Annelids (earthworms, leeches, polychaetes) have a closed circulatory system — blood flows through a network of vessels including dorsal and ventral blood vessels, capillaries, and aortic arches (pseudo-hearts).

Brain haemorrhage (haemorrhagic stroke) occurs when an artery in the brain bursts, causing bleeding into the brain tissue. This is one of two types of stroke (the other being ischaemic stroke from a blockage).

A stroke's symptoms depend on which area of the brain is affected by the blocked or ruptured blood vessel. Different brain regions control different functions (speech, movement, vision, balance), so symptoms vary accordingly.

White blood cells (leukocytes) are actually colourless — they have no haemoglobin. They appear white/colourless when separated from blood. They got their name from the white layer (buffy coat) they form when blood is centrifuged.

Passive immunisation using antitoxins/antisera is used in emergencies: anti-tetanus serum (tetanus), anti-venom (snake bites), anti-rabies immunoglobulin (rabies exposure), and anti-diphtheria serum. They provide immediate temporary protection.

The lymphatic system collects excess interstitial fluid that leaks from capillaries into tissues, filters it through lymph nodes, and returns it to the bloodstream. It also transports dietary fats and immune cells.

Chronic hypertension (high blood pressure) damages the endothelial lining of blood vessels, causing inflammation, atherosclerosis, and increasing the risk of heart attack, stroke, kidney failure, and aneurysm.

Capillaries are the only blood vessels with walls one cell thick (single layer of endothelium). This thin barrier allows rapid diffusion of O₂, CO₂, glucose, amino acids, and metabolic wastes between blood and tissues.

Atrial systole is the simultaneous contraction of both atria that pushes blood into the ventricles. It follows the diastolic filling phase and precedes ventricular systole. It lasts about 0.1 seconds in a normal cardiac cycle.

Immunoglobulins (antibodies) are plasma proteins that play a direct role in immune defence. They bind specifically to antigens on pathogens, neutralising them or marking them for destruction by other immune cells.

Blood from the legs is collected by the femoral veins, which merge to form the common iliac veins. The two common iliac veins unite to form the inferior vena cava, which carries blood to the right atrium.

Normal blood pH is maintained between 7.35-7.45 (slightly alkaline). Even small deviations cause acidosis (pH7.45), both of which are life-threatening. pH is regulated by lungs, kidneys, and buffer systems.

Plasma constitutes approximately 55% of total blood volume. The remaining 45% is the formed elements (red blood cells ~44%, white blood cells and platelets ~1%). This ratio (haematocrit) is measured in blood tests.

The lymphatic system is a second transport system in humans that collects excess interstitial fluid (as lymph), filters it through lymph nodes, and returns it to the blood via the thoracic duct. It also absorbs dietary fats from the gut.

All vertebrates (fish, amphibians, reptiles, birds, and mammals) have a closed circulatory system where blood is always enclosed within vessels. Insects and most other invertebrates have open systems.

The fish heart is a single-circuit pump — blood passes through the heart once per complete circuit: heart → gills (oxygenation) → body (deoxygenation) → back to heart. This is less efficient than the double circuit of mammals.

In Hydra, the endodermal cells digest food and nutrients diffuse outward to the ectodermal cells. The mesoglea between them allows diffusion of small molecules including glucose and amino acids from endoderm to ectoderm.

The circulatory system enables rapid mass flow of materials — much faster than diffusion alone. While diffusion over 1 mm takes about 1 second, transporting materials 1 metre by diffusion would take years. Circulation accomplishes this in minutes.

Osmosis is the movement of water (solvent) molecules across a selectively permeable membrane from a region of high water concentration (low solute concentration, high water potential) to low water concentration (high solute, low water potential).

Mycorrhizal fungi form associations with plant roots and are especially important for mineral (particularly phosphate) uptake. The extensive fungal hyphae network greatly increases the absorptive surface area for mineral ions.

AIDS (Acquired Immune Deficiency Syndrome) is caused by HIV (Human Immunodeficiency Virus). HIV specifically destroys CD4+ T helper cells, progressively collapsing the immune system and leaving the patient vulnerable to opportunistic infections.

Passive immunity is conferred by injecting pre-formed antibodies (antisera) from an immune donor into a recipient. It provides immediate but temporary protection (weeks to months) without activating the recipient's immune system.

