4. A Brief Account of Human Physiology

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CSS GSA — Paper 7, Part-I, Section II, Sub-topic 4: A Brief Account of Human Physiology

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Past-paper weightage: very high. Long questions on kidney/urine (2015, 2023), liver (2016, 2024), bile (2023), ear (2017), double circulation (2021), digestion (2018, 2023, 2023 Special "food cycle"), capillaries (2025). MCQs on bones, blood, urea and the liver appear most years.

1. Digestive System ("Food Cycle in the Body")

• Five stages: Ingestion (taking food in) → Digestion (mechanical + chemical breakdown) → Absorption (into blood/lymph through villi) → Assimilation (cells use nutrients) → Egestion (removal of faeces)
• Path: mouth → oesophagus (peristalsis) → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (colon, rectum) → anus

• Villi and microvilli give the small intestine a huge surface for absorption

1.1 Liver — "chief chemist of the body"

• Largest internal organ (≈ 1.5 kg)
• Makes bile; stores glycogen and keeps blood glucose steady
• Converts toxic ammonia into urea; detoxifies alcohol and drugs
• Makes albumin and clotting factors (fibrinogen, prothrombin)
• Stores vitamins A, D, B12 and iron; breaks down old red blood cells
• Gets blood mainly from the hepatic portal vein (food-rich blood from the gut, ≈ 75%) and the hepatic artery (oxygen)

1.2 Pancreas — both exocrine and endocrine

• Exocrine: pancreatic juice supplies bicarbonate, amylase, lipase and protease precursors such as trypsinogen; intestinal enteropeptidase activates trypsinogen to trypsin
• Endocrine: islets of Langerhans — insulin (β-cells, lowers glucose) and glucagon (α-cells, raises glucose)

1.3 Bile

• Greenish-yellow fluid made by the liver, stored in the gallbladder
• Emulsifies fat (it is not an enzyme), helps fat absorption and removes bilirubin/excess cholesterol. Pancreatic bicarbonate is the main neutraliser of gastric acid in the duodenum

2. Respiratory System

2.1 Definition and functions

The respiratory system brings oxygen into the body and removes carbon dioxide. Oxygen supports aerobic cellular respiration, which releases usable energy as ATP. Removal of carbon dioxide also helps regulate blood pH. Other functions include voice production, smell and defence against inhaled particles.

Three distinct processes: breathing (ventilation) moves air into and out of the lungs; gas exchange transfers oxygen and carbon dioxide between air, blood and tissues; cellular respiration consists of energy-releasing reactions within cells.

Aerobic respiration, simplified: glucose + oxygen → carbon dioxide + water + energy (ATP and heat).

2.2 Main organs and pathway of air

Nose/nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli.

2.3 Mechanism of breathing

Air flows down a pressure gradient. Increasing thoracic volume lowers alveolar pressure below atmospheric pressure; decreasing volume raises it.

Forced expiration recruits abdominal muscles and the interosseous parts of the internal intercostals. The lungs follow chest movement through pleural coupling; they do not actively suck in air by muscular contraction.

2.4 Gas exchange: alveoli and tissues

In the lungs: oxygen diffuses from alveolar air into blood; carbon dioxide diffuses from blood into alveoli for exhalation. Each gas moves down its own partial-pressure gradient.

At body tissues: oxygen diffuses from blood into cells; carbon dioxide produced by metabolism enters blood.

Alveoli exchange gases efficiently because they have:

• A very large combined surface area.
• Thin alveolar and capillary walls, providing a short diffusion distance.
• A moist lining in which gases dissolve.
• A rich capillary supply and continuing ventilation, maintaining gradients.
• Surfactant, which lowers surface tension and helps prevent alveolar collapse.

