Digestive System

The digestive system consists of the alimentary canal and digestive glands.

1. Alimentary Canal

It is a long and coiled duct extending from mouth to anus. It is about 9 metre long in human and is differentiated into the following parts:

1. Mouth and mouth cavity
2. Pharynx
3. Oesophagus
4. Stomach
5. Small intestine — duodenum, jejunum and ileum
6. Large intestine — caecum, colon and rectum

Mouth and Mouth Cavity

Mouth opens into a cavity called mouth cavity or oral cavity. It is bounded by upper and lower jaw and laterally by cheeks. The roof is formed by palate. A finger-like muscular tongue lies on the floor of mouth cavity. Tongue helps in mastication, swallowing and speech. The mouth cavity contains teeth and saliva helps in maceration of food.

Pharynx

Mouth cavity opens into a short and narrow common duct called pharynx. It is a common passage for air and food. It contains openings of the Eustachian tubes, internal nostrils and opening of the oesophagus.

Oesophagus

It is about 25 cm long muscular duct extending from pharynx to the stomach. It conducts food into stomach by peristaltic movement.

Stomach

It is a large J-shaped bag, about 25 cm long, situated on the left side of the upper abdominal cavity. It is divided into fundic, cardiac and pyloric regions. The wall contains gastric glands and the stomach stores food for some time and digests it by churning movement and gastric juice.

Small Intestine

The stomach opens into the small intestine through the pyloric sphincter. It measures about 6.5 m and is divided into three parts:

• Duodenum: First C-shaped part; receives bile from liver and pancreatic juice from pancreas. Maximum digestion takes place here.
• Jejunum: About 2.5 m long coiled duct. Its internal wall contains thick-walled villi.
• Ileum: Longest and highly coiled part. Its inner wall contains thick-walled finger-like projections called villi that increase the surface area for absorption. Maximum absorption occurs here.

Large Intestine

It is shorter but wider than the small intestine and measures about 1.5 m. It consists of caecum, colon and rectum.

• Caecum: First pouch-like structure. In humans, it contains a blind tube called vermiform appendix. The source notes that appendix is vestigial and may become inflamed, causing appendicitis.
• Colon: Inverted U-shaped part, about 1.2 m, differentiated into ascending, transverse, descending and sigmoid regions. It helps in formation of stool mainly by absorption of water.
• Rectum: Terminal elongated part, about 15 cm. It stores undigested food before egestion and opens outside through anus guarded by anal sphincter.

2. Digestive Glands

The source lists salivary glands, gastric glands, liver, pancreas and intestinal glands.

Salivary Glands

Three pairs of salivary glands are present around the mouth cavity. A normal person secretes about 1–1.5 litres of saliva daily.

• Salivary amylase breaks starch into maltose.
• Saliva keeps the mouth moist and cleans teeth.
• It lubricates and moistens food and helps in swallowing.
• It contains lysozyme that destroys microorganisms.

Gastric Glands and Gastric Juice

Gastric glands are present in the wall of stomach and secrete gastric juice. The source notes about 2–3 litres per day.

• HCl: Destroys microorganisms, makes food acidic, dissolves bones and helps activate pepsinogen.
• Pepsin: Breaks proteins into peptones and proteoses.
• Rennin: Converts milk into curd; source notes it is secreted in children.
• Gastric lipase: Breaks fats into fatty acids.
• Mucus: Protects the stomach wall from acid and makes food slippery.

Bile Juice

Bile is alkaline. It neutralizes the acidity of chyme and emulsifies fat into small droplets.

Pancreatic Juice

• Trypsin: protein → amino acids
• Amylase: starch → glucose/maltose as written in the source at different places
• Lipase: fat → fatty acids

Intestinal Glands

Located in the wall of jejunum and ileum. The source mentions crypts of Lieberkühn and Brunner’s glands. Intestinal juice contains peptidases, aminopeptidase, maltase, lactase, sucrase and nuclease.

Liver: Structure and Functions

Liver is described in the notes as the largest gland of the body. It lies just above the stomach, below the diaphragm in the upper abdominal cavity. It is dark red in colour and weighs about 1.5 kg. The source divides the human liver into right, left, quadrate and caudate lobes.

Histologically, liver is made of hepatic lobules. Each lobule consists of hepatocytes and a network of blood capillaries and bile ducts. Hepatic lobules are separated by Glisson’s capsule. Kupffer cells occur in the hepatic tissue and engulf microorganisms and dead RBCs/WBCs.

Functions of Liver

1. Secretion of bile: Neutralizes acidic chyme and emulsifies fat.
2. Glycogenesis: Conversion of excess glucose into glycogen with the help of insulin.
3. Glycogenolysis: Conversion of glycogen into glucose when blood glucose falls.
4. Gluconeogenesis: Formation of glucose from non-carbohydrate substances such as amino acids and fatty acids.
5. Lipogenesis: Conversion of lipid and amino acids into fat.
6. Deamination / protein metabolism: Removal of amino group from amino acids.
7. Heat production: High metabolic activity of liver produces heat and helps maintain body temperature.
8. Phagocytosis: Kupffer cells engulf microorganisms.
9. Detoxification: Converts toxic substances into harmless substances.
10. Conversion of ammonia into urea.
11. Excretion: Eliminates bile pigments such as bilirubin and biliverdin, and also cholesterol and other waste products.
12. Haematological function: During embryonic stage liver synthesizes RBCs; the source calls this haemopoiesis.
13. Clotting factors: Synthesizes factors such as fibrinogen and prothrombin.
14. Storage: Vitamins A, B, D, E, K and B₁₂; minerals such as iron, copper and potassium; and glycogen.

