Transport

Why circulation exists

Diffusion alone is fast over micrometres but hopelessly slow over centimetres, which is why organisms larger than a few millimetres need bulk transport. Punjab Textbook Chapter 15 and Campbell Chapter 42 set out the two main animal designs: open systems (arthropods, most molluscs) where haemolymph bathes tissues directly in a haemocoel, and closed systems (annelids, cephalopods, all vertebrates) where blood stays in vessels at higher pressure. Vertebrate hearts evolve from the two-chambered fish heart through the three-chambered amphibian/most-reptile heart to the four-chambered bird and mammal heart, fully separating oxygenated and deoxygenated streams.

Blood composition and haemoglobin

Adult human blood is ~ 5 L: 55% plasma (90% water plus albumin, globulins, fibrinogen, salts, glucose) and 45% formed elements (erythrocytes ~ 5 × 10⁶/mm³, leucocytes ~ 7000/mm³, thrombocytes ~ 250 000/mm³). Erythrocytes are biconcave, anucleate, and carry haemoglobin — a tetramer of two α and two β globin chains, each binding one haem with one Fe²⁺ that reversibly binds one O₂. Each erythrocyte carries ~ 280 million haemoglobin molecules, so up to 1.12 billion O₂ per cell. The sigmoidal O₂ dissociation curve reflects cooperative binding; it shifts right (Bohr effect) with low pH, high CO₂, high temperature, or high 2,3-BPG, releasing more O₂ at exercising tissues.

The human heart and cardiac cycle

The heart pumps ~ 5 L/min at rest (cardiac output = stroke volume × heart rate, roughly 70 mL × 72 bpm). The cycle is atrial systole (0.1 s, AV valves open), ventricular systole (0.3 s, semilunar valves open, AV valves shut producing the "lub"), and diastole (0.4 s, semilunar valves shut producing the "dub"). The sino-atrial (SA) node in the right atrium is the pacemaker (~ 100 bpm intrinsic, slowed to ~ 72 by vagal tone); it triggers the atrioventricular (AV) node, then the bundle of His, then Purkinje fibres. ECG waves: P (atrial depolarisation), QRS (ventricular depolarisation), T (ventricular repolarisation).

Vessels and pressure

Arteries have thick muscular and elastic walls handling ~ 120/80 mm Hg; arterioles set peripheral resistance and thus blood pressure; capillaries (~ 8 μm wide, single endothelial cell wall) are where exchange happens at ~ 30 mm Hg arterial end and ~ 15 mm Hg venous end, with plasma colloid osmotic pressure ~ 25 mm Hg pulling fluid back (Starling forces). Veins are thin-walled with valves; they hold ~ 65% of blood volume at low pressure and rely on the skeletal-muscle pump.

Plant transport: xylem and phloem

Xylem moves water and minerals upward from roots to leaves through dead, lignified tracheids and vessel elements. The driving force is the cohesion-tension (Dixon) theory: transpiration at the leaf creates negative pressure that pulls a continuous water column held together by hydrogen-bond cohesion and adhesion to xylem walls. Root pressure contributes only a few metres at most. Phloem transports sugars and amino acids in living sieve-tube elements supported by companion cells; the pressure-flow (Münch) hypothesis explains source-to-sink movement: sugar loading at sources lowers water potential, water enters from xylem, generating turgor that pushes sap to sinks where unloading occurs. Common traps: xylem is dead at maturity, phloem is alive; transpiration pull is the dominant force, not root pressure; companion cells supply ATP for active loading; guttation (not transpiration) appears as droplets at leaf margins overnight.

Lymphatic system and disorders of transport

Capillary exchange leaves about 3 L of fluid in the tissues per day that does not return directly to the venous side; the lymphatic system collects this as lymph and returns it via the thoracic duct to the left subclavian vein. Lymph nodes filter the lymph and host clonal expansion of B and T cells. Disorders MDCAT examines: atherosclerosis (lipid plaques narrow arteries, raising blood pressure and risk of myocardial infarction); hypertension (sustained > 140/90 mm Hg); thrombosis (intravascular clot) versus embolism (clot that has travelled); varicose veins (incompetent valves, blood pools); anaemias (iron-deficiency, sickle-cell from a single Glu→Val substitution at β6, thalassaemia from globin-chain imbalance); and leukaemia (uncontrolled proliferation of white cells). In plants, ringing (girdling) the bark removes the phloem while leaving the xylem intact — the classic experiment proving that organic solutes travel in phloem and water in xylem.