Unit 8: Human Biology
8.1 Digestive System
Alimentary Canal
The alimentary canal is a continuous muscular tube extending from the mouth to the anus. It consists of the mouth, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum), and anus. Each region specializes in mechanical breakdown, chemical digestion, absorption, or waste elimination.
Digestive Glands
Digestive secretions are produced by various glands:
- Salivary glands (parotid, submandibular, sublingual) secrete saliva containing salivary amylase.
- Gastric glands** in the stomach wall contain chief cells (pepsinogen), parietal cells (HCl and intrinsic factor), and mucous cells (protective mucus).
- Liver produces bile, stored in the gallbladder, which emulsifies fats.
- Pancreas releases pancreatic juice rich in enzymes (lipase, trypsin, chymotrypsin, amylase) and bicarbonate.
- Intestinal glands** (crypts of Lieberkühn) in the small intestine secrete peptidases, sucrase, lactase, maltase, and intestinal lipase.
Physiology of Digestion
Mouth: Mechanical digestion occurs via mastication (chewing). Chemical digestion begins with salivary amylase (ptyalin) catalyzing the hydrolysis of starch to maltose.
Stomach: The acidic environment (pH ≈ 2) maintained by HCl activates pepsinogen to pepsin, which hydrolyzes proteins into peptides. Mucus secreted by mucous cells protects the epithelium from autodigestion.
Small intestine: Bile from the liver emulsifies lipids, increasing surface area for enzymatic action. Pancreatic lipase hydrolyzes triglycerides to fatty acids and monoglycerides. Trypsin (activated from trypsinogen) cleaves peptide bonds. Brush‑border enzymes — maltase, lactase, sucrase — disassemble disaccharides into monosaccharides.
Absorption: The mucosal surface is enlarged by villi and microvilli, providing a massive absorptive area. Amino acids and monosaccharides enter capillaries; fatty acids and monoglycerides are re‑esterified into triglycerides, packaged into chylomicrons, and enter lymphatic lacteals.
Large intestine: Primary functions are water and electrolyte absorption, formation of feces, and housing of gut microbiota. No significant enzymatic digestion occurs here.
Figure 8.1: Schematic diagram of the alimentary canal showing the sequential organs and the locations of major digestive glands.
8.2 Respiratory System
Respiratory Organs
Air enters through the nostrils, passes the nasal cavity (where it is warmed, humidified, and filtered), then moves through the pharynx, larynx, trachea, bronchi (right and left), bronchioles, and finally reaches the alveoli — tiny sacs where gas exchange occurs.
Respiratory Mechanism
Gas exchange: Oxygen and carbon dioxide diffuse across the thin alveolar‑capillary membrane according to Fick’s law. The partial pressure gradient drives O₂ into blood and CO₂ out of blood.
Transport of gases:
- Oxygen: About 97 % binds to hemoglobin in red blood cells forming oxyhemoglobin (HbO₂); the remaining 3 % is dissolved in plasma.
- Carbon dioxide: Approximately 70 % is carried as bicarbonate (HCO₃⁻) formed by carbonic anhydrase in erythrocytes; 23 % binds to hemoglobin as carbaminohemoglobin; 7 % remains dissolved in plasma.
Regulation of respiration: The medulla oblongata houses the primary respiratory rhythm generators (dorsal and ventral groups). The pons contains the pneumotaxic and apneustic centers that modulate rhythm. Central chemoreceptors in the medulla detect changes in cerebrospinal fluid pH (reflecting CO₂), while peripheral chemoreceptors in the carotid and aortic bodies respond to arterial O₂, CO₂, and pH.
Figure 8.2: Diagram illustrating the respiratory tract from nostrils to alveoli, with inset showing gas exchange across the alveolar membrane.
8.3 Circulatory System
Double Circulation
Humans exhibit a closed double circulatory system: pulmonary circulation** (right heart → lungs → left heart) oxygenates blood, while systemic circulation** (left heart → body → right heart) delivers oxygenated blood to tissues and returns deoxygenated blood.
Heart Structure
The heart is a muscular organ with four chambers: two atria** (right and left) receiving blood, and two ventricles** (right and left) pumping blood out. Valves ensure unidirectional flow:
- Tricuspid valve** (right atrioventricular)
- Bicuspid (mitral) valve** (left atrioventricular)
- Pulmonary semilunar valve** (right ventricle → pulmonary artery)
- Aortic semilunar valve** (left ventricle → aorta)
Working Mechanism
The cardiac cycle consists of systole** (contraction) and diastole** (relaxation). Atrial systole pushes blood into ventricles; ventricular systole ejects blood into the pulmonary artery and aorta; complete diastole allows the chambers to refill. One cycle lasts about 0.8 seconds at a resting heart rate of 75 bpm.
