Human Body : Endocrine, Circulatory and Nervous Systems | ICSE Class 8 Biology Notes

This note covers chemical and nervous coordination, endocrine glands and hormones, adolescence, hygiene and stress management, blood and lymph, the heart and blood vessels, double circulation, blood groups, neurons, nerves, the brain, the spinal cord and reflex actions.

How do glands and hormones coordinate the human body?

Coordination means that organs work together so that their activities support one another. During physical exercise, muscles need more oxygen, and breathing, heartbeat and blood flow increase together. The nervous and endocrine systems help coordinate these changes.

The nervous system carries information through nerve cells. The endocrine system consists of endocrine glands and other hormone-producing cells and tissues. It provides chemical coordination through hormones carried to their targets.

Definition: A hormone is a chemical messenger produced in very small amounts that helps regulate the activities of other cells. A target cell is a cell that can respond to a particular hormone.

How do endocrine and exocrine glands differ?

A gland produces and releases a substance called its secretion. A duct is a tube carrying a gland's secretion. Endocrine glands are ductless: their hormones enter the blood. Exocrine glands release secretions through ducts onto a surface or into a body cavity.

The pancreas has both endocrine and exocrine functions. Insulin is its hormone involved in controlling blood sugar. Its pancreatic juice is a digestive secretion released into the small intestine, where digestion is completed.

The pancreas is therefore called a mixed gland. This name describes its two kinds of secretion, which follow different routes and perform different functions. Insulin travels in blood, whereas pancreatic digestive juice reaches the intestine through a duct.

Nervous communication provides rapid, directed signals along connected nerve cells. Hormonal communication can act steadily over time and reach cells through circulation. These two means of control work together in the human body.

Where are the main endocrine glands, and what do they do?

The pituitary, thyroid, adrenal glands and pancreas are hormone-producing glands with the positions and secretions listed below. Their hormones regulate growth, the use of food substances, blood sugar and responses to demanding situations. Learn each gland together with its location and function.

Metabolism means the chemical reactions occurring in the body. Thyroxine, also called thyroxin, regulates carbohydrate, protein and fat metabolism. Carbohydrates, proteins and fats are food nutrients used by the body for energy, growth and other functions.

The pituitary also releases hormones that influence other endocrine glands. Its growth hormone regulates body growth; deficiency during childhood leads to pituitary dwarfism, a condition of reduced growth caused by insufficient growth hormone.

The thyroid needs iodine, a mineral nutrient, to make thyroxine. If iodine is deficient in the diet, there is a possibility of developing goitre, an enlargement of the thyroid that can produce a swollen neck.

How does adrenaline link hormones with circulation?

Adrenaline enters the blood and acts on target tissues, including the heart. The heart beats faster, breathing rate increases, and blood is diverted towards skeletal muscles, the muscles attached to bones. These changes prepare the body for fighting or running away.

Insulin secretion is controlled by feedback, a process in which the result of an activity helps regulate that activity. When blood sugar rises, pancreatic cells respond by producing more insulin. As blood sugar falls, insulin secretion is reduced.

The male and female outlines show the pituitary in the head, thyroid in the neck, pancreas in the abdomen and adrenal glands above the kidneys. Locate and label these four glands.

See Fig. 6.7 in your NCERT textbook

What changes occur during adolescence?

Adolescence is the stage of development between childhood and adulthood, typically occurring between the ages of 10 and 19. The timing, nature and extent of changes vary among individuals. Each person experiences adolescence at their own pace.

Puberty is the stage when external and internal changes develop the body towards reproductive capability, meaning the ability to produce young ones. Hormones are mainly responsible for many changes associated with puberty, including menstruation and other signs of development.

Which physical changes become noticeable?

