B3.2 Transport

Capillaries are narrow, highly branched exchange vessels whose structure maximizes contact with tissues while minimizing diffusion distance.

Branching produces a very large total surface area. A lumen only slightly wider than a red blood cell brings blood close to the wall, and a one-cell-thick endothelium provides a short path for diffusion.

Some capillaries have fenestrations—small pores through endothelial cells—where especially rapid fluid or solute exchange is required. Their narrow diameter also slows individual red cells and increases exchange time.

In an alveolar capillary, oxygen crosses thin alveolar and capillary layers into a red blood cell; in a fenestrated capillary, pores permit faster movement of water and small dissolved substances.

Thin walls suit exchange, not high-pressure transport. Fenestrations occur in some capillary beds, not every capillary, and blood cells plus most large proteins normally remain inside.

Arteries carry blood away from the heart under higher pressure; veins return blood at lower pressure and need valves and muscle assistance.

Thick elastic and muscular artery walls absorb pulse pressure. Veins have wider lumens, thinner walls and valves that prevent backflow as surrounding muscles compress them.

Trace direction and pressure first; then use wall thickness, lumen and valves to explain the vessel’s job.

During walking, leg muscles squeeze veins and push blood past valves toward the heart; the artery on the same route carries blood away under pulse pressure.

‘Away from the heart’ defines an artery, not oxygen content; the pulmonary artery carries deoxygenated blood.

Arteries have thick muscular and elastic walls that maintain a lumen and smooth pressure pulses from the heart.

Elastic recoil helps keep blood moving between heartbeats, while smooth muscle adjusts diameter and resistance. The wall must withstand higher pressure than a vein.

Separate pulse smoothing from resistance control: which wall feature does each job?

A constricted arteriole raises resistance and can redirect blood flow, whereas elastic recoil in a large artery smooths the pulse.

A thick wall does not mean blood always flows faster; diameter, resistance and downstream demand also matter.

Pulse rate is the number of arterial pressure waves per minute, normally corresponding to ventricular contractions.

Place fingertips—not the thumb—lightly over the radial artery at the wrist or the carotid artery in the neck. Count waves for a known interval while the subject is still and use a full minute when maximum accuracy is needed.

Counting 18 pulses in 15 s gives 18 × (60/15) = 72 beats min⁻¹. Repeat after recovery and compare with a digital heart-rate sensor recorded over the same interval.

Short counts magnify counting error, and pulse rate is not cardiac output: cardiac output also depends on stroke volume. Record posture, activity and method when comparing results.

Veins use large lumens, valves and skeletal-muscle contractions to return blood to the heart despite low pressure.

A wide lumen reduces resistance, valves stop reverse flow and muscle compression raises local pressure. Breathing movements can also help draw venous blood toward the chest.

Follow one bolus of venous blood upward and identify how valves and muscle compression prevent reversal.

When calf muscles contract, a valve below the compressed region closes while the valve above opens, pushing blood upward.

Valves do not create the original pressure; they make one-way assistance effective.

Coronary artery occlusion interrupts oxygen delivery to cardiac muscle and can cause myocardial infarction: irreversible death of part of the heart muscle.

Endothelial damage can allow lipid-rich atheroma to develop beneath the artery lining. A plaque narrows the lumen, and rupture can activate platelets and fibrin formation, producing a thrombus that partly or completely blocks flow.

Reduced coronary flow causes ischaemia: aerobic ATP production falls while cardiac muscle continues to demand energy. Prolonged complete occlusion damages and kills the supplied tissue, impairing contraction.

Epidemiological data may show a positive correlation between a proposed risk factor and coronary disease. A correlation coefficient quantifies direction and strength, but confounding variables and study design must be considered.

Even a strong correlation does not by itself prove that one variable causes coronary occlusion. Distinguish gradual plaque narrowing from an acute thrombus after plaque rupture.

Transpiration creates tension—a negative pressure potential—that pulls a continuous water column from roots to leaves through xylem.

Water evaporating from moist mesophyll cell walls draws replacement water through the wall by capillary action and out of nearby xylem. This lowers pressure in the leaf xylem and transmits tension down the vessel.

Cohesion from hydrogen bonding keeps water molecules joined so the pull is transmitted through an unbroken column. Adhesion to hydrophilic xylem walls assists capillary movement and helps stabilize the column.

