Revision: Respiration and Circulation Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

It is a type of reflex whose stimulus is in the nasal passage which causes spasmodic contraction of expiratory muscles that forcefully expel the air through the nasal passage.

Difficulty or labored breathing, often described as shortness of breath.

It is a type of reflex whose stimulus is any foreign particle, resulting from deep inspiration followed by strong expiration, which forcefully expels the air through the mouth.

The trachea is commonly called a windpipe. It is a tube supported by cartilaginous rings that connect the pharynx and larynx to the lungs, allowing the passage of air. The trachea divides into right and left bronchi and enters the lungs.

The process of conversion of glucose molecules in food into energy-rich molecules, carbon dioxide and water with the help of oxygen is known as respiration.

Eupnea is the medical and physiological term for normal, unlabored, and quiet breathing in a healthy individual at rest. It represents an efficient respiratory state where the body maximizes oxygen intake while minimizing muscular effort.

Apnea is defined as the temporary cessation of breathing, marked by the absence of respiratory muscle movement and airflow.

The exchange of gases through moist skin and blood capillaries underneath is called cutaneous respiration.

The breakdown of glucose in the presence of oxygen to produce carbon dioxide, water, and energy is called aerobic respiration.

It is a process of release of energy from food substances such as glucose and fats under the control of enzymes, to carry out life processes, by the living organisms.

The breakdown of glucose in the absence of oxygen to produce alcohol or lactic acid and a small amount of energy is called anaerobic respiration.

Phagocytosis is a process in which most WBCs, particularly the neutrophils, engulf particle-like solid substances, especially bacteria.

Diapedesis is the movement of white blood cells from lymph capillary walls to neighbouring tissues, often for immunological defence.

The solid mass left behind after clotting, formed by fibrin and trapped blood cells, is called clot or thrombus.

The enzyme released by injured tissues and disintegrating platelets that initiates clotting is called thrombokinase (also known as Thromboplastin or Factor X or Stuart factor).

The heart is a hollow muscular vertebrate organ that pumps blood through rhythmic contractions.

An ARTERY is a vessel that carries blood away from the heart towards any organ.

A VEIN is a vessel that carries blood away from an organ towards the heart.

The smallest or the final branch of an artery is called an arteriole.

A venule is the smallest branch of a vein formed by the union of capillaries. It has a thin muscular coat and gradually joins with other venules to form larger veins.

Diastolic pressure is the lower limit of blood pressure, recorded when the heart is at rest and the pressure in the arteries drops after the pulse wave has passed.

Hypertension is a condition in which the blood pressure consistently remains above 140/90 mm Hg.

A sphygmomanometer is the instrument used to measure blood pressure in the arteries.

A pulse is the rhythmic expansion and contraction of arteries (especially the radial artery at the wrist) due to the pumping action of the heart.

The sound or movement of the heart as it sends blood around the body is called heartbeat. Every heartbeat causes a pulse in the arteries.

Blood pressure is the pressure that the blood flowing through the arteries exerts on their walls.

Lymph is the fluid formed from tissue fluid that enters lymph vessels. It helps in returning excess fluid to the blood and plays a role in immunity.

The graphical recording of electrical variations detected at the surface of the body during their propagation through the heart wall is called electrocardiogram.

• Living organisms need energy for life processes. Complex organic compounds (potential energy) must be converted into usable form (ATP) through respiration.
• Respiration is a biochemical process of oxidation of organic compounds in an orderly manner to release chemical energy as ATP.
• Equation — C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP
• Gaseous Exchange — Respiration involves exchange of gases (O₂ in, CO₂ out) between the organism and environment. The site where this exchange occurs is called the respiratory surface.

• Two Parts — Upper (nasal cavities, pharynx, throat) | Lower (larynx, trachea, bronchi, bronchioles, lungs).
• Nasal Cavity & Pharynx — Nasal cavity is divided into 2 chambers by the mesethmoid cartilage. Pharynx: Nasopharynx → Oropharynx (common food & air) → Laryngopharynx.
• Larynx & Trachea — Glottis covered by epiglottis (prevents food entry). Trachea held by 16–20 C-shaped cartilage rings.
• Lungs & Alveoli — Right = 3 lobes, Left = 2 lobes, covered by pleural membranes. Alveoli = site of O₂/CO₂ exchange.
• Path of Air — Nasal cavity → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli (exchange) → reverse for CO₂.
• Gas Transport — O₂ carried by haemoglobin (RBCs) | CO₂ in dissolved form in plasma.
• Other Organisms — Plants: stomata | Fish: gills (breathe faster, less O₂ in water) | Terrestrial animals: lungs.