Cytotoxic T cells (killer T cells) directly destroy cells bearing foreign antigens (virus-infected cells, cancer cells, transplanted cells) by releasing perforins and granzymes causing cell lysis.

Lymph nodes are located throughout the body, with major clusters in the neck (cervical), armpits (axillary), and groins (inguinal). They filter lymph and are sites where immune responses are initiated against pathogens.

Each antibody (immunoglobulin) molecule consists of four polypeptide chains: two identical heavy chains and two identical light chains, held together by disulphide bonds, forming a Y-shaped structure.

Immunity is the body's capacity to recognise and respond to foreign antigens (bacteria, viruses, toxins, transplants) by mobilising specific immune cells (lymphocytes) and products (antibodies) to neutralise and eliminate the threat.

Lymph nodes filter lymph (not blood directly), removing pathogens, foreign particles, and cancer cells. The spleen filters blood, removing old RBCs and pathogens. Both are important immune filtration organs.

Congestive heart failure (CHF) occurs when the weakened heart cannot pump blood efficiently. Blood backs up in both the pulmonary circulation (causing fluid in lungs) and systemic circulation (causing liver congestion and leg oedema).

Veins contain semilunar (pocket) valves that prevent backflow of blood. These one-way valves are especially important in limb veins where blood must flow upward against gravity. Arteries and capillaries lack valves.

Myocardial infarction (heart attack) occurs when a coronary artery is blocked (usually by an embolus from a ruptured plaque) cutting off blood supply to heart muscle, causing cell death (necrosis) of that area.

A thrombus is a solid mass (blood clot) that forms within a blood vessel and remains attached to the vessel wall. It is composed of platelets, fibrin, and trapped blood cells. It can obstruct blood flow causing serious complications.

Pulse (rhythmic pressure wave) can be felt in arteries where blood flows in pulses with each heartbeat. In veins and capillaries, the pulse is dampened out — blood flows smoothly without detectable pulsation.

Veins carry blood at progressively falling pressure (5-15 mmHg) back to the heart. Arterial pressure is highest (120/80 mmHg), falls across capillaries, and is lowest in veins and venae cavae (~0-5 mmHg).

Blue baby syndrome (cyanosis at birth) occurs when the foramen ovale or ductus arteriosus (foetal blood shunts) fail to close after birth, allowing mixing of oxygenated and deoxygenated blood, causing bluish skin colouration.

The middle layer (tunica media) of arteries contains smooth muscle and elastic tissue. This thick muscular layer withstands the high systolic pressure generated during ventricular contraction and helps maintain blood pressure.

Capillaries have walls consisting of a single layer of endothelial cells (one cell thick) plus a thin basement membrane. This extreme thinness allows rapid exchange of materials between blood and surrounding tissues.

Capillaries are the terminal, smallest branches of the arterial system. They form dense networks (capillary beds) in tissues where exchange of O₂, CO₂, nutrients, and wastes between blood and cells occurs.

Veins carry blood from the tissues back toward the heart. All veins (except pulmonary veins) carry deoxygenated blood. Veins have thinner walls than arteries and contain semilunar valves to prevent backflow.

An artificial pacemaker is an implanted electronic device that generates regular electrical impulses to maintain a normal heart rhythm when the natural pacemaker (SA node) or conduction system is defective.

The atrioventricular (AV) node is located in the wall between the right atrium and right ventricle. It receives the electrical impulse from the SA node and delays it briefly before relaying it to the ventricles via the Bundle of His.

One complete heartbeat (cardiac cycle) consists of one systole (contraction phase) and one diastole (relaxation phase). The normal heart rate is 60-100 beats per minute, with each cycle lasting about 0.8 seconds.

A complete cardiac cycle (heartbeat) consists of three stages: diastole (all chambers relax and fill), atrial systole (atria contract, pushing blood into ventricles), and ventricular systole (ventricles contract, ejecting blood).

The hepatic portal vein is formed by veins draining the alimentary canal (stomach, small and large intestines, pancreas, spleen). It carries nutrient-rich blood from digested food to the liver for processing.

The coronary arteries (right and left) are the very first branches of the aorta, arising from the aortic sinuses immediately above the aortic valve. They supply oxygenated blood to the heart muscle itself.

The left ventricle pumps oxygenated blood through the aorta to all parts of the body EXCEPT the lungs. The lungs receive deoxygenated blood from the right ventricle via the pulmonary artery.