When stomata open and evaporation increases, the leaf water potential becomes more negative, increasing tension in xylem and drawing water upward from roots.

The main long-distance force is tension generated at transpiring leaves, not an active pump in xylem. Cohesion transmits the pull; it does not create the initial gradient.

Mature xylem vessel elements form dead, hollow, lignified tubes adapted to carry water under tension with little resistance.

Loss of cytoplasm and organelles leaves an open lumen. End walls are absent or perforated, so aligned elements form a continuous route rather than forcing water across repeated membranes.

Lignin thickens and waterproofs the wall and prevents collapse under negative pressure. Unlignified pits allow water to enter or leave laterally, bypass a blockage, and move between xylem and surrounding tissue.

If air blocks one vessel, water can pass through pits into an adjacent vessel while lignified walls keep both tubes open under transpiration tension.

Xylem vessel elements are dead at maturity and do not actively pump water. Pits are thin wall regions for lateral movement, not open ends of the vessel.

A transverse section of a young dicot stem has an outer epidermis, cortex beneath it, vascular bundles arranged in a ring and a central pith.

Within each vascular bundle, phloem lies toward the outside, xylem toward the centre and cambium between them. The ring links transport around the stem while lignified xylem and supporting fibres add strength.

For a plan diagram, draw tissue boundaries with clear single lines and correct relative positions; do not draw individual cells. Label epidermis, cortex, vascular bundles, phloem, cambium, xylem and pith, then annotate their main functions.

Annotate xylem as water/mineral transport and support, phloem as translocation of assimilates, cortex as storage/support and epidermis as the protective outer boundary.

A plan diagram shows distribution and proportion, not cellular detail or shading. In each dicot stem bundle, xylem is inner and phloem outer; do not reverse them.

A transverse section of a young dicot root has an outer epidermis, a broad cortex and a central vascular cylinder containing xylem and phloem.

Root hairs extend from epidermal cells to increase absorption area. The endodermis forms the inner boundary of the cortex, surrounding the central vascular tissue and controlling entry to xylem.

In the centre, xylem commonly forms a star or cross; phloem occurs in groups between its arms. Draw these tissue regions in their correct relative positions with clear outlines rather than individual cells.

A suitable plan diagram runs epidermis/root hairs → cortex → endodermis → central xylem cross with phloem between the arms, with annotations for uptake and transport.

Root vascular tissue is central rather than arranged as a ring of separate bundles like a young dicot stem. A plan diagram records tissue distribution, not detailed cell anatomy.

Tissue fluid is formed when hydrostatic pressure at the arteriole end of a capillary forces some plasma out through the capillary wall.

Small solutes and water leave, but cells and most plasma proteins remain in the blood. As pressure falls and osmotic effects change along the capillary, some fluid is reabsorbed.

Compare hydrostatic and osmotic forces along the capillary; state why filtration changes with position.

Higher blood pressure at the arterial end favors filtration, while lower pressure toward the venous end favors return of fluid.

Tissue fluid is not whole blood: red cells and large proteins normally stay inside the capillary.

Tissue fluid is the extracellular liquid that bathes body cells and provides the immediate route for exchange between blood plasma and cell membranes.

It forms from filtered plasma, so it contains water and small solutes such as oxygen, glucose, amino acids and ions, but normally lacks blood cells and contains far fewer large plasma proteins.

Oxygen and nutrients diffuse from tissue fluid into cells, while carbon dioxide and other metabolic wastes diffuse out. After passing cells, tissue fluid tends to contain less oxygen and more carbon dioxide than the plasma that initially supplied it.

A respiring muscle cell lowers the local oxygen concentration and raises carbon dioxide concentration, maintaining opposite diffusion gradients between the cell and surrounding tissue fluid.

Tissue fluid is not whole blood or plasma unchanged: cells and most large proteins remain in capillaries, and metabolism alters the solute composition as fluid contacts tissues.

Excess tissue fluid enters blind-ended lymph capillaries and is returned as lymph to the blood circulation.

Lymph capillaries have very thin walls with gaps that open as tissue pressure rises, allowing fluid to enter. Overlapping wall flaps and valves prevent reverse movement.