Pancreas: Structure and Functions

Pancreas is a major digestive gland situated in the curvature of duodenum. It is yellowish, elongated and flattened, about 15–20 cm long and about 75 g in the source. Its body is differentiated into head, body and tail.

Histologically, pancreas contains pancreatic lobules and islets of Langerhans. Pancreatic lobules perform exocrine function; islets perform endocrine function.

Exocrine Function

• Trypsin: protein → amino acids
• Amylase: starch → maltose
• Lipase: fat → fatty acids

Endocrine Function

• Insulin: Secreted by β-cells; converts excess glucose into glycogen and reduces blood sugar level.
• Glucagon: Secreted by α-cells; converts glycogen into glucose and raises blood sugar level.
• Somatostatin: Secreted by δ-cells; inhibits the activity of insulin and glucagon.

Physiology of Digestion in Human

The source describes digestion under ingestion, digestion, absorption, assimilation and egestion.

1. Ingestion

The process of taking food into the mouth.

2. Digestion

The conversion of complex food molecules into simple molecules by physical and chemical processes.

Digestion in Mouth

Food is physically broken by teeth and tongue. Saliva moistens food and salivary amylase acts on starch.

Digestion in Stomach

Food is churned mechanically and acted upon by gastric juice.

Digestion in Duodenum

Partially digested semi-solid food is called chyme. Chyme enters duodenum through pyloric sphincter. Bile neutralizes acidity and emulsifies fat; pancreatic enzymes digest remaining nutrients.

Digestion in Jejunum and Ileum

Final digestion occurs in jejunum and ileum by intestinal enzymes.

3. Absorption

Absorption is the taking of digested food such as amino acids, glucose and fatty acids from the digestive tract into blood and lymph vessels. Very little absorption occurs in mouth, oesophagus, stomach, duodenum and jejunum; maximum absorption occurs in ileum because of numerous villi. Water is also absorbed in the large intestine.

Active Absorption

Absorption against concentration gradient using energy.

Passive Absorption

Absorption by simple diffusion without expenditure of energy.

4. Assimilation

The process by which absorbed food is converted into different useful products in the tissues.

5. Egestion

The removal of undigested material through anus; also called defecation in the source.

Teeth of Human

The source describes human teeth as:

• Heterodont: More than one shape/type.
• Diphyodont: Two sets during life.
• Thecodont: Fixed in sockets of jaws.
• Bunodont: Teeth with low cusps.

Types of Teeth

• Incisors: Front teeth; flat, sharp margin; cutting and biting.
• Canines: Pointed dagger-shaped teeth beside incisors; tearing food.
• Premolars: Broad margin with two cusps; grinding food.
• Molars: Broad teeth with four or five cusps; grinding. Last molars are wisdom teeth, generally developing around 18 years or later.

Dental Formula

Structure of Tooth

Teeth are made mainly of dentine. A central pulp cavity contains blood vessels and nerve fibres. The root is covered by cementing material and is fixed in a socket.

• Crown: Visible part above gum, covered by enamel. Enamel protects against dental decay and gives a shiny appearance.
• Neck: Part supported by the gum.
• Root: Part present in the bony socket, supported by cementing material.

Tongue

Tongue is a large muscular, protrusible, mobile and flattened organ lying on the floor of buccal cavity. Its upper surface contains taste buds called papillae.

Types of Papillae

• Vallate papillae: Large and situated at the base of oral part of tongue.
• Filiform papillae: Small and numerous; situated near the central part.
• Fungiform papillae: Smaller than vallate and less numerous than filiform, scattered over tongue.

Functions of Tongue

• Helps in mastication, swallowing and speech.
• Manipulates food and mixes it with saliva.
• Acts as organ of taste.

Respiratory System of Human

The respiratory system consists of nostrils and nasal chamber, pharynx, larynx, trachea, bronchi, alveoli and lungs.

Nostrils and Nasal Chamber

Nostrils are a pair of openings above the mouth. They open into a bony nasal chamber lined by ciliated pseudostratified epithelium.

• Filters dust particles.
• Warms air.
• Moistens air.
• Helps destroy microorganisms present in air.

Pharynx

Nasal chamber opens into pharynx through internal nostrils. Pharynx is a short common passage for air and food and is divided into nasopharynx, oropharynx and laryngopharynx.

Larynx

Pharynx opens into a hollow cartilaginous box-like structure called larynx. The source describes it as consisting of nine cartilages including thyroid, cricoid and arytenoid cartilages. Vocal cords produce sound. Opening into larynx is glottis and is guarded by epiglottis, which prevents entry of food.

Trachea

Also called windpipe. About 12 cm long hollow straight tube extending from larynx to thoracic cavity. It is supported by 16–20 C-shaped hyaline cartilaginous rings. Its internal wall is lined by ciliated pseudostratified epithelium and mucus which trap dust particles.

Bronchi

Trachea divides into right and left bronchi. Each bronchus enters its respective lung and divides repeatedly into bronchioles, which end in alveolar regions.