The bicuspid (mitral) valve guards the opening between the left atrium and left ventricle. It has two cusps (leaflets). When the left ventricle contracts, this valve closes to prevent blood from flowing back into the left atrium.

The human heart acts as a double pump: the right side pumps deoxygenated blood through pulmonary circulation (to lungs) and the left side pumps oxygenated blood through systemic circulation (to body). Both occur simultaneously.

Oedema is the abnormal accumulation of fluid (lymph/interstitial fluid) in tissues, causing swelling. It results from increased capillary pressure, decreased plasma protein, lymphatic obstruction, or increased capillary permeability.

Thalassaemia is an inherited haemoglobin disorder where mutations in globin genes reduce or eliminate haemoglobin chain synthesis, causing haemolytic anaemia of varying severity. It is common in Mediterranean, Middle Eastern, and Asian populations.

Leukaemia (blood cancer) is characterised by uncontrolled proliferation of abnormal white blood cells that crowd out normal blood cells. It impairs immunity, causes anaemia, and increases bleeding tendency.

Interferon is a protein produced by virus-infected cells that protects neighbouring cells by interfering with viral replication. It also stimulates natural killer cells and macrophages to fight viral infections.

Basophils (and mast cells) produce and release heparin, a powerful anticoagulant that inhibits blood clotting by inactivating thrombin and Factor Xa. Basophils also release histamine in allergic reactions.

Normal WBC count is 4,000-11,000 per cubic millimetre of blood, with an average of about 7,000-8,000/µL. Elevated WBC (leukocytosis) indicates infection or inflammation; low WBC (leukopenia) indicates immunosuppression.

Agranulocytes (lymphocytes and monocytes) are produced in lymphoid tissue (spleen, lymph nodes, thymus). Granulocytes (neutrophils, eosinophils, basophils) are produced in red bone marrow.

Lymphocytes are the primary cells of adaptive immunity. B lymphocytes produce antibodies (humoral immunity) and T lymphocytes mediate cell-based immunity. They are responsible for specific, long-lasting immune responses.

Red blood cells have a lifespan of approximately 120 days. Old or damaged RBCs are phagocytosed and broken down in the spleen (primarily) and liver (Kupffer cells). Haemoglobin is recycled.

Adult human males have approximately 4.5-5.5 million RBCs per cubic millimetre (µL) of blood. Adult females have slightly fewer (3.8-5.2 million/µL). Below normal values indicate anaemia.

Sodium chloride (NaCl) is the most abundant salt in blood plasma, making up about 58-60% of all plasma electrolytes. It is the major determinant of plasma osmolarity and is maintained at about 140 mM.

Cholesterol in plasma serves as the precursor for all steroid hormones (cortisol, aldosterone, sex hormones) and vitamin D. It is also an essential component of cell membranes. LDL transports cholesterol in blood.

Prothrombin (Factor II) is an inactive plasma protein that is converted to active thrombin by Factor Xa. Thrombin then converts fibrinogen to fibrin, forming the blood clot. Prothrombin acts as a catalyst precursor.

Inorganic ions and salts (mainly NaCl, NaHCO₃, KCl) make up about 0.9% of plasma volume. This is equivalent to isotonic saline (0.9% NaCl solution) used in medical IV fluids to replace plasma volume.

Systemic circulation is the pathway of blood from the left ventricle → aorta → body tissues → venae cavae → right atrium. It delivers oxygenated blood to all body tissues and returns deoxygenated blood to the heart.

The frog heart has 5 chambers: sinus venosus, right atrium, left atrium, single ventricle, and conus arteriosus (truncus arteriosus). The functional pumping chambers are the two atria and one ventricle (3 main chambers).

In cockroaches, the tubular heart extends into the thorax as the aorta which opens into the head haemocoel. Haemolymph is then distributed through the open body cavity (haemocoel) to all tissues.

In reptiles (except crocodilians), both aortic arches arise from the cavum venosum — the region of the single ventricle that receives oxygenated blood from the left atrium. This allows relatively oxygenated blood to reach the body.

Fish have a single-circuit heart where blood flows from heart → ventral aorta (forward, anteriorly) → gills → dorsal aorta (backward, posteriorly) → body → heart. The ventral aorta carries deoxygenated blood forward to gills.

Earthworm has three main longitudinal blood vessels: the dorsal vessel (above the gut), the ventral vessel (below the gut), and the sub-neural vessel (beneath the nerve cord). These are connected by circular vessels in each segment.

