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Excretion in Humans
ICSE Class 7 Biology • Chapter 7 (Detailed Master Notes)
Chapter Overview
Our bodies are like complex, continuously running factories. Just as any factory produces waste material during its manufacturing processes, our body cells also produce toxic waste substances during metabolic activities (like respiration). If these toxic wastes accumulate, they can poison and kill the cells. The body must constantly filter out and dispose of these waste materials. This vital life process of waste removal is called Excretion.
7.1 What is Excretion?
Excretion is the biological process by which an organism removes harmful and toxic metabolic waste products from its body. These wastes are generated by the various chemical reactions happening continuously inside the living cells.
Important Distinctions to Remember:
- Excretion: Disposing of metabolic wastes (wastes produced inside the cells), like urea, carbon dioxide, and excess salts.
- Egestion (Defecation): Disposing of undigested food (feces) from the digestive tract. Feces were never absorbed into the body cells; they simply passed straight through the digestive tube. Egestion is not excretion.
- Secretion: Producing and releasing useful substances (like saliva, enzymes, hormones, or tears) that the body needs.
7.2 Excretory Products and Organs
The human body employs several organs to eliminate different types of metabolic waste:
| Excretory Organ | Waste Product Eliminated | Form of Elimination |
|---|---|---|
| Lungs | Carbon Dioxide ($CO_2$) and water vapor. | Exhaled air during breathing. |
| Skin | Excess water, salts, and tiny amounts of urea. | Sweat produced by sweat glands. |
| Liver | Bile pigments (from dead red blood cells). Liver also converts highly toxic ammonia into less toxic Urea. | Bile pigments are passed into feces via the intestines. Urea is sent to the kidneys. |
| Kidneys (Main Excretory Organ) | Urea, excess water, excess salts, and other nitrogenous wastes. | Urine. |
7.3 The Human Urinary System
The primary excretory system in humans is the urinary system, which specifically filters the blood to remove nitrogen-based wastes (like urea) and maintains water balance.
The human urinary system consists of four main parts:
- A Pair of Kidneys: The main filtering organs. They are bean-shaped, dark red-brown organs located at the back of the abdominal cavity, one on each side of the spine. The right kidney is slightly lower than the left because of the liver.
- A Pair of Ureters: Two long, narrow muscular tubes. These originate from each kidney and transport urine downward to the urinary bladder.
- Urinary Bladder: A distensible (stretchable), hollow muscular sac located in the lower abdomen. It acts as a temporary reservoir to store urine until it is convenient to expel it.
- Urethra: A small tube leading from the urinary bladder to the outside of the body, through which urine is finally expelled. The opening is guarded by a sphincter muscle that provides conscious control over urination.
AI Image Prompt: A clear, medically accurate educational diagram of the human urinary system against a clean white background. Show two bean-shaped dark red kidneys at the top. From each kidney, draw a thin narrow tube (ureter) extending downwards to a balloon-like muscular sac (urinary bladder) at the bottom. A short tube (urethra) extends down from the bladder. Include blue and red blood vessels connecting to the kidneys. Clearly label Kidneys, Ureters, Urinary Bladder, and Urethra.
7.4 Internal Structure of the Kidney
If you slice a kidney open longitudinally, you will see it has two main regions:
- Renal Cortex: The darker, outer region of the kidney.
- Renal Medulla: The lighter, inner region.
The Nephron: The Working Unit
A kidney is not just a hollow bag; it is basically a dense mass of millions of microscopic blood filters. These tiny filtering units are called Nephrons (or uriniferous tubules). The nephron is the structural and functional unit of the kidney.
Each nephron consists of two main parts:
- Bowman’s Capsule: A cup-shaped structure located at one end. Inside this cup is a dense knot of blood capillaries called the Glomerulus. This is where actual blood filtration occurs under high pressure (like squeezing water out of a microscopic sponge). The Bowman's capsule combined with the glomerulus is called the Malpighian body.
- Renal Tubule: A long, highly coiled tube leading away from the Bowman's capsule. As the filtered liquid passes through this long tube, useful substances (like glucose, essential salts, and most of the water) are smartly reabsorbed back into the surrounding blood. The concentrated, toxic liquid left behind at the end of the tubule is Urine.
7.5 Composition of Urine
Normal human urine is a pale yellow, transparent liquid. Its composition is approximately:
- Water: 95%
- Urea: ~2.5%
- Other Wastes: ~2.5% (salts, uric acid, ammonia, etc.)
The characteristic yellowish color of urine is due to a pigment called urochrome. A healthy person excretes about 1 to 1.5 liters of urine per day, depending on fluid intake and external weather (sweating).
7.6 Common Urinary Diseases
Sometimes the urinary system may face issues:
- Urinary Tract Infection (UTI): A bacterial infection anywhere in the urinary system (kidneys, ureters, bladder, urethra). It causes a strong, persistent urge to urinate and a painful, burning sensation during urination.
- Kidney Stones (Renal Calculi): Hard, solid deposits of minerals and acid salts (often calcium oxalate) that physically crystalize and stick together inside the kidneys. Passing these stones through the narrow ureters causes excruciating back or side pain.
Dialysis (Artificial Kidney): If a person suffers complete kidney failure, their blood must be filtered artificially using a machine. This life-saving process, which artificially removes waste and excess water from the blood, is called Dialysis.
Q1. Nitrogenous waste like Urea is highly toxic. Where is it actually produced?
Answer: Urea is produced in the Liver during the breakdown of excess amino acids (proteins). The blood then carries it from the liver to the kidneys for final filtration and excretion.
Q2. Name the structural and functional unit of the human kidney.
Answer: The Nephron (or Uriniferous tubule). There are millions of these microscopic filtering units tightly packed inside each kidney.