Physiological Structure and Life Process

Your body is a living machine of organ systems that work together. While you read this page, your eyes and brain are working, your heart is pumping blood, your kidneys are cleaning it and your hormones are keeping you alert or calm. This unit studies how the nervous and endocrine systems coordinate the body, how new human beings are born, how blood circulates, how waste is removed, and how the body keeps its inner conditions steady (homeostasis).

5.1 Nervous and endocrine systems

The two systems of coordination keep the activities of the body working together and let it respond to changes (stimuli). The nervous system acts quickly through electrical signals; the endocrine system acts slowly through chemical messengers (hormones).

Nervous system	Endocrine system
Messages are electrical impulses carried by nerves	Messages are chemicals (hormones) carried by blood
Very fast, in milliseconds	Slower, from seconds to days
Effect is short-lived	Effect lasts longer
Acts on a specific organ	May act on many organs

The nervous system

The nervous system is made of neurons (nerve cells). It has two main parts:

• Central nervous system (CNS): the brain and the spinal cord.
• Peripheral nervous system (PNS): the nerves that join the CNS to all parts of the body (12 pairs of cranial nerves from the brain and 31 pairs of spinal nerves from the spinal cord).

Neuron. A neuron has a cell body (with nucleus), short branches called dendrites that receive impulses, and a long axon that carries impulses away; the axon is often covered by an insulating myelin sheath. The end of the axon meets the next neuron at a gap called a synapse, which the impulse crosses by chemicals (neurotransmitters).

Neurons are of three kinds:

Type	Function
Sensory neuron	Carries impulses from the sense organs (receptors) to the CNS
Motor neuron	Carries impulses from the CNS to muscles or glands (effectors)
Relay (connecting) neuron	Joins sensory and motor neurons inside the CNS

The brain

The brain is protected by the skull, by three membranes (meninges) and by a fluid (cerebrospinal fluid). It weighs about 1.4 kg and has three main parts.

Part	Function
Cerebrum (the largest part, with two hemispheres)	Thinking, memory, intelligence, will, speech, seeing and hearing, voluntary movement
Cerebellum (behind and below the cerebrum)	Balance of the body, posture and smooth movement
Medulla oblongata (brain stem, joins the spinal cord)	Controls involuntary actions: heartbeat, breathing, swallowing, vomiting, blood pressure

Other parts: the thalamus relays signals, and the hypothalamus controls body temperature, hunger and thirst and links the nervous system with the endocrine system through the pituitary gland.

Spinal cord, reflex action

The spinal cord is a cord of nervous tissue about 45 cm long, inside the backbone. It carries impulses between the brain and the body and is the centre of reflex actions.

Examples: pulling the hand away from a hot object; blinking when dust enters the eye; knee jerk; sneezing; coughing.

The path of a reflex action is the reflex arc:

Stimulus → receptor (skin) → sensory neuron → spinal cord (relay neuron) → motor neuron → effector (muscle) → response.

Sense organs

The five sense organs are the eye (sight), ear (hearing and balance), nose (smell), tongue (taste) and skin (touch, pain, temperature). Each has receptors that turn a stimulus into an impulse.

The endocrine system

The endocrine system is a set of ductless glands that make hormones and pour them straight into the blood. Hormones are chemical messengers; they work in tiny amounts and act on target organs.

Gland	Position	Hormone	Function / disorder
Pituitary (master gland)	Base of the brain	Growth hormone (GH)	Growth of bones. Too little: dwarfism; too much: gigantism
		TSH, ACTH, FSH, LH	Control other glands
		ADH	Water absorption in kidney; lack causes diabetes insipidus
		Oxytocin	Contraction of uterus at birth; milk release
Thyroid	Neck, in front of windpipe	Thyroxine (needs iodine)	Controls metabolism. Too little: goitre (in adults), cretinism (in children)
Parathyroid	Behind the thyroid	Parathormone	Keeps calcium level in blood
Adrenal	On top of each kidney	Adrenaline	"Fight, flight or fright": raises heartbeat and blood sugar
Pancreas	Below the stomach	Insulin, glucagon	Control blood sugar. Lack of insulin: diabetes mellitus
Ovary (female)	Lower abdomen	Oestrogen, progesterone	Female sex characters; menstrual cycle
Testis (male)	Scrotum	Testosterone	Male sex characters, sperm formation

Diseases to remember: goitre (swollen neck from lack of iodine; prevented by iodised salt), diabetes mellitus (high blood sugar, from lack of insulin), dwarfism and gigantism (growth hormone).

