Human Body Systems — Organs, Functions & Key Concepts

The human body runs on coordinated systems. Each system handles a different set of jobs, but they're all interconnected — the circulatory system delivers oxygen that the respiratory system brings in, the digestive system provides nutrients that the circulatory system distributes, and the nervous system controls all of them. Understanding each system individually, then seeing how they link up, is the key to doing well in this chapter.

The Circulatory System

Overview

The circulatory system transports oxygen, nutrients, hormones, and waste products throughout the body. Humans have a closed, double circulatory system — blood is always contained in vessels, and it passes through the heart twice in one complete circuit.

Double circulation means two loops:

1. Pulmonary circulation: Right side of heart → lungs → left side of heart (blood picks up O₂, drops off CO₂)
2. Systemic circulation: Left side of heart → body tissues → right side of heart (blood delivers O₂ and nutrients, picks up CO₂ and waste)

The Heart — Structure

The human heart is a four-chambered, muscular pump located slightly to the left of the midline in the thoracic cavity.

Four chambers:

• Right atrium (RA): Receives deoxygenated blood from the body via the superior and inferior vena cava
• Right ventricle (RV): Pumps deoxygenated blood to the lungs via the pulmonary artery
• Left atrium (LA): Receives oxygenated blood from the lungs via the four pulmonary veins
• Left ventricle (LV): Pumps oxygenated blood to the body via the aorta (the largest artery)

Valves — prevent backflow:

• Tricuspid valve: Between RA and RV (3 cusps)
• Bicuspid (Mitral) valve: Between LA and LV (2 cusps)
• Pulmonary semilunar valve: Between RV and pulmonary artery
• Aortic semilunar valve: Between LV and aorta

Chordae tendineae: Tendon-like strings attached to the cusps of the tricuspid and bicuspid valves, preventing them from inverting when ventricles contract.

Pericardium: Double-walled fibrous sac surrounding the heart. The fluid between the layers (pericardial fluid) reduces friction during heartbeat.

Heart Sounds and Cardiac Cycle

The "lub-dub" heartbeat sound:

• "Lub" (S1): Closure of the tricuspid and bicuspid (AV) valves at the start of ventricular contraction (systole)
• "Dub" (S2): Closure of the pulmonary and aortic semilunar valves at the start of ventricular relaxation (diastole)

Blood pressure: Expressed as systolic/diastolic. Normal = 120/80 mmHg.

• 120 mmHg = peak pressure during ventricular contraction (systole)
• 80 mmHg = pressure during ventricular relaxation (diastole)

Heartbeat Origin — Nodal Tissue

The heart is myogenic (generates its own electrical impulse, not dependent on nerves):

• SA node (Sinoatrial node): Located in the right atrium wall. The primary pacemaker — generates impulses at 70–75 beats/min
• AV node (Atrioventricular node): Located at the atrial–ventricular junction. Receives impulse from SA node; delays it slightly to allow atria to fully contract before ventricles
• Bundle of His: Carries impulse from AV node down the interventricular septum
• Purkinje fibres: Distribute impulse throughout the ventricles → ventricular contraction

Blood Vessels

Arterioles: Small arteries connecting arteries to capillaries; control blood flow to tissues (vasoconstriction/vasodilation).

Venules: Small veins collecting blood from capillaries.

Blood — Components

• Plasma (55%): Mostly water (~91%), proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, hormones, waste products
• Red Blood Cells/RBCs/Erythrocytes (45%): Biconcave disc; no nucleus (in mature RBCs); contain haemoglobin; carry O₂ and some CO₂; lifespan ~120 days; produced in red bone marrow
• White Blood Cells/WBCs/Leukocytes (<1%): Nucleated; part of immune system. Types: neutrophils, eosinophils, basophils, monocytes, lymphocytes
• Platelets/Thrombocytes (<1%): Cell fragments; no nucleus; essential for blood clotting

Blood groups: ABO system (A, B, AB, O) based on antigens on RBC surface. O⁻ is universal donor; AB⁺ is universal recipient.

