This MCQ module is based on: Nitrogenous Wastes Excretory System
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Nitrogenous Wastes Excretory System
🎓 Class 11
Biology
CBSE
Theory
Ch 16 – Excretory Products and their Elimination
⏱ ~14 min
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Nitrogenous Wastes and the Human Excretory System
Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na⁺, K⁺, Cl⁻, phosphate, sulphate, etc., either by metabolic activities or by other means like excess ingestion. These substances have to be removed totally or partially. This chapter is about the mechanisms of that removal, with special emphasis on the common nitrogenous wastes.
The three nitrogenous wastes
Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by the animals. Ammonia is the most toxic form and requires a large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water.
Read that sentence carefully, because the whole section follows from it. Toxicity and water cost run together: the more toxic the waste, the more water is needed to dilute it to a safe concentration. An animal's choice of waste is therefore a trade-off between the energy cost of converting ammonia into something safer and the water cost of excreting it.
Ammonotelism
The process of excreting ammonia is Ammonotelism. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature. Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role in its removal.
Ureotelism
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.
Uricotelism
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.
| Feature | Ammonotelism | Ureotelism | Uricotelism |
|---|---|---|---|
| Waste excreted | Ammonia | Urea | Uric acid |
| Toxicity | Most toxic | Intermediate | Least toxic |
| Water needed | Large amount | Moderate | Minimum loss of water |
| Where it is made | Directly from metabolism | From ammonia, in the liver | From ammonia, by further conversion |
| Route of exit | Diffusion across body surfaces or gill surfaces, as ammonium ions. Kidneys play no significant role | Filtered and excreted by the kidneys | As a pellet or paste |
| Animals | Many bony fishes, aquatic amphibians, aquatic insects | Mammals, many terrestrial amphibians, marine fishes | Reptiles, birds, land snails, insects |
Why marine fishes are ureotelic though they live in water. This looks like an anomaly until you remember that sea water is more concentrated than the fish's body fluids, so a marine fish is constantly losing water to its surroundings by osmosis. Water is therefore as precious to it as to a desert animal, and it cannot afford the large amount of water that ammonia excretion demands. The rule is not “aquatic animals are ammonotelic” but “animals with water to spare are ammonotelic”.
Excretory structures across the animal kingdom
A survey of the animal kingdom presents a variety of excretory structures. In most of the invertebrates, these structures are simple tubular forms, whereas vertebrates have complex tubular organs called kidneys.
| Structure | Found in | Function |
|---|---|---|
| Protonephridia or flame cells | Platyhelminthes (flatworms, e.g. Planaria), rotifers, some annelids and the cephalochordate — Amphioxus | Primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation |
| Nephridia | Earthworms and other annelids | Help to remove nitrogenous wastes and maintain a fluid and ionic balance |
| Malpighian tubules | Most of the insects including cockroaches | Help in the removal of nitrogenous wastes and osmoregulation |
| Antennal glands or green glands | Crustaceans like prawns | Perform the excretory function |
| Kidneys | Vertebrates | Complex tubular organs; excretion and osmoregulation |
One question to keep in mind for the exercises. Amphioxus is named specifically as a cephalochordate with flame cells — and Exercise 11(a) asks exactly for “a chordate animal having flame cells as excretory structures”. Note also that every structure in the table does two jobs, waste removal and osmoregulation. Excretion is never only about getting rid of poison; it is equally about keeping water and salts in balance.
16.1 Human Excretory System
In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
The kidneys — position and dimensions
- 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.
Internal structure of the kidney
- 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 medulla.
- The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces.
- The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.
The nephron — the functional unit
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 full structure of the nephron is taken up in Part 2; for now note only the scale of the thing. A single kidney the size of a fist contains a million independent filtering units, and between the two kidneys they process about a fifth of the body's entire cardiac output every minute.
📐 Activity 16.1 — Dissect and examine a goat kidney
What to do. Obtain a fresh goat or sheep kidney. First examine it whole: note its reddish brown, bean shape, and find the notch on the inner concave surface — the hilum — with the ureter and blood vessels entering there. Feel the tough capsule covering it and try to peel a little away. Measure its length, width and thickness.
Then cut the kidney lengthwise into two halves with a sharp blade and lay them flat. Identify the outer cortex and inner medulla, count the medullary pyramids, find the calyces into which they project, trace the renal pelvis funnelling towards the ureter, and look for the strips of cortical tissue running between the pyramids.
Predict: which zone will look darker and more streaked, the cortex or the medulla? And what are the strips of tissue between the pyramids called?
What you should see. The medulla is the darker and conspicuously striated zone, because it is packed with parallel loops of Henle, collecting ducts and vasa recta all running in the same direction. The cortex is paler and granular in appearance, because it is filled with the rounded malpighian corpuscles and the coiled PCTs and DCTs rather than straight tubes. This appearance is a direct read-out of the anatomy: the Malpighian corpuscle, PCT and DCT are situated in the cortical region, whereas the loop of Henle dips into the medulla.
The strips between the pyramids are the renal columns, called the Columns of Bertini — the cortex extending inward between the conical masses of the medulla.