Alveoli

Alveoli are tiny balloon-like air sacs and the main sites of gaseous exchange. Each human lung contains millions of alveoli. Their walls are one cell thick, they are surrounded by capillaries and their inner surfaces are moist.

Lungs

A pair of large, hollow, soft, spongy and pink organs in the thoracic cavity. They are covered by double pleural membrane with pleural fluid between layers to reduce friction. The source describes right lung with three lobes and left lung with two lobes. Alveoli make lungs the actual site of respiration.

Mechanism of Breathing

Breathing or external respiration involves inspiration and expiration.

1. Inspiration

1. External intercostal muscles contract and internal intercostal muscles relax.
2. Thoracic cage is pulled outward and upward.
3. Diaphragm is pulled downward.
4. Volume of thoracic cavity increases and lungs expand.
5. Air pressure inside lungs decreases, therefore oxygen-containing air enters lungs.

2. Expiration

1. Internal intercostal muscles contract and external intercostal muscles relax.
2. Thoracic cage moves inward and downward.
3. Diaphragm moves upward.
4. Thoracic cavity decreases in size and lungs are compressed.
5. Air pressure increases and CO₂-containing air is expelled.

Physiology of Respiration

1. External Respiration

Inhaling oxygen-containing air and expelling carbon dioxide-containing air.

2. Gaseous Exchange

Exchange of oxygen and carbon dioxide between alveoli and blood capillaries occurs by simple diffusion. Alveolar walls are made of simple squamous epithelium and are richly supplied with capillaries. Oxygen diffuses from alveoli to blood while carbon dioxide diffuses from blood into alveoli according to partial pressure differences.

3. Transport of Oxygen

• About 1% is transported dissolved in plasma.
• Most oxygen is carried as oxyhaemoglobin in RBCs.

4. Internal / Tissue Respiration

Oxyhaemoglobin dissociates in tissues and releases oxygen, which diffuses into tissue cells.

Oxidation of food in cells releases energy:

5. Transport of CO₂

• As carbonic acid / dissolved form — the source gives about 7%.
• As bicarbonate compounds — the source gives about 70%.
• As carbaminohaemoglobin — the source gives about 23%.

Pulmonary Air Volume

Bohr Effect

The source states that increase in carbon dioxide concentration increases dissociation of oxyhaemoglobin. This effect of carbon dioxide on oxyhaemoglobin is called Bohr’s effect.

Chloride Shift / Hamburger’s Phenomenon

Exchange of bicarbonate ions and chloride ions between blood plasma and RBCs, important in maintaining ionic balance of the body.

Circulatory System

Circulation is the transport of material from one part of body to another through fluid connective tissue. Human blood circulatory system is made of blood, blood vessels and heart.

Types of Circulation

• Open type: Blood circulates through open spaces/sinuses; described for most invertebrates except annelids.
• Closed type: Blood circulates through closed vessels; described for vertebrates.

Types of Heart

• Myogenic heart: Wave of contraction starts in heart muscle. Example: mammals.
• Neurogenic heart: Wave of contraction starts from nerve cells near heart. Example in source: some invertebrates.

Single and Double Circulation

• Single circulation: Blood enters heart once in one complete circulation; occurs in fishes.
• Double circulation: Blood passes through heart twice in one complete circulation; occurs in amphibians, reptiles, birds and mammals.

Human Heart: External and Internal Structure

Heart is a muscular hollow organ situated in thoracic cavity between the lungs, slightly toward the left side. The source gives weight about 250–300 g and length about 12 cm.

Heart is covered by a double pericardial membrane: outer parietal and inner visceral pericardium. Pericardial fluid between them protects against shocks and allows free movement.

Human heart has four chambers: two upper thin-walled auricles/atria and two lower thick-walled ventricles.

• Right auricle receives impure blood through superior and inferior vena cava.
• Left auricle receives pure blood through pulmonary veins.
• Right ventricle pumps impure blood through pulmonary artery to lungs.
• Left ventricle pumps pure blood through aorta to the body.

Valves

• Right atrioventricular opening: tricuspid valve.
• Left atrioventricular opening: bicuspid/mitral valve.
• Valves are connected with papillary muscles through chordae tendineae.
• Pulmonary artery and aorta are guarded by semilunar valves.

Working Mechanism of Heart

Contraction is called systole and relaxation is called diastole. During auricular contraction blood enters corresponding ventricles. During ventricular contraction, right ventricle sends blood to lungs and left ventricle sends blood to body.

Double Circulation

• Pulmonary circulation: Begins at right ventricle and ends at left auricle; deoxygenated blood becomes oxygenated in lungs.
• Systemic circulation: Begins at left ventricle and ends at right auricle; oxygenated blood is distributed to body.

Heartbeat

The source divides one heartbeat into:

1. Atrial systole — about 0.1 sec.
2. Ventricular systole — about 0.3 sec.
3. Joint diastole — about 0.4 sec.

Heart sounds are described as “lubb-dub”. The first sound is associated with closing of atrioventricular valves and second with closing of semilunar valves.

Cardiac Cycle, Heart Rate and Origin of Heartbeat

• Cardiac cycle: One complete contraction and relaxation of heart; about 0.8 sec.
• Heart rate: About 72 beats per minute in the source.
• Stroke volume: About 70 ml blood pumped in each beat.
• Cardiac output: Stroke volume × heart rate = 70 × 72 = 5040 ml, about 5 litres/minute.