The Digestive System

Overview: Alimentary Canal Path

Food travels: Mouth → Pharynx → Oesophagus → Stomach → Small Intestine (Duodenum → Jejunum → Ileum) → Large Intestine (Caecum → Colon → Rectum) → Anus

Digestion at Each Stage

Mouth (Buccal cavity):

• Mechanical digestion: chewing (mastication) by teeth
• Chemical digestion: salivary amylase (ptyalin) breaks starch → maltose (works at pH 6.8)
• Food + saliva → bolus (soft ball)
• Mucin (in saliva): lubricates food for swallowing

Oesophagus:

• Muscular tube connecting mouth to stomach
• Peristalsis (rhythmic muscular contractions) pushes bolus downward
• No digestion occurs here
• Cardiac sphincter (gastroesophageal sphincter) at the base controls entry into stomach

Stomach:

• Gastric glands in the stomach lining secrete gastric juice:
  • HCl (hydrochloric acid): kills bacteria, activates pepsinogen, provides acidic pH (~2)
  • Pepsinogen → activated by HCl → pepsin: breaks proteins → peptides
  • Mucus: protects stomach wall from self-digestion
  • Intrinsic factor (chief cells/parietal cells): needed for Vitamin B12 absorption in the ileum
• Food + gastric juice → chyme (semi-liquid)
• Pyloric sphincter regulates chyme entry into small intestine

Small Intestine: This is where most digestion and ALL absorption occurs.

Sub-parts: Duodenum (25 cm) → Jejunum (2.5 m) → Ileum (3.5 m)

Bile (from liver, stored in gall bladder):

• Bile salts: emulsify large fat globules into tiny droplets (increase surface area for lipase action)
• Bile pigments (bilirubin, biliverdin): breakdown products of haemoglobin — give colour to faeces
• No enzymes in bile (common misconception to correct!)

Pancreatic juice (from pancreas via pancreatic duct into duodenum):

• Pancreatic amylase: starch → maltose
• Pancreatic lipase: emulsified fats → fatty acids + glycerol
• Trypsinogen → activated by enterokinase (from intestinal mucosa) → trypsin: proteins → peptides
• Chymotrypsin: proteins/peptides → smaller peptides
• Carboxypeptidase: peptides → amino acids
• DNase and RNase: DNA, RNA → nucleotides
• Sodium bicarbonate (NaHCO₃): neutralises acid chyme from stomach → provides alkaline pH for intestinal enzymes

Intestinal juice (succus entericus, from intestinal glands):

• Maltase: maltose → glucose + glucose
• Sucrase: sucrose → glucose + fructose
• Lactase: lactose → glucose + galactose
• Peptidases (aminopeptidase, dipeptidase): final breakdown of peptides → amino acids
• Intestinal lipase: further fat digestion

Absorption:

• Villi: Finger-like projections of the small intestinal mucosa that increase surface area (~5 m²)
• Microvilli: Tiny projections on each villus epithelial cell (brush border) — increase surface area further
• Each villus has a capillary network (for glucose and amino acid absorption) and a lacteal (a lymph capillary for fat absorption)
• Fatty acids and glycerol → re-form triglycerides in intestinal cells → packaged into chylomicrons → enter lacteals → lymphatic system → bloodstream (via thoracic duct into subclavian vein)

Large Intestine:

• Absorbs water, some electrolytes, and vitamins (especially Vitamin K and B12 produced by bacteria)
• No digestive enzymes secreted
• Symbiotic bacteria (gut flora): break down undigested food, produce some vitamins
• Converts remaining material into faeces (undigested food, dead bacteria, dead cells, mucus)
• Rectum: Stores faeces until defecation

The Respiratory System

Pathway of Air

Nostrils → Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli

Key Structures

Nasal cavity: Filters, warms, and moistens air. Mucus and cilia trap particles. The nasal cavity is more efficient than mouth breathing for conditioning air.

Larynx (Voice box): Contains vocal cords. The epiglottis covers the laryngeal opening during swallowing — prevents food from entering the airway.

Trachea: Reinforced by C-shaped cartilage rings → prevents collapse during breathing. Lined by ciliated mucous epithelium — cilia sweep mucus and trapped particles up toward the pharynx (mucociliary escalator).

Bronchi: Trachea bifurcates into right and left primary bronchi, each entering a lung. Further divide into secondary and tertiary bronchi.

Bronchioles: Smaller branches, no cartilage. Terminal bronchioles end in alveolar ducts.

Alveoli: The functional units of the lung (~700 million in human lungs). Thin-walled air sacs surrounded by dense capillary networks. Site of gas exchange.

