Revision: Human Body and Organ System General Science SSC (English Medium) 8th Standard Maharashtra State Board

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

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.

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.

Veins are the blood vessels that carry deoxygenated (impure) blood from the body cells to the heart.

Arteries are the blood vessels in which oxygenated blood is going away from the heart to all parts of the body.

The circulatory system is a transport system moving substances throughout our body with the help of blood.

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.

The complete sequence of events that occurs during one heartbeat, including contraction and relaxation of the atria and ventricles, is called the cardiac cycle.

Blood is a fluid that circulates in the heart, arteries, capillaries, and veins of vertebrate animals. It transports nutrients and oxygen throughout the body and removes waste.

The plasma from which the protein fibrinogen has been removed is called serum. 

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

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

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

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).

Rhesus factor is a hereditary protein present in red blood cells. If present, the individual is Rh-positive; if absent, they are Rh-negative.

Blood transfusion is the process of introducing blood from a healthy donor into the bloodstream of a patient, typically through a vein, often during surgery or after heavy blood loss.

A Blood Recipient is a person who receives blood during a transfusion, requiring compatibility with the donor's blood group.

Antigens are specific proteins present on the surface of red blood cells that determine an individual’s blood group (e.g., Antigen A or Antigen B).

Antibodies are proteins present in blood plasma that react against specific antigens not found on the individual's own red blood cells, playing a key role in blood group compatibility.

A person with blood group AB is called a universal recipient because they can receive blood from all major blood groups (A, B, AB, and O) without risk of incompatibility.

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.

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

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

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

• 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).

• 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.

• 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.

• 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.

1. The cycle begins with atrial contraction (atrial systole) while the ventricles are relaxed; blood flows easily from atria into ventricles through open cuspid valves.
2. As ventricles contract (ventricular systole) and atria relax, pressure closes the cuspid valves, preventing blood from flowing back into the atria.
3. Chordae tendinae, attached to papillary muscles, hold the cuspid valve flaps in place and prevent their inversion during ventricular contraction.
4. Blood is pushed out of the ventricles into the pulmonary artery and aorta through semilunar valves, which open under pressure from below.
5. When ventricles relax again, blood tends to return, but the semilunar valves fill and close, preventing backflow into the heart.

• 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).

• ABO System: Introduced by Karl Landsteiner (1900). Based on the presence/absence of antigen A and B on RBCs, four groups: A, B, AB, O.
• Universal Donor & Acceptor: AB = universal acceptor (both antigens, no antibodies); O = universal donor (no antigens, both antibodies).
• Rh Factor: Discovered by Landsteiner & Wiener (1940). 80–85% people are Rh⁺; the rest are Rh⁻.
• HDN (Erythroblastosis Foetalis): When a Rh⁻ mother carries a Rh⁺ foetus, she produces anti-Rh antibodies (after the first delivery), which attack subsequent Rh⁺ foetuses.
• Prevention: Rh⁻ mother is injected with anti-D antibodies during all pregnancies with Rh⁺ foetus to prevent HDN.

• 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.