5.2 Reproduction

Reproduction is the production of new individuals of the same kind. Humans reproduce sexually: a male gamete (sperm) fuses with a female gamete (egg or ovum) in fertilisation.

Male reproductive system

Organ	Function
Testes (a pair, in the scrotum)	Make sperms and the hormone testosterone. The scrotum keeps them about 2–3 °C cooler than the body
Epididymis	Stores sperms and lets them mature
Vas deferens	Tube carrying sperms from the epididymis
Seminal vesicle, prostate gland	Make a fluid that feeds the sperms; with the sperm it forms semen
Urethra and penis	Carry semen (and urine) out of the body

Female reproductive system

Organ	Function
Ovaries (a pair)	Produce an egg about every 28 days; make oestrogen and progesterone
Fallopian tubes (oviducts)	Carry the egg to the uterus; fertilisation takes place here
Uterus (womb)	A pear-shaped muscular organ where the embryo grows
Cervix	The neck of the uterus
Vagina	Receives the sperm; the birth canal

Puberty, gametes and menstruation

At puberty (about 10–14 years in girls, 12–16 in boys), the sex hormones start the secondary sexual characters: in boys, deeper voice, beard and broad shoulders; in girls, breast development and wider hips. A girl begins to have the monthly menstrual cycle (first period = menarche), and it stops around the age of 45–50 (menopause).

The menstrual cycle

The cycle averages 28 days and is controlled by FSH and LH from the pituitary and oestrogen and progesterone from the ovary.

Days	Event
1–5	Menstruation: the lining of the uterus breaks down and leaves the body with blood
6–13	The uterus lining rebuilds under oestrogen; an egg ripens in the ovary
About 14	Ovulation: the ripe egg is released from the ovary (LH peak)
15–28	The uterus lining thickens under progesterone. If the egg is not fertilised, the hormone level falls and menstruation starts again

If the egg is fertilised and settles in the uterus wall (implantation), menstruation stops and pregnancy begins.

Fertilisation, pregnancy and birth

1. Fertilisation: a sperm fuses with the egg in the fallopian tube to form a zygote (2n = 46).
2. The zygote divides repeatedly while moving to the uterus and sinks into the lining (implantation) after about 6–7 days. It is now an embryo.
3. The placenta forms from the embryo and the uterine wall. Through it the mother gives food and oxygen to the baby and takes away wastes; the umbilical cord joins the baby to the placenta.
4. The baby floats in the amniotic fluid inside the amniotic sac, which protects it from shocks. After the eighth week it is called a foetus.
5. Gestation lasts about 9 months (about 280 days or 40 weeks). The birth (parturition) is helped by oxytocin; the mother's body then makes milk (prolactin).

After birth the mother's breast milk is the best food for the baby, as it has the right nutrients and antibodies.

Twins

Identical twins form when one fertilised egg splits into two embryos; they have the same sex and genes. Fraternal twins form from two eggs fertilised by two sperms; they may be of different sex and look no more alike than other brothers and sisters.

Family planning and reproductive health

Birth control (contraception) methods: barrier methods (condom), hormonal pills and injections, the intrauterine device (copper-T), and surgical methods (vasectomy in men, tubectomy in women). Condoms also protect from sexually transmitted infections (STIs) such as HIV/AIDS, syphilis and gonorrhoea. Early marriage and teenage pregnancy are dangerous to the health of both mother and child; in Nepal the legal age of marriage is 20 years. Healthy habits, correct knowledge and regular health check-ups protect reproductive health.

5.3 Circulation

The circulatory system carries oxygen, digested food, hormones and waste products around the body. It has blood (the fluid), the heart (the pump) and blood vessels (the pipes).

Blood

An adult has about 5 litres of blood, which is about 7–8% of the body mass. Blood is made of a liquid, plasma (about 55%), and cells (about 45%).

