5.4Unit 5: Respiration, Internal Environment, Coordination and Gene Technology

Homeostasis — IAL Edexcel Linear Revision Notes & Topical Questions

10 min read 12 parts Lesson 5 of 36 Unit quiz

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Lesson summary

5.4 Homeostasis — IAL EDEXCEL LINEAR notes: Explain homeostasis and feedback, thermoregulation, the structure and function of the kidney in excretion and osmoregulation, and how hormones affect gene expression.Homeostasis = maintaining the internal environment within restricted l…

Temperature
Blood glucose
Negative feedback loop
Positive feedback
Vasodilation
Flattening of hairs

Part 1 of 10Principle and importance of homeostasis

Homeostasis

Homeostasis maintaining the internal environment within restricted limits, fluctuating around a normal level (dynamic equilibrium).

Receptors detect conditions inside and outside the body.

Temperature

Temperature above about 40 °C kinetic energy breaks the hydrogen bonds holding enzymes in shape,
so the active site changes and the enzyme–substrate complex cannot form.
Below about 35 °C reactions slow dangerously (hypothermia).
The core temperature set point is about 37 °C.

Blood glucose

Blood glucose cells need a steady supply of glucose for ATP, and pancreas cells monitor its concentration.

Water

Water it makes up cytoplasm and takes part in reactions, and the kidneys regulate the water content of the blood.

Part 2 of 10Negative and positive feedback

Negative feedback loop

Negative feedback loop a receptor detects a stimulus, a coordination system transfers information and an effector (muscle or gland) responds.
- (nervous or hormonal)
A rise in a factor produces a response that lowers it, and a fall produces a response that raises it.
There are usually two corrective mechanisms, one for too high and one for too low, and correction reduces as the factor nears normal.

Positive feedback

Positive feedback the response makes the factor deviate further from normal, enhancing the original stimulus.
Labourstretch receptors in the cervix signal the pituitary to release oxytocin, which strengthens contractions and stretches the cervix further.
Blood clottingactivated platelets release chemicals that activate more platelets, so the wound closes quickly before negative feedback resumes.
Hypothermiafalling temperature slows metabolism, which lowers temperature further, and this is not homeostasis.

Part 3 of 10Cooling mechanisms

Thermoregulation

Thermoregulation maintenance of a constant internal body temperature by cooling and warming mechanisms.

Vasodilation

Vasodilation muscles in the walls of arterioles near the skin relax, so more blood flows through skin capillaries and loses heat by radiation.
Vasodilation occurs in arterioles, not capillaries, which have no muscle.

Sweating

Sweating sweat glands secrete sweat, and evaporation takes heat from the skin.
Evaporating 1 g of sweat removes about 2.4 kJ.
Sweating works less well in humid air because of a smaller concentration gradient.

Flattening of hairs

Flattening of hairs erector pili muscles relax, so hairs lie flat, no insulating air layer forms and heat leaves by radiation.

Heat exchange

Heat exchange heat is gained or lost by radiation, conduction, convection and evaporation.

Part 4 of 10Warming mechanisms and the hypothalamus

Vasoconstriction

Vasoconstriction arteriole muscles contract,
- so less blood flows through skin capillaries and blood is diverted through deeper shunt vessels,
- reducing heat loss.

Metabolic rate

Metabolic rate exothermic reactions warm the body, and thyroxine from the thyroid raises basal metabolic rate, while adrenaline may also raise it.

Shivering

Shivering a reflex nervous response in which muscles contract rapidly and generate heat.

Erection of hairs

Erection of hairs erector pili muscles contract and trapped air insulates the skin.

Less sweating

Less sweating reduces heat lost by evaporation.
Vasoconstriction, hair erection and less sweating retain heat and do not raise blood temperature.

Hypothalamus

Hypothalamus it controls hormones, sleep, growth, body temperature and blood pressure.
Thermoreceptors in the skin and mucous membranes send impulses along sensory neurones to the hypothalamus.
The hypothalamus also monitors the temperature of blood flowing through it.
It sends impulses along motor neurones to effectors.

Calculations

Q₁₀ is rate at (T + 10) °C ÷ rate at T °C, and rate of change = change in value ÷ time.

Part 5 of 10Structure of the kidney

Structure of the kidney

Functions

Functions osmoregulation and excretion of urea and excess salts.
- (water content of the blood, which protects blood pressure and prevents osmotic damage)

Position

Position two kidneys are supplied by the renal artery, drained by the renal vein and connected to the bladder by the ureter.

Gross structure

Gross structure a fibrous capsule surrounds three regions.

Region	Contents
Cortex	Glomerulus, Bowman's capsule, proximal convoluted tubule, distal convoluted tubule
Medulla	Loop of Henle and collecting duct
Renal pelvis	Receives urine from all nephrons and connects to the ureter

Structure of the kidney

Nephron

Nephron thousands of tubules per kidney form the functional unit that makes urine.
Cortical nephrons have short loops of Henle and make up about 85% of nephrons.
Juxtamedullary nephrons have long loops spanning the medulla and conserve water efficiently.

Blood supply

Blood supply the afferent arteriole from the renal artery splits into the glomerulus capillaries, which rejoin as the efferent arteriole.
Blood then flows through capillaries around the nephron and into the renal vein.