2.5 Transport and control

• Oxygen: carried mainly bound to haemoglobin in red blood cells as oxyhaemoglobin; a small amount dissolves in plasma.
• Carbon dioxide: transported mainly as bicarbonate ions, with smaller amounts bound to haemoglobin or dissolved.
• Control: respiratory centres in the medulla and pons coordinate breathing. Chemoreceptors respond to changes in carbon dioxide, pH and oxygen. Under ordinary resting conditions, rising carbon dioxide and the associated fall in pH provide a major stimulus to increase ventilation.
• During exercise: ventilation increases to support greater oxygen demand and carbon dioxide removal.

2.6 Important disorders and environmental links

2.7 CSS answer structure and diagram practice

For a 5-mark question on breathing: define ventilation → trace the air pathway briefly → compare inspiration and expiration → explain pressure changes → add a clear labelled diagram.

For a question on alveoli: connect each structural feature to its exchange advantage and state the directions of oxygen and carbon dioxide diffusion.

Diagram to practise: draw the trachea dividing into two bronchi, both lungs, branching bronchioles and the diaphragm below. Add an enlarged alveolus surrounded by a capillary; label oxygen moving towards blood and carbon dioxide towards the alveolus.

Practice questions — prepared for revision, not claimed as past-paper questions:

1. Explain inspiration and expiration with a labelled diagram.
2. How are alveoli adapted for gaseous exchange?
3. Distinguish breathing, gas exchange and cellular respiration.
4. Explain how oxygen and carbon dioxide are transported in blood.
5. Explain the effects of smoking on the respiratory system.

2.8 Quick revision / MCQ traps

• Gas exchange occurs primarily in alveoli, not the trachea.
• The diaphragm contracts during inspiration; quiet expiration is usually passive.
• Most oxygen travels with haemoglobin; most carbon dioxide as bicarbonate.
• Pulmonary artery: carries deoxygenated blood to the lungs. Pulmonary veins: return oxygenated blood to the heart.
• Exhaled air still contains oxygen; it is not pure carbon dioxide.
• Surfactant reduces surface tension; cilia help clear mucus and trapped particles.
• Respiration releases energy from nutrients; breathing supplies and removes the gases involved.

3. Circulatory System

3.1 Heart and double circulation

• Four chambers: two atria (receive) and two ventricles (pump); a septum keeps oxygenated and deoxygenated blood apart
• Pulmonary circuit: right ventricle → lungs → left atrium
• Systemic circuit: left ventricle → body → right atrium
• Left ventricle has the thickest wall; valves (tricuspid, bicuspid/mitral, semilunar) stop backflow
• Textbook examples: resting pulse ≈ 72 beats/min and blood pressure ≈ 120/80 mmHg; these are illustrative values, not fixed values for every person

3.2 Blood vessels

3.3 Blood

• Blood is made in the red bone marrow; the spleen destroys old RBCs ("graveyard of RBCs")
• Clotting happens at the site of injury inside the damaged vessel: platelets plug the hole and fibrinogen turns into fibrin threads
• Blood groups (Karl Landsteiner, 1901): A, B, AB, O; O negative = conventional universal red-cell donor; AB positive = conventional universal red-cell recipient. These rules concern red cells, not plasma or whole blood; compatibility testing is still required

4. Excretory System (Kidneys)

• Two kidneys, each with about one million nephrons
• Urine formation: glomerular filtration into Bowman's capsule → selective reabsorption (glucose, amino acids, most water) in the tubule and loop of Henle → tubular secretion (H⁺, K⁺, drugs) → collecting duct → ureter → bladder → urethra
• Urea is the main nitrogenous waste of protein metabolism in humans (made in the liver, removed by kidneys)
• Kidneys also control water/salt balance (osmoregulation, with ADH), blood pH, blood pressure (renin), and make erythropoietin for RBC production
• Other excretory organs: skin (sweat), lungs (CO₂, water vapour), liver (bile pigments)

4.1 Urine formation — explain the direction of each process

Nephron route: Bowman’s capsule → proximal convoluted tubule → loop of Henle → distal convoluted tubule → collecting duct. Urine then passes through calyces → renal pelvis → ureter → bladder → urethra.