Component	Feature	Function
Plasma	Pale yellow liquid, 90% water	Carries food, hormones, wastes (urea), carbon dioxide
Red blood cells (RBC, erythrocytes)	Biconcave discs, no nucleus, contain haemoglobin; 4.5–5.5 million per mm³; live about 120 days; made in the bone marrow	Carry oxygen (as oxyhaemoglobin)
White blood cells (WBC, leucocytes)	Larger, with a nucleus; 5,000–10,000 per mm³	Fight germs by eating them or making antibodies
Platelets (thrombocytes)	Cell fragments; 1.5–4 lakh per mm³	Help the blood to clot at a wound

Haemoglobin is an iron-containing red pigment. Lack of iron or of haemoglobin causes anaemia. Green vegetables, liver, beans and jaggery (sakhar) are rich in iron.

Blood groups. In 1901 Karl Landsteiner found the ABO blood groups, based on the antigens (A, B) on the red cells and antibodies in the plasma. A person with group O can donate to all (universal donor) and one with group AB can receive from all (universal recipient). The Rh factor is another antigen: people with it are Rh⁺, those without are Rh⁻.

Blood group	Antigen on RBC	Antibody in plasma	Can donate to	Can receive from
A	A	anti-B	A, AB	A, O
B	B	anti-A	B, AB	B, O
AB	A and B	none	AB	A, B, AB, O
O	none	anti-A and anti-B	A, B, AB, O	O

The heart

The heart is a muscular organ about the size of a fist, between the lungs, a little to the left. It has four chambers:

• two upper atria (auricles), which receive blood;
• two lower ventricles, which pump blood out. The left ventricle has a thicker wall because it pumps blood to the whole body.

Valves between atria and ventricles (the bicuspid valve on the left and the tricuspid valve on the right) and at the exits of the ventricles prevent blood from flowing backwards. The heart makes the "lub-dub" sound when the valves close.

The right side carries deoxygenated blood and the left side oxygenated blood; the septum keeps them from mixing.

Vessel	Carries blood	From → to
Vena cava (superior and inferior)	Deoxygenated	Body → right atrium
Pulmonary artery	Deoxygenated	Right ventricle → lungs
Pulmonary vein	Oxygenated	Lungs → left atrium
Aorta	Oxygenated	Left ventricle → body

Blood vessels

Feature	Artery	Vein	Capillary
Direction	Away from heart	Towards heart	Between artery and vein
Wall	Thick, elastic	Thin	One cell thick
Valves	Absent	Present	Absent
Blood pressure	High	Low	Falling
Blood (usually)	Oxygenated (except the pulmonary artery)	Deoxygenated (except the pulmonary vein)	Exchange of food, oxygen and waste with cells

Double circulation

In human beings the blood passes twice through the heart in one complete round, so it is called double circulation.

1. Pulmonary circulation: right ventricle → pulmonary artery → lungs (blood gets oxygen) → pulmonary vein → left atrium.
2. Systemic circulation: left ventricle → aorta → body → vena cava → right atrium.

William Harvey (1628) first described the circulation of blood.

Pulse, heartbeat and blood pressure

• Heartbeat. A resting adult heart beats about 72 times a minute. The pulse felt at the wrist is the stretch of an artery with each beat.
• Blood pressure is the force of blood on the vessel wall. Normal adult value is about 120/80 mm Hg (systolic/diastolic), measured with a sphygmomanometer.
• Stroke volume is the blood pumped by one ventricle in one beat (about 70 mL). Cardiac output = heart rate × stroke volume.

Heart diseases and care

• High blood pressure (hypertension): caused by stress, too much salt and fat, lack of exercise.
• Atherosclerosis and heart attack: fat deposits narrow the arteries and a clot blocks the blood flow to the heart muscle.
• Stroke: blockage or bursting of a blood vessel in the brain.

Care: regular exercise, less salt and oil, no smoking, no alcohol, healthy weight, and regular check-ups.

Lymphatic system

Some fluid leaks out of capillaries to the tissues (tissue fluid). The extra fluid returns to the blood as lymph, which flows through thin vessels with lymph nodes. Lymph carries fats from the intestine and white cells help to fight infection in the nodes.

5.4 Excretion

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

Organ	Excretory product
Kidneys	Urea, uric acid, excess water and salts (in urine)
Lungs	Carbon dioxide and water vapour
Skin	Water, salts and a little urea (in sweat)
Liver	Bile pigments (from the breakdown of haemoglobin); makes urea from ammonia

The urinary system

The urinary system has two kidneys, two ureters, one urinary bladder and one urethra.

