Excretory System Class 10 ICSE — Selina Chapter 8

The excretory system class 10 icse chapter in Selina Concise Biology explains how the body removes nitrogenous waste. It defines excretion, names the waste substances, and describes each excretory organ. The chapter then traces urine through the kidney, nephron by nephron, and ends with osmoregulation and dialysis. This page walks through every section in the book’s own order. Worked examples use fresh numbers so you can practise the pattern.

Download The Excretory System Class 10 ICSE Biology Chapter 8 PDF

Download the The Excretory System Class 10 ICSE Biology chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 9 pages, 11 sections, 36 questions.

What is inside Chapter 8

9 pages · 11 sections · 36 exercise questions

• PROGRESS CHECK: 2 questions
• PROGRESS CHECK: 2 questions
• PROGRESS CHECK: 1 questions
• REVIEW QUESTIONS: 31 questions

8.1 EXCRETION (ex : out, crete: flow)

Excretion is the removal of chemical wastes from the body, mainly nitrogenous ones. The word comes from Latin: ex means out, crete means flow. In this ICSE chapter, the definition matters because everything else builds on it.

Excretion keeps the body in a homeostatic state. Homeostasis is the steady internal condition the body maintains. If wastes build up, they poison the cells. So excretion is a continuous housekeeping process, not a one-time event.

The chapter draws a sharp line between excretion and defaecation. Defaecation is passing out faeces, which is undigested food from the intestine. That food was never part of the body’s chemistry, so it is not a chemical waste. Similarly, breathing out carbon dioxide is part of respiration, not excretion.

Because humans remove most nitrogenous waste as urine, the term urinary system fits humans better than excretory system. The organs that form, store and eliminate urine together make up this system.

8.2 SUBSTANCES TO BE GOT RID OFF

A list of five waste groups opens this section. The body must remove carbon dioxide and water, nitrogenous wastes, excess salts and vitamins, excess water, and bile pigments.

Carbon dioxide forms when cells oxidise glucose for energy. It leaves through the lungs. Nitrogenous wastes — urea, uric acid and ammonia — come mainly from the liver. Extra amino acids cannot be stored. The liver breaks them into usable glucose and urea. Urea is highly poisonous, so it must go through the kidneys.

Bile pigments deserve attention. They are breakdown products of haemoglobin from dead red blood cells. The liver extracts them and pours them into bile juice. In the intestine they colour faeces yellowish brown. Some pigments reach the urine.

Here is the waste-to-organ map the chapter gives:

8.3 THE EXCRETORY ORGANS

Humans use three organs for excretion. The kidneys are the primary ones. They throw out urea as urine. Sweat glands are incidental. Their main job is cooling the body, and sweat carries only small amounts of nitrogenous waste. Lungs release carbon dioxide in expired air.

A common exam trap sits here. Excretion is not the same as secretion. Secretion gives out a substance with some utility. Tears, sweat, saliva, milk and insulin all serve the body, so they are secretions. Urine is the only true excretion. This distinction appears in the chapter’s “What are these — excretion or secretion?” exercise, a classic trap across the Class 10 ICSE Biology collection.

The section ends with a PROGRESS CHECK of two questions. One asks you to pick waste substances from a list. The other asks whether undigested food counts as excretion. The answer is no, because faeces are not chemical wastes formed inside cells.

8.4 KIDNEYS (URINARY SYSTEM)

The kidneys are two bean-shaped organs, about 10 cm long and 6 cm wide. They sit on either side of the backbone, protected by the last two ribs. The right kidney sits slightly lower than the left, because the liver takes space above it.

Each kidney has a notch on its median surface called the hilum. The ureter arises from this notch and connects to the urinary bladder in the lower abdomen. The front end of the ureter widens inside the kidney and is called the pelvis, from the Latin for basin.

Urine flows from the kidney into the pelvis, then down the ureter to the bladder. The ureter openings into the bladder project like valves. They prevent backflow when the bladder contracts. A sphincter muscle guards the opening into the urethra. It relaxes only at urination, called micturition, under an impulse from the brain. The bladder empties intermittently through the urethra. For the official syllabus and exam format, visit the CISCE website.

8.4.1 Internal Structure of the kidney

A longitudinal section of the kidney shows two main regions. The outer cortex is dark. The inner medulla is lighter, made of finely striped substance arranged in conical pyramids. The apex of each pyramid, called the papilla, projects into the pelvis.

The whole kidney is packed with minute tubules called uriniferous tubules, nephrons, or renal tubules. These are the structural and functional units of the kidney. Every function in the rest of this chapter happens inside one nephron.

For a labelling question, remember the order from outside in: cortex, medulla, pyramids, papilla, pelvis. The pyramids point their papillae into the pelvis like spouts of funnels. This arrangement lets urine drip from many tubules into a single collecting basin.

