Human Body Systems

This topic covers the major organ systems of the human body: the circulatory, respiratory, Digestive, nervous, endocrine, immune, and excretory systems.

The Circulatory System

Structure (OL/HL)

The human circulatory system is a closed double circulatory system:

Pulmonary circulation: heart to lungs and back (deoxygenated blood to lungs, oxygenated blood returns).
Systemic circulation: heart to body and back (oxygenated blood to body, deoxygenated blood returns).

The Heart (OL/HL)

The heart has four chambers:

Right atrium: receives deoxygenated blood from the body (via vena cava).
Right ventricle: pumps deoxygenated blood to the lungs (via pulmonary artery).
Left atrium: receives oxygenated blood from the lungs (via pulmonary vein).
Left ventricle: pumps oxygenated blood to the body (via aorta).

Valves: prevent backflow of blood.

Bicuspid (mitral) valve: between left atrium and left ventricle.
Tricuspid valve: between right atrium and right ventricle.
Semilunar valves: in the aorta and pulmonary artery.

Worked Example: Tracing blood flow through the heart.

Deoxygenated blood enters the right atrium from the vena cava. The tricuspid valve opens and blood Flows into the right ventricle. The right ventricle contracts, the tricuspid valve closes, and the Semilunar valve in the pulmonary artery opens. Blood flows to the lungs.

Oxygenated blood returns from the lungs via the pulmonary vein into the left atrium. The bicuspid Valve opens and blood flows into the left ventricle. The left ventricle contracts, the bicuspid Valve closes, and the semilunar valve in the aorta opens. Blood flows to the body.

Blood Vessels (OL/HL)

Vessel	Structure	Function
Artery	Thick walls, narrow lumen, elastic	Carry blood away from heart at high pressure
Vein	Thin walls, wide lumen, valves	Return blood to the heart
Capillary	One cell thick walls	Exchange of materials with tissues

Blood Composition (OL/HL)

Plasma: liquid component (water, dissolved substances, proteins).
Red blood cells (erythrocytes): transport oxygen (contain haemoglobin). No nucleus. Biconcave disc shape.
White blood cells (leucocytes): part of the immune system (phagocytes, lymphocytes).
Platelets (thrombocytes): blood clotting.

Worked Example: Adaptations of red blood cells.

Biconcave disc shape: Increases the surface area to volume ratio for efficient gas exchange.
No nucleus: Creates more space for haemoglobin.
Flexible membrane: Allows the cell to squeeze through narrow capillaries.
Contains haemoglobin: A red pigment that binds reversibly to oxygen in the lungs and releases it in the tissues.

Cardiac Cycle (HL)

The sequence of events in one heartbeat:

Atrial systole: atria contract, pushing blood into ventricles.
Ventricular systole: ventricles contract, pushing blood into arteries.
Diastole: all chambers relax; heart fills with blood.

Cardiac output (HL):

$\mathrm{Cardiac output = \mathrm{stroke volume \times \mathrm{heart rate$

Blood Pressure (OL/HL)

Measured in mmHg. Normal: approximately 120/80.

Systolic pressure: pressure during ventricular contraction.
Diastolic pressure: pressure during relaxation.

Hypertension (high blood pressure) increases the risk of heart disease and stroke.

The Respiratory System

Structure (OL/HL)

Air path: nose $\to$ trachea $\to$ bronchi $\to$ bronchioles $\to$ alveoli.

Gas Exchange (OL/HL)

Occurs in the alveoli by diffusion.

Features of alveoli for efficient gas exchange:

Large surface area.
Thin walls (one cell thick).
Surrounded by many capillaries.
Moist surface for gas dissolution.

Oxygen diffuses from alveoli into blood (high to low concentration).

Carbon dioxide diffuses from blood into alveoli.

Breathing Mechanism (OL/HL)

Inspiration: intercostal muscles and diaphragm contract; thorax volume increases; lung pressure Decreases; air rushes in.

Expiration: intercostal muscles and diaphragm relax; thorax volume decreases; lung pressure Increases; air is pushed out.

Worked Example: Fick’s Law and gas exchange.

