Excretion in Humans Chapter Notes | Biology Class 7 ICSE PDF Download

Introduction

Excretion is the process by which the body removes harmful and unwanted waste products produced during metabolism. This chapter explores how the human body eliminates substances like excess water, salts, urea, and other wastes to maintain health. The primary organs involved are the kidneys, supported by other organs like the skin, lungs, and liver, which work together to ensure the body remains free of toxic materials.

What is Excretion?

During different metabolic activities, the body produces many substances of which some are useful while the others are useless (not required by the body). If retained in the body, the useless unwanted substances may become poisonous and cause much harm and in severe cases, even death. The organs which remove these unwanted and toxic substances from the body are called excretory organs. The process of removal of the unwanted and harmful metabolic waste substances is called excretion.

Substances Excreted from the Body

1. Urea and Uric Acid (Nitrogenous Wastes): These are byproducts of the breakdown of excess amino acids in the liver. If allowed to build up, they can be toxic to the body.
2. Bile Pigments: Formed in the liver, these pigments give urine its yellowish color. Excess bile pigments are harmful and must be expelled.
3. Water: The body takes in large amounts of water through food and beverages. While some water is necessary for bodily functions, excess water is removed through various mechanisms.
4. Salts: Minerals like sodium chloride (common salt) are required in specific amounts. Any excess is excreted, primarily through urine.
5. Vitamins: Excess water-soluble vitamins (e.g., vitamins B and C) are not stored in large amounts and are eliminated through urine. Fat-soluble vitamins are stored to some extent, but any excess may also be excreted.
6. Medications: Certain drugs, such as antibiotics, are expelled through urine when taken in excess.

The Excretory System

The human excretory system, also known as the renal system, is responsible for filtering and removing waste products from the body. It consists of the following components:

1. Kidneys: These are two reddish-brown, bean-shaped organs located toward the back of the abdomen, one on each side of the spine, near the last two ribs.
2. Ureters: Narrow tubes that transport urine from each kidney to the urinary bladder.
3. Urinary Bladder: A muscular sac in the lower abdomen that stores urine until it is ready to be excreted.
4. Urethra: A single tube that carries urine from the urinary bladder to the outside of the body. The urethra is longer in males and shorter in females.

Structure of the Kidney

• Outer darker region called the cortex.
• Inner lighter region called the medulla.
• Medulla drains urine into a funnel-shaped renal pelvis.
• Ureters originate from the renal pelvis.

Nephrons

• Millions of microscopic tubular structures called renal tubules or nephrons.
• Each nephron starts with a cup-like Bowman's capsule.
• Continues as a convoluted (twisted) tubule that opens into a collecting duct.
• Collecting ducts empty into the renal pelvis, leading to the ureter.
• Nephron is the structural and functional unit of the kidney.

Role of Kidneys in Urine Formation

• Nephrons filter blood to remove waste and form urine.
• Blood enters kidneys through the renal artery, carrying wastes and useful substances.
• Nephrons remove excess water, mineral salts, and urea from blood to form urine.
• Reabsorb useful substances like glucose, sodium, and potassium back into the blood.
• Purified blood leaves the kidney with the right balance of water and nutrients.
• Urine formed in nephrons passes to ureters and is stored in the urinary bladder.
• Urine is expelled through the urethra during urination.
• Normal urine contains water, urea, uric acid, and some mineral salts.

Accessory Excretory Organs

Skin, lungs, liver, and salivary glands assist in waste removal.

Excretory Role of Skin

• Produces sweat, which evaporates to cool the body.
• Sweat glands under the skin secrete sweat through ducts.
• Sweat contains excess water, salts, and traces of urea and uric acid.

Excretory Role of Lungs

• Remove carbon dioxide produced during respiration.
• Carbon dioxide passes from blood to lungs and is exhaled through the nose.

Excretory Role of Liver

• Breaks down amino acids to produce urea, which is sent to kidneys for excretion.
• Eliminates bile pigments, excess vitamins, and accumulated drugs.

Osmoregulation

• Kidneys maintain water and salt balance in the blood.
• This process is called osmoregulation.
• In summer, less frequent urination and more concentrated urine due to water loss through sweat.
• In winter, more frequent urination as less water is lost through sweat.

Common Disorders of the Urinary System

Kidney Stones

• Formed when substances like calcium oxalate, calcium phosphate, or uric acid crystallize in urine.
• Crystals stick together to form kidney stones, causing severe pain.
• Can block urine flow in ureters, bladder, or urethra.
• Often require surgical removal.
• Drinking sufficient water daily helps prevent stone formation.

Urinary Tract Infection (UTI)

• Infection in kidneys, ureters, bladder, or urethra caused by bacteria.
• Symptoms include frequent urge to urinate and burning sensation during urination.
• Treated with antibiotics, mild cases cure in 2-3 days.

Diabetes

• Presence of glucose in urine indicates diabetes mellitus.
• Caused by high blood sugar levels, with excess sugar excreted in urine.

Presence of Blood Cells in Urine

• May indicate infection, tumor, internal bleeding, or kidney damage.

What Happens if Kidneys Fail?

• Kidney failure occurs due to infection, injury, or damage.
• A person can live with one kidney, but failure of both leads to waste buildup in blood.
• Untreated kidney failure can be fatal.
• Dialysis is a treatment where a machine filters and cleans the blood.
• Regular dialysis is needed for complete kidney failure to maintain normal life.
• Kidney transplant from a suitable donor is an alternative, allowing both donor and recipient to live normally.

Renal Tubules

• Approximately 2 million renal tubules in both kidneys.
• Each tubule is 4-5 cm long, totaling over 16 km in length.
• Kidneys filter 1 liter of blood per minute.
• Produce 180 liters of primary urine daily, with 179 liters reabsorbed.
• Final urine output is about 1.2 liters per day.