A worked pattern for a diagram question: name the dark outer band as the cortex, the striped inner region as the medulla, and the cavity receiving urine as the pelvis. These three names carry most of the marks in section diagrams.

8.4.2 Structure of a Kidney tubule

Each kidney tubule has five parts in order. The Bowman’s capsule is a thin-walled cup, one cell thick. It holds a knot of blood capillaries called the glomerulus. Together they form the Malpighian capsule.

Next comes the proximal convoluted tubule, or PCT, lying in the cortex. “Proximal” means nearer, closer to the Bowman’s capsule. Then the Loop of Henle, a hair-pin shaped tube that is not convoluted. It runs into the medulla, turns back, and re-enters the cortex.

The distal convoluted tubule, or DCT, follows. “Distal” means farther from the capsule. It opens into a collecting duct, which receives many tubules and pours urine into the pelvis.

The chapter gives striking numbers. Both kidneys hold about 2 million tubules. Each is 4-5 cm long, giving a total length over 60 km. That length provides a huge surface for reabsorbing water. About 1 litre of blood flows through the kidneys per minute. From it, 160 litres of filtrate form in 24 hours, but only 1.2 litres of urine remain.

Blood supply follows a fixed path. The renal artery branches off the dorsal aorta. Arterioles enter the Bowman’s capsule as afferent arterioles. They break into the glomerulus capillaries. These reunite as the efferent arteriole.

The efferent arteriole runs a short distance and breaks into a secondary network called the vasa recta. This network surrounds the tubule, then rejoins into veins that form the renal vein, which empties into the posterior vena cava. All body blood passes through the kidneys 350-400 times a day.

8.4.3 Function of the Kidney-Formation of urine

Urine forms in two major steps: ultrafiltration and reabsorption.

Ultrafiltration happens in the glomerulus. The efferent arteriole is narrower than the afferent. This narrowness raises the hydrostatic pressure inside the glomerulus, much higher than in other capillaries. The pressure forces the liquid part of the blood out into the Bowman’s capsule.

This fluid, called the glomerular filtrate, contains water, urea, salts, glucose and other plasma solutes. Blood cells, proteins and large molecules stay behind in the blood, which becomes thicker. A common mistake is calling this filtrate urine; it is not, because it still holds glucose and salts.

The filtrate is not urine yet. It is an extremely dilute solution full of usable materials. As it passes down the tubule, the walls reabsorb what the body needs, in a process called selective reabsorption. The chapter’s Table 8.1 summarises each part’s job:

The filtrate left after reabsorption and tubular secretion is urine. The distal tubule passes potassium and foreign chemicals into the forming urine. This passage needs cell activity, so it is called tubular secretion.

Urine then moves into collecting ducts, to the pelvis, and down the ureter by peristalsis and gravity. Micturition expels it from the bladder through the urethra when the sphincter relaxes. All this work needs energy, so the kidney’s oxygen demand is 6 to 7 times higher than muscles.

Urine is clear yellow from urochrome, with a pH of 5 to 8 (usually about 6), volume of 1 to 1.5 litres per day, and specific gravity 1.003 to 1.035.

8.5 CONSTITUENTS OF URINE

Normal human urine is about 95% water and 5% dissolved solids. The solid share varies with food and time after eating. The main organic constituents are urea, creatinine and uric acid. The main inorganic ones are sodium chloride, potassium chloride and ammonia. Urine also carries out hormones, antibiotics and excess vitamins.

Four abnormal constituents signal disease:

The chapter adds a practical clue: the taste of urine. Normally it is saltish. When sweetish, sugar diabetes is likely, because glucose appears in the urine. When tasteless, too much water dilutes it, a condition called diabetes insipidus, from insufficient ADH hormone. “Insipid” means tasteless.

8.6 REGULATION OF URINE OUTPUT

The concentration of urine is controlled by antidiuretic hormone, or ADH. It is secreted by the posterior lobe of the pituitary gland. ADH governs how much water the tubules reabsorb. If ADH secretion falls, urine production rises. This increased output is called diuresis. Substances that raise urine formation are diuretics — liquid diets, tea, coffee and alcohol are examples.

Two conditions come from uric acid. Uric acid is relatively insoluble in water. It may crystallise and deposit in the joints, causing gout. Excessive uric acid and salts like calcium oxalate can form kidney stones when the kidneys are not fully normal.

A worked pattern for a true-false question: if a statement says sodium chloride is fully reabsorbed in the tubule, mark it false. Reabsorption is selective, not complete. Only about two-thirds of water and most glucose come back in the PCT, while some salts pass out in urine.