Origin and Conduction of Heartbeat

Heartbeat is generated by specialized tissue. The sino-atrial (SA) node near the opening of superior vena cava is described as natural pacemaker. Another node at the atrioventricular junction is AV node. From AV node, impulse passes through bundle of His and then Purkinje fibres into ventricular walls.

1. SA node releases an electrical wave.
2. Wave spreads over auricles causing atrial contraction.
3. Impulse is picked up by AV node.
4. It is conducted through bundle of His and its branches.
5. Purkinje fibres distribute impulse through ventricles, producing ventricular contraction.

Artificial Pacemaker

When natural pacemaker becomes defective and heartbeat falls below the range written in the source, an artificial pacemaker can be surgically placed and connected with the heart to deliver electrical waves and maintain normal rhythm.

Electrocardiogram (ECG)

A graphical record of electrical activity of heart is called electrocardiogram. The method is electrocardiography. ECG is used as a diagnostic tool for heart abnormalities. The source credits Einthoven with its development around 1903.

Blood Grouping, Blood Transfusion and Rh Factor

ABO blood grouping classifies blood based on presence or absence of antigens A and B on RBC membranes and antibodies in plasma. The source credits Karl Landsteiner with discovery of ABO system and de Castello/Sturli with AB group.

Blood Transfusion Compatibility

Rh Factor

Rh factor is another antigen on RBC membrane. The source associates its discovery with rhesus monkey studies around 1940 and states that a large majority of people are Rh positive.

Excretory System of Human

Excretion is the process of removing metabolic waste products from the body. The source lists lungs, liver, skin and kidneys as excretory organs. Kidneys remove nitrogenous wastes.

Kidneys

A pair of dark red, bean-shaped organs on the dorsal side of abdominal cavity. The source gives approximately 10–12 cm length, about 5–7 cm width and about 150 g weight each. The outer surface is convex and inner surface concave with a depression called hilum. Renal artery, renal vein and ureter enter/leave at hilum. Each kidney is covered by tough fibrous renal capsule.

Each kidney contains about one million blood-filtering units called nephrons/uriniferous tubules.

Ureters

Each kidney gives rise to a narrow muscular ureter, about 25 cm long and about 5 mm in diameter in the source. It carries urine to urinary bladder.

Urinary Bladder and Urethra

Urinary bladder stores urine. Urethra carries urine outside. The source notes that urethral arrangement differs between male and female.

Internal Structure of Kidney

• Renal cortex: Outer darker part, about one-third of kidney; contains Bowman’s capsules and parts of nephrons.
• Medulla: Inner lighter part, containing loops of Henle and collecting ducts arranged into renal pyramids.
• Renal pyramids open into minor calyces; minor calyces combine into major calyces; major calyces open into funnel-shaped renal pelvis, which continues as ureter.
• Connective tissue between pyramids is called renal column.

Structure of Nephron

Nephron is structural and functional unit of kidney. Each kidney contains about one million nephrons. The source gives length about 2.5–3.5 cm.

1. Bowman’s Capsule

First cup-shaped part of nephron, bounded by double-layered squamous epithelium. It contains a network of blood capillaries called glomerulus. Glomerulus remains connected with afferent and efferent renal arterioles. Bowman’s capsule with glomerulus is called Malpighian body.

2. Renal Tubule

• Proximal convoluted tubule (PCT): First coiled, comparatively thicker part.
• Loop of Henle: U-shaped narrow tube between PCT and DCT, with descending and ascending limbs.
• Distal convoluted tubule (DCT): Posterior coiled, comparatively wider part opening into collecting duct.

Mechanism of Urine Formation

The source divides urine formation into ultrafiltration, selective reabsorption and tubular secretion.

1. Ultrafiltration

Filtration of blood in Bowman’s capsule under glomerular hydrostatic pressure is called ultrafiltration. Glomerulus is connected with afferent and efferent arterioles. The source gives glomerular blood pressure about 70 mm Hg and describes opposing pressures that reduce effective filtration pressure. About 180+ litres of filtrate may form per day, but only about 1–1.5 litres of urine is finally produced.

2. Selective Reabsorption

Useful substances filtered from blood are reabsorbed from renal tubules into blood by active and passive processes.

• PCT reabsorbs maximum useful materials such as glucose, amino acids, fatty acids, vitamins, minerals and much water.
• Descending limb of loop of Henle reabsorbs large amount of water.
• Ascending limb reabsorbs salts and ions.
• DCT reabsorbs additional salts/ions; the source notes that filtrate becomes hypotonic.

3. Tubular Secretion

Some waste materials that are not removed by filtration are secreted from blood into renal tubules by active/passive transport. Collecting duct finally contains water and metabolic wastes; this mixture is urine.

Micturition

Collection and periodic removal of urine from urinary bladder is called micturition. The source notes sensation generally develops around 300–400 ml and can be postponed voluntarily to a larger volume.

Composition of Urine

Urine is pale yellow due to urochrome. The source states roughly 95% water with urea and other wastes including uric acid, creatinine, salts and ions.

• Glycosuria: Presence of glucose in urine.
• Haematuria: Presence of blood in urine.

