The Excretory System: Elimination of Body Wastes

ICSE Class 10 Biology • Chapter 08 • Detailed Master Notes

A large number of waste products are formed during metabolic activities in the body. Large amounts of $CO_2$ and $H_2O$ are produced by the metabolism of carbohydrates, fats, and proteins. Nitrogenous wastes such as ammonia, urea, uric acid, etc., are formed from proteins and other complex nitrogenous compounds. These products become toxic if retained inside the body and hence must be sent out.

8.1 What is Excretion?

Excretion: The process of removal of chemical wastes (especially nitrogenous wastes) from the body.

Excretion vs Secretion Very Important

Secretion: Giving out by a cell or a gland some substance that has utility for the body (e.g., tears, sweat, saliva, milk, insulin).
Excretion: The passing out of substances that have no further use in the body or are strictly harmful (e.g., urine).

8.2 Substances to be Eliminated

The following categories of substances must be constantly eliminated from the body:

Carbon dioxide and Water: Every living cell liberates energy by oxidizing glucose with the production of carbon dioxide and water. $CO_2$ is eliminated through the lungs. The water becomes a part of the rest of the water in the body.
Nitrogenous metabolic wastes: These include urea, uric acid, and ammonia. They are produced mainly in the liver from the dead protein remains of other tissues. Any extra amino acids cannot be stored in the body. They are broken down in the liver to produce usable glucose, and the urea that has to be excreted out.
Note: Urea is highly poisonous; if allowed to accumulate in the blood to a certain level, it causes death. It is excreted out through the kidneys.
Excess salts: Such as common salt (NaCl) and even some excess water-soluble vitamins (B and C) need to be eliminated. They are mainly given out by the kidneys.
Water: Water is taken in with food and beverages, in large quantities. The excess quantity of water is removed which also serves a useful purpose of dissolving harmful materials to carry them out.
Bile pigments: (chiefly yellow bilirubin) are breakdown products of the haemoglobin of dead RBCs. The liver cells extract it from the circulation and secrete it into the bile juice poured into the duodenum. These pigments are modified in the intestine to give faeces their yellowish brown colour. Some of these pigments are excreted in urine.

8.3 The Excretory Organs

Excretion in humans is brought about by the following organs:

Kidneys: These are the primary excretory organs throwing out excretory products (chiefly urea) in the form of urine.
Sweat glands: Excretion by sweat glands is incidental. These glands are primarily concerned with cooling (thermoregulation). Sweat passes out sweat only when required for cooling, so truly they are not excretory.
Lungs: Excretion by lungs in the form of $CO_2$ released in the expired air.

8.4 The Urinary System

The human urinary system consists of the following organs:

Two Kidneys: Bean-shaped organs, about 10 cm long and 6 cm wide, located on either side of the backbone.
Note: The right kidney is slightly lower than the left kidney because the large liver occupies considerable space on the right side.
Two Ureters: Tubes arising from the hilum of each kidney, running downwards to connect to the urinary bladder.
Urinary Bladder: A muscular sac in the lower abdomen that temporarily stores urine.
Urethra: The tube leading from the urinary bladder to the outside. Its opening is guarded by circular muscles called sphincters which relax only during urination (micturition).

Fig 8.1: The human excretory system and the associated blood vessels

Internal Structure of the Kidney

A longitudinal section of the kidney shows two main regions:

Cortex: The outer, dark red region. It appears dotted due to the presence of Malpighian corpuscles (Bowman's capsule + glomerulus).
Medulla: The inner, lighter region. It is finely striped due to the presence of straight tubes (Loops of Henle and collecting ducts).
The medulla is composed of several conical masses called Renal Pyramids. The apex of each pyramid, called the papilla, projects into the basin-like cavity called the Pelvis. The pelvis is the expanded front end of the ureter.

