from the neurohypophysis.
ADH makes water reabsorption easier in the later parts of the tubule, so it prevents diuresis. When body fluid volume rises, the osmoreceptors switch off and ADH release is suppressed, completing the feedback.
ADH also constricts blood vessels, raising blood pressure; higher blood pressure increases glomerular blood flow and so GFR.
A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release renin. Renin converts angiotensinogen in blood to angiotensin I, which is further converted to angiotensin II.
Angiotensin II is a powerful vasoconstrictor, so it raises glomerular blood pressure and GFR. It also activates the adrenal cortex to release aldosterone.
Aldosterone makes the distal parts of the tubule reabsorb more Na⁺ and water, which again raises blood pressure and GFR. This whole chain is the renin–angiotensin mechanism.
More blood flowing into the atria of the heart can release Atrial Natriuretic Factor (ANF). ANF dilates blood vessels (vasodilation) and lowers blood pressure, so it acts as a check on the renin–angiotensin mechanism.

10. Micturition

NCERT §16.6

Urine made by the nephrons is carried to the urinary bladder and stored there until the central nervous system (CNS) gives a voluntary signal.
The signal starts when the filling bladder stretches: stretch receptors in its wall send signals to the CNS.
The CNS sends back motor messages that make the smooth muscles of the bladder contract while the urethral sphincter relaxes, so urine is released.
Release of urine is micturition, and the neural mechanism behind it is the micturition reflex.
An adult passes on average 1 to 1.5 litres of urine a day. It is a light yellow, watery fluid, slightly acidic (pH 6.0), with a characteristic odour.
On average 25–30 g of urea leaves the body in urine each day.
Many conditions change the character of urine, so urine analysis helps diagnose metabolic disorders and kidney malfunction. Glucose in urine (glycosuria) and ketone bodies in urine (ketonuria) point to diabetes mellitus.

11. Role of other organs in excretion

NCERT §16.7

Besides the kidneys, the lungs, liver and skin also help eliminate excretory wastes.
The lungs remove large amounts of CO₂, approximately 200 mL a minute, and a significant amount of water every day.
The liver, the largest gland in the body, secretes bile. Bile carries bilirubin, biliverdin, cholesterol, broken-down steroid hormones, vitamins and drugs, and most of these finally leave the body with the digestive wastes.
Sweat is a watery fluid holding NaCl, a little urea, lactic acid and other substances. Its main job is to cool the body surface, but it also removes some of these wastes.
Sebaceous glands give out sterols, hydrocarbons and waxes in sebum, which forms a protective oily covering over the skin.
Small amounts of nitrogenous wastes can be eliminated through saliva as well.

12. Disorders of the excretory system

NCERT §16.8

When the kidneys malfunction, urea builds up in the blood: uremia. It is highly harmful and may lead to kidney failure.
In such patients urea can be removed by haemodialysis. Blood is drained from a convenient artery, an anticoagulant such as heparin is added, and it is pumped into a dialysing unit, the artificial kidney.
The unit holds a coiled cellophane tube bathed in dialysing fluid that has the same composition as plasma except for the nitrogenous wastes.
The porous cellophane lets molecules pass according to their concentration gradient. Because the dialysing fluid has no nitrogenous wastes, these move out freely and the blood is cleared.
The cleared blood has anti-heparin added and is pumped back into the body through a vein.
Kidney transplantation is the ultimate way to correct acute renal (kidney) failure. A working kidney from a donor, preferably a close relative, is used to reduce the chance of rejection by the host's immune system.
Renal calculi are stones: insoluble masses of crystallised salts (oxalates and others) formed inside the kidney.
Glomerulonephritis is inflammation of the glomeruli of the kidney.