Counter Current Mechanism and Osmoregulation

Counter Current Mechanism

Opposite action of the two limbs of loop of Henle is called counter current mechanism. The source describes descending limb as allowing water reabsorption while ascending limb is associated with salt reabsorption, producing hypertonic medulla and helping concentrate urine.

Osmoregulation

Regulation of water level and concentration of minerals in body fluids is called osmoregulation. Kidney helps maintain water level mainly through antidiuretic hormone (ADH) secreted by posterior pituitary.

When Excess Water is Consumed

Blood volume increases, receptors signal hypothalamus, ADH secretion decreases, permeability of distal/collecting tubules decreases, less water is reabsorbed and urine volume increases.

When Less Water is Consumed

Blood volume decreases, receptors signal hypothalamus, ADH secretion increases, tubular permeability increases, more water is reabsorbed and urine volume decreases.

The source calls substances that increase urine volume diuretic substances, giving alcoholic beverages as an example because they reduce ADH.

Other Functions of Kidney

• Regulation of water balance.
• Regulation of electrolyte concentration.
• Maintenance of acid–base balance.
• Removal of drugs and other foreign substances.
• Production of renin.
• Production of erythropoietin, which stimulates RBC formation.

Homeostasis, Deamination and Detoxification

Homeostatic Functions of Skin

• Barrier against microorganisms and chemicals.
• Regulates dehydration.
• Protects against light.
• Helps regulate body temperature.
• Helps excrete some substances.
• Absorbs substances such as vitamin D.
• Provides sensation against stimuli.

Deamination

Excess amino acids cannot be stored. They are deaminated in liver. Removal of amino group is called deamination; the remaining keto acid may enter carbohydrate or fat metabolism.

Detoxification

Conversion of highly toxic ammonia into less toxic urea by ornithine cycle is described as detoxification.

Homeostasis

Maintenance of a constant internal environment so physiological processes run at optimum rate. The source mentions regulation of body temperature, blood composition and pH, blood sugar, water, carbon dioxide, urea and blood pressure. Organs involved include skin, kidneys, liver, lungs and endocrine glands.

Nervous System

The body system which controls, coordinates and regulates proper functioning of body is called nervous system. It consists of highly specialized cells called neurons.

Functions

• Conducts impulses/messages about stimuli from body parts to nerve centres.
• Maintains response to stimuli.
• Conducts impulses from nerve centres to body organs.
• Links and coordinates organs and systems.
• Stimulates and inhibits muscles, glands and internal organs according to information received.
• Helps maintain homeostasis.

Parts of Nervous System

1. Central nervous system (CNS): brain and spinal cord.
2. Peripheral nervous system (PNS): nerves originating from CNS.
3. Autonomic nervous system (ANS): sympathetic and parasympathetic divisions.

Brain

Brain is a delicate organ weighing about 1300–1400 g in the source. It is protected by cranium and three meninges: outer dura mater, middle arachnoid and inner pia mater. Cerebrospinal fluid fills spaces between membranes.

Forebrain (Prosencephalon)

Olfactory Lobes

Anterior part of forebrain, connected with sense of smell.

Cerebrum

Largest and most complex part. Divided into right and left cerebral hemispheres connected by corpus callosum. Outer cerebral cortex has folds called convolutions/gyri and depressions called sulci. It contains motor, sensory, auditory, visual, taste and speech areas. The source notes that each hemisphere controls the opposite side of body.

Cerebrum controls conscious mental activities, memory, reasoning, feelings, emotions and also acts as dominant centre for many reflexes and involuntary acts.

Diencephalon

• Epithalamus: Contains pineal gland, which secretes melatonin.
• Thalamus: Relay centre; source mentions optic pathway crossing in this region.
• Hypothalamus: Connected with pituitary gland by infundibulum.

Diencephalon acts as relay centre between cerebrum and brain stem and helps regulate emotions, pleasure, fear and perceptions of heat, cold and pain.

Midbrain (Mesencephalon)

• Corpora quadrigemina: Four rounded lobes concerned with vision/hearing in the source.
• Crura cerebri: Bundles of nerve fibres connecting hindbrain with midbrain and acting as relay centre.

Hindbrain (Rhombencephalon)

• Cerebellum: Coordinates muscular movement, equilibrium and body posture; controls reflex action of skeletal muscles.
• Pons varolii: Relay centre and helps control body posture.
• Medulla oblongata: Contains centres for vital involuntary activities such as heartbeat, respiration, sneezing and coughing.

Ventricles

Four CSF-filled cavities: right and left lateral ventricles, third ventricle and fourth ventricle.

Brain Stem

Midbrain, pons and medulla together form brain stem, connecting forebrain with spinal cord and carrying sensory/motor pathways.

Cerebrospinal Fluid (CSF)

Transparent, slightly alkaline fluid containing water, minerals, glucose and small amount of proteins/urea/creatinine. The source gives total volume about 150 ml and says it is secreted by choroid plexus.

• Protects and supports brain and spinal cord.
• Maintains uniform pressure.
• Acts as cushion and shock absorber.
• Keeps CNS moist.
• Helps remove metabolic wastes, drugs and other substances.
• Helps supply nutrients.
• Acts as buffer and makes brain effectively lighter due to buoyancy.