• Height and body shape: Growth in height becomes more prominent. Boys' shoulders might become a bit broader and their chests may get wider. Girls also change in height and weight and develop breasts.
• Voice: The voice box, the structure in the throat that produces voice, grows. In boys this leads to hoarseness and may appear as a bump called the Adam's apple. It is not noticeable in every individual.
• Body hair: Hair develops in the armpits and pubic region, the area around the external reproductive organs. Boys often develop facial hair. The timing and extent of hair growth vary.
• Skin: Increased oily secretions can clog skin pores and lead to infections. Acne is a skin condition in which small reddish pimples appear, commonly on the face.

Secondary sexual characteristics are features, such as facial hair and breast development, that help distinguish males and females but are not directly involved in reproduction. Internal changes also occur as reproductive organs mature.

Menstruation is the phase of the menstrual cycle when blood is discharged from the body. The menstrual cycle is a recurring reproductive cycle beginning at puberty. Menstruation is a natural process; beliefs that it makes someone unclean have no scientific basis.

What emotional changes can occur?

Adolescence is often marked by stronger emotions than childhood. Mood swings, meaning changes in emotional state, and increased sensitivity may occur. Understanding how emotions affect behaviour helps a person respond thoughtfully and make responsible choices.

How can hygiene and stress management support adolescence?

Personal hygiene means practices that keep the body clean. Maintaining hygiene, particularly in the armpits and pubic region, may protect against potential infections. Cleanliness remains important alongside nutrition, physical activity and a supportive social environment.

During menstruation, proper hygiene is essential for comfort and health. Sanitary pads and reusable cloth pads designed for this purpose can be used. Used disposable pads should be wrapped in newspaper and placed in a dustbin for proper disposal.

What does managing stress mean?

Stress is a feeling of mental or physical strain when dealing with demands or difficult situations. Stress management means using constructive ways to cope with such strain. It includes recognising feelings and choosing helpful responses rather than acting without thought.

• Yoga: A practice involving postures and breathing that can form part of a routine for managing stress.
• Meditation: A practice of calmly directing attention that can help a person settle their thoughts.
• Time management: Organising available time and responsibilities to make tasks more manageable.
• Sports and hobbies: Physical activity and enjoyable interests provide constructive ways to use energy and engage the mind.
• Rational thinking: Considering facts and possible responses calmly instead of accepting an immediate fear as a fact.

Regular exercise and physical activities help keep the body and mind fit, build stamina and boost mood. A nutritious diet supports growth and development. Interests such as music, dance and sports can provide positive activities during emotional changes.

Guidance and support also matter. Staying informed and seeking help when needed can make changes easier to understand. Respectful behaviour towards others helps create a favourable and safe environment during this stage of life.

What do blood and lymph transport?

Blood is a fluid connective tissue, a tissue that links body parts by transporting substances. It carries nutrients and oxygen to cells and takes waste substances away. Its components have different roles, so blood is more than a carrier of oxygen.

Which components make up blood?

Red blood cells (RBCs) contain haemoglobin, which binds oxygen. White blood cells (WBCs) include cells that destroy invading organisms and cells involved in immune responses, the body's defence reactions. They do not perform the oxygen-carrying role of red cells.

Clotting is the formation of a clot that reduces blood loss after injury. Platelets help initiate this process. Plasma is the transport medium, whereas red cells, white cells and platelets contribute different specialised functions.

How is lymph formed?

Capillaries are the smallest blood vessels, with thin walls allowing exchange with tissues. Some water and small dissolved substances leave blood capillaries and enter spaces around cells. This forms tissue fluid, through which materials pass between blood and cells.

1. Some fluid leaves blood capillaries and enters spaces between tissue cells.
2. Nutrients and gases are exchanged between this tissue fluid and the cells.
3. Lymphatic vessels collect the fluid; inside these vessels it is called lymph.
4. The lymphatic vessels return the collected fluid to major veins, vessels carrying blood towards the heart.

The lymphatic system is this additional network of vessels that returns tissue fluid to blood. Lymph contains lymphocytes, white blood cells involved in defence. It also transports absorbed fats from the intestine and carries substances including nutrients and hormones.

How is the heart built to pump blood?