Mechanism of Breathing — Inhalation and Exhalation

Breathing is driven by pressure changes created by muscle action:

Inhalation (inspiration):

1. Diaphragm contracts → flattens downward → increases thoracic volume
2. External intercostal muscles contract → ribs move up and outward → further increases thoracic volume
3. Lung volume increases → intrapulmonary pressure drops below atmospheric pressure
4. Air flows in (high pressure → low pressure)

Exhalation (expiration) — normal breathing:

1. Diaphragm relaxes → returns to dome shape → decreases thoracic volume
2. Internal intercostal muscles relax → ribs move down and inward
3. Lung volume decreases → intrapulmonary pressure rises above atmospheric pressure
4. Air flows out

Forced exhalation (deep breathing): Internal intercostal muscles and abdominal muscles actively contract to forcibly push air out.

Gas Exchange — Alveoli to Blood

Gas exchange follows Fick's Law: diffusion rate ∝ surface area × concentration difference / membrane thickness.

Alveoli are perfect for gas exchange:

• Very large surface area (~70 m² total)
• Very thin walls (0.1–0.2 μm)
• Rich blood supply (capillaries around every alveolus)
• Short diffusion distance

In the alveolus:

• O₂ concentration is HIGH in alveolar air, LOW in capillary blood → O₂ diffuses from alveolus into blood
• CO₂ concentration is LOW in alveolar air, HIGH in capillary blood → CO₂ diffuses from blood into alveolus

Oxygen transport in blood:

• ~97% bound to haemoglobin (HbO₂ = oxyhaemoglobin) in RBCs
• ~3% dissolved in plasma

CO₂ transport in blood:

• ~70% as bicarbonate ions (HCO₃⁻) in plasma (via carbonic anhydrase reaction)
• ~23% bound to haemoglobin as carbaminohaemoglobin (HbCO₂)
• ~7% dissolved in plasma

The Nervous System

Overview

The nervous system is the body's communication and control network. It processes information and coordinates responses.

Division:

• Central Nervous System (CNS): Brain + Spinal cord
• Peripheral Nervous System (PNS): All nerves outside CNS
  • Somatic nervous system: Controls voluntary skeletal muscles
  • Autonomic nervous system: Controls involuntary functions (heart rate, digestion, breathing)
    • Sympathetic: "fight or flight" (accelerates heart rate, dilates pupils, inhibits digestion)
    • Parasympathetic: "rest and digest" (slows heart rate, constricts pupils, stimulates digestion)

The Neuron

The neuron is the structural and functional unit of the nervous system.

Parts:

• Cell body (soma/cyton): Contains nucleus and most organelles; metabolic centre
• Dendrites: Short, branching projections — receive signals from other neurons
• Axon: Long projection that carries impulse away from cell body. Covered by myelin sheath (in myelinated neurons)
• Myelin sheath: Produced by Schwann cells (PNS) or oligodendrocytes (CNS). Insulates the axon, speeds up conduction
• Nodes of Ranvier: Gaps in the myelin sheath — electrical impulse "jumps" from node to node (saltatory conduction) → much faster than unmyelinated conduction
• Synaptic knob (axon terminal): End of axon containing synaptic vesicles filled with neurotransmitters

The Reflex Arc

A reflex is an automatic, involuntary response to a stimulus that does not require conscious brain processing.

Components (in order): Receptor → Sensory (afferent) neuron → Nerve centre (spinal cord / brainstem) → Motor (efferent) neuron → Effector (muscle or gland)

Example — knee-jerk reflex:

1. Patellar tendon is tapped
2. Stretch receptor in quadriceps muscle is stimulated
3. Sensory neuron carries impulse to spinal cord
4. Interneuron (in spinal cord) relays signal
5. Motor neuron carries impulse to quadriceps muscle
6. Quadriceps contracts → lower leg kicks

Why are reflexes useful? They provide rapid responses to danger without waiting for the brain to process — withdrawal from heat, blinking, swallowing are all reflex actions.

5 Common Mistakes Students Make

Mistake 1: Saying pulmonary artery carries oxygenated blood. Pulmonary artery (right ventricle → lungs) carries DEOXYGENATED blood. Pulmonary vein (lungs → left atrium) carries OXYGENATED blood. These are the only artery/vein pair that break the usual rule. Learn this as a specific exception.

Mistake 2: Thinking bile contains digestive enzymes. Bile does NOT contain any digestive enzymes. Bile salts emulsify fats (physical process). The actual fat-digesting enzymes (lipases) come from the pancreas and intestinal glands. Many students incorrectly write "bile enzyme" in answers.

Mistake 3: Confusing tidal volume with vital capacity. Tidal volume (~500 mL) is the air in one normal, effortless breath. Vital capacity (~4600 mL) is the maximum amount you can exhale after the deepest possible inhalation. Vital capacity decreases with age, smoking, and lung disease — that's why it's measured in pulmonary function tests.