Spinal Cord

Spinal cord is part of CNS running mid-dorsally through vertebral column. It is cylindrical and elongated, extending from medulla to about second lumbar vertebra in the source. It is covered by three meninges and surrounded by CSF.

In transverse section, central H-shaped grey matter is surrounded by white matter. Grey matter forms dorsal and ventral horns. Central canal contains CSF. A pair of spinal nerves originates from each segment.

Peripheral Nervous System

Nerves originating from central nervous system constitute peripheral nervous system.

Cranial Nerves

There are 12 pairs of cranial nerves originating from brain. The source table lists their sensory/motor/mixed nature and general functions.

Spinal Nerves

There are 31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Spinal nerves are mixed nerves.

Autonomic Nervous System (ANS)

ANS controls and coordinates involuntary activities of visceral organs such as heartbeat, respiration and movement of alimentary canal. It consists of sympathetic and parasympathetic divisions.

Mechanism / Conduction of Nerve Impulse

The source divides nerve impulse transmission into electrical transmission along neuron and chemical transmission across synapse.

1. Electrical Transmission

Polarization / Resting Potential

At rest, outside of neuron membrane is positively charged and inside is negatively charged. The source explains this through Na⁺/K⁺ distribution and sodium pump and gives a potential difference around 80 mV.

Depolarization

When stimulus reaches neuron, membrane becomes permeable to sodium ions; sodium enters and charge across membrane reverses.

Action Potential

Depolarization disturbs the next membrane region and moves forward along nerve fibre. This travelling electrical change is action potential.

Repolarization

After action potential, polarization is restored; impulse thus propagates electrically.

2. Chemical Transmission at Synapse

At synapse, arrival of impulse causes synaptic vesicles to release acetylcholine. It diffuses across synaptic cleft, stimulates postsynaptic membrane and produces depolarization. Acetylcholinesterase breaks acetylcholine after transmission.

Endocrine System

Characteristics of Hormones

• Chemical messengers carrying specific messages.
• Transported by blood.
• Secreted in small amount but effects are strong.
• Act on target organs.
• Not stored for long in body; destroyed soon in the source description.
• Excess or deficiency may cause disease.

Pituitary Gland

Pituitary gland lies at base of brain below hypothalamus and is connected with it by a stalk called infundibulum. It is called master gland because its hormones control several other endocrine glands.

Parts

• Anterior lobe
• Middle lobe
• Posterior lobe

Anterior and middle lobes are grouped as adenohypophysis in the source; posterior lobe is neurohypophysis.

Hormones of Anterior Lobe

• Somatotropin / Growth Hormone (GH): Stimulates growth by cell division, especially bone growth.
• Thyroid Stimulating Hormone (TSH): Stimulates thyroid to secrete thyroxine.
• Prolactin / Lactogenic hormone: Stimulates mammary gland to produce milk.
• ACTH: Stimulates adrenal cortex.
• FSH: Stimulates ovarian follicle development in female and seminiferous tubules/sperm formation in male.
• LH: Source relates it to release of ovum in female and reproductive function in male.

Middle Lobe

MSH: Stimulates melanocytes to produce melanin.

Posterior Lobe

• Oxytocin: Promotes uterine wall contraction during childbirth and milk ejection; source also notes role in sperm transport.
• ADH / Vasopressin: Increases water reabsorption by increasing permeability of distal/collecting tubules.

Disorders of Growth Hormone

• Hyposecretion: Dwarfism.
• Hypersecretion: Gigantism; in adults, acromegaly with enlargement of feet, hands and lower jaw.

Thyroid and Parathyroid Glands

Thyroid Gland

Situated on either side behind thyroid cartilage. Source describes it as largest endocrine gland with two lobes connected by isthmus. Histologically it is made of thyroid follicles lined by cuboidal epithelium and containing colloid.

Thyroid Hormones

Thyroxine (T₄) and T₃ regulate metabolism and growth.

• General metabolism of carbohydrates and fats.
• Growth and differentiation through protein synthesis.
• Metamorphosis in larval forms in the source.

Hypothyroidism

• Cretinism in children: Physical and mental retardation, poor growth.
• Myxoedema in adults: Puffy/sluggish appearance, skin changes, low metabolic rate and related symptoms.
• Simple goitre: Iodine deficiency leads to insufficient thyroxine and enlargement of thyroid.

Hyperthyroidism

The source describes Graves’/exophthalmic goitre with enlarged/bulging eyes, long slender body, restlessness, insomnia and increased heart rate, temperature, blood pressure and BMR.

Thyrocalcitonin

Helps regulate calcium level by promoting movement of calcium from blood toward bone when blood calcium is high.

Parathyroid Glands

Four small glands embedded on dorsal surface of thyroid. Chief cells secrete parathormone (PTH).

• Regulates metabolism of calcium and phosphate.
• Releases calcium from bone and increases blood calcium.
• Calcium is important for muscle contraction and blood clotting.

Disorders

• Hypoparathyroidism: Tetany due to low calcium; uncontrolled muscle contractions in face, larynx, hands and feet.
• Hyperparathyroidism: Excess calcium mobilization and deposition problems noted in tissues such as kidney/gall bladder/ureter in source.

Adrenal / Suprarenal Glands

One adrenal gland lies on top of each kidney, so they are called suprarenal glands. Each gland has outer cortex and inner medulla.