Hormonal control: ADH increases water reabsorption in the collecting system. Aldosterone promotes sodium reabsorption and potassium secretion in the distal nephron. Kidneys also activate vitamin D.

CSS distinction: filtration is not the removal of all blood contents; reabsorption returns substances to blood, while secretion moves substances into tubular fluid. Excretion removes metabolic waste; egestion removes undigested food.

Source: NIDDK — kidney function.

5. Nervous System

• Central: brain and spinal cord; peripheral: nerves
• Cerebrum: thinking, memory, speech, voluntary action
• Cerebellum: balance and coordination
• Medulla oblongata: breathing, heartbeat, reflexes like coughing
• Neuron is the basic unit; a reflex arc gives quick automatic responses
• Dementia: loss of thinking, memory and reasoning (Alzheimer's is the commonest cause)

5.1 Coordination and reflex action

Neuron: dendrites receive signals → cell body integrates them → axon conducts impulses to terminals. Myelin speeds conduction; neurotransmitters transmit signals across most synapses.

Reflex arc: receptor → sensory neuron → central nervous system (often via an interneuron) → motor neuron → effector. Withdrawal from a hot object is a rapid protective response; conscious awareness follows processing in the brain.

Hypothalamus: coordinates temperature, thirst and endocrine regulation. Spinal cord: conducts signals and integrates many reflexes. Autonomic nervous system: sympathetic activity supports fight or flight; parasympathetic activity supports rest and digestion.

Exam trap: a hormone circulates widely but acts on cells with suitable receptors; “master gland” does not mean the pituitary acts independently of the hypothalamus.

6. Endocrine System (Hormones)

6.1 Negative feedback and common distinctions

Blood glucose: a rise stimulates pancreatic beta cells to release insulin, promoting glucose uptake in muscle/adipose tissue and storage; a fall stimulates alpha cells to release glucagon, increasing hepatic glucose output. Negative feedback opposes the initial change.

• Diabetes mellitus: impaired insulin secretion and/or action causes persistent high blood glucose.
• Diabetes insipidus: deficient ADH action causes excessive dilute urine; it is distinct from diabetes mellitus.
• Parathyroid hormone: raises blood calcium.
• Adrenal cortex: cortisol and aldosterone; adrenal medulla: adrenaline.
• Exocrine glands release through ducts; endocrine glands release hormones into surrounding fluid and blood.

7. Sense Organs

7.1 Ear

7.2 Eye

• Light path: cornea → pupil (size set by iris) → lens → retina (rods for dim light, cones for colour) → optic nerve → brain

8. Skeletal and Muscular System

• Adult has 206 bones (a newborn about 300, which fuse); femur (thigh) is the longest and strongest; stapes is the smallest
• Functions: support, protection, movement, blood formation in marrow, calcium store
• Muscles: skeletal (voluntary), smooth (involuntary, gut walls), cardiac (involuntary, rhythmic, fatigue-resistant; requires continuous oxygen supply)

8.1 Movement: link structure with function

• Tendons connect muscle to bone; ligaments connect bone to bone.
• Skeletal muscles generate pulling force and often act in antagonistic pairs, such as biceps and triceps.
• A synovial joint includes articular cartilage, a joint capsule and synovial fluid, supporting movement with reduced friction.
• Bone is living tissue; red marrow forms blood cells and bone stores calcium and phosphate.

9. MCQ Traps

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• 206 bones; femur longest; stapes smallest
• Liver's main blood supply = hepatic portal vein (no "hepatic portal artery")
• Protein waste in humans = urea (fish excrete ammonia; birds uric acid)
• Left ventricle thickest; arteries away, veins towards the heart
• Bile emulsifies fat but is not an enzyme
• Fish die out of water because gills collapse and cannot absorb oxygen from air

10. Self-Test Questions

1. "Liver is the chief chemist in the human body." Comment.
2. Draw the human ear and explain its functions.
3. How is urine formed? What is the role of the kidney in excretion?
4. Describe the food cycle (ingestion to egestion) in the human body.
5. What is double circulation? How is the heart adapted for it?
6. Describe the structure and functions of capillaries.

