IB DP Biology D3.3 Homeostasis Study Notes - New Syllabus -2025

IB DP Biology D3.3 Homeostasis Study Notes – New syllbaus

IB DP Biology D3.3 Homeostasis Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

How are constant internal conditions maintained in humans?
What are the benefits to organisms of maintaining constant internal conditions?

Standard level and higher level: 2 hours
Additional higher level: 2 hours

IBDP Biology 2025 -Study Notes -All Topics

D3.3.1 – Homeostasis: Maintenance of the Internal Environment

🧬 What is Homeostasis?

Homeostasis is the process by which an organism keeps its internal environment stable.
It involves keeping key variables within narrow, preset limits, even when the external environment changes.
This stability is vital for cells and organs to function properly.

📌 Key Homeostatic Variables in Humans

Variable	Normal Range / Set Point	Importance
Body temperature	Around 37°C	Enzymes work best at this temperature; too high or low damages cells
Blood pH	Around 7.4 (slightly alkaline)	pH affects enzyme activity and oxygen transport
Blood glucose concentration	~4-6 mmol/L (fasting)	Provides steady energy supply; too high or low is harmful
Blood osmotic concentration	~300 mOsm/L	Maintains fluid balance and cell size

🌿 How Does Homeostasis Work?

Body uses feedback systems to monitor and adjust these variables.
Typically involves three parts:

Receptor: Detects change (e.g., temperature sensors in skin).
Coordinator (Control center): Usually brain or endocrine glands; processes info.
Effector: Produces response to restore balance (e.g., sweat glands, muscles).

🔍 Examples of Homeostasis in Action

Variable	Change	Response	Effect
Body temperature	Rises (hot environment)	Sweat production, vasodilation of skin blood vessels	Heat loss, temperature returns to normal
Blood glucose	Increases after meal	Insulin released by pancreas	Cells take up glucose, blood level lowers
Blood pH	Drops (acidosis)	Increased breathing rate to expel CO₂	Raises blood pH back to normal
Blood osmotic concentration	Increases (dehydration)	Thirst stimulated, ADH hormone released	Water intake and retention restore balance

🧠 Why Is Homeostasis Important?

Ensures enzymes and cells function optimally.
Protects against harmful extremes (too hot/cold, acidic/basic).
Maintains fluid and electrolyte balance.
Supports overall health and survival.

D3.3.2 – Negative Feedback Loops in Homeostasis

🧬 What is Negative Feedback?

Negative feedback is a control mechanism that reverses any change in a homeostatic variable to bring it back to its normal level (set point).
It acts like a thermostat in your home: if the temperature goes too high or too low, it triggers actions to restore balance.
Negative feedback maintains stability by correcting deviations both above and below the set point.

🌿 Why Negative Feedback?

It prevents extreme fluctuations in the body.
Keeps variables within narrow, safe limits.
Ensures a stable internal environment, which is essential for cell function.
Positive feedback, in contrast, amplifies changes, which is usually unstable and rare in homeostasis (except in special cases like blood clotting or childbirth).

🔍 How Negative Feedback Works: Basic Steps

Step	Description	Example
1. Stimulus	Change occurs, e.g., body temperature rises	Heat from environment
2. Receptor	Detects change, e.g., temperature sensors in skin	Thermoreceptors
3. Control center	Processes info, e.g., hypothalamus in brain	Receives signal
4. Effector	Produces response to reverse change	Sweat glands activate to cool skin
5. Response	Variable returns toward set point	Body temperature lowers to normal

📌 Example: Blood Glucose Regulation

If blood glucose rises after eating:

Pancreas releases insulin.
Insulin promotes glucose uptake by cells.
Blood glucose falls back to normal.

If blood glucose drops:

Pancreas releases glucagon.
Glucagon triggers glucose release from liver.
Blood glucose rises to normal.