Fig 8.2: A kidney in longitudinal section

8.5 Microscopic Structure of the Kidney (The Nephron)

The kidney is composed of an enormous number of minute tubules called Uriniferous tubules, Renal tubules, or Nephrons. There are over 1 million nephrons in each kidney. They are the structural and functional units of the kidney.

URINIFEROUS TUBULES: Tiny, so many, and for so much!

Total number in both kidneys: Approximately 2 million
Each single tubule: 4-5 cm long
Total length of all tubules together: more than 60 km. This great length provides a huge surface for reabsorption of usable substances, especially water.
Blood flowing through kidneys per minute: 1 litre
Glomerular filtrate produced in 24 hours: 160 litres
Urine produced from glomerular filtrate after reabsorption per day: 1.2 litre

Structure of a Renal Tubule (Nephron)

Each nephron consists of the following parts:

Bowman's Capsule: A thin-walled, cup-like structure at the upper end of the nephron. Its wall consists of a single layer of squamous epithelium. It encloses a knot of blood capillaries called the Glomerulus.
Note: Bowman's Capsule + Glomerulus = Malpighian Capsule (or Renal Corpuscle).
Proximal Convoluted Tubule (PCT): The starting, highly coiled region of the tubule. It lies in the cortex. ("proximal" means nearer to Bowman's capsule).
Loop of Henle: A middle hair-pin shaped loop running down into the medulla and turning back up. It is not convoluted.
Distal Convoluted Tubule (DCT): The end, coiled region of the tubule lying in the cortex. ("distal" means farther away). It opens into a collecting duct.
Collecting Duct: Receives the contents of many nephrons and pours it as urine into the pelvis of the kidney.

Fig 8.3: A single renal (uriniferous) tubule

8.6 Blood Supply to the Kidney Tubules

A pair of Renal arteries branch off from the dorsal aorta to enter the respective kidneys. Each renal artery branches and rebranches several times to give rise to arterioles.

An Afferent arteriole (incoming) enters the Bowman's capsule and breaks up into a network of capillaries called the Glomerulus.
The reuniting capillaries of the glomerulus form an Efferent arteriole (outgoing) which leaves the Bowman's capsule.
Crucial structural feature: The Afferent arteriole is wider than the Efferent arteriole.
The Efferent arteriole then forms a secondary capillary network (the Vasa recta) surrounding the renal tubule. These capillaries reunite to form a vein, which eventually joins the Renal Vein taking blood away to the posterior vena cava.

350 times a day through kidneys!

All the body blood passes through the kidneys 350-400 times a day at the rate of 1-2 litres per minute.

Fig 8.4: Diagrammatic sketch of one malpighian capsule

8.7 Physiology of Urine Formation

The formation of urine occurs in three major steps:

Step 1: Ultrafiltration

Blood flows through the glomerulus under great pressure. This is because the efferent arteriole is narrower than the afferent arteriole, creating a high Hydrostatic Pressure. This pressure forces the liquid part of the blood (plasma, along with urea, glucose, amino acids, and salts) out through the capillary walls into the Bowman's capsule.

This fluid is called the Glomerular Filtrate. It is essentially plasma minus the larger proteins and blood cells (which cannot pass through the capillary pores).

Step 2: Selective Reabsorption

The glomerular filtrate entering the renal tubule is not urine. It is an extremely dilute solution containing a lot of usable materials including glucose and some salts such as those of sodium. As the filtrate passes down the tubule, much of the water is reabsorbed together with the usable substances. But their reabsorption is only to the extent that the normal concentration of the blood is not disturbed. This is called selective absorption.

Note: Glucose is completely reabsorbed in a healthy person. If not reabsorbed, the person would suffer from rapid dehydration and nutrient loss.

Step 3: Tubular Secretion

Certain substances like potassium ions ($K^+$) in the normal course, and a large number of foreign chemicals including drugs like penicillin are passed into the forming urine in the distal convoluted tubule. This passage involves the activity of the cells of the tubular wall, and hence it is called tubular secretion.

The filtrate left after reabsorption and tubular secretion is called urine.