$\mathrm{Rate of diffusion \propto \frac{\mathrm{Surface area \times \mathrm{Concentration difference}{\mathrm{Diffusion distance}$

The alveoli are adapted to maximise the rate of diffusion:

Large surface area: Millions of alveoli provide about 70 m $^2$ of surface area.
Steep concentration gradient: Breathing constantly refreshes the air in the alveoli, maintaining a high concentration of oxygen and a low concentration of carbon dioxide. Blood flow also maintains the gradient by carrying oxygenated blood away.
Short diffusion distance: The alveolar wall and capillary wall together are only about 1 micrometre thick.

The Digestive System

Structure (OL/HL)

Mouth $\to$ oesophagus $\to$ stomach $\to$ small intestine (duodenum, ileum) $\to$ large intestine $\to$ rectum $\to$ anus.

Accessory organs: liver, pancreas, gall bladder.

Digestion (OL/HL)

Mechanical digestion: chewing (mouth), churning (stomach), peristalsis.

Chemical digestion: breakdown by enzymes.

Location	Enzyme	Substrate	Product
Mouth	Amylase	Starch	Maltose
Stomach	Pepsin	Proteins	Peptides
Duodenum	Trypsin	Proteins	Amino acids
Duodenum	Lipase	Lipids	Fatty acids + glycerol
Ileum	Maltase	Maltose	Glucose

Absorption (OL/HL)

Occurs in the small intestine (ileum).
Villi increase surface area.
Features of villi: large surface area, thin walls, dense capillary network, lacteal (for fat absorption).
Glucose and amino acids absorbed into blood capillaries.
Fatty acids and glycerol absorbed into lacteals (lymphatic system).

The Liver (OL/HL)

Functions:

Produces bile (emulsifies fats).
Detoxification (breaks down alcohol, drugs).
Storage of glycogen, iron, vitamins.
Deamination of excess amino acids.
Regulation of blood glucose levels.

The Nervous System (OL/HL)

Neurons

Sensory neurons: carry impulses from receptors to the CNS.
Motor neurons: carry impulses from the CNS to effectors.
Relay neurons: connect sensory and motor neurons within the CNS.

Structure of a Motor Neuron

Cell body with nucleus.
Dendrites (receive impulses).
Long axon (transmits impulses).
Myelin sheath (insulates axon, speeds up transmission).
Synaptic terminals (at the ends of axons).

Synapse (OL/HL)

The junction between two neurons.

Electrical impulse arrives at the synaptic terminal.
Neurotransmitter vesicles fuse with the membrane.
Neurotransmitter is released into the synaptic cleft.
Neurotransmitter binds to receptors on the post-synaptic membrane.
A new electrical impulse is generated in the next neuron.

Reflex Arc (OL/HL)

An involuntary, rapid response to a stimulus.

Pathway: receptor $\to$ sensory neuron $\to$ relay neuron $\to$ motor neuron $\to$ effector (muscle/gland).

Example: touching a hot object:

Heat receptors detect the stimulus.
Sensory neuron transmits impulse to spinal cord.
Relay neuron connects to motor neuron.
Motor neuron stimulates effector (arm muscle to withdraw hand).

The Endocrine System (HL)

Hormones

Chemical messengers secreted by endocrine glands into the bloodstream, targeting specific organs.

Gland	Hormone	Function
Pituitary	ADH	Regulates water reabsorption in kidneys
Pituitary	FSH/LH	Control reproductive system
Thyroid	Thyroxine	Regulates metabolism
Pancreas	Insulin	Lowers blood glucose
Pancreas	Glucagon	Raises blood glucose
Adrenal	Adrenaline	”Fight or flight” response
Ovary	Oestrogen	Female secondary sexual characteristics
Testis	Testosterone	Male secondary sexual characteristics

Blood Glucose Regulation (HL)

High blood glucose: pancreas secretes insulin.

Liver and muscle cells take up glucose.
Glucose is converted to glycogen (glycogenesis).

Low blood glucose: pancreas secretes glucagon.

Glycogen is converted back to glucose (glycogenolysis).
Fat is broken down (lipolysis).

Negative Feedback (HL)

Hormone levels are regulated by negative feedback. When the level rises above the set point, Mechanisms reduce it. When it falls below, mechanisms increase it.