📊 Negative Feedback vs Positive Feedback

Feature	Negative Feedback	Positive Feedback
Effect on change	Reverses change	Amplifies change
Role in homeostasis	Maintains stability	Rare, triggers rapid events
Examples	Temperature, blood glucose	Blood clotting, childbirth contractions
Outcome	Returns variable to set point	Drives process to completion

D3.3.3 – Regulation of Blood Glucose: Hormonal Control in Homeostasis

🧬 Why Regulate Blood Glucose?

Glucose is the main energy source for cells.
Blood glucose levels must stay within a narrow range (~4–6 mmol/L).
Too high or low glucose levels can disrupt cell function and be harmful.

🌿 Key Hormones Involved

Hormone	Source	Role in Blood Glucose Regulation
Insulin	Beta cells of the pancreas	Lowers blood glucose
Glucagon	Alpha cells of the pancreas	Raises blood glucose

🔬 How Insulin Works

Released when blood glucose is high (e.g., after eating).
Transported in blood to target cells (muscle, liver, fat).
Effects on target cells:

Increases glucose uptake (muscle and fat cells).
Stimulates conversion of glucose to glycogen (glycogenesis) in the liver.
Promotes fat storage.

Result: Blood glucose levels decrease back to normal.

🧪 How Glucagon Works

Released when blood glucose is low (e.g., between meals or fasting).
Travels via blood to the liver (main target).
Effects:

Stimulates breakdown of glycogen to glucose (glycogenolysis).
Promotes formation of glucose from non-carbohydrate sources (gluconeogenesis).

Result: Blood glucose levels increase back to normal.

📍 Control of Hormone Secretion

Pancreatic endocrine cells detect blood glucose changes directly.
High glucose → stimulates beta cells to release insulin.
Low glucose → stimulates alpha cells to release glucagon.
Both hormones work antagonistically to maintain glucose balance.

🔍 Feedback Loop Summary

Change in Blood Glucose	Hormonal Response	Effect on Blood Glucose
Blood glucose rises	Insulin secreted	Lowers glucose by uptake & storage
Blood glucose falls	Glucagon secreted	Raises glucose by glycogen breakdown & synthesis

🧠 Why Is This Important?

Maintains a constant energy supply to cells, especially the brain.
Prevents damage caused by hyperglycemia (high glucose) and hypoglycemia (low glucose).
Illustrates hormonal control as a key homeostatic mechanism.

🧠 Summary Box:
Blood glucose is tightly regulated by insulin and glucagon, secreted by pancreatic endocrine cells.
Insulin lowers glucose by increasing uptake and storage.
Glucagon raises glucose by breaking down glycogen and making new glucose.
This negative feedback system keeps blood glucose within a healthy range.

D3.3.4 – Physiological Changes in Type 1 and Type 2 Diabetes

🧠 What is Diabetes?

Diabetes is a chronic condition where the body cannot regulate blood glucose properly.
Results in high blood glucose (hyperglycemia).
Two main types: Type 1 and Type 2 diabetes.

🌿 Type 1 Diabetes

Aspect	Details
Cause	Autoimmune destruction of beta cells in the pancreas
Effect	Little or no insulin production
Result	Blood glucose remains high because glucose can’t enter cells
Risk Factors	Genetic predisposition, autoimmune triggers (unknown exact cause)
Common Age of Onset	Usually childhood or adolescence
Treatment	Requires insulin injections to regulate blood glucose
Prevention	Currently no known prevention

🌿 Type 2 Diabetes

Aspect	Details
Cause	Insulin resistance: body cells respond poorly to insulin Eventually, pancreas may produce less insulin
Effect	High blood glucose due to ineffective glucose uptake
Risk Factors	Obesity, sedentary lifestyle, poor diet, age, genetics
Common Age of Onset	Usually adults, but increasing in younger people due to lifestyle
Treatment	Lifestyle changes (diet, exercise), oral medications, sometimes insulin
Prevention	Healthy diet, regular physical activity, weight management

🔬 Physiological Changes in Both Types

Change	Type 1 Diabetes	Type 2 Diabetes
Insulin levels	Low or absent	Normal, high initially, then may decrease
Blood glucose	High	High
Glucose uptake by cells	Severely reduced	Reduced due to resistance
Effects on metabolism	Cells starve for glucose, use fats → weight loss, ketoacidosis risk	Cells starve for glucose but less severe initially

🔍 Why Diabetes is Dangerous

High blood glucose damages blood vessels and nerves.
Leads to complications: heart disease, kidney failure, blindness, poor wound healing.
Managing blood glucose is crucial to prevent these.

