Everything

Homeostasis

What is homeostasis?

Homeostasis is the maintenance of a relative constant internal environment despite changes in the internal or external environment.

The internal environment includes the condition surrounding cells, particularly the blood and tissue fluid.

Homeostasis is important because cells and enzymes require particularly conditions to function effectively.

Body doesn't keep variables at exactly one value. Instead, they fluctuate around an optimum/set point within acceptable limits.

Variables controlled by homeostasis

• Core body temperature

• Blood glucose concentration

• Blood water potential

• Blood ion concentrations

• Carbon dioxide concentration

• Blood pH

Why homeostasis is important

Change in internal conditions can affect the functions of cells.

Example (temperature):

• Enzymes have an optimum temperature

• If temperature decreases:

  • Molecules have less kinetic energy

  • There are fewer successful collisions between enzyme and substrate

  • Enzyme-controlled reactions occur more slowly

• If temperature becomes too high:

  • Bonds maintaining protein structure can be disrupted

  • The enzyme’s tertiary structure changes

  • The active site changes shape

  • Substrate may no longer bind

  • The enzyme becomes denatured

• Therefore, maintaining a suitable temperature allows metabolic reactions to occur at appropriate rates.

Negative Feedback

Most homeostatic mechanisms involve negative feedback.

Definition

Negative feedback occurs when a change in a variable produces a response that reverses or counteracts the original change.

General pathway

Stimulus/change

↓

Receptor detects change

↓

Coordination centre

↓

Effector

↓

Response

↓

Response opposes original change

↓

Variable returns towards normal

Example

Body temperature increases

→ thermoreceptors detect increase

→ hypothalamus coordinates response

→ sweating + vasodilation

→ increased heat loss

→ temperature decreases towards normal.

Receptors, Coordination Centers & Effectors

Stimulus → Receptor → Coordination center → Effector → Response

Receptors

Receptors detect changes in the internal or external environment.

Examples:

• Thermoreceptors → Temperature

• Osmoreceptors → water potential

• Chemoreceptors → chemicals

Coordination center

The coordination center receives information from Receptors and coordinates an appropriate response.

Examples:

• Hypothalamus

• Brain

• Pancreas

Effectors

Effectors carry out the response.

Examples:

• Muscles

• Glands

• Liver cells

• Kidney cells

Nervous & Endocrine Control

Homeostasis involves communication between different parts of the body.

Nervous system

Uses electrical impulses along neurones.

Advantages

• Very rapid

• Precise

• Short-lived

• Specific target

Endocrine system

Uses hormones released into the blood.

Advantages

• Can affect many target cells

• Effects can last longer

• Useful for longer-term regulation

Comparison

The hypothalamus is particularly important because it links nervous and endocrine control.

Temperature Control

Humans are endotherms.

This means they generate significant amounts of heat internally through metabolic reactions and can regulate their core temperature.

Core temperature is maintained around approximately 37°C, although it naturally fluctuates.

Endotherms

• Generate heat internally through metabolic reactions.

• Examples:

  • Humans

  • Mammals

  • Birds

• They use physiological mechanisms to regulate body temperature.

• Advantages

  • Can remain active over a wide range of environmental temperatures

  • Enzyme activity can remain close to optimum

  • Can inhibt colder climates

• Disadvantage

  • Maintaining body temperature requires a lot of energy

  • Need loads of food

  • May have slow growth

Ectotherms

• Their body temperature is strongly influenced by the environment

• Examples

  • Reptiles

  • Amphibians

  • Many fish

  • Many invertebrates

• They rely heavily on behavioural responses

• Advantages

  • Need less food as less energy is used

  • Can survive in difficult habitats where food is in short supply

• Disadvantages

  • More vulnerable to predators when cold

  • Less active

the original change.

General pathway

Stimulus/change

↓

Receptor detects change

↓

Coordination centre

↓

Effector

↓

Response

↓

Response opposes original change

↓

Variable returns towards normal

Example

Body temperature increases

→ thermoreceptors detect increase

→ hypothalamus coordinates response

→ sweating + vasodilation

→ increased heat loss

→ temperature decreases towards normal.

Receptors

Receptors detect changes in the internal or external environment.

