Ch 3 · Blood Vessels · O Level Biology 5090
Topic 3 of 4 in Transport in Animals — O Level Biology 5090 — 35 lessons.
Introduction
Blood leaves the heart at high pressure in surges, seeps through tissues slowly enough to exchange materials, and returns at low pressure. Three kinds of vessel handle these jobs. You must identify them, describe their structure and function and explain each feature in terms of blood pressure. You also need the names of the main vessels serving the heart, lungs, kidneys and liver.
Key ideas — Arteries, veins and capillaries
Arteries carry blood away from the heart at high pressure: thick wall containing muscle and elastic tissue; narrow lumen relative to the wall; round in section; no valves.
🧑🏫 Teacher's explanation
Think of blood vessels like a city's water system. Arteries are the high-pressure main water pipes leaving the pumping station; they need thick, stretchy walls to handle the sudden bursts of pressure every time the pump (the heart) kicks in. Veins are like the return drainage pipes; the pressure is much lower, so the walls can be thinner, but they need 'one-way trapdoors' (valves) to make sure the water doesn't flow backward due to gravity. Capillaries are like the tiny, microscopic drip irrigation tubes that reach every single plant; they are so thin that water can pass right through their walls to reach the roots.
📖 Definitions
🔍 Worked example — Easy
A student observes a cross-section of a blood vessel under a microscope. The vessel has a very thick wall consisting of muscle and elastic tissue and a narrow lumen. Identify the vessel type.
🔍 Worked example — Standard
Compare the structure of an artery and a vein in terms of their walls and lumen size.
🔍 Worked example — Exam-style
Explain how the structure of a capillary is specifically adapted to its function in the exchange of substances.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the type of blood vessel. [1] (b) Explain how the structure of this vessel is related to the pressure of the blood it carries. [2] [3 marks]
Key ideas — Linking structure to function and pressure
🧑🏫 Teacher's explanation
Think of your circulatory system as a city's water supply. Arteries are like high-pressure fire hoses; they need thick, reinforced rubber to handle the sudden bursts of water every time the pump turns on. Capillaries are like thin, porous garden hoses laid out everywhere; they are very thin so water and nutrients can seep through the walls easily. Veins are like wide, slow-moving drainage pipes; they don't need much strength because the pressure is low, but they need to be wide to keep the flow moving easily without getting clogged.
📖 Definitions
🔍 Worked example — Easy
A student observes a blood vessel under a microscope. It has a very thick muscular and elastic wall and a narrow lumen. Identify the vessel type and state its primary function.
🔍 Worked example — Standard
Calculate the percentage change in the diameter of an artery if it narrows from 5.0 mm to 4.2 mm due to muscle contraction.
🔍 Worked example — Exam-style
Explain why capillaries are only one cell thick and have a very narrow lumen.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Naming the main vessels
Arteries carry blood away from the heart; veins carry it towards the heart.
Key ideas — Exchange at the capillaries: tissue fluid
transfer between blood, tissue fluid and cells. As blood flows through a capillary under pressure, some plasma leaks out through the thin wall. This fluid, now called tissue fluid, bathes every cell. Red blood cells and large proteins stay in the blood.
🧑🏫 Teacher's explanation
Think of the circulatory system like a city's water supply. The arteries are like the high-pressure main water pipes leaving the treatment plant; they need thick, tough walls to handle the sudden surges of pressure every time a pump turns on. The veins are like the drainage pipes; the pressure is much lower, so the walls can be thinner, and they often have valves to ensure water doesn't flow backward.
📖 Definitions
🔍 Worked example — Easy
A student observes a cross-section of a blood vessel under a microscope. The vessel has a very narrow lumen and a very thick layer of smooth muscle and elastic tissue. Identify the vessel type and explain why the thick wall is necessary.
🔍 Worked example — Standard
Compare the structure of a capillary to an artery in terms of its wall thickness and explain how this relates to its function.
🔍 Worked example — Exam-style
Calculate the percentage change in the diameter of an artery if it expands from 2.0 mm to 2.5 mm during a surge in blood pressure.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe two structural features of a capillary that allow it to be efficient at exchanging substances. [2] (b) Explain how the structure of an artery is related to the pressure of the blood it carries. [2] [4 marks]
Worked example
Question. A photomicrograph shows vessel A with a thick wall and small round lumen, and vessel B with a thin wall and large irregular lumen. (a) Identify A and B, giving a reason for each. [2] (b) Describe two ways in which a capillary is suited to its function. [2] (c) Explain why vessel A has elastic tissue in its wall. [2] (d) Name the vessel that carries blood from the aorta to a kidney. [1]
Common mistakes
Practice
Answers