IB DP Biology: Transport: blood vessels, heart, xylem and phloem (DP Biology)

This study guide covers IB Diploma Programme Biology, section B3.2 (Transport), as set out in the IB Biology guide, first assessment 2025. Statements B3.2.1 to B3.2.10 are standard level and higher level. Statements B3.2.11 to B3.2.18 are additional higher level, so they are HL only and are labelled that way below.

Use it with the revision notes and the practice questions. Wider theme: Theme B. Course hub: IB DP Biology. Checklist: printable checklist. To find weak spots first, take the free 10-minute diagnostics.

What this unit covers

Section	What you must be able to do	Level
B3.2.1	Link capillary structure to exchange	SL and HL
B3.2.2, B3.2.3, B3.2.5	Identify arteries and veins; explain how each wall suits its job	SL and HL
B3.2.4	Measure pulse rate	SL and HL
B3.2.6	Causes and consequences of coronary occlusion; use epidemiological data and correlation	SL and HL
B3.2.7, B3.2.8	Transpiration pull, cohesion, xylem structure	SL and HL
B3.2.9, B3.2.10	Plan diagrams of dicot stem and root	SL and HL
B3.2.11 to B3.2.13	Tissue fluid formation, exchange, lymph	HL only
B3.2.14	Single versus double circulation	HL only
B3.2.15, B3.2.16	Heart structure and the cardiac cycle	HL only
B3.2.17	Root pressure	HL only
B3.2.18	Phloem sieve tubes and companion cells	HL only

B3.2.1 Capillaries

Capillaries are where blood exchanges materials with tissues.

• Branching and narrow diameter. Dense networks give a large total surface area. A lumen only about as wide as one red blood cell holds cells close to the wall and slows flow, giving time for exchange.
• Thin walls. The wall is a single layer of endothelial cells, so diffusion distances are tiny.
• Fenestrations. Some capillaries have pores through the endothelial cells. They are found where exchange must be especially rapid, such as the kidney glomerulus, intestinal villi and endocrine glands.

Exchange with the external environment also happens in capillaries, for example around alveoli.

B3.2.2, B3.2.3 and B3.2.5 Arteries and veins

Feature	Artery	Vein
Wall thickness	Thick	Thin
Lumen compared with wall	Narrow	Wide
Muscle and elastic tissue	Plentiful	Little
Valves	None (except at heart exits)	Present
Blood pressure inside	High	Low

Arteries (B3.2.3). Each ventricular contraction sends a high-pressure surge. Elastic fibres let the wall stretch, then recoil, which keeps pushing blood onwards between beats and smooths flow. Smooth muscle strengthens the wall and can narrow or widen the artery.

Veins (B3.2.5). Pressure is low, so a thick wall is unnecessary. The thin, flexible wall is squeezed by nearby skeletal muscle, which pushes blood towards the heart. Valves close against backflow.

Worked example: identifying vessels from a micrograph. A micrograph has a scale bar of 20 mm that stands for 100 µm. Vessel A has a lumen 30 mm across and a wall 6 mm thick. Vessel B has a lumen 44 mm across and a wall 3 mm thick.

1. Magnification = 20 mm ÷ 100 µm = 20 000 µm ÷ 100 µm = ×200.
2. Vessel A: lumen = 30 ÷ 200 mm = 150 µm; wall = 6 ÷ 200 mm = 30 µm; wall ÷ lumen = 0.20.
3. Vessel B: lumen = 220 µm; wall = 15 µm; wall ÷ lumen = 0.068.
4. A has the relatively thicker wall, so it is the artery. B has a wide lumen and a thin wall, so it is the vein.

Compare the ratio, not absolute size, because a small artery can have a narrower lumen than a large vein.

B3.2.4 Measuring pulse rate

The pulse is the wave of stretch and recoil that passes along an artery after each ventricular contraction. Use the carotid pulse (side of the neck, beside the windpipe) or the radial pulse (inner wrist, thumb side).