Unicellular organisms use all three transport mechanisms. Simple substances (gases, water) enter and leave by diffusion and osmosis. Nutrients may require facilitated diffusion or active transport against concentration gradients.

Simple aquatic multicellular organisms use all available transport mechanisms: diffusion (gases, small molecules), facilitated diffusion (glucose via carriers), and active transport (ions against gradients). No circulatory system is needed.

Unlike vertebrates where haemoglobin is inside red blood cells, earthworm haemoglobin is dissolved directly in the blood plasma (not in cells). This is an important distinction in invertebrate physiology.

In Planaria (flatworms), the movement of materials into and out of cells is primarily by diffusion. Their flat, thin body and extensively branched gut keep all cells within short diffusion distances of the environment.

The blood vascular system (cardiovascular system) is the transport system in complex multicellular animals. It consists of the heart, blood vessels, and blood, transporting nutrients, gases, hormones, and wastes.

Sucrose loading into phloem sieve tubes primarily uses the symplast pathway — moving through plasmodesmata (connections between cell cytoplasms) from mesophyll cells through companion cells into sieve tubes.

Water molecules adhere to xylem vessel walls through adhesion (hydrogen bonding with cellulose). This adhesion, combined with cohesion between water molecules, creates the continuous water column and contributes to capillary rise.

Aphids (plant lice) are used to study phloem composition. Their stylets penetrate phloem sieve tubes and when the stylet is severed, phloem sap continues to flow and can be collected and analysed.

The pressure-flow (mass flow) hypothesis proposed by Ernst Münch (1930) is the most accepted theory for phloem transport. High turgor pressure at the source (leaves) drives phloem sap to low-pressure sinks.

Phloem is the vascular tissue responsible for transporting organic solutes (primarily sucrose, also amino acids, hormones, and some minerals) from source (leaves) to sink (roots, fruits, growing regions) by translocation.

Root pressure is least effective during the day when transpiration is high. Active transpiration creates strong tension in xylem that far exceeds root pressure, making root pressure relatively insignificant during daytime.

Hugo von Mohl (1856) proposed the starch-sugar hypothesis: in light, starch in guard cells converts to sugar, decreasing water potential, causing water to enter by osmosis, making guard cells turgid and stomata open.

Guttation occurs through hydathodes — specialised water pores at the margins and tips of leaves. Root pressure forces xylem sap out through these hydathodes when transpiration is low (at night, high humidity).

CO₂ concentration is the most direct signal for stomatal opening/closing — low CO₂ (from photosynthesis) opens stomata and high CO₂ closes them. Light, temperature, and ABA also affect stomata but more indirectly.

Abscisic acid (ABA), the 'stress hormone', causes stomatal closure during water stress. It stops K⁺ pumping into guard cells (which normally drives stomatal opening), causing K⁺ to leave guard cells and stomata to close.

Guard cells are the only epidermal cells that contain chloroplasts. These chloroplasts produce ATP via photosynthesis to drive K⁺ pumps for stomatal opening. Regular epidermal cells lack chloroplasts.

At night, stomata close but cuticular transpiration continues through the waxy cuticle. Cuticular transpiration is responsible for 5-10% of total transpiration and occurs regardless of stomatal status.

Lenticels are oval or round pores in the bark of woody stems and roots, filled with loose complementary cells with large intercellular spaces. They allow gas exchange between internal tissues and the atmosphere.

There are three types of transpiration: stomatal transpiration (through stomata, 90-95%), cuticular transpiration (through the cuticle, 5-10%), and lenticular transpiration (through lenticels in bark, very small %).

The theoretical tensile strength of the xylem water column is sufficient to lift water up to about 200 metres. The tallest trees (coast redwoods, ~115m) are well within this limit, supporting the cohesion-tension theory.

Transpiration pull (evaporation from leaves) is the primary driving force for water ascent in xylem. It creates negative pressure (tension) in xylem that pulls the continuous water column upward. Root pressure and capillarity contribute less.

At incipient plasmolysis, the cell is flaccid — turgor pressure is zero and the protoplast just begins to pull away from the cell wall. The cell has lost enough water that it can no longer maintain turgor. DPD = osmotic pressure.

Facilitated diffusion uses specific protein carriers (channel proteins or carrier proteins) embedded in the membrane to transport molecules (glucose, amino acids) down their concentration gradient without energy expenditure.