• Breathing — Physical process of gaseous exchange between the atmosphere and the lungs involving the thoracic cage, ribs, sternum, intercostal muscles and diaphragm.
• Two Phases — Breathing has two phases: Inspiration (air in) and Expiration (air out).
• Inspiration (Active) — External intercostal muscles and diaphragm contract. Ribs & sternum move up and outward, diaphragm flattens downward → thoracic volume increases → lung pressure decreases → air rushes in.
• Expiration (Passive) — Intercostal muscles and diaphragm relax. Ribs & sternum move down and inward, diaphragm arches upward (dome-shaped) → thoracic volume decreases → lung pressure increases → air is expelled out.
• Key Difference — Inspiration = active (needs muscle contraction) | Expiration = passive (muscles simply relax).

• Exchange of gases (O₂ & CO₂) between alveolar air and blood via simple diffusion, driven by pressure gradients.
• O₂ Exchange — O₂ diffuses from alveoli (pO₂ = 104 mmHg) → blood (pO₂ = 40 mmHg), raising blood pO₂ to 95 mmHg.
• CO₂ Exchange — CO₂ diffuses from blood (pCO₂ = 45 mmHg) → alveoli (pCO₂ = 40 mmHg), lowering blood pCO₂ to 40 mmHg.
• Driving Force — Exchange occurs by simple diffusion from high pressure → low pressure (no energy needed).

• Meaning — O₂ from blood is delivered to cells/tissues, and CO₂ from cells passes into the blood.
• O₂ Transport — 97% as oxyhaemoglobin (HbO₂) via RBCs, 3% dissolved in plasma. One Hb molecule has 4 Fe²⁺ ions, each binding one O₂: Hb + 4O₂ → Hb(O₂)₄
• Bohr Effect — Rise in CO₂ / lower pH / higher temperature → reduces Hb-O₂ affinity (curve shifts right) → O₂ released to tissues.
• Haldane Effect — Binding of O₂ with Hb displaces CO₂ from blood (curve shifts left, higher Hb-O₂ affinity).
• CO₂ Transport — 70% as bicarbonate ions (HCO₃⁻) in plasma | 23% as carbaminohaemoglobin | 7% dissolved in plasma.
• Chloride Shift (Hamburger's Phenomenon) — When CO₂ enters blood, Cl⁻ moves into RBCs (Na⁺ stays behind). When CO₂ leaves, Cl⁻ moves back out. This alternate Cl⁻ movement maintains electrical balance.

• Cellular respiration is the process where food (glucose) is oxidised inside the cell to release energy, stored as ATP via phosphorylation.
• Oxidation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 686 Kcal | Phosphorylation: ADP + iP + 7.3 Kcal → ATP
• First step occurs in cytoplasm — glucose breaks down into pyruvate (3-carbon molecule), releasing some energy.
• Without O₂ (Anaerobic) — Pyruvate → Ethanol + CO₂ (yeast) or Lactic acid (muscles). Less energy released.
• With O₂ (Aerobic) — Pyruvate breaks down in mitochondria into CO₂ + H₂O, releasing a large amount of energy as ATP.

• Respiration is controlled by both nervous and chemical signals.
• Nervous regulation centres are located in medulla oblongata (DRG & VRG) and pons Varolii (pneumotaxic & apneustic centres).
• Medulla centres — DRG (Dorsal Respiratory Group) and VRG (Ventral Respiratory Group) control the basic rhythm of breathing.
• Chemical regulation is done by chemoreceptors — Peripheral (in carotid bodies & aortic bodies) and Central (in brain stem) — detect changes in CO₂, O₂ and pH levels in blood.
• Hering-Breuer reflex controls the depth and rhythm of respiration to prevent over-inflation of lungs.

• Transportation moves oxygen, nutrients and waste between the organism and the environment, and within the body.
• Two methods — Diffusion (passive, no energy) and Active transport (against gradient, needs energy).
• Cyclosis/Streaming movement — circular movement of cytoplasm for intracellular transport. e.g. Paramecium, Amoeba, root hairs in plants.
• Intracellular — transport within the cell (e.g. Sponges, Coelenterates) | Extracellular — transport outside the cell (e.g. Roundworm).
• In higher organisms, transportation is carried out by specialised circulatory systems using blood and lymph.

• In higher animals, circulation is carried out by blood and lymph through the blood vascular system.
• Open Circulation — Blood flows through haemocoelis (open body cavities/sinuses), not enclosed in vessels. No respiratory pigment. e.g. Arthropods and Molluscs.
• Closed Circulation — Blood circulates through a closed network of blood vessels throughout the body. e.g. Annelids, Vertebrates.
• Single Circulation — Blood passes through the heart once per cycle. The heart has 2 chambers (venous heart). e.g. Fishes.
• Double Circulation — Blood passes through the heart twice per cycle. The heart has 4 chambers. Involves pulmonary (heart → lungs → heart) and systemic (heart → body → heart) circulation. e.g. Mammals, Birds.

