B3.2 Transport

Capillaries are narrow, thin-walled vessels that bring blood close to cells for exchange.

A one-cell-thick endothelium shortens diffusion distance, while branching creates a large total area and slows flow enough for exchange. This combination raises the chance that gradients can drive useful transfer.

Use the exchange job: why does this vessel favour diffusion rather than pressure transport?

Oxygen leaves a capillary across its thin wall into nearby tissue cells while carbon dioxide enters the blood.

A capillary is not a high-pressure transport pipe; its thin wall is suited to exchange, not resisting large pressure.

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 detected per unit time, usually linked to ventricular contractions.

Each heartbeat ejects blood into elastic arteries, creating a wave that can be felt or measured. Rate changes with exercise, temperature, hormones and measurement conditions.

Read the pulse as a pressure wave, then distinguish rate from stroke volume and cardiac output.

Counting 18 pulses in 15 seconds and multiplying by four gives an estimated rate of 72 beats per minute.

Pulse rate is not identical to cardiac output: stroke volume can change while rate stays the same.

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.

A coronary artery occlusion reduces blood flow to cardiac muscle, limiting oxygen delivery and potentially causing myocardial damage.

The heart cannot rely on blood inside its chambers for oxygen; it needs coronary vessels. A blocked vessel lowers aerobic respiration and ATP supply in the affected tissue.

Link the blocked vessel to oxygen delivery, respiration and ATP demand in the cardiac muscle it supplies.

If a plaque narrows a coronary artery, exercise may expose oxygen shortage because cardiac demand rises while supply cannot increase enough.

Occlusion is not simply high blood pressure. It is a restriction of coronary flow; diagnosis requires evidence of the vessel and tissue effect.

Water enters roots and moves through xylem toward leaves as a continuous pathway driven mainly by transpiration pull.

Root hairs provide area for uptake, and cohesion keeps water connected in narrow xylem tubes. Evaporation at leaves lowers water potential and transmits tension downward.

Trace the continuous water column from leaf evaporation back to root uptake; do not stop at root absorption.

On a sunny day, evaporation from leaves can increase the pull on the xylem column and raise water delivery from roots.

Root uptake alone does not explain tall-tree transport; the leaf-to-root water-potential gradient is essential.

Xylem vessels are adapted for long-distance water transport by being hollow, strengthened and connected into low-resistance tubes.

Dead vessel elements lack internal contents that would obstruct flow. Lignified walls prevent collapse under tension, and end openings allow a continuous column.

Match each vessel feature to flow resistance or collapse under tension, then identify the driving force.

A lignified xylem vessel can remain open while water is pulled upward through it under negative pressure.

Xylem is not a pump that actively pushes water all the way to leaves; transpiration and cohesion provide the main driving mechanism.

Stem tissues arrange vascular bundles so xylem and phloem connect roots, leaves and growing regions while support tissues maintain the stem.

Xylem position and lignification support water flow and mechanical strength; phloem provides living conduits for assimilate movement. Arrangement changes with organ and growth pattern.

Identify tissue by wall structure and cargo before assigning its direction through the stem.

A vascular bundle can carry water upward in xylem while phloem distributes sucrose from a source leaf to a growing sink.

A stem cross-section is not interpreted by labels alone: identify tissue by wall structure, contents and direction of transported material.

Root hairs increase surface area for water and mineral uptake, while the endodermis controls entry into the vascular cylinder.

Thin root-hair walls shorten diffusion distance. The endodermal barrier forces water and ions through selective membranes before they reach xylem, helping regulate the internal supply.

Separate increased surface area from selective entry: where do transport proteins and the endodermis matter?

A mineral ion can enter a root hair by a transporter, move through cortex cells and be checked at the endodermis before xylem loading.

More root hairs increase potential uptake but do not remove the need for concentration gradients and transport proteins.

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.

Cells receive oxygen and solutes from tissue fluid and release wastes into it by diffusion across short distances.

Tissue fluid is the immediate aqueous environment around cells. Gradients between capillary fluid, tissue fluid and cells determine net movement, while metabolism continually changes those gradients.

Start with the local concentration gradient around the cell, not with an assumed active delivery route.

Oxygen diffuses from tissue fluid into a respiring cell; carbon dioxide diffuses out along the opposite gradient.

Tissue fluid does not actively deliver every molecule. A gradient and a permeable cell membrane are still required.

Lymph ducts collect excess tissue fluid and return it to the blood, helping maintain fluid balance and supporting immune transport.

Some filtered fluid is not reabsorbed at capillaries. One-way lymph vessels and valves collect it, while movement from muscles and body pressure drives it toward large veins.

Ask what happens when filtration continues but lymph return is blocked; use the pressure and volume change.

If lymph drainage is blocked, fluid accumulates in tissues and swelling develops because filtration continues without adequate return.

Lymph is not a second arterial circulation; it is a low-pressure drainage route with immune functions.

A double circulatory system sends blood through the lungs and then the body in two linked circuits, allowing each circuit to operate at a suitable pressure.

The right side of the mammalian heart pumps to lungs at lower pressure, while the left side pumps oxygenated blood to the body at higher pressure. Separation prevents mixing.

Trace the two linked circuits and compare their pressures before explaining the advantage of separation.

Blood follows body tissues → right heart → lungs → left heart → body tissues, completing two circuits in one journey.

A single circuit would not automatically provide the same pressure separation; the advantage is the linked two-pump arrangement.

The mammalian heart uses chambers, valves and a muscular septum to maintain one-way flow and separate oxygenated from deoxygenated blood.

Atria receive blood, ventricles eject it, valves close when pressure reverses and the septum prevents mixing. Ventricular wall thickness matches the pressure required by each circuit.

Use pressure changes to predict which valve opens, then check chamber identity and oxygenation.

The left ventricle has a thicker wall because it must generate systemic rather than pulmonary pressure.

A thicker wall does not mean the chamber receives more oxygenated blood by definition; trace the circuit and valve sequence.

The cardiac cycle is the timed sequence of atrial and ventricular contraction and relaxation that fills chambers and ejects blood.

Pressure changes open and close valves. During diastole chambers fill; atrial systole tops up ventricles; ventricular systole raises pressure and opens outflow valves.

At each phase, identify filling or ejection from chamber pressure and valve state rather than memorising names.

When ventricular pressure exceeds arterial pressure, semilunar valves open and blood leaves the ventricle; when it falls, they close.

Systole does not mean every chamber contracts simultaneously; atrial and ventricular events are offset.

Root pressure is positive pressure generated when roots load ions into xylem, lowering water potential and drawing water into the vessels.

Ion accumulation causes osmotic water entry, producing pressure that can push xylem sap upward. It is most evident when transpiration is low, such as at night.

Contrast root pressure with transpiration pull and state when each can contribute to upward movement.

If roots continue ion loading overnight, water may be forced toward leaves and emerge as guttation droplets at leaf margins.

Root pressure can contribute to short-distance upward movement but cannot alone explain water transport to the tops of tall trees.

Phloem transports sucrose and other assimilates from source tissues to sink tissues by pressure-driven mass flow.

Loading sucrose into sieve tubes lowers water potential, draws water in from xylem and raises hydrostatic pressure. Unloading at a sink lowers pressure, driving bulk flow along the tube.

Locate the source and sink first; then explain how loading, water entry and pressure difference drive flow.

A mature leaf can be a source exporting sucrose to a developing fruit, which acts as a sink and unloads the assimilate.

Phloem flow is not always upward: direction depends on source and sink locations and can change with growth.