• Kidneys: bean-shaped organs about 10 cm long, on either side of the backbone, just under the ribs; they filter the blood.
• Ureter: a tube from each kidney to the bladder.
• Urinary bladder: a muscular bag that stores urine (up to about 400–500 mL comfortably).
• Urethra: carries urine out of the body.

The nephron

Each kidney has about one million nephrons, the structural and functional units of the kidney. A nephron has:

• the Bowman's capsule, a cup around a cluster of capillaries called the glomerulus;
• a long coiled renal tubule: the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule, which opens into a collecting duct.

Urine formation

Urine is made in three steps.

1. Ultrafiltration (in the glomerulus): blood in the glomerulus is under high pressure. Water, glucose, salts, amino acids and urea pass into Bowman's capsule. Blood cells and big proteins remain in the blood. About 180 litres of filtrate is formed per day.
2. Selective reabsorption (in the tubule): useful substances (all the glucose, most of the water, amino acids and salts) go back into the blood.
3. Tubular secretion: extra wastes (urea, excess salts, drugs) are added to the filtrate by the tubule.

The fluid that reaches the collecting duct is urine: about 1.5 litres per day, pale yellow (the colour is from the pigment urochrome), made of about 95% water, 2% urea and 3% salts and other wastes.

Kidney problems and dialysis

• Kidney stones: hard deposits of salts (calcium oxalate) in the kidney or ureter; they cause severe pain. Drinking enough water helps to prevent them.
• Kidney failure: the kidneys stop cleaning the blood. Urea builds up in the blood. Treatment is dialysis (haemodialysis), where the patient's blood passes through a machine with a membrane that removes urea and extra salts and returns the clean blood, or a kidney transplant from a donor. Nepal has dialysis and transplant centres, such as the Tribhuvan University Teaching Hospital in Kathmandu.

Role of the skin and liver

The skin sweats to cool the body, and removes some water and salts. The liver changes harmful ammonia into urea and breaks down old red cells.

5.5 Homeostasis

Homeostasis works by negative feedback: when a condition goes away from its normal value, the body reacts to bring it back. Receptors detect the change, a control centre (often the brain) processes it, and effectors (muscles, glands) correct it.

Temperature regulation

The normal human body temperature is about 37 °C (98.6 °F). The control centre is the hypothalamus.

When it is hot	When it is cold
Sweat glands make more sweat; evaporation cools the skin	Sweating stops
Blood vessels in the skin widen (vasodilation), so more heat is lost	Blood vessels in the skin narrow (vasoconstriction), so less heat is lost
Hair lies flat	Hairs stand up (goose bumps) to trap warm air
Less food and activity	Shivering makes heat; adrenaline and thyroxine increase metabolism

Control of blood sugar

The normal blood glucose level is about 70–110 mg per 100 mL (fasting). The pancreas keeps it steady:

Condition	Pancreas releases	Effect
Blood sugar high (after a meal)	Insulin	Liver and muscles take up glucose and store it as glycogen; sugar falls
Blood sugar low (hungry)	Glucagon	Liver changes glycogen back to glucose; sugar rises

If insulin is lacking or does not work, blood sugar stays high: diabetes mellitus. Signs: much urine, thirst, tiredness, weight loss. Treatment: diet, exercise, tablets or insulin injections.

Water and salt balance

The kidney controls water balance under the control of ADH from the pituitary. When the body has too little water (hot day, little drinking), the blood becomes concentrated and the pituitary releases more ADH; the kidney reabsorbs more water and passes a small amount of dark urine. When we drink much water, less ADH is released and a large amount of pale urine is passed.

Other homeostatic organs

Organ	Role
Brain (hypothalamus)	Temperature, thirst, hunger
Kidney	Water, salts, pH, removal of urea
Pancreas	Blood glucose
Liver	Stores glucose; removes toxins
Lungs	Oxygen and carbon dioxide levels
Skin	Temperature, water loss