The heart is a muscular pumping organ between the lungs, slightly tilted to the left. It is about the size of a clenched fist. The pericardium, a double-walled protective bag enclosing fluid, surrounds it.

The heart has four chambers. The two upper chambers are called atria, with one chamber called an atrium. The two lower chambers are ventricles. Ventricular walls are thicker than atrial walls because the ventricles pump blood out of the heart.

What do septa and valves do?

A septum is a partition between chambers; septa is the plural. The interatrial septum separates the two atria, and the interventricular septum separates the two ventricles. These partitions keep blood in the right and left sides separate.

Valves are structures that prevent backward blood flow. The tricuspid valve, with three flaps, guards the opening between the right atrium and right ventricle. The bicuspid or mitral valve guards the opening between the left atrium and left ventricle.

Semilunar valves guard the ventricular exits into the pulmonary artery and aorta. Pulmonary means related to the lungs. The pulmonary artery carries blood towards the lungs; the aorta is the main artery carrying blood to the body. An artery carries blood away from the heart.

What starts the heartbeat?

The sinoatrial node is a specialised patch of heart muscle in the upper right atrium. It produces electrical signals that start and maintain rhythmic contraction. It is called the heart's natural pacemaker, meaning the structure that sets the rhythm of beating.

Contraction means the shortening of muscle; relaxation follows contraction. The repeated contraction and relaxation of the heart chambers moves blood through the circulation. The atria pass blood to the ventricles, and the ventricles pump it into outgoing arteries.

The cutaway shows right and left atria above the ventricles. Labels identify the vena cava, aorta, pulmonary artery, pulmonary veins, sinoatrial node and interventricular septum. Trace each chamber to its connected vessel.

See Fig. 15.2 in your NCERT textbook

How do the main blood vessels and heart conditions differ?

Arteries carry blood away from the heart, veins carry it towards the heart, and capillaries connect the smallest branches of these vessels. Arterial walls are thick and elastic to withstand blood arriving under pressure. Veins have thinner walls and valves that help prevent backflow.

Oxygenated blood is blood enriched with oxygen after passing through the lungs. Deoxygenated blood has given up some oxygen to tissues. These terms describe oxygen content; artery and vein describe the direction of blood flow relative to the heart.

Vena cava is singular; venae cavae is plural. Coronary refers here to vessels serving the heart muscle. The main vessels can be identified by their connections and the direction in which blood travels.

The muscle doing the pumping also needs its own blood supply. The coronary vessels serve the heart muscle itself, rather than replacing the vessels carrying blood between the heart, lungs and rest of the body.

Which heart-related terms should be distinguished?

Palpitations are a noticeable sensation of the heartbeat, which may feel fast, pounding or irregular. Cardiac arrest means the heart stops beating. Hypertension means blood pressure higher than normal; blood pressure is the pressure blood exerts on vessel walls.

These terms describe different conditions. Cardiac arrest is also different from a heart attack, in which heart muscle is suddenly damaged by inadequate blood supply. A sensation of palpitations does not, by itself, mean that the heart has stopped beating.

How does double circulation move blood around the body?

Double circulation means that blood passes through the heart twice during one complete journey through the lungs and body. It consists of pulmonary circulation, the route between heart and lungs, and systemic circulation, the route between heart and body tissues.

What is the complete route?

1. Deoxygenated blood from body tissues returns through veins and the venae cavae to the right atrium.
2. Blood passes through the tricuspid valve into the right ventricle.
3. The right ventricle pumps blood through the pulmonary outlet valve into the pulmonary trunk and pulmonary arteries towards the lungs.
4. In the lungs, blood releases carbon dioxide and takes up oxygen.
5. Pulmonary veins return oxygenated blood to the left atrium.
6. Blood passes through the bicuspid valve into the left ventricle.
7. The left ventricle pumps blood through the aortic outlet valve into the aorta and body arteries.
8. At tissue capillaries, substances are exchanged with body cells; blood then returns through veins towards the right atrium.

