How is each blood vessel adapted to its job, and how is tissue fluid formed and returned?
3.1.2 Transport in animals: the structure and functions of arteries, arterioles, capillaries, venules and veins; the formation of tissue fluid from plasma at the arterial end of a capillary bed and its return at the venous end and via the lymphatic system, explained in terms of hydrostatic and oncotic (osmotic) pressure.
A focused answer to the OCR H420 3.1.2 dot point on blood vessels and tissue fluid. Covers the structure and function of arteries, arterioles, capillaries, venules and veins, and how hydrostatic and oncotic pressure form tissue fluid at the arterial end and return it at the venous end and via the lymph.
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What this dot point is asking
OCR wants you to relate the structure of arteries, arterioles, capillaries, venules and veins to their functions, and to explain the formation of tissue fluid at the arterial end of a capillary bed and its return at the venous end and through the lymphatic system, using hydrostatic and oncotic (osmotic) pressure.
The answer
The blood vessels and their adaptations
Blood leaves the heart in arteries, passes through arterioles into networks of capillaries, and returns through venules and veins. Each is built for the pressure it carries.
- Artery. Thick wall with abundant elastic tissue and smooth muscle, and a relatively narrow lumen. The elastic tissue stretches during ventricular systole and recoils during diastole, smoothing the pulsatile flow and helping maintain a high, steady pressure. There is a smooth endothelium lining.
- Arteriole. Smaller, with proportionally more smooth muscle; contracting (vasoconstriction) or relaxing (vasodilation) the muscle controls blood flow into different capillary beds.
- Capillary. Wall is a single layer of flattened endothelial cells (one cell thick) with small gaps, giving a short diffusion distance and a large total surface area for exchange. The lumen is just wide enough for red blood cells to squeeze through, slowing flow for efficient exchange.
- Venule and vein. Thin walls with little muscle or elastic tissue, a wide lumen (low resistance), and valves to stop backflow. Blood at low pressure is squeezed along by contraction of nearby skeletal muscles.
Formation of tissue fluid
Tissue fluid bathes the body cells and is formed from blood plasma. Two opposing pressures decide the direction of fluid movement across the capillary wall:
- Hydrostatic pressure of the blood pushes fluid out of the capillary.
- Oncotic pressure (from plasma proteins, which cannot leave) draws water in by osmosis.
At the arterial end, the heart gives the blood a high hydrostatic pressure that exceeds the oncotic pressure, so the net outward pressure forces water and small solutes (glucose, ions, oxygen) out through the gaps in the capillary wall, forming tissue fluid. Cells and plasma proteins are too large to leave and stay in the blood.
Return of tissue fluid
As blood flows along the capillary, hydrostatic pressure falls (fluid has been lost and there is frictional resistance). Meanwhile the plasma proteins are now more concentrated, so the oncotic pressure stays high. At the venous end, oncotic pressure exceeds hydrostatic pressure, so most of the water is reabsorbed into the capillary by osmosis, carrying dissolved waste (such as carbon dioxide and urea) back into the blood.
About 90 percent of the fluid is reabsorbed this way. The remaining excess drains into blind-ended lymph capillaries as lymph, which is slowly returned to the blood near the heart. This is why a blockage of the lymphatic system or a fall in plasma proteins causes fluid to accumulate (oedema).
Examples in context
Example 1. Why a cut artery spurts but a cut vein oozes. The high, pulsatile pressure in an artery makes blood spurt in time with the heartbeat, whereas the low, steady pressure in a vein makes blood flow out smoothly, a direct consequence of their different wall structures.
Example 2. Filariasis and elephantiasis. Parasitic worms blocking the lymph vessels prevent the return of excess tissue fluid, causing severe swelling (oedema), which illustrates the role of the lymphatic system in fluid balance.
Try this
Q1. Describe one structural difference between a capillary and an artery, and explain its functional importance. [2 marks]
- Cue. A capillary wall is a single endothelial cell thick (an artery is thick-walled), giving a short diffusion distance for rapid exchange of substances with the tissues.
Q2. State the two pressures that determine the movement of fluid across a capillary wall. [2 marks]
- Cue. Hydrostatic pressure (pushing fluid out) and oncotic (osmotic) pressure from plasma proteins (drawing water in).
Q3. Explain why plasma proteins remain in the capillary while glucose leaves to form tissue fluid. [1 mark]
- Cue. Plasma proteins are too large to pass through the gaps in the capillary wall, whereas small molecules such as glucose can.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H420/01 20204 marksExplain how the structure of an artery is related to its function, and how this differs from the structure of a vein.Show worked answer →
Pair each structural feature with the function it serves, then contrast the vein.
