B3.2 Transport
Transport systems move materials through animal blood vessels, plant xylem and phloem, and heart-driven circuits using specialised structures and pressure gradients.
- Syllabus
- First assessment 2025
- Topic
- B3.2
- Level
- HL
Learning objectives
B3.2.1Capillary adaptations• Capillaries are narrow, highly branched, and close to cells• One-cell-thick endothelial walls reduce diffusion distance• Fenestrations in some capillaries allow rapid exchange and tissue fluid formationB3.2.2Artery and vein structure• Arteries and veins have endothelium, smooth muscle, elastic tissue, and collagen• Arteries have thicker walls and smaller lumens to withstand high pressure• Veins have wider lumens and thinner walls for low-pressure returnB3.2.3Artery adaptations• Thick artery walls and collagen prevent rupture under high pressure• Elastic fibres stretch and recoil to even out pulse pressure and maintain flow• Smooth muscle in arteries and arterioles regulates blood distributionB3.2.4Pulse rate measurement• Ventricular contraction creates a pressure wave felt as a pulse• Pulse rate can be measured at radial or carotid arteries• Counting for a full minute is most accurate; shorter counts can be scaledB3.2.5Vein adaptations• Veins have valves that prevent backflow toward capillaries• Thin flexible walls allow surrounding muscles to compress veins• Large lumens reduce friction during low-pressure blood returnB3.2.6Coronary artery occlusion• Atherosclerosis forms plaques beneath damaged coronary artery endothelium• Plaque rupture can trigger thrombosis and occlude coronary arteries• Reduced oxygen supply can kill cardiac muscle and cause myocardial infarctionB3.2.7Water transport in plants• Transpiration from leaf mesophyll creates tension in xylem water columns• Cohesion between water molecules transmits tension from leaves to roots• Adhesion to xylem walls helps maintain an unbroken transpiration streamB3.2.8Xylem vessel adaptations• Mature xylem vessels are dead, hollow tubes with absent or perforated end walls• Lignified walls resist collapse under negative pressure and waterproof the vessel• Pits allow lateral movement of water between vessels and surrounding tissuesB3.2.9Stem tissue distribution• Dicot stems have epidermis, cortex, pith, and vascular bundles in a ring• Each vascular bundle contains xylem, phloem, cambium, and supporting fibres• Plan diagrams show tissue positions without drawing individual cellsB3.2.10Root tissue distribution• Dicot roots have epidermis with root hairs, cortex, endodermis, and central vascular tissue• Xylem forms a central cross with phloem between its arms• The Casparian strip blocks apoplast flow and forces selective symplast entry to xylemB3.2.11(HL)—Tissue fluid in capillaries• Hydrostatic pressure at arteriole ends forces plasma fluid out by ultrafiltration• Plasma proteins remain in blood and maintain osmotic pull• Reduced pressure near venule ends allows about 90% of tissue fluid to re-enter capillariesB3.2.12(HL)—Exchange between tissue fluid and cells• Tissue fluid bathes body cells and mediates exchange with blood• Oxygen, glucose, amino acids, ions, and wastes diffuse between cells and tissue fluid• Tissue fluid has fewer proteins, less oxygen, and more carbon dioxide than blood plasmaB3.2.13(HL)—Lymph ducts• Lymph capillaries drain excess tissue fluid that does not re-enter blood capillaries• Lymphatics use smooth muscle, body movement, and valves to move lymph• Lymph nodes filter debris and contain immune cells before lymph returns to veinsB3.2.14(HL)—Single vs. double circulation• Bony fish have single circulation: heart to gills to body and back• Mammals have double circulation with separate pulmonary and systemic circuits• Double circulation keeps oxygenated and deoxygenated blood separate and maintains high systemic pressureB3.2.15(HL)—Mammalian heart adaptations• Four chambers and a septum separate right pulmonary and left systemic flow• Valves and tendinous cords ensure one-way blood movement• Thick left ventricular muscle, coronary arteries, and myogenic cardiac muscle support pressurized pumpingB3.2.16(HL)—Cardiac cycle stages• The sinoatrial node initiates excitation, followed by atrial systole• The atrioventricular node delays conduction before ventricular systole• Ventricular systole opens semilunar valves; diastole allows refilling and recoveryB3.2.17(HL)—Root pressure generation• Endodermal cells actively pump mineral ions into xylem• This lowers xylem water potential so water enters by osmosis• Positive root pressure can push water upward when transpiration is lowB3.2.18(HL)—Phloem adaptations• Phloem sieve tube elements are living tubes with sieve plates and reduced organelles• Companion cells contain many mitochondria and connect by plasmodesmata• Active loading at sources and unloading at sinks drive pressure-flow translocation of sucrose and amino acids
Capillaries Slow Blood to Enable Exchange
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.
Valves and Muscles Return Low-Pressure Venous Blood
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.
Dead, Lignified Xylem Forms a Low-Resistance Tube
Mature xylem vessel elements form dead, hollow, lignified tubes adapted to carry water under tension with little resistance.
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.
A Dicot Root Combines Absorption with Selective Entry
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.
Hydrostatic Pressure Filters Plasma into Tissue Spaces
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.
Tissue Fluid Is the Immediate Exchange Medium
Tissue fluid is the extracellular liquid that bathes body cells and provides the immediate route for exchange between blood plasma and cell membranes.
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.
Lymph Returns the Fluid That Capillaries Do Not Reabsorb
Excess tissue fluid enters blind-ended lymph capillaries and is returned as lymph to the blood circulation.
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.