8.1 The Circulatory System
- Syllabus
- 9700–2028–2029
- Topic
- 8.1
- Level
- AS
Learning objectives
8.1.1Closed double circulation• Closed double circulation: heart, blood, blood vessels8.1.2Blood vessels• Blood vessels: arteries, arterioles, capillaries, venules, veins8.1.3Main blood vessels• Main blood vessels: pulmonary artery/vein, aorta, vena cava8.1.4Blood vessel recognition• Recognise vessels from microscope slides and photomicrographs8.1.5Plan diagrams of arteries and veins• Plan diagrams: arteries and veins in TS and LS8.1.6Structure-function relationships• Structure-function relationships: muscular arteries, elastic arteries, veins, capillaries8.1.7Blood cells• Blood cells: red blood cells, monocytes, neutrophils, lymphocytes8.1.8Water as main blood component• Water as main blood component: role in transport (solvent action, high specific heat capacity)8.1.9Tissue fluid• Tissue fluid functions and formation in capillary networks
A closed double circulation links lung and body circuits
Mammals have a closed double circulation: blood remains inside the heart and vessels, and a complete journey passes through the heart twice in pulmonary and systemic circuits.
- Right heart pumps blood to lung capillaries in the pulmonary circuit.
- Blood returns to the left heart after gas exchange.
- Left heart pumps blood to body tissues in the systemic circuit.
- Blood returns from tissues to the right heart.
- The system contains heart, blood and arteries, arterioles, capillaries, venules and veins.
Two serial circuits let blood be re-pressurised after passing through lung capillaries before high-pressure delivery to body tissues, while vessel containment maintains a closed network.
Double means two linked circuits, not two hearts or disconnected blood systems. Arteries carry blood away from the heart and veins return it; oxygen content does not define the vessel type.
Five vessel classes form a continuous flow pathway
Blood flows away from the heart through arteries and arterioles, exchanges in capillaries, and returns through venules and veins.
| Vessel | Position in flow | Main network job |
|---|---|---|
| artery | leaves heart | carries high-pressure blood towards organs |
| arteriole | branches from artery | distributes blood into capillary beds and controls entry by smooth muscle |
| capillary | between arteriole and venule | provides a thin exchange surface close to cells |
| venule | drains capillary bed | collects blood into progressively larger return vessels |
| vein | returns to heart | carries low-pressure blood back, assisted by valves |
The reusable sequence is heart to artery to arteriole to capillary to venule to vein to heart. Vessel names follow direction and network position rather than oxygen content.
Do not omit arterioles or venules: they connect the large vessels to exchange beds. Pulmonary artery and vein show why oxygen status does not define artery versus vein.
Named vessels form the pulmonary and systemic route
The named vessels connect the two linked circuits of a closed double circulation. Capillary beds are the exchange networks between the large vessels and the lungs or body tissues; arteries carry blood away from the heart and veins carry it towards the heart.
- Pulmonary outflow: right side of heart → pulmonary artery → lung capillary beds. Deoxygenated blood reaches the lungs for gas exchange.
- Pulmonary return: lung capillary beds → pulmonary vein → left side of heart. Blood returns oxygenated before entering the systemic circuit.
- Systemic outflow: left side of heart → aorta → systemic arteries and arterioles → body-tissue capillary beds. Oxygenated blood is delivered for exchange with tissues.
- Systemic return: body-tissue capillary beds → systemic venules and veins → vena cava → right side of heart. Deoxygenated blood returns, completing the route back to the pulmonary circuit.
Whole route cue: heart right side → pulmonary artery → lungs → pulmonary vein → heart left side → aorta → body capillaries → vena cava → heart right side.
Do not define a vessel by oxygen content alone: the pulmonary artery carries deoxygenated blood away from the heart, while the pulmonary vein carries oxygenated blood towards it. The vessel names above describe direction and route; capillaries are the exchange beds, not an extra third circulation.
Recognise vessels from multiple structural clues
Vessel recognition from slides, photomicrographs or electron micrographs requires section orientation, scale, lumen and wall evidence; no single colour or shape is sufficient.
- Identify transverse/longitudinal/partial view and use scale.
- Compare lumen width with total wall thickness.
- Artery evidence: relatively thick muscular/elastic wall and smaller regular lumen.
- Vein evidence: thinner wall, wider often irregular/collapsed lumen and valves when visible.
- Capillary evidence: tiny lumen, one-cell-thick endothelial wall and close association with tissues.
- State at least two observed clues and qualify any feature hidden by section angle or resolution.
Do not identify from colour, one measurement or oxygen content. Oblique sections can change apparent lumen/wall shape, so use scale and multiple compatible clues.