One Pascal (Pa) is the pressure from a force of 1 Newton on an area of 1 m². One kilopascal (kPa) = 1000 Pa. Water potential in plants is measured in kilopascals or megapascals (MPa = 1000 kPa).

Pressure potential (turgor pressure/Ψp) is the physical pressure exerted on the protoplast by the cell wall as the cell fills with water. In turgid cells it is positive; in wilted cells it is zero or negative (tension).

The apoplast (cell wall) pathway becomes blocked at the endodermis by the Casparian strip — a band of suberin (waterproof wax) in the endodermal cell walls. Water and solutes must enter through the symplast (cytoplasm) here.

Osmosis is specifically the movement of water (solvent) across a selectively permeable membrane from a region of higher water potential (more dilute) to lower water potential (more concentrated). It requires no energy input.

Plants need all three for nutrition: CO₂ (fixed in photosynthesis to make organic compounds), water (hydrogen source in photosynthesis, transport medium, reactant), and mineral nutrients (for enzymes, nucleic acids, chlorophyll).

Materials cross cell membranes by multiple processes: diffusion (gases, lipid-soluble substances), osmosis (water), facilitated diffusion (glucose, amino acids via carriers), and active transport (ions against gradients using ATP).

Mycorrhizal fungi are particularly effective at absorbing phosphate, which is relatively immobile in soil. The fungal hyphae extend far beyond the root hair zone, dramatically increasing the plant's phosphate uptake.

Ions dissolved in water can move by mass flow (bulk flow) — when water moves en masse through vessels driven by pressure differences, dissolved ions are carried along with it passively, without individual diffusion.

Root hairs are the primary sites of water and mineral absorption. They are tiny tubular extensions of root epidermal cells that enormously increase the absorptive surface area. They are found 1-10 cm behind the root tip.

The cohesion-tension theory explains that water rises in xylem because: transpiration from leaves creates negative pressure (tension), and cohesion (hydrogen bonding between water molecules) gives water sufficient tensile strength to be pulled up as a continuous column.

The Eustachian valve (valve of the inferior vena cava) is a small crescentic fold in the right atrium at the opening of the postcaval (inferior vena cava). It directs blood through the foramen ovale in the fetus.

Blood is the body's multipurpose transport medium: carries O₂ and CO₂ (respiratory gases), glucose and amino acids (nutrients), urea and CO₂ (nitrogenous/metabolic wastes), hormones, heat, and immune cells.

The three main cellular components of blood are: red blood cells (erythrocytes — O₂ transport), white blood cells (leukocytes — immunity), and platelets (thrombocytes — blood clotting).

Suppressor T cells (regulatory T cells) inhibit the immune response by suppressing the activity of other T and B cells, preventing excessive immune reactions and autoimmunity. They help maintain immune homeostasis.

Cardiac muscle is striated (cross-striped) like skeletal muscle but is involuntary like smooth muscle. Cardiac muscle cells are branched and connected by intercalated discs, allowing synchronised contraction.

Root pressure is most significant when transpiration is very low — at night or when temperature falls dramatically. In cold conditions, low transpiration reduces cohesion-tension, making root pressure relatively more important.

Active transport of minerals requires ATP energy, which comes from cellular respiration. Similarly, muscle contraction requires ATP from respiration. Respiration is the common energy-providing process for both functions.

Hydathodes are specialised water-secreting structures at leaf margins and tips. Water is forced out through hydathodes during guttation when root pressure drives xylem sap out of the plant during low transpiration periods.

Plants absorb mineral nutrients as both cations (K⁺, Ca²⁺, Mg²⁺, NH₄⁺) and anions (NO₃⁻, H₂PO₄⁻, SO₄²⁻) depending on the mineral. Both types are actively transported into root cells against concentration gradients.

On very hot, humid days plants may wilt despite adequate soil water, because the rate of transpiration exceeds the rate of water absorption. The cells lose turgor and become flaccid, causing the plant to wilt.

Transpiration removes water from leaf cells, increasing solute concentration, which increases osmotic pressure (decreases water potential). This creates a water potential gradient that pulls water up from roots through xylem.

Imbibition is the phenomenon where solid colloid materials (seeds, wood, cellulose, proteins) absorb water and swell due to adsorption of water molecules onto their surface and between their particles.