Worked Example: Negative feedback in blood glucose regulation.

Blood glucose rises after a meal.
Beta cells in the pancreas detect the increase.
Beta cells secrete insulin.
Insulin stimulates cells to take up glucose and the liver to convert glucose to glycogen.
Blood glucose falls.
When blood glucose drops below the set point, alpha cells detect the decrease.
Alpha cells secrete glucagon.
Glucagon stimulates the liver to convert glycogen back to glucose.
Blood glucose rises back to the set point.

The Immune System (OL/HL)

Non-Specific Defences

Skin: physical barrier.
Mucus membranes: trap pathogens.
Stomach acid: destroys pathogens.
Phagocytes: engulf and digest pathogens (phagocytosis).

Specific Defences (HL)

Lymphocytes:

B cells: produce antibodies. Each B cell produces one type of antibody specific to one antigen.
T cells: destroy infected cells (T killer cells) and coordinate the immune response (T helper cells).

Antibodies:

Y-shaped proteins.
Bind to specific antigens on pathogens.
Mark pathogens for destruction by phagocytes.
Cause agglutination (clumping) of pathogens.

Memory cells: remain after infection, providing long-term immunity.

Vaccination (OL/HL)

Introduces a harmless form of the pathogen (or antigen).
Stimulates the production of antibodies and memory cells.
Provides active artificial immunity.

Worked Example: How vaccination provides immunity.

A vaccine contains a harmless form of the pathogen (e.g., dead or weakened pathogens, or isolated antigens).
The antigens are introduced into the body.
Macrophages present the antigens to T helper cells and B cells.
B cells produce antibodies specific to the antigen.
Some B cells and T cells become memory cells.
If the real pathogen enters the body in the future, memory cells recognise the antigen immediately.
A rapid secondary immune response is triggered, producing large quantities of antibodies.
The pathogen is destroyed before it can cause disease.

The Excretory System (OL/HL)

Kidneys (OL/HL)

Functions: remove urea, regulate water balance, regulate ion concentrations.

Structure: cortex (contains glomeruli and Bowman’s capsules), medulla (contains loops of Henle And collecting ducts).

Urine Formation (HL)

Ultrafiltration: in the glomerulus, high blood pressure forces small molecules (water, glucose, urea, ions) into the Bowman’s capsule. Large molecules (proteins, blood cells) remain.
Selective reabsorption: in the proximal convoluted tubule, all glucose, amino acids, and most water and ions are reabsorbed back into the blood.
Water regulation: in the loop of Henle and collecting duct, water is reabsorbed under the control of ADH (antidiuretic hormone).

ADH and Water Balance (HL)

High blood osmolarity (dehydration): more ADH is released $\to$ more water reabsorbed $\to$ concentrated urine.
Low blood osmolarity (overhydration): less ADH released $\to$ less water reabsorbed $\to$ dilute urine.

Worked Examples

See the examples integrated throughout the sections above.

Common Pitfalls

Arteries carry oxygenated blood — except the pulmonary artery (carries deoxygenated).
Veins carry deoxygenated blood — except the pulmonary vein (carries oxygenated).
Oxygen diffuses from alveoli to blood — down the concentration gradient.
Insulin lowers blood glucose — glucagon raises it.
Antibodies are produced by B cells — not T cells.
Ultrafiltration vs reabsorption — know what is filtered and what is reabsorbed in the kidney.
Confusing systole and diastole — systole is contraction; diastole is relaxation.
Forgetting that the heart is myogenic — it generates its own electrical impulses via the SAN (sinoatrial node).

Practice Questions

Ordinary Level

Describe the path of blood through the heart, naming all four chambers and the major blood vessels.
Explain how gas exchange occurs in the alveoli.
Name three enzymes involved in digestion, their substrates, and products.
Describe the reflex arc for withdrawing a hand from a hot object.