The pulmonary route runs from the right ventricle through the lungs to the left atrium. The systemic route runs from the left ventricle through body tissues to the right atrium. Connecting these routes through the heart completes the circulation.

Separation of the right and left sides prevents oxygenated and deoxygenated blood from mixing in the heart. This allows an efficient supply of oxygen to the body. The lungs replenish oxygen, while systemic circulation delivers it to tissues.

The schematic places the lungs above the heart and body parts below it. Arrows show two connected loops. Labels identify pulmonary vessels, aorta, vena cava and capillaries, linking lung circulation with body circulation.

See Fig. 15.4 in your NCERT textbook

When tracing the diagram, begin at a named chamber and follow the arrows. A complete explanation should return to that chamber. Naming the lungs and body tissues without showing their connections to the chambers leaves out the essential sequence.

What do the ABO blood groups mean?

The ABO system classifies human blood into groups A, B, AB and O. These letters are group names. The grouping depends on the presence or absence of substances called A and B antigens on red blood cells.

An antigen is a substance that can trigger an immune response. An antibody is a protein produced in response to an antigen. In this grouping, anti-A means antibody directed against A, and anti-B means antibody directed against B.

Nil means none. In the O row, “anti-A, B” means both anti-A and anti-B antibodies. A donor gives blood, and a recipient receives it. A blood transfusion is the transfer of donated blood to a recipient.

Within this ABO compatibility scheme, group O is called the universal donor, and group AB the universal recipient or universal acceptor. Group O lacks A and B antigens; group AB lacks the corresponding antibodies in plasma.

Note: Blood must be carefully matched before transfusion. The Rh system, another grouping based on a red-cell antigen, must also be considered. The classroom ABO labels do not remove the need for proper compatibility testing.

An incompatible transfusion can cause red cells to clump and be destroyed. The donor column therefore records which groups can supply a recipient in the ABO scheme, rather than listing groups that the recipient can donate to.

How do neurons and different kinds of nerves carry information?

A neuron is a nerve cell specialised for receiving and transmitting information. A nerve impulse is an electrical signal travelling along a neuron. A motor neuron carries signals from the central nervous system towards an organ that responds.

The central nervous system (CNS) consists of the brain and spinal cord. The spinal cord is the long nervous structure running from the brain inside the backbone. The peripheral nervous system comprises nerves connecting the CNS with other body parts.

What are the parts of a motor neuron?

The cell body contains the nucleus, the cell structure containing genetic material, and surrounding cytoplasm, the cell's internal material. Dendrites are short branched processes that carry impulses towards the cell body. The axon is a long process carrying impulses away.

The end of an axon branches into nerve endings. Some axons have a myelin sheath, a covering around the axon. Gaps between adjacent portions of this sheath are called nodes of Ranvier.

A synapse is a junction through which a neuron communicates with another cell. At a chemical synapse, chemicals released from the axon ending carry the message across the gap. These messenger chemicals are called neurotransmitters.

The drawing shows a branched cell body with a nucleus, short dendrites, a long axon and branching nerve endings. Follow the parts from dendrites through the cell body to the axon endings.

See Fig. 6.1a in your NCERT textbook

How are nerves classified?

A nerve is a bundle of nerve fibres. Sensory nerves carry information towards the CNS. Motor nerves carry commands away from the CNS. Mixed nerves contain both sensory and motor fibres, allowing communication in both directions through different fibres.

Cranial nerves arise from the brain; humans have 12 pairs. Spinal nerves arise from the spinal cord; humans have 31 pairs. This classification describes origin, whereas sensory, motor and mixed describe the functions of the fibres carried.

Information therefore has a direction: sensory input reaches the CNS, and motor output travels towards responding organs. Calling a nerve “mixed” does not mean that every individual fibre carries impulses in both directions.

What are the functions of the brain and spinal cord?

The brain is the main centre for processing information and coordinating activities. The skull protects it, and fluid around it provides cushioning. The backbone protects the spinal cord. Together these structures form the central nervous system.

