BIO101 – Topic 13: Excretion and Homeostasis
Course Code: BIO 101
Course Name: General Biology
© Lesuyai Organic Guides 2025
(Original | Educational | Professional | Plagiarism-Free | A4 Format)
13.1 Introduction to Excretion
Excretion is the biological process by which living organisms remove metabolic wastes and
harmful substances produced within the body. It is vital for maintaining a stable internal
environment (homeostasis). Metabolism produces by-products such as carbon dioxide, urea,
ammonia, and salts that can be toxic if accumulated. Excretion ensures these wastes are removed
safely. Examples: In humans: kidneys excrete urea; lungs excrete carbon dioxide; skin excretes
sweat. In plants: oxygen (from photosynthesis) and carbon dioxide (from respiration) diffuse out
through stomata.
13.2 Excretory Organs in Humans and Their Functions
Organ Main Excretory Product(s) Function
Kidneys Urea, salts, water Filter blood to form urine
Lungs Carbon dioxide, water vapor Remove gaseous waste
Skin Sweat (water, salts, urea) Regulates body temperature and excretes small wastes
Liver Bile pigments Converts toxic substances like ammonia into urea
13.3 Structure of the Human Urinary System
The urinary system consists of: - Kidneys – bean-shaped organs that filter blood - Ureters – tubes
carrying urine from kidneys to bladder - Urinary Bladder – stores urine - Urethra – carries urine out
of the body Text Diagram Description: Two kidney-shaped organs on either side of the spine, each
connected by a thin tube (ureter) leading to a central bladder, with a single tube (urethra) exiting
downward.
13.4 The Nephron – Structural and Functional Unit of the Kidney
Each kidney contains about one million nephrons. Parts of a Nephron: - Bowman’s capsule –
receives filtrate from blood - Glomerulus – network of capillaries for filtration - Proximal tubule –
reabsorbs water, glucose, amino acids - Loop of Henle – concentrates urine by reabsorbing water
and salts - Distal tubule & Collecting duct – further reabsorption and final urine formation
Functions of the Nephron: 1. Filtration of blood in the glomerulus 2. Reabsorption of essential
materials (glucose, water, ions) 3. Secretion of unwanted substances into the tubule 4. Excretion –
final removal of urine
13.5 Homeostasis
Homeostasis is the process by which living organisms maintain a constant internal environment
despite external changes. Examples: - Regulation of body temperature - Control of blood sugar
levels - Water and salt balance - pH regulation Main organs involved: kidneys, liver, brain
(hypothalamus), pancreas, and skin.
13.6 Temperature Regulation in Humans
Condition Body Response Result
Course Code: BIO 101
Course Name: General Biology
© Lesuyai Organic Guides 2025
(Original | Educational | Professional | Plagiarism-Free | A4 Format)
13.1 Introduction to Excretion
Excretion is the biological process by which living organisms remove metabolic wastes and
harmful substances produced within the body. It is vital for maintaining a stable internal
environment (homeostasis). Metabolism produces by-products such as carbon dioxide, urea,
ammonia, and salts that can be toxic if accumulated. Excretion ensures these wastes are removed
safely. Examples: In humans: kidneys excrete urea; lungs excrete carbon dioxide; skin excretes
sweat. In plants: oxygen (from photosynthesis) and carbon dioxide (from respiration) diffuse out
through stomata.
13.2 Excretory Organs in Humans and Their Functions
Organ Main Excretory Product(s) Function
Kidneys Urea, salts, water Filter blood to form urine
Lungs Carbon dioxide, water vapor Remove gaseous waste
Skin Sweat (water, salts, urea) Regulates body temperature and excretes small wastes
Liver Bile pigments Converts toxic substances like ammonia into urea
13.3 Structure of the Human Urinary System
The urinary system consists of: - Kidneys – bean-shaped organs that filter blood - Ureters – tubes
carrying urine from kidneys to bladder - Urinary Bladder – stores urine - Urethra – carries urine out
of the body Text Diagram Description: Two kidney-shaped organs on either side of the spine, each
connected by a thin tube (ureter) leading to a central bladder, with a single tube (urethra) exiting
downward.
13.4 The Nephron – Structural and Functional Unit of the Kidney
Each kidney contains about one million nephrons. Parts of a Nephron: - Bowman’s capsule –
receives filtrate from blood - Glomerulus – network of capillaries for filtration - Proximal tubule –
reabsorbs water, glucose, amino acids - Loop of Henle – concentrates urine by reabsorbing water
and salts - Distal tubule & Collecting duct – further reabsorption and final urine formation
Functions of the Nephron: 1. Filtration of blood in the glomerulus 2. Reabsorption of essential
materials (glucose, water, ions) 3. Secretion of unwanted substances into the tubule 4. Excretion –
final removal of urine
13.5 Homeostasis
Homeostasis is the process by which living organisms maintain a constant internal environment
despite external changes. Examples: - Regulation of body temperature - Control of blood sugar
levels - Water and salt balance - pH regulation Main organs involved: kidneys, liver, brain
(hypothalamus), pancreas, and skin.
13.6 Temperature Regulation in Humans
Condition Body Response Result