Examples:

• Thermoreceptors → temperature

• Osmoreceptors → water potential

• Chemoreceptors → chemicals

Coordination centre

The coordination centre receives information from receptors and coordinates an appropriate response.

Examples:

• Hypothalamus

• Brain

• Pancreas

Effectors

Effectors carry out the response.

Examples:

• Muscles

• Glands

• Liver cells

• Kidney cells

⭐ Exam chain

Stimulus → receptor → coordination centre → effector → response

• longer-term regulation

Comparison

The hypothalamus is particularly important because it links nervous and endocrine control.

Thermoreceptors

Thermoreceptors detect changes in temperature.

Peripheral thermoreceptors

Found mainly in the skin.

They detect changes in the temperature of the external environment/skin.

Central thermoreceptors

Associated with the hypothalamus.

They detect changes in the temperature of the blood and therefore provide information about core temperature.

Remember:

Peripheral → skin

Central → internal/core temperature

Hypothalamus

The hypothalamus is an important coordination centre for thermoregulation.

It receives information about temperature and coordinates responses.

If temperature is too high:

→ mechanisms increase heat loss.

If temperature is too low:

→ mechanisms reduce heat loss and increase heat production.

Body temperature is too high

When core temperature rises above the normal range:

Main responses:

1. Vasodilation

2. Increased sweating

3. Reduced heat production

4. Behavioural responses

VASODILATION

Vasodilation = widening of blood vessels supplying the skin.

Mechanism

Temperature rises

↓

Hypothalamus detects change

↓

Arterioles supplying skin dilate

↓

More blood flows through capillaries near the skin

↓

More thermal energy is transferred from the blood to the surroundings

↓

Heat loss increases

↓

Temperature decreases towards normal.

• Vasodilation increases blood flow near the skin surface, increasing heat transfer from the blood to the surroundings.

Sweating

Sweat is released by sweat glands onto the skin.

The sweat contains water.

The water then evaporates.

Evaporation requires energy.

This energy is taken from the skin.

Therefore:

Sweating → evaporation → energy loss from skin → cooling

It is primarily the evaporation, not simply the presence of sweat, that causes cooling.

Body Temp too low

When core temperature falls:

The body needs to:

• Reduce heat loss

and

• Increase heat production

Responses include:

• Vasoconstriction

• Shivering

• Increased metabolic rate

• Piloerection

• Behavioural responses

VASOCONSTRICTION

Vasoconstriction = narrowing of blood vessels supplying the skin.

Temperature falls

↓

Arterioles constrict

↓

Less blood flows near the skin surface

↓

Less thermal energy transferred to surroundings

↓

Heat loss decreases

SHIVERING

Shivering consists of rapid, involuntary contractions of skeletal muscles.

Muscle contraction requires ATP.

ATP is produced through respiration.

Some energy released during respiration is dissipated as heat.

Therefore:

Shivering → increased muscle activity → increased respiration → increased heat production

PILOERECTION

Hair erector muscles contract.

This causes hairs to become more upright.

The hairs can trap a layer of air close to the skin.

Air is a poor conductor of heat.

Therefore this can reduce heat loss.

This response is much less significant in humans because humans have relatively little body hair.

INCREASED METABOLIC RATE

Increasing metabolic activity increases heat production.

For example, increased activity of skeletal muscles increases respiration and therefore heat production.

Hormones can also influence metabolic rate.

BEHAVIOURAL RESPONSES

Behaviour can also help thermoregulation.

When cold:

• Put on more clothing.

• Move into a warmer environment.

• Increase physical activity.

• Curl up/reduce exposed surface area.

When hot:

• Remove clothing.

• Move into shade.

• Reduce physical activity.

• Drink water.

• Increase exposure to moving air.

Endotherm vs ectotherm

Behavioural responses are particularly important in ectotherms.

COMPLETE THERMOREGULATION

Temperature too high

↑ Core temperature

↓

Thermoreceptors

↓

Hypothalamus

↓

Effectors

↓

Vasodilation + sweating

↓

↑ heat loss

↓

Temperature returns towards normal.