• Blood = Plasma (55%) + Formed Elements (44%). Serum = Plasma − clotting factors.
• Plasma — Straw-coloured, viscous fluid. 90% water + 7–8% proteins. Contains fibrinogen (clotting), globulins (defence), albumins (osmotic balance), minerals and glucose.
• RBCs (Erythrocytes) — Count = 5–5.5 million/mm³. Matured RBCs are enucleated. Lifespan = 120 days. Produced in red bone marrow (erythropoiesis). Destroyed in the liver and spleen.
• WBCs (Leucocytes) — Colourless, nucleated, fewer in number. Count = 5000–11000/mm³. Short-lived. Defend the body against infections.
• Platelets (Thrombocytes) — Count = 2.5–4.5 lakh/mm³. Release substances for blood clotting/coagulation. Low count = Thrombocytopenia.

• Platelets are oval-shaped, enucleated cell fragments found in mammals only. Normal count = 2.5–4.5 lakh/mm³. Lifespan = 3–5 days, destroyed mainly in the spleen.
• At injury site, platelets disintegrate → release thrombokinase (thromboplastin/Factor X) → initiates clotting cascade. Vitamin K is essential for the synthesis of prothrombin in the liver.
• Thrombokinase + Ca²⁺ ions → converts inactive prothrombin → active thrombin.
• Thrombin converts soluble fibrinogen → insoluble fibrin threads → mesh traps blood cells → forms a clot sealing the wound.
• Clot contracts → squeezes out serum → leaves behind a solid mass called thrombus. Low platelet count = Thrombocytopenia (excessive bleeding).

• WBCs are colourless, nucleated cells lacking haemoglobin. Normal count = 5000–11000/mm³. Produced in red bone marrow, lymph nodes, liver and spleen. Short-lived (~2 weeks).
• Two types — Granulocytes (Neutrophils, Eosinophils, Basophils) and Agranulocytes (Lymphocytes, Monocytes). Most WBCs are amoeboid and enter tissues via diapedesis.
• Neutrophils (70%) and Monocytes are phagocytic — engulf germs and damaged cells. Eosinophils initiate an allergic response. Basophils release histamine, heparin and serotonin.
• Lymphocytes (25–30%) provide immunity — B-lymphocytes (antibody-mediated) and T-lymphocytes (cell-mediated). Form the basis of immunity and vaccination.
• WBC count increases during infection, stress or leukaemia (leukocytosis) and decreases in viral illness or bone marrow disorders (leukopenia).

• Platelets are oval-shaped, enucleated cell fragments found in mammals only. Normal count = 2.5–4.5 lakh/mm³. Lifespan = 3–5 days, destroyed mainly in the spleen.
• At injury site, platelets disintegrate → release thrombokinase (thromboplastin/Factor X) → initiates clotting cascade. Vitamin K is essential for the synthesis of prothrombin in the liver.
• Thrombokinase + Ca²⁺ ions → converts inactive prothrombin → active thrombin.
• Thrombin converts soluble fibrinogen → insoluble fibrin threads → mesh traps blood cells → forms a clot sealing the wound.
• Clot contracts → squeezes out serum → leaves behind a solid mass called thrombus. Low platelet count = Thrombocytopenia (excessive bleeding).

• Heart is conical-shaped, of mesodermal origin, located in mediastinum (between two lungs), tilted slightly to the left. Enclosed in pericardium (a double-walled membrane with pericardial fluid to reduce friction).
• Three layers — Epicardium (outer, protective), Myocardium (middle, contraction/relaxation), Endocardium (inner, protective).
• Four chambers — 2 atria (thin-walled, receiving blood) + 2 ventricles (thick-walled, pumping blood). Left ventricle is the thickest as it pumps blood to the entire body.
• Septa & External Features — Atria separated by inter-atrial septum (has fossa ovalis). Ventricles are separated by the interventricular septum. Atria and ventricles are separated externally by the coronary sulcus. Aorta = ascending → aortic arch → descending.
• Blood Flow & Conducting System — Right ventricle → deoxygenated blood → lungs | Left ventricle → oxygenated blood → body. Heart is myogenic; pacemaker (SA node) → AV node → Bundle of His → Purkinje fibres → coordinated contraction.