Mistake 4: Saying exhalation is always passive (relaxation only). Normal, quiet exhalation is passive (diaphragm and external intercostals relax). But forced exhalation (during exercise, coughing, singing) involves active contraction of internal intercostal muscles and abdominal muscles. The question "which muscles are involved in forced exhalation?" requires you to name internal intercostals and abdominal muscles.

Mistake 5: Confusing the SA node and AV node. SA node = pacemaker (sets heart rate, generates initial impulse). AV node = relay station (receives impulse from atria, delays it slightly, sends it to ventricles). If SA node fails, AV node can take over as a secondary pacemaker (but at a lower rate of ~40–60 beats/min). An artificial pacemaker replaces SA node function.

Practice Questions

Q1. What is double circulation and why is it advantageous in humans?

Q2. Trace the journey of a glucose molecule from food entering the mouth to reaching a muscle cell in the arm.

Q3. Explain why the left ventricle wall is thicker than the right ventricle wall.

Q4. What is peristalsis? In which parts of the digestive system does it occur?

Q5. Why is the residual volume important? What would happen if the residual volume were zero?

Q6. Differentiate between the somatic and autonomic nervous systems.

Q7. What is enterokinase and why is it important?

Q8. How does the nervous system control the heartbeat, and what role does the vagus nerve play?

Frequently Asked Questions

Why does blood appear red?

Blood appears red because of haemoglobin in red blood cells. Haemoglobin contains iron in the form of haem groups. When iron binds oxygen (oxyhaemoglobin), it reflects light in the red wavelength — bright red. Deoxygenated haemoglobin (deoxy-Hb) absorbs red light more and appears dark red/maroon. The common idea that "deoxygenated blood is blue" is incorrect — it's never blue, just a darker shade of red. Veins look bluish through skin because skin absorbs different light wavelengths, making the dark red blood appear blue.

Why don't we digest our own stomach lining?

The stomach secretes mucus (from goblet cells and mucous cells) that coats the entire stomach lining and forms a protective barrier against HCl and pepsin. The mucus layer is constantly replaced. Prostaglandins stimulate mucus secretion and bicarbonate production to neutralise any acid reaching the lining. When this protective mechanism fails (due to H. pylori infection, NSAIDs, excessive alcohol), peptic ulcers result — the stomach literally begins digesting its own wall.

What is the difference between breathing and respiration?

Breathing (pulmonary ventilation) is the mechanical process of moving air in and out of the lungs — a physical process involving muscles and pressure changes. Respiration is the biochemical process of breaking down glucose to produce ATP, occurring in every cell. Aerobic cellular respiration = glucose + O₂ → CO₂ + H₂O + ATP (in mitochondria). These are related (breathing provides O₂ for respiration and removes CO₂), but they are fundamentally different processes.

What is the difference between the sympathetic and parasympathetic nervous systems?

Sympathetic: prepares the body for activity — increases heart rate, dilates bronchioles, diverts blood to muscles, dilates pupils, inhibits digestion. Uses noradrenaline as neurotransmitter at effectors. Ganglia close to spinal cord. Parasympathetic: prepares for rest and digestion — slows heart rate, constricts bronchioles, stimulates digestive activity, constricts pupils, promotes glandular secretion. Uses acetylcholine as neurotransmitter. Ganglia close to or within target organs. Both branches operate continuously — the net heart rate reflects their relative balance.

How is carbon dioxide transported in blood?

About 70% of CO₂ is transported as bicarbonate ions (HCO₃⁻) in plasma. In RBCs: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (catalysed by carbonic anhydrase in RBCs). HCO₃⁻ moves out of RBCs into plasma (Cl⁻ moves in — the "chloride shift"). About 23% binds to haemoglobin's amino groups (carbaminohaemoglobin). About 7% dissolves directly in plasma.

What would happen if the epiglottis didn't work?

The epiglottis is a cartilage flap that folds over the larynx (glottis) during swallowing, directing food into the oesophagus and preventing it from entering the trachea. If it fails to close properly, food or liquid enters the trachea → aspiration. Aspiration triggers violent coughing (a reflex to expel the foreign material). Repeated or severe aspiration can cause aspiration pneumonia (infection from aspirated material in the lungs). In medical conditions (stroke, intoxication), impaired epiglottis function is a serious concern requiring protective measures (positioning, feeding tubes).