How are the brain's main regions organised?

The brain has three major regions: the forebrain, midbrain and hindbrain. The forebrain includes the cerebrum, thalamus and hypothalamus. The hindbrain includes the pons, cerebellum and medulla. Each of these parts contributes to coordination. Voluntary means under conscious control, while involuntary means occurring without conscious direction.

Voluntary actions are actions based on conscious decisions, such as writing or moving a chair. Involuntary actions occur without conscious direction. Cardiovascular means related to the heart and blood vessels; reflexes are rapid automatic responses to stimuli.

A stimulus is a change detected by the body. The cerebrum interprets sensory inputs and supports decisions about responses. The cerebellum helps make movements precise and maintains balance, including during walking in a straight line or riding a bicycle.

What does the spinal cord do?

The spinal cord carries information between body parts and the brain. It also provides connections for reflex actions such as withdrawing a hand from a hot object. Its function therefore includes both communication and rapid local coordination.

The section shows the cerebrum occupying the upper region, the cerebellum towards the back and the spinal cord continuing below. Labels also identify the midbrain, hypothalamus, pons and medulla inside the skull.

See Fig. 6.3 in your NCERT textbook

How does a reflex arc produce a quick response?

A reflex action is a rapid, automatic response to a stimulus without waiting for a conscious decision. A response is the resulting activity. Withdrawing a hand from a hot object is an example that helps explain the nervous pathway.

A receptor detects a stimulus. Receptors are usually located in sense organs, and receptors in the skin detect heat and pain. An effector is a muscle or gland that carries out the response to a nervous signal.

What is the sequence in hand withdrawal?

1. Heat or pain stimulates receptors in the skin when the hand touches a hot object.
2. A sensory neuron carries the resulting nerve impulse towards the spinal cord.
3. A relay neuron, a neuron connecting other neurons within the CNS, passes the signal towards a motor neuron.
4. The motor neuron carries an impulse from the spinal cord towards an arm muscle.
5. The muscle acts as the effector and contracts, withdrawing the hand as the response.

The reflex arc is the pathway followed by the nervous signals producing a reflex response. In this example, the connection is made in the spinal cord. Information also reaches the brain, so a spinal reflex does not mean that the brain receives no information.

A hand touches a hot vessel. Arrows connect skin receptors, a sensory neuron, a relay neuron in the spinal cord, a motor neuron and an arm muscle. A separate arrow indicates a message travelling to the brain.

See Fig. 6.2 in your NCERT textbook

How do automatic and learned responses differ?

Blinking when a hand moves in front of the face and movement of the foot when the knee is tapped are other reflex examples. Their pathways should not be assumed to contain every detail of the hand-withdrawal example.

A conditioned reflex is a response acquired through learning. In Pavlov's dog experiment, repeated association of a bell with food led to salivation in response to the bell. Salivation means the release of saliva; the bell had become a learned signal for food.

Glossary

• Hormone — A chemical messenger produced in small amounts that regulates activities in responsive target cells.
• Endocrine gland — A ductless gland that releases hormones into blood for transport to target tissues.
• Exocrine gland — A gland whose secretion passes through ducts onto a surface or into a body cavity.
• Adolescence — The stage between childhood and adulthood, involving physical, emotional and behavioural changes.
• Puberty — The stage during which external and internal changes develop the body towards reproductive capability.
• Lymph — Fluid within lymphatic vessels that returns to blood and participates in transport and defence.
• Septum — A partition separating chambers, including the partitions between the right and left heart chambers.
• Pacemaker — The structure setting the heartbeat rhythm, normally the sinoatrial node in the right atrium.
• Double circulation — Circulation in which blood passes through the heart twice during a complete lung-and-body circuit.
• Neuron — A specialised nerve cell that receives and transmits information through nervous signals.
• Synapse — A junction through which a neuron communicates with another neuron or a responding cell.
• Receptor — A specialised structure that detects a stimulus and initiates a signal in a sensory pathway.
• Effector — A muscle or gland that produces a response after receiving a nervous signal.
• Reflex arc — The pathway followed by nervous signals from a receptor towards an effector during a reflex.