An artery carries blood away from the heart at high pressure, so it has a thick wall with a lot of elastic tissue that stretches as the heart contracts and recoils between beats to smooth the flow and maintain pressure. It also has smooth muscle to control diameter, and a narrow lumen that helps keep pressure high.
A vein returns blood at low pressure, so it has a thinner wall with less muscle and elastic tissue, a wide lumen to reduce resistance, and valves to prevent backflow as blood is moved by the contraction of surrounding skeletal muscles.
Markers reward elastic tissue and recoil for the artery, and a wide lumen plus valves for the vein.
OCR H420/01 20215 marksExplain how tissue fluid is formed at the arterial end of a capillary bed and how most of it is returned at the venous end.Show worked answer →
Compare hydrostatic and oncotic pressure at each end of the capillary.
At the arterial end: the high hydrostatic pressure of the blood (from the heart) is greater than the oncotic pressure (the osmotic pull of the plasma proteins). The net outward pressure forces water and small solutes out through the capillary wall, forming tissue fluid. Large plasma proteins and cells stay in the capillary.
At the venous end: hydrostatic pressure has fallen (because fluid has left and there is resistance), while the plasma proteins left behind have raised the oncotic pressure. Now oncotic pressure exceeds hydrostatic pressure, so water moves back into the capillary by osmosis.
The excess fluid not reabsorbed drains into the lymphatic system and is eventually returned to the blood. Markers reward the pressure comparison at each end and the role of the lymph.
Related dot points
- 3.1.2 Transport in animals: the structure of the mammalian heart and the events of the cardiac cycle (atrial systole, ventricular systole and diastole), the pressure and volume changes that open and close the valves, and the myogenic control of heart rate by the SAN, AVN, bundle of His and Purkyne tissue, including the interpretation of electrocardiograms (ECGs).
A focused answer to the OCR H420 3.1.2 dot point on the mammalian heart. Covers the heart's structure, the three stages of the cardiac cycle, how pressure changes open and close the valves, myogenic control by the SAN, AVN, bundle of His and Purkyne tissue, and how to read an ECG.
- 3.1.2 Transport in animals: the role of haemoglobin in transporting oxygen, the oxygen dissociation curve and cooperative binding, the Bohr effect, the higher oxygen affinity of fetal haemoglobin, and the transport of carbon dioxide including the formation of hydrogencarbonate ions and the chloride shift.
A focused answer to the OCR H420 3.1.2 dot point on oxygen transport. Covers haemoglobin and cooperative binding, the sigmoidal oxygen dissociation curve, loading and unloading, the Bohr effect, fetal haemoglobin, and carbon dioxide transport as hydrogencarbonate with the chloride shift.
- 3.1.1 Exchange surfaces: the need for specialised exchange surfaces as size and metabolic rate increase and surface-area-to-volume ratio falls; the features of an efficient exchange surface; the structure and function of the mammalian gas-exchange system, the counter-current system in fish gills, and the tracheal system of insects.
A focused answer to the OCR H420 3.1.1 dot point on exchange surfaces and gas exchange. Covers why surface-area-to-volume ratio drives the need for exchange surfaces, the features of an efficient surface, the mammalian lung, the fish counter-current system and the insect tracheal system.
- 3.1.3 Transport in plants: the structure and function of xylem and phloem; the cohesion-tension theory of water transport in the xylem and the factors affecting transpiration; the mass flow hypothesis of translocation in the phloem from source to sink; and the adaptations of xerophytes for reducing water loss.
A focused answer to the OCR H420 3.1.3 dot point on transport in plants. Covers xylem and phloem structure, the cohesion-tension theory of transpiration, the factors affecting transpiration rate, the mass flow hypothesis of translocation, and xerophyte adaptations.
- 5.1.2 Homeostasis and excretion: the principles of homeostasis and negative feedback; the role of the liver in deamination and detoxification; the structure of the nephron and the processes of ultrafiltration and selective reabsorption; and osmoregulation by ADH acting on the collecting duct.
A focused answer to the OCR H420 5.1.2 dot point on homeostasis and the kidney. Covers negative feedback, the liver's role in deamination and detoxification, the nephron, ultrafiltration and selective reabsorption, and osmoregulation by ADH acting on the collecting duct.
Sources & how we know this
- OCR A Level Biology A (H420) Specification — OCR (2023)