📌 Summary Table: Differences Between Type 1 & Type 2 Diabetes

Feature	Type 1 Diabetes	Type 2 Diabetes
Cause	Autoimmune beta cell destruction	Insulin resistance + eventual insulin deficiency
Insulin Production	None or very low	Initially normal/high, then decreases
Onset Age	Usually young	Usually adult (but younger cases rising)
Risk Factors	Genetics, autoimmunity	Obesity, lifestyle, genetics
Treatment	Insulin injections	Lifestyle changes, meds, insulin if needed
Prevention	No known prevention	Healthy lifestyle can prevent/delay

🧠 Summary Box:
Type 1 diabetes is caused by loss of insulin production due to immune attack.
Type 2 diabetes is caused mainly by cells becoming resistant to insulin.
Both cause high blood glucose but differ in causes, risk factors, and treatment.
Prevention focuses on lifestyle changes for type 2; type 1 currently has no prevention.
Managing blood glucose is vital to avoid serious health complications.

D3.3.5 – Thermoregulation as an Example of Negative Feedback Control

🧬 What is Thermoregulation?

Thermoregulation is the process by which the body maintains a stable internal temperature (~37°C), despite changes in the external environment.

It is a classic example of negative feedback control.

🌿 Key Components of Thermoregulation

Component	Role
Peripheral thermoreceptors	Detect temperature changes in the skin and send signals to the brain
Hypothalamus	Acts as the control center; compares signals with the set point and coordinates responses
Pituitary gland	Releases hormones like thyroid-stimulating hormone (TSH) to regulate metabolism
Thyroxin	Hormone from the thyroid gland that increases metabolic rate, producing heat

🔬 How the Body Responds to Temperature Changes

When Body Temperature Rises (Too Hot):

Peripheral thermoreceptors detect increased skin temperature.
Signals sent to hypothalamus trigger cooling mechanisms:
Sweat glands produce sweat → evaporative cooling.
Vasodilation of skin blood vessels → more heat lost through skin.
Metabolic rate may decrease to reduce heat production.

When Body Temperature Falls (Too Cold):

Peripheral thermoreceptors detect low temperature.
Hypothalamus activates heat-producing responses:
Muscle shivering: rapid muscle contractions generate heat.
Vasoconstriction: skin blood vessels constrict to reduce heat loss.
Thyroxin secretion increases, raising metabolic rate for more heat.
Brown adipose tissue (brown fat) generates heat by burning fats (non-shivering thermogenesis).

📍 Role of Effectors: Muscle and Adipose Tissue

Effector	Function
Skeletal muscles	Shivering produces heat by rapid contractions
Brown adipose tissue	Specialized fat that generates heat without muscle movement (especially in infants)

🔍 Summary of Negative Feedback Loop in Thermoregulation

Step	Description
1. Stimulus	Body temperature deviates from set point
2. Receptors	Peripheral thermoreceptors detect change
3. Control center	Hypothalamus processes info and coordinates response
4. Effectors	Sweat glands, muscles, blood vessels, thyroid gland respond
5. Response	Body temperature returns to normal

🧠 Why Is Thermoregulation Important?

Keeps enzymes working optimally.
Prevents damage from extreme temperatures.
Maintains overall homeostasis and survival.

🧠 Summary Box:
Thermoregulation is controlled by a negative feedback system involving sensors (thermoreceptors), control center (hypothalamus), and effectors.
Sweating, vasodilation, shivering, vasoconstriction, and hormone regulation work together to maintain temperature.
Thyroxin increases metabolic heat production.
Muscle activity and brown fat play important roles in generating heat when cold.