Must-know facts

Ammonia: most toxic, needs most water. Uric acid: least toxic, least water. Urea sits between.
Ammonotelic: many bony fishes, aquatic amphibians, aquatic insects. Ureotelic: mammals, many terrestrial amphibians, marine fishes. Uricotelic: reptiles, birds, land snails, insects.
Urea is made from ammonia in the liver; kidneys play no significant role in removing ammonia.
Protonephridia (flame cells): Planaria, rotifers, some annelids, Amphioxus. Nephridia: earthworm. Malpighian tubules: insects. Green (antennal) glands: prawns.
Kidney: 10–12 cm × 5–7 cm × 2–3 cm, 120–170 g, between last thoracic and third lumbar vertebra.
Columns of Bertini: cortex extending between medullary pyramids.
Nearly one million nephrons per kidney; glomerulus + Bowman's capsule = Malpighian body (renal corpuscle).
Corpuscle, PCT and DCT in cortex; loop of Henle dips into medulla; juxta medullary nephrons have long loops and a well-developed vasa recta.
Kidneys filter 1100–1200 mL of blood per minute, about 1/5 of each ventricle's output.
GFR ≈ 125 mL/min = 180 L/day; urine ≈ 1.5 L/day, so nearly 99% of filtrate is reabsorbed.
Filtration barrier: glomerular endothelium, basement membrane, Bowman's capsule epithelium (podocytes with filtration slits); all plasma except proteins passes.
PCT reabsorbs nearly all nutrients and 70–80% of electrolytes and water.
Descending limb: permeable to water, almost impermeable to electrolytes. Ascending limb: impermeable to water, electrolytes leave.
Medullary gradient 300 → about 1200 mOsmol L⁻¹, made mainly by NaCl and urea; urine up to about 4× as concentrated as filtrate.
ADH (vasopressin), from neurohypophysis: more water reabsorbed, less diuresis. Renin → angiotensinogen → angiotensin I → angiotensin II → aldosterone. ANF: vasodilation, checks renin–angiotensin.
Urine: 1–1.5 L/day, pH 6.0, 25–30 g urea/day. Glycosuria and ketonuria indicate diabetes mellitus.
Lungs remove about 200 mL CO₂ per minute.
Haemodialysis: heparin before, anti-heparin after; dialysing fluid = plasma minus nitrogenous wastes.

Common traps

Saying the ascending limb of Henle's loop is permeable to water.

Descending limb: water out, electrolytes stay. Ascending limb: electrolytes out, water stays. Down concentrates, up dilutes.

Calling marine fishes ammonotelic because they live in water.

Many bony fishes are ammonotelic, but NCERT lists marine fishes as ureotelic.

Thinking urea is made in the kidney.

The liver converts ammonia into urea; the kidneys filter it out of the blood.

Putting the loop of Henle in the cortex.

Malpighian corpuscle, PCT and DCT are cortical; the loop of Henle dips into the medulla.

Mixing up the kidney's blood flow and GFR.

1100–1200 mL of blood is filtered per minute, but only about 125 mL of filtrate forms per minute (GFR).

Thinking ADH makes more, dilute urine.

ADH increases water reabsorption and prevents diuresis, so urine becomes less and more concentrated.

Saying ANF works with the renin–angiotensin mechanism.

ANF causes vasodilation and lowers blood pressure; it is a check on the renin–angiotensin mechanism.

Believing the vasa recta is well developed in cortical nephrons.

Cortical nephrons have no vasa recta, or only a highly reduced one; it is well developed beside the long loops of juxta medullary nephrons.

Forgetting which drug goes in at which end of dialysis.

Heparin (anticoagulant) is added as blood leaves the artery; anti-heparin before it returns through a vein.

Key terms

Ammonotelism: Excreting nitrogen mainly as ammonia, which needs plenty of water.
Ureotelism: Excreting nitrogen mainly as urea, made from ammonia in the liver.
Uricotelism: Excreting nitrogen as uric acid pellets or paste with little water loss.
Protonephridia: Flame-cell excretory structures, mainly for osmoregulation, as in Planaria.
Hilum: The notch on the kidney's concave side where ureter, vessels and nerves enter.
Columns of Bertini: Extensions of the renal cortex between the medullary pyramids.
Nephron: The functional unit of the kidney: a glomerulus plus a renal tubule.
Malpighian body: Glomerulus together with Bowman's capsule; also called the renal corpuscle.
Vasa recta: A U-shaped capillary running beside Henle's loop, from the peritubular network.
Podocytes: Bowman's capsule epithelial cells whose arrangement leaves filtration slits.
Ultrafiltration: Filtration so fine that everything in plasma except proteins passes.
GFR: Volume of filtrate formed by the kidneys per minute, about 125 mL.
JGA: Sensitive region where DCT and afferent arteriole meet; releases renin when GFR falls.
Counter current mechanism: Opposite flows in Henle's loop and vasa recta that keep the medullary osmotic gradient.
ADH: Antidiuretic hormone (vasopressin): increases water reabsorption in the later tubule.
Aldosterone: Adrenal cortex hormone, released under angiotensin II, that increases distal Na⁺ and water reabsorption.
ANF: Atrial Natriuretic Factor: dilates vessels and lowers blood pressure, checking renin–angiotensin.
Micturition: Release of urine from the bladder, driven by the micturition reflex.
Uremia: Build-up of urea in blood when the kidneys fail.
Haemodialysis: Clearing urea from blood through a cellophane membrane in an artificial kidney.
Renal calculi: Kidney stones of crystallised salts such as oxalates.
Glomerulonephritis: Inflammation of the glomeruli.

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