11. CSS Exam Perspective

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• Most repeated areas: liver and pancreas, kidney and urine formation, digestion, heart and circulation, ear
• Additional revision priorities: nervous system, endocrine glands, blood composition and groups, and breathing. These are study priorities, not predictions of a future paper.
• Tip: use accurate labelled diagrams where relevant, then explain structure and function; marks depend on the question and answer quality

Syllabus completion — a brief account of the whole body

Homeostasis and coordination

Homeostasis maintains internal variables within workable ranges through receptors, a control centre and effectors. Negative feedback opposes a change: rising glucose stimulates insulin; falling glucose stimulates glucagon. Increased blood concentration stimulates ADH release, increasing collecting-duct water reabsorption. ADH is synthesised in the hypothalamus and released from the posterior pituitary.

A neuron has dendrites, a cell body and an axon. Myelin speeds conduction; synapses use chemical messengers to communicate. A simple reflex pathway is receptor → sensory neuron → central nervous system → motor neuron → effector. The autonomic system controls many involuntary activities; sympathetic and parasympathetic divisions often have complementary effects.

Respiratory and circulatory mechanisms

Alveoli provide a large, moist exchange surface with thin walls and nearby capillaries. Oxygen and carbon dioxide diffuse down partial-pressure gradients. Haemoglobin carries most oxygen; most carbon dioxide travels as bicarbonate. Breathing moves air; gas exchange moves gases; cellular respiration releases usable energy from nutrients.

Full blood route: body → venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve/artery → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve/aorta → body. The sinoatrial node initiates the heartbeat. Coronary vessels supply the heart muscle.

Capillary answer: connect the one-cell-thick endothelium to a short diffusion distance, extensive networks to large exchange area, and large total cross-sectional area to slow average flow. Not every tissue is vascular; cartilage and the cornea are important exceptions.

Digestive and excretory precision

• Bile emulsifies fat and supports absorption; pancreatic bicarbonate is the principal neutraliser of acid entering the duodenum.
• The pancreas releases trypsinogen; intestinal enteropeptidase activates it to trypsin. Pancreatic lipase mainly produces monoglycerides and free fatty acids.
• Glucose and amino acids enter portal blood. Most absorbed long-chain fats are packaged into chylomicrons and enter lacteals before blood.
• The nephron filters blood, reabsorbs needed materials and secretes selected substances. The loop of Henle supports a concentration gradient; ADH adjusts collecting-duct water permeability.
• Excretion removes metabolic waste such as urea; egestion removes undigested food.

Systems missing from the brief overview

FSH and LH regulate the gonads. In the ovarian cycle, follicle development precedes ovulation; the corpus luteum produces progesterone to support the endometrium. Pregnancy involves placental exchange of gases, nutrients and wastes without normal direct mixing of maternal and fetal blood.

Immunity: innate defence includes barriers, inflammation and phagocytes. Adaptive defence involves B cells/antibodies, T cells and memory. Vaccination induces active immunity; transferred antibodies provide passive protection.

Movement and senses: skeletal muscles pull on bones through tendons, commonly working in antagonistic pairs; ligaments connect bones. In the eye, lens shape changes for near/far focusing; retinal rods favour dim light and cones colour/detail. Cochlear hair cells detect sound; the vestibular apparatus supports balance.

Answer practice linked to the saved papers

For liver/pancreas (2016/2024), separate liver metabolism from pancreatic exocrine/endocrine roles. For kidney (2015/2023), explain filtration, reabsorption and secretion with a nephron diagram. For ear (2017), label the parts and explain both hearing and balance. For double circulation (2021), trace both circuits and relate chamber/valve structure to function. For food cycle (2023 Special), include all five stages. For capillaries (2025), pair each feature with its exchange function.

Additional practice—not past-paper: Explain negative feedback using blood glucose; compare nervous and hormonal coordination; trace a red cell through one complete double circuit.