Part 6 of 10Urea and ultrafiltration

Deamination

Deamination liver cells (hepatocytes) remove the amino group from excess amino acids, producing ammonia and a keto acid.
The keto acid can enter the Krebs cycle or be converted to glucose, glycogen or fat.

Ornithine cycle

Ornithine cycle toxic ammonia is converted to less toxic urea: ammonia + carbon dioxide → urea + water.

Ultrafiltration

Ultrafiltration blood pressure in the glomerulus is high because the afferent arteriole is wider than the efferent arteriole.
Small molecules are forced from the capillaries into the Bowman's capsule as glomerular filtrate.
Filtrate contains water, glucose, amino acids, urea and salts (Na⁺ and Cl⁻).
Blood cells, platelets and large proteins stay in the blood.

Filtration barrier

Filtration barrier two cell layers with a basement membrane between them separate blood from the capsule lumen.
Gaps between capillary endothelial cells let fluid through.
The mesh-like basement membrane stops large proteins.
Gaps between the epithelial cells of the Bowman's capsule let small molecules pass.

Part 7 of 10Selective reabsorption and the loop of Henle

Selective reabsorption and the loop of Henle

Selective reabsorption

Selective reabsorption useful substances are reabsorbed into the blood as filtrate passes along the nephron, mostly in the proximal convoluted tubule.
- (water, salts, glucose, amino acids)

Proximal tubule adaptations

Proximal tubule adaptations microvilli increase surface area, co-transporter proteins, many mitochondria for active transport and tightly packed cells.
Sodium ions are actively transported into surrounding tissue and chloride ions follow by diffusion.
Glucose and amino acids enter by co-transporter proteins that also carry sodium.
The lower water potential of the tissue draws water out by osmosis, and urea diffuses out.
All these substances pass into nearby capillaries.

Loop of Henle

Loop of Henle a countercurrent multiplier that makes urine more concentrated than blood and so conserves water.

Sodium and chloride ions leave the ascending limb into the medulla, by diffusion in the first part and active transport in the second part.

The ascending limb is impermeable to water, so water stays in the filtrate and its water potential rises.

The medulla has a low water potential, so water leaves the permeable descending limb by osmosis.

The descending limb has few transport proteins, so filtrate loses water and its water potential falls.

Water and ions pass into the capillary network.

Part 8 of 10Osmoregulation and ADH

Osmoregulation and ADH

Osmoreceptors

Osmoreceptors they are in the hypothalamus and monitor the water potential of the blood.

Low blood water potential (dehydration from low intake, sweating or diarrhoea) — the posterior pituitary releases antidiuretic hormone (ADH).

ADH reaches the distal convoluted tubule and collecting duct.
It binds to receptors and triggers a cascade that moves vesicles carrying aquaporins to the luminal membrane.
More aquaporins make the walls more permeable to water, so more water is reabsorbed.
A small volume of concentrated urine is produced and blood water content rises.

High blood water potential

High blood water potential the hypothalamus no longer stimulates ADH release, so ADH falls.
Fewer aquaporins make the walls less permeable, so less water is reabsorbed.
A large volume of dilute urine is produced and blood water content falls.

Part 9 of 10Hormones and gene expression

Hormones and gene expression

Transcription factors

Transcription factors about 10% of human genes code for these proteins that bind DNA to switch genes on (activators) or off (repressors).

Steroid and thyroid hormones

Steroid and thyroid hormones they are lipid soluble, cross the cell surface membrane and bind transcription factors in the nucleus.
At normal temperature the thyroid hormone receptor bound to DNA reduces expression of a gene for a protein that raises metabolic rate.
In the cold, thyroxine binds the receptor, which lets RNA polymerase bind, so the gene is switched on and more heat is generated.

Peptide hormones

Peptide hormones adrenaline, insulin, glucagon and ADH cannot cross the membrane and bind to cell surface receptors.
Binding activates second messengers such as cyclic AMP (cAMP), formed from ATP.
The messengers activate protein kinases, which start a cascade that may change transcription factor activity and gene expression.

Part 10 of 10More on this topic

Positive feedback

Positive feedback it amplifies a change in special processes but is not the usual way of keeping a stable internal environment.

Confusing vasoconstriction and vasodilation — dilation widens skin arterioles to lose heat and constriction narrows them to keep heat.

Saying positive feedback maintains a stable internal environment — negative feedback does.

Key terms

Term	Meaning
Homeostasis	Maintenance of the internal environment within restricted limits.
Negative feedback	A response that reverses a change and returns a factor to normal.
Thermoregulation	Maintenance of a constant internal body temperature.
Nephron	The functional unit of the kidney that forms urine.
Ultrafiltration	Filtering of small molecules from blood into the Bowman's capsule under high pressure.
Antidiuretic hormone (ADH)	A hormone that increases water reabsorption in the collecting duct.
Aquaporin	A water channel protein in the membrane.
Second messenger	A molecule such as cAMP that relays a hormone signal inside a cell.

Avoid these mistakes

Saying vasoconstriction occurs in capillaries — it occurs in arterioles supplying the skin capillaries.

Saying muscles constrict or arterioles contract — muscles contract and arterioles constrict.

Saying the kidney filters by active transport — ultrafiltration is driven by high blood pressure.

Saying peptide hormones enter the nucleus — they act through cell surface receptors and second messengers.

Last updated: 18 October 2026