1. Press two fingertips lightly on the artery. Do not use your thumb, which has a pulse of its own.
2. Count beats for a timed period, with the person rested, then convert to beats per minute.

Worked example. 38 beats are counted in 30 s: 38 × 2 = 76 beats per minute. Three counts in 15 s give 22, 24 and 23. The mean is 23, so 23 × 4 = 92 beats per minute. Averaging repeats reduces miscounting error.

Digital methods, such as a wrist monitor or a finger sensor that detects blood volume changes with light, can be compared with this manual method. They remove counting error but can misread when the sensor moves.

B3.2.6 Coronary occlusion

The coronary arteries supply cardiac muscle with oxygen and glucose.

Causes. Fatty deposits (plaque) build up beneath the artery lining. This narrows the lumen, a condition called atherosclerosis. A plaque can rupture and trigger a blood clot (thrombus) that blocks the artery. Risk rises with smoking, high blood pressure, high blood cholesterol, a diet high in saturated fat, inactivity and inherited factors.

Consequences. Less blood reaches the muscle, so aerobic respiration falls. Partial blockage may cause chest pain on exertion (angina). Complete blockage kills the muscle beyond it, a heart attack (myocardial infarction), which can bring irregular rhythm or heart failure.

Epidemiological data and correlation. Compare rates, not counts, because populations differ in size.

Worked example. City P has 312 new cases of coronary heart disease in a population of 240 000. City R has 126 in 180 000.

1. P: 312 ÷ 240 000 × 100 000 = 130 per 100 000.
2. R: 126 ÷ 180 000 × 100 000 = 70 per 100 000.
3. P’s rate is 130 ÷ 70 = 1.86 times R’s.

A correlation coefficient (−1 to +1) measures how closely two variables vary together. Suppose eight countries give saturated fat intake (g per day) of 8, 11, 13, 15, 18, 21, 24 and 27, with CHD deaths per 100 000 of 38, 45, 41, 60, 57, 72, 66 and 84. These invented data give r = +0.94, strongly positive. A strong correlation still does not prove that fat intake causes disease, because other factors may differ between countries. A weak or absent correlation counts as evidence against a proposed link.

B3.2.7 and B3.2.8 Water transport in xylem

Transpiration pull. Water evaporates from the moist cell walls of leaf mesophyll cells into air spaces and escapes through stomata. This lowers the water potential in the walls, so water is drawn out of xylem vessels and through the walls by capillary action. The pull puts the water in the xylem under tension, a negative pressure potential. Hydrogen bonds between water molecules (cohesion) keep the column unbroken, and adhesion to the vessel wall helps hold it up.

Worked example. Soil water potential is −0.2 MPa and a leaf cell wall is at −1.5 MPa. Water moves towards the lower value, down a gradient of 1.3 MPa.

Xylem vessel adaptations.

• No cell contents, and end walls that are incomplete or missing, so water flows without obstruction.
• Walls strengthened with lignin, which stops them collapsing under tension.
• Pits, thin unlignified gaps, let water enter or leave sideways.

B3.2.9 and B3.2.10 Stem and root sections

A plan diagram shows tissue regions only, with no individual cells. Use clear outlines and ruled label lines, add a title and magnification, and annotate labels with functions (xylem carries water and minerals; phloem carries sugars).

Region	Dicot stem	Dicot root
Epidermis	Outer layer; protection and limiting water loss	Outer layer; root hair cells absorb water and minerals
Cortex	Between epidermis and bundles; storage and support	Wide; storage and a route for water inwards
Vascular tissue	Separate bundles in a ring; xylem towards the centre, phloem towards the outside	Central core; xylem often star-shaped, with phloem between its arms

B3.2.11 to B3.2.13 Tissue fluid and lymph (HL only)

Formation (B3.2.11). Blood enters capillaries from arterioles at higher pressure. This forces plasma through the thin wall, forming tissue fluid. Large plasma proteins mostly stay in the blood. At the venule end the pressure is lower, so much of the fluid drains back into the capillary.