Table 8.1 Steps in Urine Formation

Part of renal tubule	Activity
1. GLOMERULUS	Ultrafiltration
2. BOWMAN'S CAPSULE	Receives glomerular filtrate
3. PROXIMAL CONVOLUTED TUBULE	Reabsorbs most water (about two-thirds), and much of glucose and sodium and chloride ions
4. LOOP OF HENLE	Some absorption of water and sodium ions
5. DISTAL CONVOLUTED TUBULE	Reabsorption of remaining chlorides and some water. Walls secrete potassium and foreign chemicals such as penicillin and other drugs into the forming urine

Urine excretion - Final urine passes into collecting ducts to the pelvis and through the ureter into the urinary bladder by ureteral peristalsis (waves of constriction in the ureters) and due to gravity. Urine is expelled from the urinary bladder through the urethra (in the penis in males, and directly in females) by relaxation of the sphincter muscles located at the opening of the urinary bladder into the urethra under impulse from the nervous system. Such a process is called micturition.

8.8 Properties and Composition of Urine

Physical Properties

Colour: Clear yellow. This is due to the presence of a pigment called Urochrome (formed from the breakdown of haemoglobin).
Volume: Normally 1 to 1.5 litres per day.
pH: 5.0 to 8.0. It is slightly acidic (approx. pH 6). A protein-rich diet makes it more acidic, while a vegetable diet makes it alkaline.
Odour: Faint odour. On standing for some time, it gives a strong ammonia-like smell due to bacterial activity breaking down urea into ammonia.
Specific Gravity: 1.003 to 1.035.

Chemical Composition

Normal urine consists of about 95% water and 5% solid wastes.

Table 8.2 Constituents of Urine (g/L)

Constituent	Amount (g/L)	Constituent	Amount (g/L)
Urea	2.3	Sodium chloride	9.0
Creatinine	1.5	Potassium chloride	2.5
Uric acid	0.7	Ammonia	0.6
Others	2.6	Others	2.5

Abnormal Constituents in Urine

Glucose: Condition is called Glycosuria. Suggests Diabetes mellitus (due to lack of insulin).
Blood: Condition is called Haematuria. Suggests infection, kidney stones, or a tumour in the urinary tract.
Albumin (Protein): High blood pressure or increased permeability of Bowman's capsule membrane.
Bile pigments: Suggests anaemia, hepatitis, or liver cirrhosis.

Taste of urine?
Normally, urine is saltish. If it is sweetish, it suggests "sugar diabetes" (Diabetes mellitus). If it is tasteless, it is due to diabetes insipidus (insufficient ADH).

GOUT AND KIDNEY STONES

Uric acid is relatively less soluble in water and may crystallize and get deposited in the joints causing gout. Excessive uric acid and certain salts like calcium oxalate may be the source of kidney stones.

8.9 Regulation of Urine Output (Osmoregulation)

Osmoregulation: The process of maintaining the constant osmotic pressure of the blood and tissue fluids by regulating the amount of water and salts in the body.

The kidney acts as a primary osmoregulatory organ. This is controlled by a hormone called Antidiuretic Hormone (ADH) or Vasopressin, secreted by the posterior lobe of the pituitary gland.

If water content is low: More ADH is secreted, causing the DCT and collecting ducts to reabsorb more water back into the blood. Urine becomes highly concentrated.
If water content is high: Less ADH is secreted, reducing reabsorption. Urine becomes dilute and voluminous.

Diuresis and Diuretics:
Diuresis is the condition of producing increased amounts of urine. Substances that promote this are called diuretics (e.g., tea, coffee, alcohol). This is why you feel the urge to urinate more often after consuming these beverages.

Why do we urinate more frequently in winter than in summer?

ICSE GIVE REASON In tropical climates, we drink a lot of water during summer yet we urinate fewer times in summer than in winter and the urine passed is generally thicker. The reason is that we lose a considerable part of water through perspiration (sweat for cooling). To conserve water and maintain the osmotic pressure of blood, the kidneys reabsorb more water. In winter, sweating is negligible, so excess water is eliminated largely via the kidneys, increasing urine output.