Hormones of Adrenal Cortex

• Glucocorticoids: Stimulate synthesis of glucose from amino acids and fatty acids (gluconeogenesis) and influence WBC activity.
• Mineralocorticoids: Regulate mineral metabolism, especially sodium and potassium balance.
• Sex steroids: Concerned with sexual organs and secondary sexual characters.

Disorders of Adrenal Cortex

• Addison’s disease: Source attributes to deficiency of mineralocorticoids and glucocorticoids; low sodium/high potassium, weakness, anaemia, diarrhoea, dark bronze skin, low BP.
• Cushing’s disease: Source attributes to hypersecretion of glucocorticoid; high blood sugar, abdominal obesity/wasting of limb muscles, liver glycogen deposition.
• Adrenal virilism: Hypersecretion of sex steroid hormones; male-type secondary characters in female and menstrual changes.
• Gynaecomastia: Source associates with excess female sex hormone in male; female-like voice, reduced facial hair, enlargement of breasts.

Hormones of Adrenal Medulla

Adrenaline / Emergency Hormone

Increases during cold, heat and emotional excitement. It helps convert glycogen to glucose and prepares body for emergency.

• Increases heartbeat, blood pressure and pulse rate.
• Converts glycogen to glucose for quick energy.
• Causes bronchodilation; the source notes therapeutic use in asthma.

Noradrenaline

The source says it helps bring body back toward normal after excitation and affects blood pressure and heart rate.

Male Reproductive System

1. Testes

Primary male sex organs; paired, oval bodies situated in scrotum outside abdominal cavity so their temperature remains about 2–3°C below body temperature, important for sperm production. Each testis contains many lobules with seminiferous tubules. Seminiferous tubules open via rete testis and vasa efferentia toward epididymis. Interstitial/Leydig cells secrete male sex hormones.

2. Epididymis

Highly coiled tube outside testis, made from vasa efferentia. Source differentiates caput, corpus and cauda. It stores sperm and secretes nourishing fluid.

3. Vas Deferens

Also called seminal duct; thick muscular tube carrying sperm from epididymis toward seminal vesicle/ejaculatory region.

4. Seminal Vesicles

Paired muscular pouches behind urinary bladder. Secrete yellowish slightly alkaline viscous fluid rich in fructose; store/nourish sperm.

5. Ejaculatory Duct

Short duct formed in the region of seminal vesicle and urethra; conducts semen into urethra.

6. Urethra

Common tube for passage of urine and semen.

7. Penis

Erectile copulatory organ composed of columns of erectile tissue. The source names corpora cavernosa and corpus spongiosum, and describes glans penis at tip covered by prepuce.

8. Accessory Glands

• Prostate gland: Single gland below urinary bladder; secretes milky slightly acidic fluid helping sperm mobility and nourishment; source gives about 15–20% semen volume.
• Cowper’s glands: Paired pea-sized glands below prostate; secrete alkaline viscous fluid that lubricates urethra and helps movement of penis during copulation.

Female Reproductive System

1. Ovaries

Primary female sex organs; paired small oval bodies in pelvic/abdominal region, attached near uterus and supported by ligaments/mesovarium in source.

Each ovary is described as having outer germinal epithelium, middle tunica albuginea and inner connective tissue stroma. Germinal epithelium forms ovarian follicles. Follicles mature through primary/secondary stages to Graafian follicle. After ovulation, remaining follicle forms corpus luteum, which secretes progesterone.

2. Fallopian Tubes / Oviducts

Paired muscular ciliated tubes about 10–12 cm long in source. They conduct ovum toward uterus and secrete nutritive fluid.

• Isthmus: Narrow part attached to uterus.
• Ampulla: Wider curved part.
• Infundibulum: Funnel-shaped end near ovary with fimbriae; site of fertilization in source.

3. Uterus

Hollow thick-walled muscular pear-shaped organ attached to body wall by broad ligament. Wall consists of outer perimetrium, middle myometrium and inner endometrium. Source states uterine cavity can expand greatly during pregnancy.

• Fundus — upper dome-shaped part.
• Body — broad middle part.
• Cervix — lower narrow part connecting to vagina.

4. Vagina

Fibromuscular tubular female copulatory organ, about 12 cm in source, extending from cervix to exterior.

• Passage for menstrual flow.
• Receives sperm during sexual intercourse.
• Acts as birth canal.

5. Vulva / External Genitalia

Source lists mons pubis, clitoris, labia majora and labia minora.

6. Glands

• Bartholin’s glands: Bean-shaped glands near vaginal opening, secrete viscous lubricating fluid.
• Mammary glands: Rounded glands in breasts; secrete milk after pregnancy under influence of prolactin.

Menstrual Cycle in Human

Cyclic changes in uterus and ovaries of female occurring approximately every 28 days from puberty to menopause are called menstrual cycle. Menstruation itself lasts about 3–5 days in source.

1. Menstruation phase
2. Follicular phase
3. Ovulation
4. Luteal / secretory phase

1. Menstruation Phase

Decrease in progesterone and oestrogen causes contraction/degeneration of endometrium; blood and endometrial tissue are discharged through vagina.

2. Follicular Phase

Occurs roughly day 6–12 in source. FSH from anterior pituitary stimulates ovarian follicle development. Growing follicle produces oestrogen, which repairs/develops uterine wall and contributes to female secondary sexual characters.