Common errors and misconceptions

• Misconception: Every gland sends its secretion through a duct. Correct: Endocrine glands lack ducts and release hormones into blood; exocrine glands use ducts.
• Misconception: All adolescents develop at the same pace. Correct: The timing, nature and extent of physical changes vary among individuals, and these variations are normal.
• Misconception: Every artery carries oxygenated blood. Correct: An artery carries blood away from the heart. Pulmonary arteries carry deoxygenated blood to the lungs.
• Misconception: Double circulation means blood passes through the lungs twice in one circuit. Correct: Blood passes through the heart twice, connecting the lung and body circuits.
• Misconception: Universal donor means compatibility checks are unnecessary. Correct: The ABO label is a classroom grouping description; donated blood must still be carefully matched, including its Rh group.
• Misconception: The cerebrum and cerebellum have identical functions. Correct: The cerebrum supports thinking and interpretation, whereas the cerebellum coordinates precise movement, posture and balance.
• Misconception: The brain receives no information during a spinal reflex. Correct: The immediate connection is made in the spinal cord, but information also travels to the brain.
• Misconception: Palpitations and cardiac arrest are the same condition. Correct: Palpitations are noticeable heartbeats; cardiac arrest means the heart stops beating.

Exam-style questions with model answers

Q1. Distinguish endocrine and exocrine glands by stating the route taken by each type of secretion. [2 marks]

1. Endocrine glands are ductless and release their hormones into blood for transport to target tissues.
2. Exocrine glands release their secretions through ducts onto a surface or into a body cavity.

Q2. For each gland, name the hormone studied and give its function: pituitary, thyroid, adrenal gland and pancreas. [4 marks]

1. The pituitary secretes growth hormone, which regulates growth and development of the body, including growth during childhood.
2. The thyroid secretes thyroxine, which regulates carbohydrate, protein and fat metabolism and helps coordinate the body's growth.
3. The adrenal gland secretes adrenaline, which prepares the body for an emergency by increasing heartbeat and breathing rate.
4. The pancreas secretes insulin, which helps regulate blood sugar by lowering its level when it has risen.

Q3. Explain three changes during adolescence: growth in height, voice changes and emotional changes. Retain the differences in timing and extent between individuals. [3 marks]

1. Growth in height becomes more prominent during adolescence, but the timing and extent of growth vary from person to person.
2. The voice box grows. In boys, this leads to hoarseness and may produce a visible Adam's apple, though it is not noticeable in every individual.
3. Adolescence is often marked by stronger emotions than childhood. Mood swings and increased sensitivity may occur, influencing behaviour and responses.

Q4. Starting with blood returning from body tissues, trace double circulation in six stages. Include the chambers, main vessels, gas exchange in the lungs and return from body tissues. [6 marks]

1. Deoxygenated blood returns from body tissues through veins and the venae cavae to the right atrium of the heart.
2. Blood passes through the tricuspid valve into the right ventricle, which pumps it into the pulmonary trunk and pulmonary arteries.
3. The pulmonary arteries carry blood to the lungs, where it releases carbon dioxide and takes up oxygen before returning towards the heart.
4. Pulmonary veins carry oxygenated blood into the left atrium; it then passes through the bicuspid valve into the left ventricle.
5. The left ventricle pumps blood into the aorta, from which arteries deliver it towards capillaries in the body tissues.
6. Substances are exchanged with body cells at the tissue capillaries. Blood returns through veins, completing a circuit that passes through the heart twice.

Q5. In the ABO scheme, a group AB recipient has neither anti-A nor anti-B antibodies in plasma, and group O red cells have neither A nor B antigens. Name the universal recipient and universal donor in this scheme. [2 marks]

1. Group AB is the universal recipient in the ABO scheme because it lacks the stated antibodies against A and B antigens.
2. Group O is the universal donor in this scheme because its red cells lack both A and B antigens.