D3.3.6 – Thermoregulation Mechanisms in Humans

🧬 How Humans Regulate Body Temperature

Humans maintain a stable internal temperature (~37°C) through a mix of physiological and behavioural responses.

These responses help balance heat gain and heat loss to adapt to changing environments.

🌿 Physiological Mechanisms of Thermoregulation

Mechanism	Description	Effect on Body Temperature
Vasodilation	Blood vessels near the skin widen (dilate)	Increases blood flow to skin → more heat lost by radiation and convection → cools body
Vasoconstriction	Blood vessels near the skin narrow (constrict)	Reduces blood flow to skin → less heat lost → conserves heat
Shivering	Rapid involuntary muscle contractions	Generates heat through increased metabolic activity
Sweating	Sweat glands release sweat onto skin	Evaporation of sweat removes heat, cooling the body
Hair erection (piloerection)	Tiny muscles at hair follicles contract, making hairs stand up	Traps a layer of air for insulation → reduces heat loss
Uncoupled respiration in brown adipose tissue (BAT)	BAT cells use fat to generate heat instead of ATP	Produces heat without muscle movement (non-shivering thermogenesis)

🔬 How These Mechanisms Work Together

Condition	Response	Purpose
Too hot	Vasodilation, sweating, hair lies flat	Lose excess heat
Too cold	Vasoconstriction, shivering, hair stands up, BAT activation	Generate and conserve heat

📍 Additional Behavioural Adaptations (Brief Mention)

Seeking shade or shelter
Wearing or removing clothing
Changing activity levels (e.g., resting when hot)

🧠 Why These Mechanisms Are Important

Keep internal conditions stable for enzyme function.
Protect against heat stress or hypothermia.
Ensure overall homeostasis and survival.

🧠 Summary Box:
Humans use vasodilation, vasoconstriction, sweating, shivering, hair erection, and brown fat activity to control temperature.
These are mainly physiological responses regulated by the nervous and endocrine systems.
Behavioural changes also help but are outside the scope of detailed study.
Together, these keep body temperature within narrow, safe limits.

D3.3.7 – Role of the Kidney in Osmoregulation and Excretion

🧠 Key Definitions

Excretion: The process of removing metabolic waste products from the body (e.g., urea, carbon dioxide).
Osmoregulation: The process of maintaining the balance of water and dissolved solutes (osmotic concentration) in body fluids.

🌿 Osmotic Concentration

Osmotic concentration is measured in osmoles per litre (osmol L⁻¹).
It refers to the total concentration of dissolved particles (solutes) in a solution.
Cells and tissues must keep osmotic concentration within limits to maintain proper hydration and cell function.

🔬 Kidney Functions

Function	Description	Importance
Excretion	Removes nitrogenous wastes like urea from the blood by filtering and producing urine	Prevents toxic build-up
Osmoregulation	Controls the water and salt balance by adjusting the volume and concentration of urine	Maintains blood osmotic concentration within narrow limits

📍 How Kidneys Perform Osmoregulation

Blood enters the kidney and is filtered through the glomerulus.
Useful substances (glucose, salts, water) are reabsorbed into the blood as needed.
Antidiuretic hormone (ADH) regulates the amount of water reabsorbed in the collecting ducts:
High ADH → more water reabsorbed → concentrated urine → less water lost.
Low ADH → less water reabsorbed → dilute urine → more water lost.
This adjusts the osmotic concentration of blood.

🔍 Summary of Kidney Roles

Process	What It Controls	Outcome
Excretion	Removes metabolic waste (urea)	Keeps blood clean and non-toxic
Osmoregulation	Regulates water and solute balance	Keeps blood osmotic concentration stable

🧠 Why This is Important

Maintaining osmotic balance prevents cell shrinkage or swelling.
Removing wastes prevents toxicity and damage to organs.
Kidneys are vital for overall homeostasis and health.