3. Ovulation

Around 14th day, mature Graafian follicle ruptures and releases ovum. Source associates this with rise of LH.

4. Luteal / Secretory Phase

From about day 15 to 28, remaining follicle forms corpus luteum, which secretes progesterone and prepares uterus/endometrium for pregnancy.

• If egg is fertilized, corpus luteum remains active for continued hormonal support according to source.
• If egg is not fertilized, corpus luteum degenerates around end of cycle; hormone levels fall and cycle repeats.

Sense Organs

Sense organs perceive changes inside and outside body. They contain specialized receptor/sensory cells.

Source lists eye for vision, ear for sound, tongue for taste, nose for smell, and skin for touch/heat/cold.

Human Eye

Eye is organ of sight. Human eyeball is approximately spherical and lies in eye socket of skull, protected by bones and eyelids. Six pairs/sets of muscles help rotation in different directions.

Accessory Glands

• Meibomian glands: Modified sebaceous glands along eyelid margins, secrete oily material for lubrication.
• Lacrimal / tear glands: Present near upper outer side of eyeball, secrete tears containing lysozyme; tears keep eye moist/clean and drain to nasal chamber.

Coats of Eyeball

1. Outer Coat

• Sclera: White opaque fibrous protective layer; maintains shape.
• Cornea: Transparent anterior part through which light enters; refracts light.
• Conjunctiva: Thin transparent protective layer covering front region in source.

2. Middle Coat

• Choroid: Thick, vascular, pigmented; absorbs stray light and provides nutrition.
• Ciliary body: Lies near junction of sclera/cornea; ciliary muscles and suspensory ligaments change lens shape and hold lens.
• Iris: Muscular pigmented opaque diaphragm in front of lens. Central opening is pupil. Iris controls pupil size and amount of light entering eye; pigment determines eye colour.

3. Retina

Photosensitive inner coat containing rods and cones.

• Rods: Function in dim light and black-and-white vision; contain rhodopsin formed from vitamin A derivative and opsin in source description.
• Cones: Function in bright light and colour vision; source associates with iodopsin.
• Fovea centralis / yellow spot: Most sensitive region with many cones, gives sharp image.
• Blind spot: Area where optic nerve leaves retina; rods and cones absent, so no image is formed.

Lens

Large flexible transparent biconvex crystalline structure behind iris. It focuses object image on retina according to object distance.

Aqueous Chamber

Small chamber between cornea and lens filled with clear aqueous humour containing water, nutrients and ions. Nourishes lens and helps regulate pressure.

Vitreous Chamber

Large chamber between lens and retina filled with jelly-like vitreous humour. It supports retina and lens, helps refraction and maintains eyeball shape.

Working of Eye / Image Formation

Cornea, aqueous humour, lens and vitreous humour form optical apparatus. Light passes through cornea and pupil; lens adjusts shape to focus rays on retina. This adjustment is accommodation. Image formed on retina is inverted; visual impulses are carried by optic nerve to brain, where they are interpreted as upright.

Defects and Diseases of Eye

• Hypermetropia / long-sightedness: Image tends to form behind retina; corrected with convex lens.
• Myopia / short-sightedness: Image tends to form in front of retina; corrected with concave lens.
• Astigmatism: Unequal curvature of refractive surfaces; corrected by cylindrical lens.
• Cataract (Motibindu): Lens becomes opaque; source says corrected by surgical removal/replacement.
• Glaucoma (Jalbindu): Source associates with impaired drainage/canal of Schlemm and increased intraocular pressure; may damage optic nerve.
• Presbyopia: Age-related accommodation defect; source notes bifocal lenses.
• Conjunctivitis: Inflammation of conjunctiva, also called pink eye.
• Trachoma: Chronic conjunctival disease caused by Chlamydia trachomatis according to source.

Human Ear (Stato-Acoustic Organ)

Ear is organ of hearing and equilibrium and is described as stato-acoustic organ. It is divided into external ear, middle ear and internal ear.

Middle Ear Ossicles

The source labels malleus, incus and stapes. These transmit vibrations from tympanic membrane to inner ear.

Internal Ear

Internal ear is a complex delicate membranous labyrinth inside a bony cavity. Bony labyrinth contains perilymph.

1. Vestibule

Contains utriculus and sacculus. Both contain sensory areas called maculae, concerned with balancing during stationary condition — static equilibrium.

2. Semicircular Canals

Three semicircular canals arranged in different planes. Each has a swollen ampulla containing sensory spot called crista. Cristae are concerned with balance during dynamic/rotational movement.

3. Cochlea

Main hearing organ, a spirally coiled tube arising near sacculus. Source gives about 2.5–3 turns.

Cochlea contains three fluid-filled chambers:

• Scala vestibuli: Filled with perilymph and connected toward middle ear through oval window/fenestra ovalis.
• Scala tympani: Filled with perilymph and associated with round window/fenestra rotunda.
• Scala media: Filled with endolymph, bounded by Reissner’s membrane above and basilar membrane below. Organ of Corti lies on basilar membrane and receives vibrations.

Functions of Ear

1. Hearing

Source summarizes pathway as:

2. Equilibrium

• Dynamic equilibrium: Ampullae/cristae of semicircular canals detect rotational movement.
• Static equilibrium: Maculae in utriculus and sacculus of vestibule detect position of body at rest.