Chapter summary

• The CNS (brain + spinal cord) and the PNS (nerves) coordinate quickly using electrical impulses; the endocrine glands coordinate slowly using hormones.
• Neuron: dendrites, cell body, axon, myelin sheath. Reflex arc: receptor → sensory neuron → spinal cord → motor neuron → effector.
• Brain: cerebrum (thinking), cerebellum (balance), medulla (involuntary actions). Pituitary is the master gland; thyroxine needs iodine; insulin controls blood sugar.
• Human reproduction: sperm and egg meet in the fallopian tube; the zygote implants in the uterus; gestation about 280 days. Menstrual cycle 28 days, ovulation about day 14.
• Blood = plasma + RBC (oxygen) + WBC (defence) + platelets (clotting). Blood groups A, B, AB, O.
• Heart has four chambers; double circulation (pulmonary and systemic). Cardiac output = heart rate × stroke volume; normal BP 120/80 mm Hg.
• Nephron: filtration (glomerulus), reabsorption (tubule), secretion; about 180 L filtered and 1.5 L urine per day. Dialysis treats kidney failure.
• Homeostasis: constant internal conditions by negative feedback: body temperature 37 °C (hypothalamus), blood glucose (insulin and glucagon), water (ADH).

Practice questions

Group A — Objective questions (1 mark each)

1. The part of the brain that controls balance of the body is the [1]acerebrumbcerebellumcmedulla oblongatadthalamus
2. The hormone that lowers blood sugar is [1]aglucagonbadrenalinecinsulindthyroxine
3. Fertilisation in human beings takes place in the [1]auterusbovarycfallopian tubedvagina
4. The blood vessel that carries oxygenated blood from the lungs to the heart is the [1]apulmonary arterybpulmonary veincaortadvena cava
5. The functional unit of the kidney is the [1]aneuronbnephroncglomerulusdureter
6. The blood group known as the universal donor is [1]aAbBcABdO

Answers to Group A

1. (b) — the cerebellum controls balance and posture.

2. (c) — insulin lowers blood glucose; glucagon raises it.

3. (c) — the egg is fertilised in the fallopian tube.

4. (b) — the pulmonary vein is the one vein with oxygenated blood.

5. (b) — each kidney has about a million nephrons.

6. (d) — group O has no A or B antigens.

Group B — Short answer (2 marks each)

1. What is a reflex action? Give two examples. [2]
2. Why is the pituitary called the master gland? [2]
3. Write the functions of the placenta. [2]
4. Differentiate between an artery and a vein (two points). [2]
5. Define homeostasis. How does sweating help in homeostasis? [2]

Answers to Group B

1. A reflex action is a quick, automatic response to a stimulus controlled by the spinal cord without thinking. Examples: pulling the hand from a hot object; blinking.

2. It controls the work of many other endocrine glands (thyroid, adrenal, gonads) through its hormones.

3. It supplies food and oxygen from the mother to the foetus, removes carbon dioxide and wastes, and makes hormones.

4. An artery carries blood away from the heart; its wall is thick and it has no valves. A vein carries blood to the heart; its wall is thin and it has valves.

5. Homeostasis is the keeping of a constant internal environment. Sweat evaporates from the skin and takes heat from the body, so the temperature stays near 37 °C.

Group C — Long answer (4 marks each)

1. Draw and label a neuron. Describe the path of a reflex arc. [4]
2. Describe the 28-day menstrual cycle. [4]
3. A person's heart beats 75 times per minute and pumps 68 mL per beat. Calculate the cardiac output in litres per minute and the blood pumped in one hour. [4]
4. Explain how urine is formed in the nephron. [4]
5. Describe how blood sugar is kept constant in the body. [4]

Answers to Group C

1. Neuron: dendrites, cell body with nucleus, axon with myelin sheath, axon terminals. Reflex arc: stimulus → receptor → sensory neuron → spinal cord (relay neuron) → motor neuron → effector (muscle) → response.

2. Days 1–5: menstruation. Days 6–13: the uterus lining rebuilds and an egg ripens under FSH and oestrogen. Day 14: ovulation. Days 15–28: the lining thickens under progesterone; if there is no fertilisation, hormone levels fall and menstruation begins again.

3. Cardiac output = 75 × 68 = 5,100 mL = 5.1 L per minute. In one hour: 5.1 × 60 = 306 litres.

4. Ultrafiltration of blood in the glomerulus to Bowman's capsule (about 180 L/day); selective reabsorption of glucose, water, amino acids and salts in the tubule; tubular secretion of extra wastes. The remaining fluid is urine (about 1.5 L/day), sent to the bladder via the ureter.

5. When blood sugar rises after a meal, the pancreas releases insulin and the liver stores glucose as glycogen. When blood sugar falls, the pancreas releases glucagon and the liver turns glycogen into glucose. This is negative feedback; lack of insulin causes diabetes.