The pyramids and calyces. You should count a few conical medullary pyramids, each with its blunt tip projecting into a calyx. The calyces merge into the broad funnel of the renal pelvis, which narrows into the ureter at the hilum. Trace that path with a probe and you have traced the route of every drop of urine leaving the kidney.
The measurements should come close to 10–12 cm long, 5–7 cm wide and 2–3 cm thick for a human kidney; a goat's will be somewhat smaller but the proportions are similar.
One thing you will not see. The nephrons themselves. Each kidney has nearly one million of them, but a single nephron is far too fine to make out with the naked eye — the striations of the medulla are the closest you get to seeing them. That is worth noting, because it explains why the functional unit of the kidney was understood only after the microscope.
🎯 Interactive: Which waste, which structure?
Waste form / structure: Ammonotelic — gill surfaces
Many bony fishes are ammonotelic. Ammonia, being readily soluble, is generally excreted by diffusion through the gill surfaces as ammonium ions, and the kidneys do not play any significant role in its removal.
🎯 Competency-Based Questions
Scenario: A researcher studies four animals: a freshwater fish, a desert-dwelling lizard, a camel, and a tadpole that is undergoing metamorphosis into an adult frog that will live on land. She measures the nitrogenous waste each excretes and the volume of water lost with it.
Q1. Predict the chief nitrogenous waste of the fish and the lizard, and justify each. L3 Apply
The freshwater fish is ammonotelic. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic in nature, and this works because ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces as ammonium ions. Although ammonia is the most toxic form and requires a large amount of water for its elimination, a fish surrounded by fresh water has unlimited water, so there is no reason to spend energy converting it.
The desert lizard is uricotelic. 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. For an animal whose water supply is scarce, the least toxic waste, needing the least water, is worth the extra cost of making it.
The desert lizard is uricotelic. 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. For an animal whose water supply is scarce, the least toxic waste, needing the least water, is worth the extra cost of making it.
Q2. What change would you expect in the tadpole's excretion as it becomes an adult frog, and why? L4 Analyse
It should shift from ammonotelism to ureotelism. The tadpole is an aquatic amphibian, and such animals are ammonotelic, excreting ammonia by diffusion across the body and gill surfaces. The adult frog lives on land, and the chapter states the principle exactly: terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Many terrestrial amphibians mainly excrete urea and are called ureotelic animals, in which ammonia produced by metabolism is converted into urea in the liver and released into the blood, which is filtered and excreted out by the kidneys. So the metamorphosis involves not only limbs and lungs but a change of liver biochemistry and of the organ doing the excreting — from body surface to kidney. This single animal demonstrates the whole logic of the section within one lifetime.
Q3. Fill in the blanks: Each kidney measures ______ cm in length and weighs ______ g. The notch on its inner surface is the ______, inner to which lies the ______ with projections called ______. The cortex extending between the medullary pyramids forms the ______. L1 Remember
10–12; 120–170; hilum; renal pelvis; calyces; Columns of Bertini (renal columns).
Q4. The camel excretes urea, not uric acid, although it lives in a desert. Does this contradict the water-conservation argument? L4 Analyse
No — it shows that the mode of excretion is set by ancestry as much as by habitat. The camel is a mammal, and the chapter states plainly that mammals... mainly excrete urea and are called ureotelic animals. No mammal is uricotelic; the enzymatic pathway that converts ammonia to uric acid rather than urea is simply not part of the mammalian inheritance.
What a desert mammal does instead is conserve water at the kidney rather than change its waste product. It produces a highly concentrated urine, using the long loops of Henle and the counter current mechanism taken up in Part 3, and the chapter notes that in ureotelic animals some amount of urea may be retained in the kidney matrix to maintain a desired osmolarity — the very mechanism that makes concentration possible. So the camel achieves the same end by a different route: not a cheaper waste, but a better kidney. The wider lesson is that adaptation works within the limits of what an animal's lineage already possesses.
What a desert mammal does instead is conserve water at the kidney rather than change its waste product. It produces a highly concentrated urine, using the long loops of Henle and the counter current mechanism taken up in Part 3, and the chapter notes that in ureotelic animals some amount of urea may be retained in the kidney matrix to maintain a desired osmolarity — the very mechanism that makes concentration possible. So the camel achieves the same end by a different route: not a cheaper waste, but a better kidney. The wider lesson is that adaptation works within the limits of what an animal's lineage already possesses.
Q5. “Excretion simply means getting rid of poisons, so an excretory organ is essentially a waste disposal unit.” Evaluate. L5 Evaluate
The definition is half right and therefore misleading.
What is right. Removal of toxic substances is certainly part of it: animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na⁺, K⁺, Cl⁻, phosphate, sulphate, etc., and these substances have to be removed. Ammonia in particular is the most toxic form.
What the definition misses. (i) Excretion is also osmoregulation. Look at the survey of structures: protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation; nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance; Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation. Every one of them is described as doing two jobs. (ii) Substances are removed “totally or partially” — the chapter's own phrase. Water and Na⁺ are not poisons; they are removed only in whatever excess exists at that moment, which is regulation, not disposal. (iii) Useful material is deliberately retained. In ureotelic animals some amount of urea may be retained in the kidney matrix to maintain a desired osmolarity — a waste product kept on purpose because it is useful. A pure disposal unit would never do that. (iv) The choice of waste is a water-economy decision, not a toxicity decision alone: terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes for conservation of water.