Since nitrate concentration is higher in root cells than in soil, ions must be transported against their concentration gradient. This requires active transport using ATP-driven carrier proteins (ion pumps) in root hair cell membranes.

Transpiration is the loss of water vapour from any exposed aerial surface of a plant — mainly through stomata on leaves but also through cuticle, lenticels of stems, and even fruits. Not limited to lower leaf surface only.

Guttation is the exudation of liquid water droplets from hydathodes at leaf margins and tips, driven by root pressure when transpiration is low (at night, in humid conditions). These are drops of xylem sap, not pure water.

All vertebrates (fish, frog, toad, and humans) have closed circulatory systems. Grasshoppers (insects) have open circulatory systems. So the answer should be 'all vertebrates' but fish is the first option listed correctly.

Blood performs all three functions: transports hormones from endocrine glands to target organs, carries digested nutrients (glucose, amino acids, fatty acids) from gut to tissues, and transports O₂ and CO₂.

The sinus venosus (in amphibians and some animals) is formed by the union of both precaval veins (superior venae cavae) and the postcaval vein (inferior vena cava), collecting all deoxygenated blood returning to the heart.

Blood platelets (thrombocytes) are cell fragments that aggregate at the site of vessel injury, forming a platelet plug and releasing clotting factors that initiate the coagulation cascade to stop bleeding.

Lacteals are lymphatic capillaries in intestinal villi that absorb fats (as chylomicrons). Amino acids and sugars are absorbed into blood capillaries, not lacteals. Fats enter the lymphatic system and eventually the blood.

The spleen is called the graveyard of red blood cells. Old or damaged RBCs (after ~120 days) are removed from circulation and broken down by macrophages (Kupffer cells) in the spleen and liver.

Lymph is essentially blood plasma minus red blood cells and most platelets. It contains lymphocytes, proteins, and dissolved substances. Lymph is formed when plasma filters out of capillaries into tissue spaces.

Pulmonary veins carry oxygenated blood from the lungs to the left atrium — unusually, veins carrying oxygenated blood. All other veins carry deoxygenated blood. Pulmonary arteries carry deoxygenated blood to lungs.

Rabbits (and all mammals) have a single left aortic arch. However the question refers to the number arising — one aorta arises from the left ventricle. In fish there are multiple pairs of aortic arches.

Both tuberculosis (Mycobacterium tuberculosis) and leprosy (Mycobacterium leprae) are intracellular bacterial infections. Cell-mediated immunity (T cells and macrophages) is the primary defence against these pathogens.

The heart wall has three layers: epicardium (outer, smooth visceral pericardium), myocardium (thick middle layer of cardiac muscle), and endocardium (inner smooth lining). All three layers contribute to heart structure and function.

Erythroblastosis foetalis (haemolytic disease of the newborn) is caused by Rh incompatibility between mother and fetus — it is an alloimmune (not autoimmune) disorder. Autoimmune disorders involve immune attack on self tissues.

Cell-mediated immunity (T cells) is particularly important against intracellular pathogens like viruses and Mycobacterium tuberculosis that are not accessible to antibodies. It also protects against cancer and organ transplant rejection.

Normal human blood pH is maintained at 7.35-7.45 (slightly alkaline). Blood pH is regulated by buffer systems (bicarbonate, phosphate, protein), the lungs (CO₂ removal), and kidneys (H⁺ excretion).

Serum is plasma with fibrinogen and other clotting factors removed. It is the yellowish fluid that remains after blood clots and the clot is removed. Serum contains antibodies and is used for diagnostic tests.

Antibodies (immunoglobulins) are glycoproteins — proteins with sugar chains attached. They are Y-shaped molecules with variable regions that specifically recognise and bind to antigens with high specificity.

Haemopoietic (blood-forming) tissue is found primarily in the red bone marrow of long bones and flat bones. All blood cells (RBCs, WBCs, platelets) are produced from haematopoietic stem cells in the bone marrow.

Vaccines produce active immunity — they stimulate the person's own immune system to make antibodies and memory cells. Passive immunity uses ready-made antibodies from another organism injected into the recipient.

When T cells encounter antigens, they proliferate and differentiate into: cytotoxic (killer) T cells (destroy infected cells), helper T cells (coordinate immune response), and memory T cells (provide long-term immunity).

Lymphocytes (B cells and T cells) are the primary cells of the adaptive immune response. B cells produce antibodies (humoral immunity) and T cells mediate cell-based immunity. Neutrophils and macrophages provide innate immunity.