Larger lymph ducts use one-way valves, smooth-muscle contraction and compression by body movement to move lymph at low pressure. The ducts eventually empty into large veins near the heart.

When a leg muscle contracts it compresses a lymph vessel; the valve behind closes and the valve ahead opens, moving lymph toward its return to venous blood.

Lymph ducts return fluid rather than pumping it in an arterial circuit. If drainage is blocked, excess tissue fluid accumulates and causes oedema.

Bony fish have a single circulation, whereas mammals have a double circulation with separate pulmonary and systemic circuits.

Fish route: heart → gills → body tissues → heart. Blood passes through the heart once per complete circuit and pressure falls as it passes the gill capillaries before reaching the body.

Mammal route: right heart → lungs → left heart → body → right heart. Passing through the heart twice allows low pulmonary pressure that protects lung capillaries and high systemic pressure for rapid delivery, while the septum prevents mixing.

A red blood cell in a mammal is re-pressurized by the left ventricle after leaving lung capillaries; in a fish it travels directly from gills to body tissues without returning to the heart first.

Single and double refer to how many times blood passes through the heart per full circuit, not to the number of chambers or whether the animal has one or two hearts.

The mammalian heart is a myogenic, four-chambered double pump adapted to deliver pressurized blood unidirectionally to pulmonary and systemic arteries.

Atria receive blood; ventricles eject it. The septum separates oxygenated and deoxygenated sides, the thicker left ventricular cardiac muscle generates systemic pressure, and coronary vessels supply the metabolically active myocardium.

The sinoatrial pacemaker initiates each beat. Atrioventricular valves prevent return to atria, semilunar valves prevent return from arteries, and tendinous cords stop AV valves inverting under ventricular pressure.

Trace flow: venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve/artery → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic semilunar valve/aorta.

Valves open and close because of pressure differences; they do not actively pull blood. The right side pumps deoxygenated blood to lungs, while the left side pumps oxygenated blood to the body.

The cardiac cycle is a pressure-driven sequence initiated by the sinoatrial node: atrial systole, ventricular systole and diastolic refilling.

The SAN excitation spreads across atria, causing atrial systole. After a conduction delay, ventricles contract; left ventricular pressure closes the AV valve and then opens the aortic semilunar valve when it exceeds aortic pressure.

As ventricles relax, pressure falls, semilunar valves close and the AV valves reopen when atrial pressure exceeds ventricular pressure. During diastole the chambers refill before the next SAN impulse.

Systolic arterial pressure is the peak reached during ventricular ejection; diastolic pressure is the lower arterial pressure during ventricular relaxation. A reading is reported systolic over diastolic in mmHg.

Systole and diastole must be assigned to a chamber: atrial and ventricular systole are offset. Valve movements follow pressure gradients rather than causing the pressure changes.

Root pressure is a positive pressure potential generated when root cells actively load mineral ions into xylem and water follows by osmosis.

ATP-powered ion transport raises solute concentration and lowers xylem water potential. Water enters from surrounding root tissue, creating hydrostatic pressure that can push xylem sap upward.

Root pressure is most useful when transpiration pull is weak—for example during high humidity, at night, or in spring before leaves of deciduous plants have opened.

Continued ion loading during a humid night can create enough positive pressure for xylem sap to emerge as guttation droplets at leaf margins.

Root pressure supplements transport when transpiration is insufficient but cannot by itself account for water reaching the tops of tall trees; it is positive pressure, unlike transpiration tension.

Phloem translocates sucrose, amino acids and other carbon compounds as sap from sources to sinks through sieve tube elements supported by companion cells.

Sieve elements align end-to-end with perforated sieve plates. They retain only a thin layer of cytoplasm, have few organelles and no nucleus, reducing resistance to mass flow while remaining living cells.

Companion cells contain many mitochondria for ATP-dependent loading and unloading and connect to sieve elements by plasmodesmata. Source loading draws water from xylem and raises pressure; sink unloading lowers pressure, driving bulk flow.

A mature leaf loads sucrose into nearby phloem, water enters from xylem, and high hydrostatic pressure drives sap toward a developing fruit where sucrose is unloaded.

Phloem movement is source-to-sink, not always upward. Sieve elements lack a nucleus but are alive because companion cells maintain them through plasmodesmata.