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12. Past-Paper Long Questions — Exam-Ready Answers

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How to use these answers: These are focused model answers for the 5-mark questions recorded below. Suggested coverage splits are revision guides, not official FPSC marking schemes. Answer every command in the question; use a short definition, clear headings and relevant labelled diagrams. The years and question references come from the existing notes and remain unverified against original papers. The previously combined liver/pancreas/bile entry is separated into topic variants; these variants are not claimed to reproduce separate original questions.

• CSS 2025 Q3(b): Describe the structure and functions of capillaries in the human blood circulatory system. (5 marks)

  Suggested coverage: definition and structure 2; functions linked to adaptations 3.

  Model answer

  Capillaries are microscopic blood vessels that connect arterioles with venules and form the main exchange surface between blood and tissues.

  Structure: Their walls consist of a single layer of endothelial cells supported by a thin basement membrane. They lack the thick muscular coat of arteries. Their narrow lumen often allows red blood cells to pass in single file. Extensive branching forms networks with a large exchange surface. The networks’ large total cross-sectional area produces slow average blood flow, allowing time for exchange.

  Functions:

  • Oxygen and nutrients pass from blood to tissues, while carbon dioxide and metabolic wastes enter the blood.
  • Pulmonary capillaries exchange oxygen and carbon dioxide with alveolar air.
  • Intestinal capillaries absorb water-soluble nutrients, including glucose and amino acids.
  • Specialised glomerular capillaries filter water and small solutes during urine formation.
  • Skin capillary blood flow contributes to heat exchange.

  Thus, thin walls, extensive networks and slow flow make capillaries efficient exchange vessels.

  Diagram cue: arteriole → capillary network beside tissue cells → venule; show oxygen/nutrients moving out and carbon dioxide/wastes moving in.

• CSS 2023 Special Q4(c): Write a comprehensive note on the “Food Cycle in our Body”. (5 marks)

  Suggested coverage: introduction and pathway 1; five stages with correct mechanisms 4.

  Model answer

  The food cycle is the processing of food through ingestion, digestion, absorption, assimilation and egestion.

  1. Ingestion: Food enters the mouth, where teeth break it into smaller pieces. Saliva moistens it, and salivary amylase begins starch digestion. Swallowing and oesophageal peristalsis move it to the stomach.
  2. Digestion: The stomach churns food; hydrochloric acid provides an acidic medium for pepsin to digest proteins. In the duodenum, bile emulsifies fats, while pancreatic amylase, proteases and lipase digest carbohydrates, proteins and fats. Intestinal brush-border enzymes complete carbohydrate and protein digestion.
  3. Absorption: Small-intestinal villi and microvilli provide a large surface. Glucose and amino acids enter blood capillaries and travel to the liver through the hepatic portal vein. Most long-chain dietary fats enter lacteals as chylomicrons and later reach the blood.
  4. Assimilation: Cells use absorbed nutrients for energy, growth and repair; surplus nutrients may be stored as glycogen or fat.
  5. Egestion: The large intestine absorbs remaining water and salts. Faeces are stored in the rectum and expelled through the anus.

  Diagram cue: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus; add liver, gallbladder and pancreas beside the duodenum.

  Precision: Egestion removes undigested material; excretion removes metabolic wastes.

• Liver, pancreas and bile — references recorded together: CSS 2016 Q5(a), 2024 Q4(b), 2023 Q3(a)(ii)

  The old notes combined several question prompts under one 5-mark label. Use the relevant variant below according to the actual question and its stated marks.

  Variant A: “Liver is the chief chemist in the human body.” Comment. — 5-mark practice answer

  The liver is called the chief chemist because it carries out numerous reactions essential for metabolism, storage, detoxification and excretion.