White blood cells (WBCs/leukocytes) divide by mitosis, like all somatic cells. They are produced in bone marrow (granulocytes, monocytes) and lymphoid tissue (lymphocytes) by mitotic division of stem cells.

Blood transports: O₂ (haemoglobin), CO₂ (bicarbonate ions), nutrients (glucose, amino acids, fatty acids), hormones, metabolic wastes (urea, CO₂), heat, and immune cells. It is the body's primary transport medium.

The spleen is called the blood bank of the body because it stores a reserve of blood (especially RBCs and platelets) that can be released into circulation during emergencies (blood loss, exercise, stress).

Antibodies (immunoglobulins) are proteins produced by B cells/plasma cells that specifically bind to and neutralise antigens from pathogens (bacteria, viruses), toxins, and foreign substances, thereby fighting infection.

Helper T cells (CD4+ T cells) are crucial immune coordinators that stimulate B cells to produce antibodies and activate cytotoxic T cells and macrophages. HIV specifically destroys helper T cells, causing immune failure.

Immunoglobulins (antibodies) are glycoproteins found in blood plasma (and other body fluids). They are produced by plasma cells (activated B lymphocytes) and circulate in the plasma to neutralise antigens.

The thymus is largest in childhood and involutes (shrinks) after puberty. It is where T lymphocytes mature and develop self-tolerance. Despite shrinking, the thymus-derived T cells provide lifelong cellular immunity.

Water potential (Ψ) = solute potential (Ψs, osmotic) + pressure potential (Ψp, turgor) + matric potential (Ψm, surface adsorption). All three components contribute to total water potential in plant cells.

Leukaemia is a cancer of blood-forming tissues (bone marrow, lymphatic system) characterised by overproduction of abnormal white blood cells. It impairs normal blood cell production and immune function.

Both B and T lymphocytes originate from stem cells in the bone marrow. B cells mature in the bone marrow; T cells migrate to the thymus for maturation. They then populate lymphoid organs (spleen, lymph nodes).

Antiserums (antisera) contain antibodies produced by immunised animals or humans. They are used for passive immunisation to provide immediate protection against toxins (antitoxins) and snake venoms.

Passive immunity is obtained by injecting ready-made antibodies (antiserum/immunoglobulins) from another immune organism. It provides immediate but temporary protection. Active immunity develops after vaccination or infection.

Imbibition involves the diffusion of water molecules into colloidal materials (like seeds, wood, starch). Water moves by diffusion along the matric potential gradient without any energy expenditure.

Water potential (Ψ) = pressure potential (Ψp) + solute potential (Ψs). Solutes lower water potential (more negative Ψs) by diluting water. Positive pressure increases water potential; negative pressure (tension) lowers it.

Stomatal transpiration accounts for about 90-95% of all water loss from plants. Cuticular transpiration contributes about 5-10%, and lenticular (through lenticels) is negligible. Guard cells control stomatal opening.

Humoral immunity is mediated by B lymphocytes (B cells) that produce antibodies. When activated by antigens, B cells differentiate into plasma cells that secrete specific immunoglobulins into the blood and lymph.

White blood cells (leukocytes) are the immune cells of the blood. They protect the body against all types of pathogens — bacteria, viruses, fungi, parasites — through phagocytosis, antibody production, and cell-mediated immunity.

Basophils are not phagocytic — they release histamine and heparin in allergic reactions and inflammation. Neutrophils and monocytes are highly phagocytic (engulf pathogens). Lymphocytes produce antibodies and kill infected cells.

A heart murmur is an abnormal sound heard during auscultation caused by turbulent blood flow through a defective heart valve (stenotic or incompetent). Normal heart sounds are caused by valve closure.

A thrombus is a blood clot that forms inside a blood vessel and remains attached to the vessel wall. Thrombosis is the process of thrombus formation. If a thrombus detaches, it becomes an embolus.

Water constitutes about 91-92% of blood plasma. The remaining 8-9% consists of plasma proteins (7-8%), glucose, lipids, amino acids, hormones, metabolic wastes, and dissolved gases.

Veins contain semilunar valves that prevent backflow of blood, especially in limbs where blood must flow upward against gravity. Arteries have high enough pressure that backflow does not occur; capillaries have no valves.