Temperature too low

↓ Core temperature

↓

Thermoreceptors

↓

Hypothalamus

↓

Effectors

↓

Vasoconstriction + shivering

↓

↓ heat loss + ↑ heat production

↓

Temperature returns towards normal.

THE LIVER

The liver is an important organ involved in:

• Metabolism

• Homeostasis

• Storage

• Detoxification

• Excretion

FUNCTIONS OF THE LIVER

Carbohydrate metabolism

The liver:

• Converts glucose to glycogen.

• Breaks down glycogen.

• Can produce glucose from other compounds.

Protein metabolism

The liver:

• Deaminates excess amino acids.

• Converts ammonia into urea.

Detoxification

The liver metabolises potentially harmful substances.

Storage

The liver stores:

• Glycogen

• Certain vitamins

• Iron and other minerals

Other functions

• Produces bile.

• Produces plasma proteins.

• Processes absorbed nutrients.

LIVER STRUCTURE

The liver is divided into lobes and contains many functional units called lobules.

Hepatocytes

These are the main functional cells of the liver.

They carry out many of the liver's metabolic functions.

Liver lobules

A lobule consists of plates/cords of hepatocytes arranged around a central vein.

Between the hepatocytes are sinusoids.

BLOOD SUPPLY TO THE LIVER

BLOOD GLUCOSE HOMEOSTASIS

Blood glucose concentration needs to be controlled.

Glucose is required for:

• Respiration

• ATP production

Too much glucose can cause problems, while too little glucose means cells may not have enough substrate for respiration.

The pancreas detects changes in blood glucose concentration and releases hormones.

The liver is a major target organ.

26. GLYCOGENESIS

Glycogenesis = conversion of glucose into glycogen.

When blood glucose is too high:

Glucose

↓

Converted to

Glycogen

↓

Stored mainly in the liver and muscles.

This helps reduce blood glucose concentration.

27. GLYCOGENOLYSIS

Glycogenolysis = breakdown of glycogen.

When blood glucose concentration is too low:

Glycogen

↓

Broken down

↓

Glucose released

↓

Blood glucose concentration increases.

28. GLUCONEOGENESIS

Gluconeogenesis is the production of glucose from non-carbohydrate sources.

This can occur when blood glucose is low and glycogen stores are insufficient.

Examples of substrates include:

• Glycerol

• Certain amino-acid-derived compounds

The liver is an important site of gluconeogenesis.

🧪 29. AMINO ACIDS AND NITROGENOUS WASTE

Proteins are digested into amino acids.

Amino acids are used to:

• Make proteins.

• Make other nitrogen-containing compounds.

However, excess amino acids cannot be stored in the same way that glucose can be stored as glycogen.

Therefore, excess amino acids are broken down.

This produces nitrogenous waste.

30. DEAMINATION

Deamination is the removal of the amino group (-NH₂) from an amino acid.

It occurs in the liver.

Simplified process

Amino acid

↓

Amino group removed

↓

Ammonia produced

Carbon-containing compound

The carbon-containing part can be used in metabolism or converted into other compounds.

☠ 31. AMMONIA

Ammonia is highly toxic.

It can interfere with cell metabolism and is particularly dangerous at high concentrations.

Therefore, ammonia needs to be converted into a less toxic compound.

In mammals, the liver converts ammonia into urea.

32. UREA

Urea is much less toxic than ammonia.

It is produced in the liver and transported in the blood to the kidneys.

Overall pathway

Excess amino acids

↓

Deamination in liver

↓

Ammonia

↓

Urea formed in liver

↓

Blood

↓

Kidney

↓

Urine

↓

Excretion

⭐ OCR point

You need to understand the formation of urea, but you do not need to memorise every individual reaction of the ornithine cycle.

☠ 33. DETOXIFICATION

The liver detoxifies potentially harmful substances.

For example:

• Alcohol

• Some drugs

• Other toxic compounds

Hepatocytes contain enzymes involved in modifying these substances.

The products can then be further metabolised or excreted.

Key distinction

Detoxification = making harmful substances less harmful.

Excretion = removing metabolic waste from the body.

🚽 34. EXCRETION

Definition

Excretion is the removal of metabolic waste products from the body.