Origin and Conduction of Heart Beat
Electrical impulse originates at the sinoatrial (SA) node** (the pacemaker) in the right atrium, travels to the atrioventricular (AV) node**, then via the Bundle of His** and its left/right bundle branches to the Purkinje fibers**, which disseminate the impulse throughout ventricular myocardium, causing coordinated contraction.
Cardiac Cycle
Sequence: atrial systole → ventricular systole → complete diastole. During ventricular systole, the semilunar valves open; during diastole, the AV valves open to allow filling.
Cardiac Output
Cardiac output (CO) is the volume of blood pumped per minute:
CO = SV × HR
where SV = stroke volume (mL/beat) and HR = heart rate (beats/min). Typical resting values: SV ≈ 70 mL, HR ≈ 75 bpm → CO ≈ 5.25 L/min.
Arterial System
The aorta** is the main artery, giving rise to:
- Ascending aorta → coronary arteries
- Aortic arch → brachiocephalic trunk (right subclavian & right common carotid), left common carotid, left subclavian
- Descending thoracic aorta → intercostal, bronchial arteries
- Abdominal aorta → celiac trunk, superior/inferior mesenteric arteries, renal arteries, gonadal arteries, common iliac arteries
Venous System
Venous return is via:
- Superior vena cava** (draining head, neck, upper limbs)
- Inferior vena cava** (draining lower limbs, abdomen)
- Pulmonary veins** (four, carrying oxygenated blood from lungs to left atrium)
- Systemic veins: jugular, renal, iliac, etc.
Blood Grouping
The ABO system** is based on the presence of antigens A and B on red blood cells:
| Blood Group | Antigen(s) | Antibody(ies) in Plasma |
|---|---|---|
| A | A | anti‑B |
| B | B | anti‑A |
| AB | A and B | none |
| O | none | anti‑A and anti‑B |
The Rh factor** (D antigen) determines positivity (+) or negativity (–). Universal donor: O– (no A, B, or D antigens). Universal recipient: AB+ (has A, B, D antigens, no corresponding antibodies).
Blood Pressure
Blood pressure is measured as systolic over diastolic (mmHg). Normal adult values: systolic ≈ 120 mmHg**, diastolic ≈ 80 mmHg**. Mean arterial pressure (MAP) can be approximated by:
MAP ≈ Diastolic + ⅓(Systolic – Diastolic)
Hypertension: sustained ≥140/90 mmHg. Hypotension: <90/60 mmHg.
Figure 8.3: Diagram of the heart showing chambers, valves, and major blood vessels; inset illustrating the cardiac conduction pathway.
8.4 Excretory System
Modes of Excretion
Organisms excrete nitrogenous waste according to availability of water:
- Ammonotelism:** excretion of ammonia (NH₃) – typical of bony fish, aquatic amphibians (highly toxic, requires large water volume).
- Ureotelism:** excretion of urea – predominant in mammals, amphibians (less toxic, water‑conserving).
- Uricotelism:** excretion of uric acid – seen in birds, reptiles, insects (insoluble, minimizes water loss).
Excretory Organs
Primary excretory organ: the kidneys**. Additional routes include the lungs** (CO₂), skin** (sweat containing water, salts, urea), and liver** (conversion of ammonia to urea via the urea cycle).
Formation of Urine
Urine formation involves three key processes:
- Glomerular filtration:** Blood pressure in the glomerular capillaries forces plasma (minus large proteins) into Bowman's capsule. The filtration rate (GFR) is given by:
GFR = Kf × (PGC – PBS – πGC)
where Kf = filtration coefficient, PGC = glomerular capillary hydrostatic pressure, PBS = Bowman's capsule hydrostatic pressure, πGC = glomerular capillary oncotic pressure.
- Tubular reabsorption:** Useful substances (glucose, amino acids, Na⁺, Cl⁻, water) are reabsorbed from the filtrate back into peritubular capillaries, primarily in the proximal convoluted tubule.
- Tubular secretion:** Waste ions (H⁺, K⁺, NH₄⁺) and certain drugs are secreted from peritubular blood into the tubular lumen, mainly in the distal convoluted tubule and collecting duct.
The countercurrent multiplier system** in the loop of Henle creates a medullary interstitial gradient that enables the kidney to produce concentrated urine via water reabsorption in the collecting duct under ADH influence.
Figure 8.4: Diagram of a nephron illustrating glomerular filtration, tubular reabsorption, secretion, and the countercurrent mechanism.