Lymphatic vessels contain one-way valves that prevent backflow of lymph. Lymph is moved through these vessels by skeletal muscle contractions, breathing movements, and smooth muscle contractions in larger vessels.

Lymph originates from blood plasma that leaks out of capillaries into the interstitial (tissue) spaces due to hydrostatic pressure. It is collected by lymphatic capillaries and returned to the blood via the thoracic duct.

Fibrinogen is a plasma protein that plays a critical role in blood clotting (coagulation). It is converted to fibrin by thrombin, forming a mesh that traps blood cells to create a blood clot and stop bleeding.

Systolic pressure (the higher reading, ~120 mmHg) represents the maximum arterial pressure when the ventricles contract (systole) and forcefully eject blood into the aorta and pulmonary artery.

Red blood cells (erythrocytes) contain haemoglobin — the iron-containing protein that binds oxygen in the lungs and releases it in tissues. Mature RBCs have no nucleus, maximising haemoglobin content.

Blood cells (RBCs, WBCs, and platelets) constitute approximately 45% of blood volume — this is called the haematocrit. The remaining 55% is plasma. In anaemia, haematocrit falls below 45%.

After water and proteins, the next important plasma constituents are inorganic salts/electrolytes (Na⁺, K⁺, Cl⁻, HCO₃⁻ — maintaining pH and osmolarity) and glucose (energy source for all cells).

Plasma proteins constitute about 7-8% of plasma volume. They include albumins (maintain osmotic pressure), globulins (antibodies, transport proteins), and fibrinogen (blood clotting).

Plasma constitutes approximately 55% of blood volume. Blood cells (red blood cells, white blood cells, platelets) make up the remaining 45% (haematocrit). Plasma is mostly water (91-92%) with dissolved proteins, salts, and nutrients.

Diastolic blood pressure (the lower reading) represents pressure in arteries when the heart is relaxing and filling with blood. Normal diastolic range is 60-80 mmHg; values of 70-85 are considered normal.

Blood pressure is the force (pressure) exerted by circulating blood against the walls of blood vessels. It is created by the pumping action of the heart and maintained by the elasticity of artery walls.

The right subclavian artery supplies the right arm (right forelimb) and continues as the axillary artery. It also gives branches to the right side of the neck and right thoracic wall.

The hepatic portal vein carries nutrient-rich blood from the intestines and other digestive organs to the liver. The liver processes these nutrients — storing glucose as glycogen, detoxifying harmful substances, and producing bile.

The hepatic portal vein is formed by the confluence of veins draining all parts of the alimentary canal: gastric veins (stomach), duodenal veins, mesenteric veins (intestine), and splenic vein.

The superior vena cava (anterior vena cava) receives blood from the upper body including the thoracic wall via intercostal veins. Azygos and hemiazygos veins drain the thoracic wall into the superior vena cava.

The dorsal aorta (descending aorta) runs along the posterior side of the body, giving off branches to various organs, and terminates as the median caudal artery supplying the tail/coccyx region.

The iliac arteries (common iliac arteries) arise from the bifurcation of the abdominal aorta and supply the pelvic organs, perineum, and lower limbs. The internal iliac supplies pelvic walls and viscera.

The superior mesenteric artery (anterior mesenteric in some animals) supplies the small intestine and most of the large intestine — the majority of the intestinal tract. It is one of the largest branches of the abdominal aorta.

Intercostal arteries (arising from the thoracic aorta) supply the muscles and other structures of the thoracic wall (intercostal muscles, ribs, skin). There are 9 pairs of posterior intercostal arteries.

The coronary arteries are the very first branches of the aorta, arising from the aortic sinuses just above the aortic valve. They supply oxygenated blood to the heart muscle (myocardium) itself.

Arteries are blood vessels that carry blood away from the heart. They have thick, elastic, muscular walls to withstand the high pressure generated by ventricular contraction. All arteries (except pulmonary artery) carry oxygenated blood.

The three arteries from the aortic arch are: brachiocephalic trunk (divides into right subclavian and right common carotid), left common carotid, and left subclavian artery.

Three main arteries arise from the anterior (highest) point of the aortic arch: the brachiocephalic trunk (right common carotid + right subclavian), left common carotid, and left subclavian artery.

Blood flows from capillaries → venules → veins → back to the heart. Veins carry deoxygenated blood from tissues toward the heart (except pulmonary veins which carry oxygenated blood from lungs to heart).