Examples:

Carbon dioxide

Produced during respiration.

→ removed by lungs.

Urea

Produced from excess amino acids.

→ removed by kidneys.

Excess water and ions

→ removed mainly by kidneys.

Don't confuse:

Excretion ≠ egestion

Egestion is the removal of undigested food from the digestive tract.

THE KIDNEY

The kidneys are major organs of homeostasis.

They regulate:

• Blood water potential

• Ion concentrations

• Removal of nitrogenous waste

• Blood composition

They produce urine, which contains substances that the body needs to remove.

KIDNEY STRUCTURE

A kidney can be divided into:

Cortex

The outer region.

Contains:

• Glomeruli

• Bowman's capsules

• Convoluted tubules

Medulla

The inner region.

Contains:

• Loops of Henle

• Collecting ducts

Renal pelvis

The central cavity that collects urine before it enters the ureter.

Ureter

Carries urine from the kidney to the bladder.

Urethra

Carries urine from the bladder to outside the body.

BLOOD SUPPLY TO THE KIDNEY

Blood enters the kidney through the:

Renal artery

Blood leaves through the:

Renal vein

The renal artery branches into smaller vessels supplying nephrons.

THE NEPHRON

The nephron is the functional unit of the kidney.

The nephron is involved in:

• Filtration

• Selective reabsorption

• Water regulation

• Ion regulation

• Urine formation

Main regions

1. Bowman's capsule

2. Glomerulus

3. Proximal convoluted tubule

4. Loop of Henle

5. Distal convoluted tubule

6. Collecting duct

NEPHRON BLOOD SUPPLY

Blood flows:

Renal artery

↓

Afferent arteriole

↓

Glomerulus

↓

Efferent arteriole

↓

Capillary network around nephron

↓

Renal vein

Important terms

Afferent = into the glomerulus

Efferent = away from the glomerulus

The efferent arteriole has a smaller diameter than the afferent arteriole.

This contributes to the high pressure in the glomerulus.

GLOMERULUS & BOWMAN'S CAPSULE

The glomerulus is a network of capillaries inside Bowman's capsule.

It is where ultrafiltration occurs.

The high pressure forces small molecules from the blood into Bowman's capsule.

ULTRAFILTRATION

Definition

Ultrafiltration is the filtration of small molecules from the blood into Bowman's capsule due to high hydrostatic pressure.

Why is pressure high?

Two important factors:

1. Afferent arteriole is wider

Blood enters through a relatively wide arteriole.

2. Efferent arteriole is narrower

Blood leaves through a narrower arteriole.

Therefore:

Resistance to outflow → high hydrostatic pressure in glomerulus

THE FILTRATION BARRIER

The filtration barrier prevents large substances from entering the filtrate.

It includes:

1. Fenestrated endothelium

Capillary endothelial cells contain pores.

Allows small molecules through.

2. Basement membrane

Acts as a filtration barrier.

Restricts larger molecules, especially proteins.

3. Podocytes

Specialised cells around the capillaries.

They have filtration slits between their extensions.

Together these structures allow small molecules through while retaining cells and most plasma proteins.

WHAT IS FILTERED?

Small substances can pass into Bowman's capsule.

Examples:

• Water

• Glucose

• Amino acids

• Urea

• Mineral ions

What is NOT normally filtered?

• Red blood cells

• Most plasma proteins

• Other large molecules

⭐ Important

The filtrate initially contains many substances that the body still needs.

That's why selective reabsorption is necessary.

ULTRAFILTRATION — STEP BY STEP

High blood pressure in glomerulus

↓

Forces plasma out of capillaries

↓

Through filtration barrier

↓

Into Bowman's capsule

↓

Filtrate enters proximal convoluted tubule

The filtrate contains water and small dissolved substances.

SELECTIVE REABSORPTION

The body cannot afford to lose all the useful substances filtered into the nephron.

Selective reabsorption returns useful substances from the filtrate back into the blood.

Most selective reabsorption occurs in the proximal convoluted tubule (PCT).

Substances reabsorbed include:

• All/virtually all glucose

• Amino acids

• Many ions

• Water

PROXIMAL CONVOLUTED TUBULE

The PCT is adapted for efficient reabsorption.