Worked example. Take a hydrostatic pressure of 4.3 kPa at the arteriole end and 1.6 kPa at the venule end, with the pull from plasma proteins as 3.3 kPa. Net outward force at the arteriole end = 4.3 − 3.3 = +1.0 kPa (fluid leaves). At the venule end = 1.6 − 3.3 = −1.7 kPa (fluid returns). The protein pull is supplied here as data.

Exchange (B3.2.12). Oxygen, glucose, amino acids and ions pass from tissue fluid into cells; carbon dioxide and urea pass out. Tissue fluid is like plasma but has far less protein and no red blood cells.

Lymph (B3.2.13). Excess tissue fluid enters lymph capillaries, which have thin walls with gaps. Valves prevent backflow in lymph vessels. Lymph is returned to the blood circulation near the heart.

B3.2.14 Single and double circulation (HL only)

• Bony fish: heart (one atrium, one ventricle) → gill capillaries → body capillaries → heart. Blood passes through the heart once per circuit.
• Mammal: right ventricle → lung capillaries → left atrium → left ventricle → body capillaries → right atrium. Blood passes through the heart twice per circuit.

Pressure drops sharply in the narrow gill capillaries, so blood reaches the body slowly. In mammals the heart restores pressure after the lungs, and the lung circuit can run at lower pressure, protecting thin alveolar walls.

B3.2.15 and B3.2.16 The heart (HL only)

Structure	Form and function
Cardiac muscle	Branching cells linked together contract in a coordinated way; many mitochondria resist fatigue
Pacemaker (sinoatrial node)	Cells in the right atrium start each heartbeat
Atria	Thin walls; receive blood and push it into ventricles
Ventricles	Thick walls; the left is thicker because it drives blood round the whole body
Atrioventricular valves	Stop blood flowing back into atria during ventricular contraction
Semilunar valves	Stop blood flowing back from arteries into ventricles
Septum	Keeps oxygenated and deoxygenated blood apart
Coronary vessels	Supply the cardiac muscle itself

Pathway (traced on a frontal-plane diagram of the heart): vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle → aorta.

Cardiac cycle (left side).

1. The pacemaker fires; the atrium contracts and tops up the ventricle.
2. Pressure in the ventricle exceeds that in the atrium, so the atrioventricular valve closes.
3. Ventricular contraction raises pressure above that in the aorta, so the semilunar valve opens and blood is ejected.
4. The ventricle relaxes; its pressure falls below aortic pressure and the semilunar valve closes.
5. When ventricular pressure drops below atrial pressure, the atrioventricular valve opens and filling begins.

Pressure data. Systolic pressure is the peak in the artery; diastolic is the minimum. A reading of 126/84 mmHg converts (× 0.133) to 16.8/11.2 kPa, and the pulse pressure is 42 mmHg or 5.6 kPa. A cycle of 0.8 s gives 60 ÷ 0.8 = 75 beats per minute.

B3.2.17 Root pressure (HL only)

Root cells actively transport mineral ions into the xylem. This lowers the water potential there, so water enters by osmosis and builds a positive pressure potential. Root pressure matters when transpiration is insufficient, for example in humid air or in spring before leaves open on deciduous plants. Guttation, droplets forced from leaf edges, shows it.

B3.2.18 Phloem (HL only)

Cell	Adaptation	Benefit
Sieve tube element	Sieve plates with pores	Sap passes between cells
Sieve tube element	Reduced cytoplasm and organelles, no nucleus	Little obstruction to flow
Companion cell	Many mitochondria	ATP for active loading
Companion cell	Plasmodesmata to the sieve tube	Materials pass between the two cells

At a source, sucrose is loaded into sieve tubes, water follows by osmosis, and pressure rises. At a sink, sucrose is unloaded, water leaves and pressure falls. The pressure difference moves sap from source to sink.

Official syllabus

This resource follows the IB Diploma Programme Biology guide, first assessment 2025, published by the International Baccalaureate Organization.