  • Carbohydrate metabolism: It stores glucose as glycogen, breaks glycogen down when required and forms glucose from non-carbohydrate sources, helping maintain blood glucose.
  • Protein metabolism: It removes amino groups from excess amino acids and converts toxic ammonia into urea for renal excretion. It also synthesises albumin and many clotting factors.
  • Fat metabolism: It processes fatty acids and synthesises cholesterol and lipoproteins.
  • Detoxification: It metabolises many drugs, alcohol and other potentially harmful substances.
  • Bile formation: It produces bile, supporting fat digestion and absorption and providing a route for bilirubin and excess cholesterol excretion.
  • Storage: It stores glycogen, iron and vitamins such as A and B12.

  These coordinated functions explain its central role in maintaining the chemical balance of blood.

  Variant B: Discuss the functioning of liver and pancreas. — 5-mark practice answer

  The liver and pancreas are accessory digestive organs that also regulate metabolism.

  Liver: It produces bile, whose salts emulsify fat and aid absorption. It regulates blood glucose through glycogen storage and breakdown, converts ammonia into urea, metabolises drugs and alcohol, synthesises albumin and clotting factors, and stores vitamins and iron.

  Pancreas—exocrine role: It releases pancreatic juice into the duodenum. Bicarbonate neutralises acidic chyme; amylase digests starch; lipase digests fat; protease precursors support protein digestion. Trypsinogen is activated to trypsin in the small intestine.

  Pancreas—endocrine role: Beta cells release insulin, promoting glucose uptake and storage and lowering blood glucose. Alpha cells release glucagon, increasing hepatic glucose production when blood glucose falls.

  Together, these organs support digestion, nutrient processing and glucose homeostasis.

  Suggested balance: approximately half the answer on each organ; include both pancreatic roles.

  Variant C: Write a note on bile. — 5-mark practice answer

  Bile is a yellowish-green digestive fluid produced by the liver, stored and concentrated in the gallbladder, and released through bile ducts into the duodenum.

  Composition: It contains water, bile salts, bile pigments such as bilirubin, cholesterol and electrolytes. It contains no digestive enzymes.

  Functions:

  • Bile salts emulsify large fat globules into small droplets, increasing the surface available to pancreatic lipase.
  • They form micelles that facilitate absorption of fat digestion products and fat-soluble vitamins A, D, E and K.
  • Bile provides an excretory route for bilirubin from haemoglobin breakdown and for excess cholesterol.

  Bile therefore assists fat digestion physically rather than enzymatically. Pancreatic bicarbonate is the principal neutraliser of gastric acid entering the duodenum.

  Flow cue: liver → bile ducts ↔ gallbladder → common bile duct → duodenum.

• CSS 2015 Q3(b), 2023 Q3(b): Discuss the function of kidneys. How is urine formed? (5 marks)

  Suggested coverage: kidney functions 2; urine formation and outflow 3.

  Model answer

  The kidneys maintain the internal environment by filtering blood and producing urine. Their functional units are nephrons.

  Functions: They remove urea and other metabolic wastes, regulate water and electrolyte balance, and maintain acid–base balance. They also help regulate blood pressure through renin, stimulate red-cell production through erythropoietin, and activate vitamin D.

  Urine formation:

  1. Glomerular filtration: Blood pressure forces water and small dissolved substances from glomerular capillaries into Bowman’s capsule. Blood cells and most large proteins remain in the circulation.
  2. Selective reabsorption: Useful substances, including glucose, amino acids, much of the filtered salts and most water, return from the tubules to blood. The loop of Henle helps establish the concentration gradient needed to conserve water.
  3. Tubular secretion: Substances such as hydrogen ions, potassium and certain drugs move from blood into tubular fluid, helping regulate pH and composition.
  4. Final concentration: ADH increases collecting-duct water permeability, allowing more water to return to blood when water must be conserved.

  Urine passes through collecting ducts to the renal pelvis, then ureter → bladder → urethra.

  Diagram cue: label glomerulus, Bowman’s capsule, proximal tubule, loop of Henle, distal tubule and collecting duct; distinguish blood flow from filtrate flow.

• CSS 2017 Q3(c): Draw the structure of the human ear and briefly explain its functions. (5 marks)

  Suggested coverage: labelled anatomical diagram 2; structure and functions 3. The split is a practice guide.