🧠 Summary Box:
Excretion removes harmful metabolic wastes; osmoregulation controls water and solute balance.
Kidneys filter blood, selectively reabsorb water and salts, and produce urine.
ADH hormone plays a key role in adjusting urine concentration to maintain blood osmotic balance.
Osmotic concentration is measured in osmoles per litre (osmol L⁻¹).

D3.3.8 – Role of Glomerulus, Bowman’s Capsule, and Proximal Convoluted Tubule in Excretion

🧬 Key Structures in Kidney Excretion

Structure	Location	Function
Glomerulus	Network of capillaries inside Bowman’s capsule	Filters blood plasma under high pressure (ultrafiltration)
Bowman’s capsule	Cup-shaped structure surrounding the glomerulus	Collects the filtrate (filtered fluid) from blood
Proximal convoluted tubule (PCT)	Twisted tubule after Bowman’s capsule	Reabsorbs useful substances back into the blood

🌿 Ultrafiltration at the Glomerulus

Blood enters the glomerulus at high pressure.
Small molecules (water, glucose, ions, urea) pass through capillary walls into Bowman’s capsule.
Large molecules (proteins, blood cells) stay in the blood.
This process is called ultrafiltration—it filters plasma based on size.

🔍 Filtrate in Bowman’s Capsule

The filtrate contains:

Water
Glucose
Ions (Na⁺, K⁺, Cl⁻)
Urea and other wastes
This fluid moves from Bowman’s capsule into the proximal convoluted tubule.

🧪 Reabsorption in the Proximal Convoluted Tubule

Most useful substances are actively and passively reabsorbed into blood:

Glucose (all of it, normally)
Ions (sodium, chloride, potassium)
Water (by osmosis)
Waste products like urea and toxins remain in the filtrate.
Reabsorption keeps body fluids balanced and prevents loss of valuable molecules.

📌 Summary of Filtration and Reabsorption

Process	What Happens	Purpose
Ultrafiltration	Blood plasma filtered through glomerulus into Bowman’s capsule	Removes plasma fluid and small solutes from blood
Reabsorption	Useful molecules reabsorbed in proximal tubule	Retains nutrients and balances ions and water
Excretion	Waste and excess substances remain in filtrate to be excreted	Removes toxins from body

🧠 Why These Steps Are Important

Prevents loss of valuable substances like glucose and ions.
Ensures removal of metabolic wastes and toxins.
Maintains fluid and chemical balance in the body.

🧠 Summary Box:
Glomerulus and Bowman’s capsule perform ultrafiltration to remove plasma fluid and small solutes.
Proximal convoluted tubule reabsorbs useful substances back into blood.
This process separates waste from useful molecules for excretion in urine.
Essential for maintaining body’s chemical balance and health.

D3.3.9 – Role of the Loop of Henle

🧠 What is the Loop of Henle?

A U-shaped part of the nephron tubule in the kidney.

Key role in concentrating urine and conserving water.

🌿 Structure of the Loop of Henle

Part	Feature	Function
Descending limb	Permeable to water, impermeable to salts	Water leaves by osmosis into surrounding tissue
Ascending limb	Impermeable to water, actively transports sodium ions out	Pumps out Na⁺ to create high osmotic concentration in medulla

🔬 How the Loop of Henle Works

Active transport of sodium ions (Na⁺) in the ascending limb pumps Na⁺ into the medulla.
This creates a high osmotic concentration in the surrounding medulla tissue.
The high osmolarity draws water out of the descending limb and collecting ducts by osmosis.
This mechanism allows kidneys to reabsorb water, producing concentrated urine and conserving water.

📍 Key Points

The active transport of Na⁺ is essential to maintain the osmotic gradient.
Water reabsorption happens where membranes are permeable (descending limb and collecting duct).
Helps in water balance and preventing dehydration.

🧠 Summary Box:
The loop of Henle establishes a high osmotic concentration in the medulla by pumping out sodium ions in the ascending limb.
This gradient allows water to be reabsorbed from the descending limb and collecting ducts.
Critical for concentrating urine and conserving body water.