Higher Level

Explain how insulin and glucagon regulate blood glucose levels, including negative feedback.
Describe the roles of B cells and T cells in the immune response.
Explain the process of urine formation in the kidney, including ultrafiltration, selective reabsorption, and the role of ADH.
Describe the structure of a synapse and explain how a nerve impulse is transmitted across it.
Explain the cardiac cycle, including the roles of the valves and the electrical control of the heartbeat.
A person has a blood pressure of 160/100 mmHg. Explain why this is classified as hypertension and describe the associated health risks.
Describe the process of phagocytosis and explain how it provides non-specific defence against pathogens.
Explain why a person with type 1 diabetes must inject insulin rather than taking it orally.
Compare the structure and function of arteries, veins, and capillaries.
Describe the role of the liver in digestion, including bile production and deamination.

Review: The Excretory System in Detail

The kidney is the primary organ of excretion in humans. It filters the blood, removes waste Products, and regulates water and ion balance.

Structure of the kidney:

Cortex: Outer region containing glomeruli and convoluted tubules.
Medulla: Inner region containing the loops of Henle and collecting ducts.
Renal pelvis: Funnel-shaped area where urine collects before flowing into the ureter.
Ureter: Carries urine from the kidney to the bladder.
Bladder: Stores urine until it is expelled through the urethra.
Urethra: Tube through which urine is expelled from the body.

Nephron structure and function:

The nephron is the functional unit of the kidney. Each kidney contains approximately 1 million Nephrons.

Bowman’s capsule and glomerulus (ultrafiltration): Blood enters the glomerulus (a knot of capillaries) at high pressure. Water, glucose, amino acids, urea, and ions are forced out of the blood through the basement membrane into the Bowman’s capsule. Large proteins and blood cells are retained.
Proximal convoluted tubule (selective reabsorption): All glucose, all amino acids, most water and ions, and some urea are reabsorbed. Glucose and amino acids are reabsorbed by active transport; water follows by osmosis.
Loop of Henle (countercurrent multiplication): The descending limb is permeable to water but not ions. The ascending limb actively pumps out $\mathrm{Na^+$ and $\mathrm{Cl^-$ . This creates a concentration gradient in the medulla, which is essential for producing concentrated urine.
Distal convoluted tubule: Fine-tuning of water and ion balance under hormonal control (ADH and aldosterone).
Collecting duct: Water reabsorption is controlled by ADH. In the presence of ADH, the collecting duct is permeable to water, and water is reabsorbed into the concentrated medulla, producing concentrated urine. Without ADH, the collecting duct is impermeable to water, and dilute urine is produced.

Worked Example: Diabetes insipidus.

A patient produces very large volumes of dilute urine and is constantly thirsty. This is a symptom Of diabetes insipidus, which is caused by a lack of ADH (or by the kidneys not responding to ADH). Without ADH, the collecting ducts remain impermeable to water, so water is not reabsorbed. The Result is large volumes of dilute urine (polyuria) and excessive thirst (polydipsia) as the body Tries to replace the lost water.

Review: The Nervous System — Neurones and Synapses

Types of neurones:

Sensory neurones: Transmit impulses from receptors to the central nervous system (CNS). They have a long axon and a cell body located off to the side of the axon.
Motor neurones: Transmit impulses from the CNS to effectors (muscles and glands). They have a cell body in the CNS, a long axon, and short dendrites.
Relay neurones (interneurones): Connect sensory and motor neurones within the CNS. They have many short dendrites and a short axon.

Synaptic transmission:

An action potential arrives at the pre-synaptic membrane.
Voltage-gated calcium channels open, and $\mathrm{Ca^{2+}$ ions flow into the pre-synaptic neurone.
The influx of $\mathrm{Ca^{2+}$ causes synaptic vesicles to fuse with the pre-synaptic membrane, releasing neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the post-synaptic membrane.
This binding opens ion channels, causing a change in membrane potential (depolarisation for an excitatory synapse, hyperpolarisation for an inhibitory synapse).
If the depolarisation reaches threshold, an action potential is generated in the post-synaptic neurone.
The neurotransmitter is broken down by enzymes (e.g., acetylcholinesterase breaks down acetylcholine) or taken back up into the pre-synaptic neurone (reuptake), terminating the signal.

Worked Example: How drugs affect synaptic transmission.

