Excretory System – NDA Biology Notes

What is the Excretory System?

The body produces waste every minute. Cells produce carbon dioxide, urea, uric acid, and excess salts. If these wastes stay in the body, they become toxic. The excretory system removes these wastes.

The main organ of excretion in humans is the kidney. The kidneys filter blood, remove waste, and produce urine.

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Organs of Excretion in Humans

The human body removes waste through multiple organs.

• Kidneys — remove urea, excess salts, and water through urine
• Lungs — remove carbon dioxide and water vapour
• Skin — removes some salts and water through sweat
• Liver — converts harmful ammonia into urea (less toxic), which is then removed by kidneys

The kidneys are the primary excretory organs. They are the main focus of NDA/CDS questions.

The Kidneys — Structure and Function

Humans have two kidneys. They are located on either side of the backbone in the abdominal cavity.

Each kidney is made up of millions of tiny filtering units called nephrons. The nephron is the functional unit of the kidney. [NDA tested via CDS]

The Nephron — How It Works

Each nephron performs filtration in a specific sequence.

Step 1 — Filtration at the Bowman’s Capsule:

Blood enters the nephron through a tiny ball of capillaries called the glomerulus. The glomerulus sits inside a cup-shaped structure called the Bowman’s capsule.

Blood is filtered here under pressure. Water, glucose, urea, salts, and small molecules pass from the blood into the Bowman’s capsule. This filtered liquid is called the glomerular filtrate. [NDA tested via CDS]

Blood cells and large proteins do NOT pass through — they stay in the blood.

Step 2 — Reabsorption in the Tubule:

The filtrate then passes through a long tubule. Here, useful substances are reabsorbed back into the blood.

Glucose is completely reabsorbed. Most water is reabsorbed. Salts are selectively reabsorbed.

Step 3 — Urine Formation:

What remains after reabsorption is urine. Urine contains water, urea, salts, and waste products. Urine is NOT the same as glomerular filtrate — most useful substances have been removed.

Component	In Glomerular Filtrate	In Urine
Urea	Present	Present (concentrated)
Glucose	Present	Absent (reabsorbed)
Water	Present	Present (reduced)
Salts	Present	Present (partially)
Blood cells	Absent	Absent
Proteins	Absent	Absent

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Urea — The Main Nitrogenous Waste

Proteins are broken down during digestion and metabolism. This produces ammonia — a highly toxic compound.

The liver converts ammonia into urea, which is much less toxic. Urea travels in the blood to the kidneys and is removed in urine.

This process is called ureagenesis or the urea cycle. [NDA tested via CDS]

Urea is the main nitrogenous waste in humans.

• Ammonia → most toxic → produced in liver → converted to urea
• Urea → less toxic → excreted in urine by kidneys
• Uric acid → least toxic → excreted in urine → also the waste product in birds and reptiles

Uric acid is the nitrogenous waste product in birds and reptiles. This is an important distinction for NDA/CDS. [NDA tested via CDS]

Animal Group	Nitrogenous Waste
Humans / Mammals	Urea
Birds / Reptiles	Uric acid
Aquatic animals (fish, amphibia)	Ammonia

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ADH — Antidiuretic Hormone

The amount of water in urine is controlled by a hormone called ADH (Antidiuretic Hormone), also called vasopressin.

ADH is produced by the hypothalamus and released by the pituitary gland.

When the body is dehydrated, more ADH is released. ADH tells the kidney tubules to reabsorb more water back into the blood. This produces less urine (concentrated urine).

When the body has enough water, less ADH is released. The kidney reabsorbs less water. This produces more urine (dilute urine). [NDA tested via CDS]

Alcohol suppresses ADH. This is why drinking alcohol causes frequent urination and dehydration. [NDA tested via CDS]

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Dialysis — Artificial Kidney

When kidneys fail, the body cannot filter blood on its own. A machine called a dialysis machine (artificial kidney) does the filtering instead. [NDA 2021-I]

The process is called haemodialysis. Blood is taken from the patient, passed through the dialysis machine which removes urea and waste, and returned to the patient.

The membrane used in dialysis is a semi-permeable membrane — it allows small molecules (urea, salts) to pass but not blood cells or proteins.

Bowman’s Capsule is NOT Part of the Respiratory System

This was directly tested in NDA 2018.

A question listed: Bronchi, Bowman’s capsule, Diaphragm, Trachea — and asked which does NOT take part in breathing.

The answer is Bowman’s capsule — it is a kidney structure, part of the excretory system. It has nothing to do with breathing or the lungs. [NDA 2018-I]

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Skin as an Accessory Excretory Organ

The skin removes some waste through sweat. Sweat contains water, salts, and a small amount of urea.

The skin is called an accessory excretory organ — not the primary one. The kidneys are primary.

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Uric Acid — Gout Connection

Uric acid is a waste product of the breakdown of nucleic acids (DNA and RNA).

When uric acid levels in blood become too high, uric acid crystals deposit in joints. This causes a painful condition called gout. [NDA tested via CDS]

Gout = high uric acid in blood = crystal deposits in joints.

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Water Reabsorption — Where it Happens

Water is absorbed back into the body at two main sites:

• Renal tubules in kidneys — major reabsorption of water
• Large intestine — absorbs water from undigested food

Both sites were confirmed correct in CDS 2015. [NDA tested via CDS]

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