The better formulation: an excretory organ is a regulator of the composition of body fluids, which removes wastes as one part of that task. That is why the kidney's design is a compromise, and why Part 4 of this chapter is about hormonal control rather than about plumbing.
What is right. Removal of toxic substances is certainly part of it: animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na⁺, K⁺, Cl⁻, phosphate, sulphate, etc., and these substances have to be removed. Ammonia in particular is the most toxic form.
What the definition misses. (i) Excretion is also osmoregulation. Look at the survey of structures: protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation; nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance; Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation. Every one of them is described as doing two jobs. (ii) Substances are removed “totally or partially” — the chapter's own phrase. Water and Na⁺ are not poisons; they are removed only in whatever excess exists at that moment, which is regulation, not disposal. (iii) Useful material is deliberately retained. In ureotelic animals some amount of urea may be retained in the kidney matrix to maintain a desired osmolarity — a waste product kept on purpose because it is useful. A pure disposal unit would never do that. (iv) The choice of waste is a water-economy decision, not a toxicity decision alone: terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes for conservation of water.
The better formulation: an excretory organ is a regulator of the composition of body fluids, which removes wastes as one part of that task. That is why the kidney's design is a compromise, and why Part 4 of this chapter is about hormonal control rather than about plumbing.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): Terrestrial animals are generally ureotelic or uricotelic rather than ammonotelic.
Reason (R): Ammonia is the most toxic form and requires a large amount of water for its elimination, so terrestrial adaptation necessitated less toxic wastes for conservation of water.
Answer: A. Both are true and the reason is exactly the chapter's explanation. Water economy, not toxicity alone, drives the choice.
Assertion (A): The kidneys of an ammonotelic fish are the main route for removing its nitrogenous waste.
Reason (R): Ammonia is readily soluble and is generally excreted by diffusion across body surfaces or gill surfaces as ammonium ions.
Answer: D. The assertion is false — kidneys do not play any significant role in ammonia removal. The reason is true and is precisely why the kidneys are not needed: the gill surface does the job by simple diffusion.
Assertion (A): Uricotelic animals lose very little water in excretion.
Reason (R): Uric acid, being the least toxic, can be removed in the form of a pellet or paste with a minimum loss of water.
Answer: A. Both are true and the reason explains the assertion. This is why birds, reptiles and land snails, which must either fly, live in dry places or seal themselves in an egg, are uricotelic.
Coming next. Part 2 takes up the detailed structure of the nephron and Section 16.2 — glomerular filtration as ultra filtration, the podocytes and filtration slits, the glomerular filtration rate of 125 mL per minute, the juxta glomerular apparatus, and the reabsorption and secretion that follow.
Frequently Asked Questions - Nitrogenous Wastes and the Human Excretory System
What are the three major nitrogenous wastes and how do they differ?
Ammonia, urea and uric acid. Ammonia is the most toxic form and requires a large amount of water for its elimination, while uric acid, being the least toxic, can be removed with a minimum loss of water. Urea is intermediate.
What are ammonotelism, ureotelism and uricotelism?
Ammonotelism is excreting ammonia, seen in many bony fishes, aquatic amphibians and aquatic insects. Ureotelism is excreting urea, seen in mammals, many terrestrial amphibians and marine fishes. Uricotelism is excreting uric acid as a pellet or paste, seen in reptiles, birds, land snails and insects.
Why are terrestrial animals not ammonotelic?
Because ammonia is the most toxic nitrogenous waste and requires a 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.
How is urea formed and excreted in ureotelic animals?
Ammonia produced by metabolism is converted into urea in the liver, released into the blood, and then 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.
Name the excretory structures of different animal groups.
Protonephridia or flame cells in Platyhelminthes such as Planaria, rotifers, some annelids and the cephalochordate Amphioxus. Nephridia in earthworms and other annelids. Malpighian tubules in most insects including cockroaches. Antennal glands or green glands in crustaceans like prawns. Kidneys in vertebrates.
What does the human excretory system consist of?
A pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
Describe the position and size of the human kidney.
Kidneys are reddish brown, bean shaped structures situated between the levels of the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity. Each measures 10 to 12 cm in length, 5 to 7 cm in width and 2 to 3 cm in thickness, with an average weight of 120 to 170 g.
Describe the internal structure of the kidney.
The hilum is a notch on the inner concave surface through which the ureter, blood vessels and nerves enter. Inner to it lies the renal pelvis, a broad funnel-shaped space with projections called calyces. The outer layer is a tough capsule, and inside are two zones, an outer cortex and an inner medulla. The medulla is divided into conical medullary pyramids projecting into the calyces, and the cortex extends between them as renal columns called Columns of Bertini.
What is a nephron?
A nephron is one of the nearly one million complex tubular structures in each kidney that are its functional units. Each nephron has two parts, the glomerulus and the renal tubule.
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