Q6. State one main function each of red blood cells, white blood cells and platelets, identifying how their roles differ. [3 marks]

1. Red blood cells contain haemoglobin and carry oxygen from the lungs towards body tissues. Their main role here is oxygen transport.
2. White blood cells help defend the body against foreign organisms. Different white cells contribute to protection, including destruction of invaders and immune responses.
3. Platelets release substances involved in blood clotting after injury. This helps reduce loss of blood, rather than providing oxygen transport or defence against invading organisms.

Q7. A hand touches a hot vessel. In this withdrawal reflex, the pathway contains skin receptors, a sensory neuron, a spinal relay neuron, a motor neuron and an arm muscle. Explain the response in five ordered stages. [5 marks]

1. Heat or pain stimulates the receptors in the skin of the hand. The receptors detect the stimulus and initiate a nervous signal.
2. The sensory neuron carries the nerve impulse from the skin receptors towards the spinal cord, bringing information into the central nervous system.
3. A relay neuron within the spinal cord passes the signal towards a motor neuron, providing the connection for a rapid response.
4. The motor neuron carries an impulse away from the spinal cord towards the arm muscle that will perform the response.
5. The arm muscle is the effector. It contracts and withdraws the hand from the hot vessel, without waiting for a conscious decision.

Q8. Distinguish the functions of the cerebrum, cerebellum and medulla, giving one example of the activity associated with each. [3 marks]

1. The cerebrum processes sensory information and supports thinking and voluntary activity. Deciding to write is an example of an activity involving conscious control.
2. The cerebellum coordinates precise movements and maintains posture and balance. It helps coordinate the movements involved in walking in a straight line.
3. The medulla controls involuntary functions, including breathing and cardiovascular reflexes. Breathing and regulation of heart activity continue without a conscious decision directing each individual action.

Key takeaways

• Endocrine glands release hormones into blood, whereas exocrine glands send their secretions through ducts.
• The pituitary, thyroid, adrenal glands and pancreas help regulate growth, metabolism, emergency responses and blood sugar.
• Adolescence involves physical, emotional and behavioural changes whose timing and extent vary among individuals.
• Hygiene, nutritious food, physical activity, constructive interests and guidance support healthy development during adolescence.
• The heart has four chambers; septa separate its sides, and valves prevent backward blood flow.
• Double circulation connects the pulmonary and systemic routes, with blood passing through the heart twice per complete circuit.
• Red cells carry oxygen, white cells contribute to defence, platelets help clotting, and lymph supports fluid return and transport.
• The brain and spinal cord coordinate nervous activity; reflex arcs provide pathways for rapid automatic responses.

Test yourself

Why is the pancreas called a mixed gland?

It has endocrine tissue that secretes hormones such as insulin and exocrine tissue that produces pancreatic digestive juice.

Why does the thyroid need iodine?

Iodine is needed to make thyroxine. Dietary iodine deficiency creates a possibility of goitre, with enlargement of the thyroid.

What does the natural pacemaker do?

The sinoatrial node in the right atrium produces signals that initiate and maintain the rhythmic contractions of the heart.

Why is the pulmonary artery an artery despite carrying deoxygenated blood?

It carries blood away from the heart towards the lungs. Vessel names depend on flow direction, rather than oxygen content.

What is the difference between cranial and spinal nerves?

Cranial nerves arise from the brain and number 12 pairs; spinal nerves arise from the spinal cord and number 31 pairs.

What do mixed nerves contain?

They contain sensory fibres carrying information towards the central nervous system and motor fibres carrying commands away from it.

Does a spinal withdrawal reflex prevent information reaching the brain?

No. Its immediate nervous connection is made in the spinal cord, but information also travels onwards to the brain.

Name four approaches used to manage stress.

Yoga, meditation, time management and sports are four approaches. Hobbies and rational thinking are other constructive ways to tackle stress.