• Cardiac Cycle Duration — One complete cardiac cycle = 0.8 sec (Atrial systole + Ventricular systole + Joint diastole).
• Atrial Systole — SA node generates impulse → atria contract → blood pushed into ventricles (increases flow by ~30%). Impulse travels: SA node → AV node → Bundle of His → Purkinje fibres → ventricles.
• Ventricular Systole (0.3 sec) — Ventricles contract → cuspid valves close (Lubb sound) → right ventricle pumps deoxygenated blood to pulmonary trunk, left ventricle pumps oxygenated blood to aorta. Atria undergo diastole simultaneously.
• Ventricular Diastole (0.5 sec) — Ventricles relax → semilunar valves close (Dubb sound) → prevents backflow from pulmonary trunk and aorta.
• Joint Diastole — All four chambers relax simultaneously for 0.4 sec. This is called complete/joint diastole.
• Heart Sounds — Lubb = cuspid valves closing (ventricular systole) | Dubb = semilunar valves closing (ventricular diastole).
• Cardiac Output — Stroke Volume × Heart Rate = 70 × 72 = 5040 mL/min. Regulated by ANS (neural) and adrenaline/acetylcholine (hormonal).

• Arteries — Carry oxygenated blood from heart to body. Have no valves. Smallest arteries = arterioles → branch into capillaries. Exception: Pulmonary artery carries deoxygenated blood to the lungs.
• Veins — Carry deoxygenated blood to the heart. Have valves for unidirectional flow. Exception: Pulmonary veins carry oxygenated blood from lungs to left atrium.
• Capillaries — Tiniest vessels with one-cell-thick walls. Allow exchange of nutrients, gases, waste and hormones between blood and cells.
• Key Rule — Arteries = away from heart | Veins = towards heart. Valves present only in veins (except pulmonary veins).
• Pulse — Pressure waves produced by ventricular systole, felt in superficial arteries. Normal pulse = 72/min. Easily felt at the radial (wrist) and carotid (neck) arteries.

• Blood Pressure (BP) — Hydrostatic pressure of blood. Normal BP = 120/80 mmHg (systolic/diastolic). Measured from the brachial artery using a sphygmomanometer.
• Hypertension — BP above 140/90 mmHg. Extreme readings (180/120 mmHg) damage vital organs like brain and kidneys.
• Coronary Artery Disease (CAD) — Caused by deposition of calcium, fat, cholesterol and fibrous tissue in coronary arteries, narrowing them.
• Angina Pectoris — Insufficient O₂ to heart muscles causes acute chest pain. Angiography (X-ray imaging) is used to locate blockages in cardiac blood vessels.
• Heart Transplantation — Severely damaged heart replaced by a healthy heart from a brain-dead/recently dead donor.
• Silent Heart Attack — Lacks classic symptoms like extreme chest pain or hypertension, making it difficult to detect.

• ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. The machine used to record it is called an electrocardiograph.
• P-wave — Represents depolarisation of atria, causing contraction of both atria.
• QRS Complex — Represents depolarisation of the ventricles, initiating ventricular contraction. Contraction begins immediately after Q → marks the start of systole.
• T-wave — Represents repolarisation of ventricles. End of T-wave = end of systole.
• Key intervals — P-Q interval = 0.16 sec | Q-T interval = represents full ventricular activity | RR interval = time between two heartbeats.
• Important distinction — Electrocardiograph = the instrument | Electrocardiogram = the recording/graph produced.

• Lymphatic system consists of lymph, lymphatic vessels, organs and tissues. The word "lymph" means clear water.
• Lymph is similar to blood but lacks RBCs, platelets and some proteins. It contains mainly WBCs (lymphocytes), 94% water and 6% solids (proteins & fats).
• Lymph is formed from tissue fluid and flows through lymph vessels by contraction of nearby muscles. It drains into veins via thoracic duct and right lymphatic duct.
• Lymph supplies nutrients and O₂ to deep tissues where blood cannot reach, and drains excess tissue fluid, waste and proteins back into the bloodstream.
• Lymph absorbs fats from the intestine and defends the body by removing bacteria and housing immune cells (lymphocytes).

1. Control of Breathing
   Respiration is under dual control—nervous and chemical, and normal breathing is an involuntary process regulated by respiratory centres.
2. Respiratory Centres
   The respiratory centres are located in the medulla and pons and include the dorsal group (inspiration), ventrolateral group (inspiration and expiration), and pneumotaxic centre (limits inspiration).
3. Hering–Breuer Reflex
   Inflation of lungs stimulates stretch receptors which send impulses via vagus nerve to inhibit inspiration, initiating expiration; this reflex controls depth and rhythm of breathing.
4. Voluntary Control
   Respiratory centres are connected to the cerebral cortex, allowing voluntary control of breathing, but this is limited by rise in carbon dioxide levels in blood.