D3.3.10 – Osmoregulation by Water Reabsorption in the Collecting Ducts

🧬 Key Concept: Osmoregulation via Water Reabsorption

The collecting ducts in the kidney control water reabsorption to maintain blood osmotic concentration.

This process is regulated by the hormone antidiuretic hormone (ADH).

🌿 Role of Osmoreceptors in the Hypothalamus

Osmoreceptors are specialized nerve cells in the hypothalamus.
They detect changes in blood osmotic concentration (how concentrated the blood is).
If blood becomes too concentrated (high osmolarity), osmoreceptors send signals to increase ADH secretion.
If blood is too dilute (low osmolarity), ADH secretion is reduced.

🔬 ADH Secretion by the Pituitary Gland

ADH is released from the posterior pituitary gland into the bloodstream.
The amount of ADH secreted depends on signals from osmoreceptors.
More ADH → more water reabsorbed; less ADH → less water reabsorbed.

🧪 Aquaporins and Water Permeability

Cells in the collecting ducts have aquaporin proteins that form water channels.
Without ADH, aquaporins are mostly stored inside cells in intracellular vesicles → low water permeability → dilute urine.
With ADH, aquaporins move to the cell membrane, increasing water permeability → more water reabsorbed → concentrated urine.

📍 How This Process Works

Condition	Osmoreceptor Response	ADH Level	Aquaporins	Urine	Blood Osmolarity
High blood osmolarity (dehydrated)	Stimulated	Increased	Inserted into membrane	Concentrated (less water)	Decreases toward normal
Low blood osmolarity (overhydrated)	Inhibited	Decreased	Removed from membrane	Dilute (more water)	Increases toward normal

🧠 Why This Mechanism Is Vital

Prevents dehydration by conserving water.
Prevents overhydration by excreting excess water.
Maintains stable osmotic concentration of blood for normal cell function.

🧠 Summary Box:
Osmoreceptors in the hypothalamus detect blood osmolarity changes and control ADH release.
ADH regulates water permeability of collecting duct cells by moving aquaporins to/from the membrane.
This system adjusts urine concentration to maintain water balance and osmotic homeostasis.

D3.3.11 – Changes in Blood Supply to Organs with Activity

🧬 Why Does Blood Supply Change?

Blood flow adjusts to meet the changing demands of different organs during various states (sleep, rest, exercise).
This ensures organs receive enough oxygen and nutrients and removes waste efficiently.

🌿 Patterns of Blood Supply to Key Organs

Organ	During Sleep	During Wakeful Rest	During Vigorous Physical Activity
Skeletal muscles	Low blood flow (low demand)	Moderate blood flow	High blood flow to support increased activity
Gut (digestive system)	Moderate blood flow	Moderate to high blood flow	Reduced blood flow as digestion slows
Brain	Maintains relatively constant blood flow	Maintains steady blood flow	Blood flow remains constant to support brain function
Kidneys	Moderate blood flow (normal filtration)	Moderate blood flow	Reduced blood flow as blood is prioritized elsewhere

🔍 Explanation of Changes

Skeletal muscles:
- At rest or sleep, muscles need less oxygen → less blood flow.
- During exercise, muscles require more oxygen → vasodilation increases blood flow.
Gut:
- Blood flow supports digestion, so it’s higher during eating and rest.
- During exercise, blood supply decreases as digestion slows and blood is redirected to muscles.
Brain:
- Brain blood flow is kept constant to maintain vital functions regardless of activity.
Kidneys:
- Blood flow supports filtration and waste removal.
- Reduced during exercise to prioritize muscles, but quickly returns to normal at rest.

📌 Summary Table: Blood Flow Changes

Organ	Blood Flow Change	Reason
Skeletal muscle	Increases with activity	More oxygen and nutrients needed
Gut	Decreases during exercise	Digestion slows, blood redirected
Brain	Remains constant	Continuous function required
Kidneys	Decreases during exercise	Blood prioritized for muscles