8.5 Nervous System
Central Nervous System (CNS)
The CNS comprises the brain** and spinal cord**. It integrates sensory information, coordinates motor output, and mediates higher functions such as learning and memory.
Peripheral Nervous System (PNS)
The PNS consists of cranial nerves** (12 pairs) and spinal nerves** (31 pairs). Cranial nerves (e.g., olfactory I, optic II, oculomotor III, trochlear IV, trigeminal V, abducens VI, facial VII, vestibulocochlear VIII, glossopharyngeal IX, vagus X, accessory XI, hypoglossal XII) primarily serve the head and neck. Spinal nerves emerge from the spinal cord and innervate the trunk and limbs (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal).
Autonomic Nervous System (ANS)
The ANS regulates involuntary visceral activities. It has two divisions:
- Sympathetic:** “fight or flight” – increases heart rate, dilates pupils, inhibits digestion; neurotransmitter mainly norepinephrine.
- Parasympathetic:** “rest and digest” – decreases heart rate, stimulates salivation and digestion; neurotransmitter acetylcholine.
Structure and Function of the Brain
- Cerebrum:** Largest part; responsible for conscious thought, memory, sensory perception, voluntary motor control.
- Cerebellum:** Located posterior to the brainstem; coordinates movement, maintains balance and posture.
- Medulla oblongata:** Controls vital autonomic functions — respiration, heart rate, blood pressure, reflexes such as vomiting and coughing.
- Hypothalamus:** Regulates homeostasis — temperature, hunger, thirst, circadian rhythms; controls pituitary hormone release.
- Thalamus:** Acts as a relay station, directing sensory signals (except olfaction) to appropriate cortical areas.
Origin and Conduction of Nerve Impulse
At rest, a neuron maintains a resting potential** of about –70 mV, with high intracellular K⁺ and extracellular Na⁺. An action potential** is triggered when the membrane reaches threshold:
- Depolarization:** Voltage‑gated Na⁺ channels open; Na⁺ influx drives the membrane potential toward +40 mV.
- Repolarization:** Na⁺ channels close; voltage‑gated K⁺ channels open; K⁺ efflux restores the negative resting potential.
Propagation along the axon occurs via local currents. In myelinated fibers, the impulse jumps from node to node (saltatory conduction**), greatly increasing speed.
At the synapse, the arriving action potential triggers vesicle fusion, releasing neurotransmitters (e.g., acetylcholine, norepinephrine) into the cleft. These bind to receptors on the postsynaptic membrane, generating excitatory or inhibitory postsynaptic potentials.
Figure 8.5: Diagram of a neuron showing resting and action potentials, nodes of Ranvier, and synaptic transmission.
8.6 Sense Organs
Eye
The eye is the organ of vision. Its main parts are:
- Cornea:** Transparent outer layer that refracts light.
- Iris:** Pigmented diaphragm controlling pupil size.
- Pupil:** Opening that admits light.
- Lens:** Flexible, biconvex structure that fine‑focuses light onto the retina.
- Retina:** Contains photoreceptor cells — rods (scotopic vision) and cones (photopic, color vision).
- Optic nerve:** Carries visual signals to the brain.
- Ciliary body:** Contains muscle that changes lens shape for accommodation.
Accommodation:** For near vision, ciliary muscles contract, relaxing the zonular fibers, allowing the lens to become more convex. For distant vision, the muscles relax, zonular fibers tense, and the lens flattens.
Refractive defects:
- Myopia (near‑sighted):** Elongated eyeball or overly curved cornea focuses light in front of the retina; corrected with diverging (concave) lenses.
- Hypermetropia (far‑sighted):** Short eyeball or insufficient corneal power focuses light behind the retina; corrected with converging (convex) lenses.
- Astigmatism:** Irregular corneal curvature causes multiple focal points; corrected with cylindrical lenses.
- Presbyopia:** Age‑related loss of lens elasticity, reducing accommodation; corrected with reading glasses.
Photochemistry:** Rods contain the pigment rhodopsin**, which undergoes bleaching in dim light, enabling low‑light vision. Cones contain iodopsin** variants (photopsins) sensitive to short (S), medium (M), or long (L) wavelengths, providing color vision.
Figure 8.6: Labeled diagram of the eye showing optical pathway from cornea to retina and optic nerve.
Ear
The ear performs hearing and balance.
- Outer ear:** Pinna collects sound waves; auditory canal directs them to the tympanic membrane (eardrum).
- Middle ear:** Air‑filled cavity containing the ossicles — malleus (hammer), incus (anvil), stapes (stirrup) — which amplify and transmit vibrations to the inner ear. The Eustachian tube equalizes pressure with the atmosphere.
- Inner ear:** Consists of the cochlea (hearing) and vestibular apparatus