  Model answer

  The ear detects sound and helps maintain balance. It consists of outer, middle and inner regions.

  Outer ear: The pinna collects sound waves and directs them through the auditory canal to the tympanic membrane (eardrum).

  Middle ear: Eardrum vibrations pass through the malleus, incus and stapes to the oval window. The ossicles increase pressure at the oval window for effective transmission into inner-ear fluid. The Eustachian tube connects the middle ear with the pharynx and equalises pressure across the eardrum.

  Inner ear—hearing: Vibrations create waves in cochlear fluid, moving the basilar membrane and bending hair cells in the organ of Corti. These cells convert mechanical stimulation into electrical signals carried through the cochlear nerve to the brain.

  Inner ear—balance: Semicircular canals detect rotational acceleration. The utricle and saccule detect linear acceleration and head position relative to gravity, supporting posture and equilibrium.

  Required diagram: Use the anatomical ear figure later on this page as a drawing reference. Label pinna, auditory canal, eardrum, malleus, incus, stapes, oval window, cochlea, semicircular canals, vestibule, auditory nerve and Eustachian tube. A flowchart alone does not satisfy “draw the structure.”

• CSS 2021 Q3(d): What is meant by double circulation? Describe how the heart is adapted to keep blood flowing in a double circulation. (5 marks)

  Suggested coverage: definition and circuits 2; adaptations linked to function 3.

  Model answer

  Double circulation means that blood passes through the heart twice during one complete journey through the lungs and body.

  Pulmonary circuit: right ventricle → pulmonary arteries → lung capillaries → pulmonary veins → left atrium. Blood releases carbon dioxide and gains oxygen.

  Systemic circuit: left ventricle → aorta → body tissues → venae cavae → right atrium. Blood supplies tissues with oxygen and nutrients and collects wastes.

  Adaptations of the heart:

  • Four chambers and a complete septum keep oxygenated and deoxygenated blood separate.
  • A thick muscular left ventricle generates the high pressure required to supply the whole body.
  • A thinner right ventricular wall pumps through the shorter pulmonary circuit at lower pressure, protecting delicate lung capillaries.
  • Atrioventricular and semilunar valves prevent backflow and maintain one-way movement.
  • Coordinated rhythmic contraction, initiated by the sinoatrial node, allows the atria to fill the ventricles before ventricular ejection.

  This arrangement supplies oxygenated blood efficiently while maintaining suitable pressures in the two circuits.

  Diagram cue: body → right atrium → right ventricle → lungs → left atrium → left ventricle → body. Add the named vessels and arrows.

13. Past-Paper MCQ Quiz (FPSC)

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MCQs recorded in these notes as FPSC questions, newest first. Original papers and official keys have not been independently checked in this review. Explanations support revision; ambiguous wording is flagged.

1. (CSS 2024) Fishes die outside the water due to:

• (A) Carbon monoxide (B) Oxygen (C) Sunlight (D) Non-absorption of oxygen
• ✅ Answer: (D) Non-absorption of oxygen — gill filaments need water to stay apart; in air they collapse and stick together, so the fish suffocates even though air has plenty of oxygen.

2. (CSS 2024) In dementia a person:

• (A) Loses weight (B) Loses thinking and writing (C) Loses thinking, remembering and reasoning (D) All of these
• ✅ Answer: (C) — dementia is a decline in memory, thinking and reasoning severe enough to disturb daily life.

3. (CSS 2019) Blood contains:

• (A) Red blood cells (B) White blood cells (C) Platelets (D) All of these
• ✅ Answer: (D) All of these — all three float in plasma (which is the fourth component).

4. (CSS 2019) The number of bones in the human body is:

• (A) 200 (B) 202 (C) 204 (D) 206
• ✅ Answer: (D) 206 — in an adult; babies have about 300 that later fuse.

5. (CSS 2019) Which vessel provides blood to the liver?