In certain diseases like cholera, the patient suffers from vomiting and watery bowels. His intestines are unable to absorb water into the blood. The result is that his kidneys reabsorb almost all the water from the forming urine, and even then, the patient may die due to the poisoning by the accumulation of high quantities of urea in his body (uremia). The immediate treatment of such a patient is to replenish water by administering oral rehydration solution (ORS).

The water balance in human body

There is about 40 litres of water in a normal human body (about 60% of the body weight). The average daily loss and gain is roughly outbalanced as follows:

Loss (ml)	Gain (ml)
Urine: 1500	Drinking water & beverages: 1500
Sweat: 500	Food: 700
Breath (lungs): 400	Metabolic water: 300
Faeces: 100
Total: 2500	Total: 2500

8.10 Artificial Kidney (Dialysis)

When both kidneys fail to function permanently due to disease or injury, nitrogenous wastes build up in the blood to toxic levels (Uraemia), leading to death. The patient is put on an Artificial Kidney machine (Dialysis).

Blood is led from a radial artery in the patient's arm through tubes made of cellophane (a semi-permeable membrane).
These tubes are suspended in a dialysing fluid.
Urea and excess salts diffuse out of the blood into the fluid.
The purified blood is safely returned to a vein in the same arm.
This process is usually repeated for about 12 hours twice a week.

Table 8.3 A Summary of Excretion in Humans

Substances excreted	Excretory Organs
Carbon dioxide	Lungs
Mineral salts / Nitrogenous wastes (urea, etc.)	Kidneys, Skin
Water	Kidneys, Skin, Lungs
Bile pigments	Liver

Exam Practice Questions (ICSE PYQ Trends)

Frequently Asked (1 Mark)

NAME THE FOLLOWING The functional unit of the kidney.
Ans: Nephron (or Uriniferous tubule).
NAME THE FOLLOWING The knot of blood capillaries inside the Bowman's capsule.
Ans: Glomerulus.
NAME THE FOLLOWING The yellow pigment that imparts colour to the urine.
Ans: Urochrome.
NAME THE FOLLOWING The condition in which blood passes along with the urine.
Ans: Haematuria.
NAME THE FOLLOWING The hormone that regulates the reabsorption of water in the kidney tubules.
Ans: ADH (Antidiuretic Hormone) or Vasopressin.

Give Biological Reasons

REASONING Answer the following:

Why is the right kidney slightly lower than the left kidney?
Ans: Because the large liver occupies considerable space on the right side of the abdominal cavity above it.
Why is the efferent arteriole narrower than the afferent arteriole?
Ans: To create a high hydrostatic pressure backlog in the glomerulus, which is strictly necessary for the process of Ultrafiltration to occur.
Why does urine give a strong ammonia smell upon standing for some time?
Ans: Because bacteria present in the environment break down the urea present in the urine into ammonia gas, which has a strong, pungent odour.
Why is selective reabsorption necessary in the nephron?
Ans: The glomerular filtrate contains essential nutrients (like glucose, amino acids, and vital salts) and large amounts of water. If not selectively reabsorbed, the body would quickly lose these nutrients and suffer from severe dehydration.

Differentiate

DIFFERENCES Differentiate between the following pairs:

Excretion and Secretion: Excretion is the removal of harmful or useless metabolic wastes (like urea in urine). Secretion is the production and release of substances that are highly useful for the body (like sweat, tears, enzymes, and hormones).
Cortex and Medulla of kidney: The cortex is the outer, darker region containing Malpighian corpuscles (which give it a dotted appearance). The medulla is the inner, lighter region containing the loops of Henle and collecting ducts (giving it a striped appearance).
Afferent and Efferent arteriole: Afferent arteriole is wider and brings blood into the glomerulus. Efferent arteriole is narrower and takes blood away from the glomerulus.