A PROGRESS CHECK of one question tests this section. It asks you to judge true or false statements about glomerular pressure, filtrate contents, sodium reabsorption, vitamins and kidney stones.

8.7 OSMOREGULATION

Osmoregulation is the regulation of the osmotic pressure of the blood. The kidney controls the percentage of water and salts while removing urea. Drinking enough water helps the kidneys work properly.

In a tropical climate, we drink a lot in summer but urinate fewer times. The urine is also thicker. The reason: perspiration removes a considerable part of the water, so the kidneys reabsorb more water from the urine, making it more concentrated.

Cholera shows why this matters. The patient vomits and passes watery stools. The intestines cannot absorb water into the blood. The kidneys then reabsorb almost all the water from the urine, and even urea with it. High urea in the blood causes uremia, which can kill. Immediate treatment is a glucose-saline drip or oral rehydration solution.

The adult body holds nearly 40 litres of water, about 60% of body weight. The chapter gives a daily water balance table with loss and gain channels.

Here is a worked pattern with fresh numbers. Suppose an adult loses 3000 mL in a day: 1700 mL in urine, 700 mL in sweat, 500 mL in breath, 100 mL in faeces. The urine share is \frac{1700}{3000} \approx 56.7\%. Sweat is \frac{700}{3000} \approx 23.3\%. Breath is \frac{500}{3000} \approx 16.7\%. Faeces are \frac{100}{3000} \approx 3.3\%. The shares must total 100%.

8.8 MISCELLANEOUS INFORMATION

One healthy kidney is enough for normal life. If one is damaged or removed, the other meets all excretory needs. Failure of both kidneys leads to death.

An artificial kidney is a dialysis machine. The patient’s blood is led from the radial artery in the arm through the machine. There, urea and excess salts are removed, and the purified blood returns to a vein in the same arm. Permanent kidney damage needs dialysis for about twelve hours, twice a week.

The chapter closes with a summary table of excretion across organs. Blood carries these wastes from the tissues, a path set out in the circulatory system chapter.

The table links every waste to its organ. Carbon dioxide leaves by the lungs. Nitrogenous wastes and salts leave by kidneys and skin. Water leaves by kidneys, skin and lungs. Bile pigments leave by the liver into the intestines. When you trace each waste to its route, the whole chapter folds into one map.

What to practise from the Excretory System Class 10 ICSE chapter

Four exercise blocks close this chapter. Work through them in the order the book presents.

• PROGRESS CHECK after section 8.3 — 2 questions on waste substances and whether undigested food is excretion
• PROGRESS CHECK after section 8.4.3 — 2 questions naming kidney parts and sequencing the nephron
• PROGRESS CHECK after section 8.6 — 1 true-false question on urine formation
• REVIEW QUESTIONS — 31 questions in four types: A. Multiple Choice, B. Very Short Answer, D. Long Answer, and E. Structured/Application/Skill

The structured questions test diagram reading. You must label a kidney section, trace the nephron with its blood supply, and answer on the malpighian capsule. Practise drawing the tubule in order: Bowman’s capsule, PCT, Loop of Henle, DCT, collecting duct.

Excretory System Class 10 ICSE frequently asked questions

What is the difference between excretion and defaecation in the excretory system class 10 icse?

Excretion removes chemical wastes formed inside cells, mainly nitrogenous ones. Defaecation passes out faeces, which are undigested food from the intestine. Faeces were never part of the body’s chemistry, so defaecation is not excretion.

Which part of the nephron reabsorbs the maximum amount of water?

The proximal convoluted tubule (PCT). It takes back about two-thirds of the water from the glomerular filtrate, along with much of the glucose and sodium and chloride ions.

Why is urine thicker in summer than in winter?

In summer, perspiration removes a considerable part of the body’s water. The kidneys then reabsorb more water from the urine, making it more concentrated and thicker.

What happens if ADH secretion is reduced?

Reduced ADH means less water is reabsorbed in the tubules, so urine production increases. This is called diuresis.

What is the difference between glycosuria and diabetes insipidus?

Glycosuria is glucose in the urine from diabetes mellitus, where insulin is insufficient. Diabetes insipidus is tasteless, diluted urine from too much water, caused by insufficient ADH hormone.

Reference: ICSE Class 10 Biology textbook material, chapter 8: The Excretory System — CISCE-aligned.

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• Chapter 3: Genetics Some Basic Fundamentals
• Chapter 4: Absorption By Roots the Processes Involved
• Chapter 5: Transpiration
• Chapter 6: Photosynthesis
• Chapter 7: The Circulatory System
• Chapter 9: The Nervous System
• Chapter 10: Endocrine Glands
• Chapter 11: The Reproductive System
• Chapter 12: Population
• Chapter 13: Aids to Health
• Chapter 14: Health Organisations
• Chapter 15: Pollution