• (A) Hepatic portal artery (B) Veins (C) Arteries (D) None of these
• Ambiguous item as transcribed: the liver receives blood from both the hepatic portal vein and hepatic artery, so both broad options “veins” and “arteries” describe real supplies. Do not treat (D) as a verified key. There is no hepatic portal artery.

6. (CSS 2019) What is the protein metabolism waste in humans?

• (A) Urea (B) Ammonia (C) Magnesium (D) None of these
• ✅ Answer: (A) Urea — the liver turns toxic ammonia from amino acids into urea, which the kidneys remove.

7. (CSS 2017) The longest and largest bone in the human body is:

• (A) Spinal cord (B) Humerus (C) Fibula (D) Femur
• ✅ Answer: (D) Femur — the thigh bone. The spinal cord is nerve tissue, not bone.

8. (CSS 2017) Blood clotting happens in which part of the human body?

• (A) Veins (B) Heart (C) Brain (D) None of these
• ✅ Answer: (D) None of these (key) — clotting happens at the site of injury in any damaged blood vessel, not in one fixed organ. Clotting factors are made in the liver.

Labelled figures and corrected numerical revision

Double circulation — follow the arrows

flowchart TD
 A["Body tissues"] -->|"venae cavae"| B["Right atrium"]
 B -->|"tricuspid valve"| C["Right ventricle"]
 C -->|"pulmonary valve and arteries"| D["Lung capillaries"]
 D -->|"pulmonary veins"| E["Left atrium"]
 E -->|"mitral valve"| F["Left ventricle"]
 F -->|"aortic valve and aorta"| A

Blood passes through the heart twice per complete circuit. Artery/vein names refer to direction relative to the heart, not oxygen content. OpenStax heart anatomy.

Nephron — distinguish blood flow from filtrate flow

flowchart TD
 A["Afferent arteriole"] --> B["Glomerular blood"]
 B -->|"blood stays in vessels"| C["Efferent arteriole"]
 C --> D["Peritubular capillaries / vasa recta"]
 B -->|"filtration"| E["Bowman's capsule: filtrate"]
 E --> F["Proximal tubule"]
 F --> G["Loop of Henle"]
 G --> H["Distal tubule"]
 H --> I["Collecting duct"]
 I --> J["Renal pelvis then ureter"]
 F -->|"reabsorption to blood"| D
 D -->|"secretion into tubule"| H

This schematic shows representative exchange arrows; reabsorption and secretion occur at multiple tubular sites. Blood cells and most large proteins remain in blood. ADH increases water permeability in the collecting system. NIDDK kidney function.

Ear — anatomical figure

External, middle and inner ear — OpenStax Anatomy and Physiology 2e, Figure 14.5

Source: OpenStax sensory perception, Figure 14.5. Sound: pinna → canal → eardrum → malleus → incus → stapes → oval window → cochlear fluid/hair cells → auditory nerve. Semicircular canals detect angular acceleration; utricle/saccule detect linear acceleration and head position. The Eustachian tube equalises pressure, not sound transduction.

Numbers to revise with their meaning

Sources: OpenStax blood, NIDDK kidneys, OpenStax teeth.

Additional supporting gap — common vision defects: myopia focuses distant images in front of the retina and is corrected with a diverging/concave lens; hyperopia tends to focus behind the retina and uses a converging/convex lens; presbyopia is age-related loss of accommodation; astigmatism involves unequal optical curvature and needs appropriate cylindrical correction.

Short-part answer — bile

Saved CSS 2023 Q3(a)(ii): short part of a combined question. The hub records 2.5 marks as a planning allocation; check the printed paper. The 5-mark bile variant above is extended practice.

Bile is a yellowish-green fluid produced by the liver and stored and concentrated in the gallbladder. Released into the duodenum, bile salts emulsify fat into small droplets and support micelle formation, assisting lipase action and fat absorption. Bile also carries bilirubin and excess cholesterol for excretion. It contains no digestive enzymes; pancreatic bicarbonate is the principal neutraliser of incoming gastric acid.

Answer focus: source/storage → fat-processing role → excretory role.