Many drugs work by affecting synaptic transmission:

Prozac (fluoxetine): A selective serotonin reuptake inhibitor (SSRI). It blocks the reuptake of serotonin into the pre-synaptic neurone, increasing serotonin concentration in the synaptic cleft. This is used to treat depression.
Nicotine: Binds to acetylcholine receptors on post-synaptic membranes, mimicking the effect of acetylcholine and causing continuous stimulation.
Organophosphates: Inhibit acetylcholinesterase, preventing the breakdown of acetylcholine. This causes continuous stimulation of post-synaptic membranes, leading to muscle spasms and can be fatal (used as nerve agents).

Review: The Endocrine System — Hormones and Negative Feedback

Key hormones and their functions:

Hormone	Source	Target	Function
Insulin	Pancreas (beta)	Liver, cells	Lowers blood glucose; promotes glucose uptake
Glucagon	Pancreas (alpha)	Liver	Raises blood glucose; stimulates glycogenolysis
ADH	Pituitary	Kidney	Increases water reabsorption
Adrenaline	Adrenal medulla	Heart, lungs	Increases heart rate, breathing rate
Thyroxine	Thyroid	All cells	Increases metabolic rate
FSH	Pituitary	Ovaries	Stimulates follicle development
LH	Pituitary	Ovaries	Triggers ovulation
Oestrogen	Ovaries	Uterus	Thickens uterine lining
Progesterone	Corpus luteum	Uterus	Maintains uterine lining
Testosterone	Testes	Body	Male secondary sexual characteristics

Negative feedback: The mechanism by which the body maintains homeostasis. When a condition Deviates from its set point, the body initiates a corrective response that brings the condition back To normal, then the corrective response is switched off.

Worked Example: Blood glucose regulation.

After a meal, blood glucose rises above the normal range (approximately 4—6 mmol/L). The pancreas Detects this and secretes insulin. Insulin stimulates cells (especially liver and muscle cells) to Take up glucose from the blood and convert it to glycogen (glycogenesis). Blood glucose falls back To normal, and insulin secretion decreases.

During fasting or exercise, blood glucose falls below normal. The pancreas secretes glucagon. Glucagon stimulates the liver to break down glycogen into glucose (glycogenolysis) and, if Necessary, to convert amino acids and lipids into glucose (gluconeogenesis). Blood glucose rises Back to normal, and glucagon secretion decreases.

Thermoregulation:

The body maintains its core temperature at approximately 37 $\degree$ C. This is controlled by the Hypothalamus, which acts as the body’s thermostat.

Heat gain mechanisms (when body temperature drops):

Vasoconstriction: Arterioles near the skin surface narrow, reducing blood flow to the skin and minimising heat loss by radiation.
Piloerection: Hair erector muscles contract, causing body hair to stand up and trap a layer of insulating air next to the skin.
Shivering: Rapid involuntary contractions of skeletal muscles generate heat as a by-product of respiration.
Increased metabolic rate: The thyroid gland secretes more thyroxine, which increases the basal metabolic rate, generating more heat.
Behavioural responses: Curling up, putting on clothes, seeking shelter.

Heat loss mechanisms (when body temperature rises):

Vasodilation: Arterioles near the skin surface widen, increasing blood flow to the skin and maximising heat loss by radiation.
Sweating: Sweat glands secrete sweat onto the skin surface. The evaporation of sweat absorbs latent heat from the skin, cooling the body. Each gram of water that evaporates absorbs approximately 2.4 kJ of heat energy.
Decreased metabolic rate: Less thyroxine is secreted, reducing heat production.
Behavioural responses: Removing clothing, seeking shade, drinking cold fluids.

The skin in thermoregulation:

The skin is the largest organ in the body and plays a central role in thermoregulation:

Structure	Function in thermoregulation
Epidermis	Barrier to water loss; contains melanin for UV protection
Dermis	Contains blood vessels, sweat glands, hair follicles
Arterioles	Dilate or constrict to control blood flow to skin
Sweat glands	Produce sweat for evaporative cooling
Hair erector muscles	Cause piloerection to trap insulating air
Sensory receptors	Detect temperature changes and send signals to hypothalamus

Negative feedback in thermoregulation:

When the core temperature rises above 37 $\degree$ C, thermoreceptors in the skin and hypothalamus Detect the change. The hypothalamus sends signals via the autonomic nervous system to activate Cooling mechanisms (vasodilation, sweating). As the temperature returns to 37 $\degree$ C, the Thermoreceptors send fewer signals, and the cooling mechanisms are gradually switched off. This is Negative feedback in action.