Draw proportional artery and vein plans in TS or LS
A vessel plan diagram records visible outline, lumen and relative wall regions without individual cells, texture, colour or invented layers.
- Establish TS or LS, orientation and scale from the source.
- Draw a large continuous outer boundary and lumen preserving actual proportions and irregularity.
- Add only major wall regions distinguishable in the image, with their relative thickness.
- In LS preserve lengthwise structure and any visible valve; do not force a circular TS representation.
- Use no shading or individual cell boundaries.
- Add ruled non-crossing labels ending on observed structures and include scale/magnification only when supported.
A plan diagram is not a detailed histological drawing. Do not idealise every artery as perfectly round, add unseen layers or infer oxygen content from the section.
Four vessel structures solve distinct circulation problems
Elastic arteries smooth high-pressure pulses, muscular arteries distribute flow, veins return low-pressure blood and capillaries create exchange surfaces.
| Vessel | Key structure | Function link |
|---|---|---|
| elastic artery | very thick wall rich in elastic tissue near heart | stretches during systole and recoils during diastole, resisting pressure and smoothing flow |
| muscular artery | thick smooth-muscle layer with elastic/collagen support | changes lumen diameter to distribute blood to organs and withstand pressure |
| vein | wide lumen, relatively thin wall, less muscle/elastin, valves | low-resistance low-pressure return; valves prevent backflow |
| capillary | one-cell-thick endothelium, narrow lumen, extensive network | short diffusion distance, close cell contact, large total area and slower flow for exchange |
Do not merge elastic and muscular arteries into one generic list: their relative elastic/smooth-muscle specialisations match different jobs. Vessel type is not defined by oxygen content.
Recognise and draw four required blood-cell types
Use cell outline, nucleus shape, cytoplasm and scale to recognise red blood cells, monocytes, neutrophils and lymphocytes, then draw only visible evidence.
| Cell | Diagnostic image evidence | Highest-value drawing labels |
|---|---|---|
| red blood cell | small biconcave disc, no nucleus, pale centre in suitable views | cell surface, biconcave form/no nucleus when supported |
| monocyte | largest listed white cell, abundant cytoplasm, kidney/bean-shaped nucleus | cell boundary, cytoplasm, indented nucleus |
| neutrophil | multi-lobed nucleus with narrow connections, granular cytoplasm | lobed nucleus, cytoplasm, cell boundary |
| lymphocyte | large round dark nucleus occupying most of cell, thin cytoplasm rim | nucleus, thin cytoplasm, cell boundary |
Check image type and scale; choose at least two diagnostic clues; draw large single clear outlines in correct proportions with no shading; label only resolved structures using ruled lines; qualify any nucleus boundary hidden by section or resolution.
Platelets are not part of this exact drawing list. Do not identify white cells by colour or size alone: nuclear morphology is decisive when visible.
Water in blood supports solute and heat transport
Water is the main liquid component of plasma and tissue fluid, so its properties make the circulation a useful transport medium. The key applications here are solvent action and heat distribution.
- Solvent → transport of dissolved substances: water dissolves solutes in plasma → they can be carried in solution through blood and tissue fluid → glucose can move from the small intestine to cells for respiration, while urea can move from the liver to the kidneys for excretion.
- High specific heat capacity → temperature distribution: water can absorb substantial heat with relatively little temperature change → plasma and tissue fluid can take up heat from warmer, active regions and redistribute it in the circulation → body temperature is kept more stable, supporting conditions suitable for enzyme activity.
Boundary: these are circulation applications of water’s solvent and thermal properties; they do not require a catalogue of every property of water or invented plasma chemistry.
Do not describe water as merely filling the vessels: connect each property to the transported material or heat. The carrier is plasma/tissue fluid, whose water-rich phase supports these functions. No image generated or bound.
Capillary filtration forms tissue fluid for cell exchange
Tissue fluid is plasma-derived fluid outside capillaries that bathes cells and forms the exchange medium between blood and tissues.
- At the arterial end, capillary hydrostatic pressure is high and forces water and small dissolved substances through gaps in the wall.
- Blood cells and most large plasma proteins remain in the capillary because they cannot pass through the gaps.
- The resulting tissue fluid delivers oxygen and nutrients to cells and receives carbon dioxide and other wastes.
- Hydrostatic pressure falls along the capillary; plasma proteins left in blood lower its water potential.
- Water moves back into the capillary by osmosis towards the venous end, carrying dissolved substances with it.
Hydrostatic pressure drives filtration out; the water-potential difference caused by retained plasma proteins supports re-entry. Tissue fluid is therefore similar to plasma but lacks blood cells and most large proteins.
Tissue fluid is not whole blood and does not normally contain red blood cells or most plasma proteins. This objective concerns its formation and exchange functions, not clinical oedema causes or immune roles of lymph.