Adaptations

Microvilli

→ large surface area

Many mitochondria

→ ATP for active transport

Thin epithelial cells

→ short diffusion distance

Close association with blood capillaries

→ efficient transport into blood

GLUCOSE REABSORPTION

Normally, essentially all glucose is reabsorbed from the filtrate.

Process

Sodium ions are actively transported out of PCT cells.

This creates a sodium ion gradient.

Sodium ions then move back into PCT cells from the filtrate through co-transport proteins.

Glucose is transported into the cell at the same time.

Glucose then moves from the PCT cells into the blood.

Simplified:

Na⁺ pumped out of PCT cell

↓

Na⁺ concentration gradient created

↓

Na⁺ moves from filtrate into cell

↓

Glucose co-transported into cell

↓

Glucose moves into blood

48. ROLE OF ATP

Active transport requires energy.

ATP is required for the sodium-potassium ion pump in the PCT cell membrane.

The pump actively transports sodium ions out of the cell.

This maintains the sodium gradient needed for sodium-glucose co-transport.

Therefore:

Respiration → ATP → active transport → sodium gradient → glucose reabsorption

49. WATER REABSORPTION

Water is reabsorbed by osmosis.

Solute reabsorption changes the water potential of different parts of the nephron and surrounding tissue.

Water moves:

From higher water potential

→ to lower water potential

through partially permeable membranes.

Water can then enter surrounding capillaries.

50. WHY SELECTIVE REABSORPTION IS IMPORTANT

Without selective reabsorption:

Useful substances would be lost in urine.

For example, losing large quantities of:

• Glucose

• Amino acids

• Water

• Ions

would seriously disrupt homeostasis.

Selective reabsorption ensures that useful substances are returned to the blood.

🌀 51. LOOP OF HENLE

The Loop of Henle is a U-shaped part of the nephron.

It is essential in establishing a water potential gradient in the medulla.

This gradient allows the kidney to produce concentrated urine when necessary.

The loop has two main limbs:

Descending limb

• Permeable to water

• Relatively less permeable to ions

Ascending limb

• Impermeable/relatively impermeable to water

• Allows ions to leave

• Active transport is involved, particularly in the thicker ascending limb

52. DESCENDING LIMB

As filtrate moves down the descending limb:

The surrounding medulla has a lower water potential than the filtrate.

Therefore:

Water moves out of the descending limb by osmosis.

Water enters the surrounding tissue and is then taken up by blood vessels.

Result:

The filtrate becomes more concentrated as it moves down.

⭐ Remember

Descending limb = water leaves

53. ASCENDING LIMB

The ascending limb behaves differently.

It is not permeable to water.

Therefore water cannot follow the ions out of the tubule.

Ions are moved out of the filtrate.

In the thick ascending limb, active transport contributes to the movement of ions such as:

• Na⁺

• Cl⁻

out of the tubule.

Result:

The surrounding medulla becomes more concentrated.

The filtrate becomes more dilute as it travels upwards.

⭐ Remember

Ascending limb = ions leave, water doesn't

54. COUNTERCURRENT MECHANISM

The descending and ascending limbs contain fluid flowing in opposite directions.

This is called countercurrent flow.

The movement of ions out of the ascending limb creates a high solute concentration in the surrounding medulla.

Water then moves out of the descending limb.

As this occurs repeatedly along the loop, a concentration gradient is established.

Overall:

Ascending limb

→ ions leave

→ medulla becomes more concentrated.

Descending limb

→ water leaves by osmosis

→ filtrate becomes more concentrated.

This helps establish the water potential gradient through the medulla.

55. LOOP OF HENLE — THE BIG PICTURE

Starting at the PCT:

Filtrate enters the Loop of Henle.

Descending limb:

Water leaves by osmosis

↓

Filtrate becomes more concentrated.

Ascending limb:

Na⁺ and Cl⁻ leave

↓

Water cannot follow.

↓

Surrounding medulla becomes more concentrated.

Result:

A water potential gradient is established from the cortex towards the inner medulla.

This gradient becomes important later when water reabsorption is regulated by ADH in the collecting tube.

KEY TERMS