Osmoregulation:

The body must maintain the water potential of the blood within narrow limits. The kidneys are the Main organs of osmoregulation.

How the kidneys regulate water balance:

Filtration: Blood is filtered in the glomerulus of each nephron. Water, glucose, urea, ions, and other small molecules pass into the renal tubule, while large proteins and blood cells remain in the blood.
Selective reabsorption: Useful substances (all glucose, the required amount of water and ions) are reabsorbed from the tubule back into the blood.
Formation of urine: The remaining water, excess ions, and urea form urine, which passes to the bladder for storage and excretion.

Role of antidiuretic hormone (ADH):

ADH is released from the posterior pituitary gland when the blood water potential is too low (blood Is too concentrated). ADH increases the permeability of the collecting duct to water by causing Aquaporin channels to be inserted into the cell membranes. More water is reabsorbed from the Filtrate into the blood, producing a smaller volume of more concentrated urine.

When the blood water potential is too high (blood is too dilute), less ADH is released. The Collecting duct becomes less permeable to water, less water is reabsorbed, and a larger volume of More dilute urine is produced.

Worked Example: Explaining osmoregulation after drinking a large volume of water.

After drinking 1 litre of water, the water is absorbed into the blood from the small intestine. This Lowers the solute concentration (increases the water potential) of the blood. Osmoreceptors in the Hypothalamus detect the change and send fewer nerve impulses to the posterior pituitary gland. Less ADH is released into the bloodstream. The collecting ducts in the kidneys become less permeable to Water. Less water is reabsorbed from the filtrate, so a larger volume of dilute urine is produced. This restores the blood water potential to normal.

Worked Example: Diabetes insipidus.

A patient produces very large volumes of dilute urine (polyuria) and feels constantly thirsty (polydipsia). These symptoms are caused by a lack of ADH (or a failure of the kidneys to respond to ADH). Without ADH, the collecting ducts remain impermeable to water, and the kidneys cannot Concentrate the urine. Treatment involves administering a synthetic analogue of ADH (desmopressin).

The nervous system:

The nervous system allows the body to detect and respond to changes in the internal and external Environment. It is divided into:

Central nervous system (CNS): The brain and spinal cord. The CNS processes information and coordinates responses.
Peripheral nervous system (PNS): All other nerves. Divided into sensory neurons (carrying impulses from receptors to the CNS) and motor neurons (carrying impulses from the CNS to effectors).

Types of neurons:

Neuron type	Function	Structure feature
Sensory	Carry impulses from receptors to the CNS	Long axon, cell body off-centre
Relay	Connect sensory and motor neurons in the CNS	Short axon, many dendrites
Motor	Carry impulses from the CNS to effectors	Long axon, cell body at start

Neuron structure:

Cell body: Contains the nucleus and most of the cytoplasm. Coordinates the metabolic activity of the neuron.
Dendrites: Branched extensions that receive nerve impulses from other neurons or receptors.
Axon: A long, thin extension that carries nerve impulses away from the cell body. Surrounded by a fatty myelin sheath (in myelinated neurons) that insulates the axon and speeds up transmission.
Nodes of Ranvier: Gaps in the myelin sheath where the axon membrane is exposed. In myelinated neurons, the action potential jumps from node to node (saltatory conduction), greatly increasing the speed of transmission.
Motor end plates: The endings of motor neurons that form synapses with muscle fibres or glands.

Transmission of nerve impulses:

At rest, the inside of the neuron is negatively charged relative to the outside (resting potential Of approximately -70 mV). This is maintained by the sodium-potassium pump, which pumps 3 sodium ions Out and 2 potassium ions in for each ATP hydrolysed.

When a stimulus depolarises the membrane to the threshold level (approximately -55 mV), Voltage-gated sodium channels open. Sodium ions rush in, causing further depolarisation. This is the Action potential (approximately +40 mV). The action potential travels along the axon as a wave of Depolarisation.

