Diffusion gets you only so far. Beyond about a millimetre, you need a circulation — and an engine to drive it.
Every sub-topic below feeds at least one of these questions.
What adaptations facilitate transport of fluids in animals and plants?
What are the differences and similarities between transport in animals and plants?
The required syllabus content for B3.2, in order. Each card is one lesson-sized checkpoint.
Adaptations should include a large surface area due to branching and narrow diameters, thin walls, and fenestrations in some capillaries where exchange needs to be particularly rapid.
Structure of arteries and veins
Adaptations of arteries for the transport of blood away from the heart
Measurement of pulse rates
Adaptations of veins for the return of blood to the heart
Causes and consequences of occlusion of the coronary arteries
Transport of water from roots to leaves during transpiration
Adaptations of xylem vessels for transport of water
Distribution of tissues in a transverse section of the stem of a dicotyledonous plant
Distribution of tissues in a transverse section of the root of a dicotyledonous plant
Arteries deliver. Veins return. Capillaries are where blood actually exchanges materials with the tissues — and their structure is exquisitely adapted for that one job.
Capillaries are densely branched throughout tissues, providing huge total surface area for exchange.
Walls are a single layer of squamous endothelial cells — minimum possible diffusion distance.
Lumen just wide enough for one red blood cell at a time — slows flow and maximises exchange time. Some capillaries also have fenestrations (small pores) for even faster exchange.
Arteries carry blood away from the heart at high pressure. Their wall structure has evolved to withstand and even smooth that pressure.
In a micrograph, you can tell an artery from a vein at a glance: arteries have relatively thick walls and narrow lumens; veins have thin walls and wide lumens.
Each heartbeat sends a pressure wave through the arteries — the pulse. You can feel it where an artery runs close to the skin.
Modern oximeters and smartwatches measure pulse optically (photoplethysmography). Worth comparing your manual count to a digital reading — both have failure modes.
By the time blood reaches the veins, it has lost most of its pressure. Veins are adapted to keep it moving anyway.
Coronary arteries feed the heart muscle itself. Their occlusion by atherosclerotic plaque causes heart attacks — and most of the risk factors are modifiable.
Atherosclerosis develops in stages:
Some genes increase risk. Older people have more damaged arteries. Males have higher risk on average.
Obesity raises blood pressure and damages walls. Inactivity contributes to obesity. Smoking raises blood pressure and damages endothelium directly. High saturated fat / cholesterol diets contribute to plaque formation.
The Seven Countries Study showed a correlation between saturated fat intake and coronary heart disease deaths. Correlation coefficients quantify this — but correlation alone never proves causation. Many additional lines of evidence (mechanisms, randomised trials) support the causal link.
The cohesion-tension theory in five steps. The driving force isn't push — it's pull.
This is the transpiration pull. It's strong enough to raise water more than 100 m to the canopy of a redwood tree.
Xylem vessels are highly specialised — they don't even have to be alive to do their job.
In a transverse section of a dicotyledonous plant, vascular tissue (xylem + phloem) is arranged differently in stem vs root — and you should be able to draw a plan diagram of both.
Working from outside in: epidermis (protection) → cortex (support, starch storage) → vascular bundles arranged in a ring. Within each bundle, xylem is on the inside and phloem on the outside.
Working from outside in: epidermis (often with root hairs for absorption) → cortex (broad layer for storage and water passage) → central vascular cylinder with xylem in a star or X pattern surrounded by phloem.
An extra 8 sub-topics for HL — same syllabus, deeper mechanism.
Release and reuptake of tissue fluid in capillaries
Exchange of substances between tissue fluid and cells in tissues
Drainage of excess tissue fluid into lymph ducts
Differences between the single circulation of bony fish and the double circulation of mammals
Adaptations of the mammalian heart for delivering pressurized blood to the arteries
Stages in the cardiac cycle
Generation of root pressure in xylem vessels by active transport of mineral ions
Adaptations of phloem sieve tubes and companion cells for translocation of sap
At the arterial end, fluid is forced out of capillaries into the tissues. At the venous end, most is drawn back in. The excess is collected by lymph vessels.
High blood pressure (hydrostatic) pushes fluid out through the thin capillary walls into the tissue spaces. The fluid (lacking large proteins) becomes tissue fluid bathing the cells.
Blood pressure has fallen. Plasma proteins (mainly albumin) still inside the capillary draw water back in by osmosis — oncotic pressure. Most tissue fluid returns to the blood here.
In tissue fluid, substances exchange between blood and cells: O₂, CO₂, glucose, amino acids, ions, hormones — all diffuse or are actively transported between capillaries and the surrounding cells.
The fluid that doesn't return to capillaries (~10%) drains into lymph ducts. Lymph is returned to the bloodstream via the thoracic duct, which empties into a large vein near the heart.
Fish have single circulation — blood passes through the heart once per circuit. Mammals (and birds) have double circulation — blood passes through twice.
Blood is pumped from the heart to the gills (gas exchange) and then directly on to the body, before returning to the heart. Pressure drops dramatically through the gill capillaries, so systemic flow is relatively slow.
Right side of heart pumps deoxygenated blood to the lungs (pulmonary circuit). Oxygenated blood returns to the left side, which pumps it to the body (systemic circuit) at high pressure. Each circuit gets full pump pressure — efficient systemic delivery to active metabolism.
The mammalian heart is two pumps in one organ — and is precisely engineered to deliver pressurised blood to the arteries.
The cardiac cycle is the repeating sequence of contractions and relaxations that constitutes a heartbeat. It is myogenic — generated by the heart itself.
Pacing is set by the sinoatrial (SA) node in the right atrium — the heart's intrinsic pacemaker. Impulses spread through the atria, pause at the AV node, then race down the bundle of His and Purkinje fibres to trigger ventricular contraction.
In addition to being pulled from above by transpiration, water in the xylem is sometimes pushed from below by root pressure.
In root endodermis cells, mineral ions are actively transported into the xylem against their concentration gradient. The resulting high solute concentration in xylem sap draws water in by osmosis — creating positive pressure that pushes water upward.
Root pressure is most evident at night when transpiration is low — visible as guttation (water droplets on leaf tips) in many plants. By day, the much greater transpiration pull dominates.
Phloem transports dissolved organic compounds (mainly sucrose, also amino acids) from places where they're made (sources) to places where they're used or stored (sinks).
Mass flow: sucrose loaded at the source raises solute concentration → water enters by osmosis → high pressure pushes sap through sieve tubes toward sinks → at the sink, sucrose is unloaded; water follows back out. Pressure-driven mass flow.
If you can't define one of these in a sentence, that's where to revise next.
“How do pressure differences contribute to the movement of materials in an organism?”
“What processes happen in cycles at each level of biological organization?”