Chapter 16 of NCERT Class 11 Biology, titled Excretory Products and Their Elimination, explains how the human body removes harmful metabolic waste. The main focus is on the excretory system, especially the kidneys, their structure, urine formation, and how different organisms get rid of nitrogenous waste. This chapter also talks about kidney disorders and how dialysis works, which is especially relevant in real-life medical situations.
I am writing on this topic because many students tend to skip or rush through this chapter assuming it’s less important. But in reality, it’s not only a high-weightage topic in school exams, but also crucial for NEET preparation. Understanding how the kidneys filter blood and how our body keeps a balance of salts and water is essential to know how our system stays clean from the inside. Also, it’s interesting to see how different animals excrete in different forms – like urea, ammonia or uric acid. So, if you’ve ever wondered why kidneys are so important or what exactly happens in dialysis, this chapter gives you all the answers in a structured way.
Excretion: Meaning and Types
Excretion means removal of metabolic waste from the body. It’s not the same as defecation (which is undigested food). The major waste in humans is nitrogenous – like ammonia, urea and uric acid.
Types of Excretory Products
- Ammonotelic (e.g., fish, amphibians): Excrete ammonia
- Ureotelic (e.g., humans, mammals): Excrete urea
- Uricotelic (e.g., birds, reptiles): Excrete uric acid
Human Excretory System
The human excretory system mainly includes:
- A pair of kidneys
- A pair of ureters
- Urinary bladder
- Urethra
Each kidney is made up of about 1 million nephrons, which are the basic filtration units.
Structure of Nephron
Nephron has two parts:
- Glomerulus + Bowman’s capsule = Renal corpuscle
- Tubule part = PCT, loop of Henle, DCT, and collecting duct
Urine Formation
Three main steps:
- Glomerular Filtration: Blood is filtered in the glomerulus (ultrafiltration)
- Tubular Reabsorption: Useful substances like glucose, water and amino acids are reabsorbed
- Tubular Secretion: Ions and other waste are secreted into the tubule
Average urine formed per day = 1.5 litres
Functions of Kidneys
- Maintain water balance (osmoregulation)
- Remove nitrogenous waste
- Maintain ionic balance
- Regulate blood pressure through Renin-Angiotensin system
Hormones Involved
- ADH (Vasopressin): Increases water reabsorption
- Aldosterone: Helps retain sodium and water
Micturition
Micturition is the process of releasing urine. The nervous system plays a role in this by signalling the bladder when it is full.
Disorders of Excretory System
- Uremia: High level of urea in blood
- Renal failure: Kidneys stop functioning
- Kidney stones: Crystals block urine passage
- Glomerulonephritis: Inflammation of glomeruli
- Dialysis: Artificial filtration when kidneys fail
Dialysis
Used when both kidneys are not working properly. Blood is taken out, passed through a dialysis machine (which acts like a nephron), and sent back into the body. This removes urea, extra salts and water.
Excretion in Other Animals
- Earthworms: Use nephridia
- Insects: Malpighian tubules
- Amphibians and fish: Ammonia
- Reptiles and birds: Uric acid (to conserve water)
Download PDF
Click below to get the official NCERT PDF of Chapter 16
Download NCERT Class 11 Biology Chapter 16 PDF