After the action potential passes, voltage-gated potassium channels open and potassium ions flow Out, repolarising the membrane. The sodium-potassium pump restores the resting potential (refractory Period), ensuring the impulse travels in one direction only.

Synapses:

A synapse is the junction between two neurons (or between a neuron and an effector). When an action Potential arrives at the presynaptic membrane:

Calcium ions enter the presynaptic neuron through voltage-gated calcium channels.
Vesicles of neurotransmitter fuse with the presynaptic membrane and release the neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane.
This triggers the opening of ion channels, which may depolarise the postsynaptic membrane (excitatory synapse, e.g., acetylcholine) or hyperpolarise it (inhibitory synapse, e.g., GABA).
The neurotransmitter is broken down by enzymes (e.g., acetylcholinesterase breaks down acetylcholine) to prevent continuous stimulation.

The reflex arc:

A reflex arc is the pathway taken by nerve impulses in an automatic, involuntary response (a Reflex). Reflexes are fast and protect the body from harm. The pathway is:

Receptor — sensory neuron — relay neuron — motor neuron — effector

Worked Example: The pain withdrawal reflex.

If you touch a hot object:

Pain receptors in the skin detect the high temperature and generate nerve impulses.
Sensory neurons transmit the impulses to the spinal cord.
A relay neuron in the spinal cord passes the impulse to a motor neuron.
The motor neuron stimulates the biceps muscle in the arm to contract (via a neuromuscular junction), pulling the hand away from the hot object.
Simultaneously, a relay neuron sends impulses to the brain, so you become conscious of the pain after the reflex has already occurred.

This reflex is protective because it does not require conscious processing by the brain, making the Response faster.

Comparison of nervous and endocrine systems:

Feature	Nervous system	Endocrine system
Signal type	Electrical impulses	Chemical (hormones)
Speed of response	Very fast (milliseconds)	Slower (seconds to hours to days)
Duration of effect	Short-lived	Long-lasting
Target organs	Specific effectors (muscles, glands)	Widespread (any cell with receptors)
Transmission method	Neurons and synapses	Bloodstream
Example	Reflex arc (withdrawal reflex)	Insulin and blood glucose regulation

Hormones and their effects:

Hormones are chemical messengers secreted by endocrine glands into the bloodstream. They travel Throughout the body but only affect specific target cells that have the appropriate receptors.

Key endocrine glands and hormones:

Gland	Hormone(s)	Function
Pituitary	FSH, LH, ADH, TSH	Controls reproduction, water balance, metabolism
Thyroid	Thyroxine	Increases metabolic rate, regulates growth
Adrenal	Adrenaline, cortisol	Fight or flight response, stress response
Pancreas	Insulin, glucagon	Regulates blood glucose concentration
Ovaries	Oestrogen, progesterone	Female secondary sexual characteristics, menstrual cycle
Testes	Testosterone	Male secondary sexual characteristics, sperm production

Worked Example: Hormonal control of the menstrual cycle.

The menstrual cycle is controlled by four hormones:

FSH (follicle-stimulating hormone, from the pituitary) stimulates follicle development in the ovary and stimulates oestrogen production.
Oestrogen (from the ovaries) stimulates the thickening of the uterine lining and inhibits FSH production (negative feedback). At high concentrations, oestrogen stimulates LH release (positive feedback).
LH (luteinising hormone, from the pituitary) triggers ovulation (release of the egg from the follicle) on approximately day 14.
Progesterone (from the corpus luteum) maintains the thickened uterine lining throughout the second half of the cycle. It inhibits FSH and LH production. If the egg is not fertilised, the corpus luteum degenerates, progesterone levels drop, and the uterine lining is shed (menstruation).

Summary

This topic covers the biological principles of human body systems, including key concepts, experimental evidence, and real-world applications.

Key concepts include:

pathogens and transmission
the immune response (innate and adaptive)
antibodies and vaccination
HIV/AIDS and autoimmune diseases
antigen-antibody interactions

Success requires the ability to recall specific factual content, apply knowledge to novel scenarios, and evaluate experimental evidence critically.

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