🔬 Structure
Capillary walls are only one cell thick. The vessels are very narrow, and their walls are permeable, which means substances can pass through them.
Animal Tissues, Organs and Systems » Blood Vessels and the Lungs
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.2.2.2
- How arteries, veins and capillaries are built for their jobs
- How to calculate the rate of blood flow
- The path air takes through the trachea, bronchi and alveoli
- How the lungs are adapted for gaseous exchange
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Unlock This CourseThe three types of blood vessel
Blood travels around your body inside tubes called blood vessels. The body has three different types, and each one has a structure that matches its job.
Key terms:
- Artery: a blood vessel that carries blood away from the heart.
- Vein: a blood vessel that carries blood towards the heart.
- Capillary: a very small blood vessel that links arteries to veins and lets substances pass into and out of the blood.
- Lumen: the hollow space in the middle of a blood vessel that the blood flows through.
Arteries
Blood leaves the heart in the arteries. The heart pumps it out hard, so the blood is under high pressure. The artery has to cope with this.
- The walls are thick and contain muscle and elastic fibres. These make the wall strong, and the elastic fibres let it stretch and spring back with each heartbeat.
- The lumen is narrow, which helps keep the pressure high.
Veins
Veins bring blood back to the heart. By the time blood reaches the veins, the pressure is much lower, so the veins do not need such strong walls.
- The walls are thinner than artery walls.
- The lumen is larger, so blood can flow easily at low pressure.
- Veins have valves. These stop the blood flowing backwards.
Capillaries
Capillaries are the smallest blood vessels. They run close to almost every cell in the body, so substances can pass between the blood and the cells.
⚡ Function
Because the wall is so thin, the distance is short. Oxygen and glucose can move out of the blood to the cells, and carbon dioxide and other waste can move into the blood.
Quick comparison
Artery: thick wall, narrow lumen, high pressure, away from the heart.
Vein: thinner wall, large lumen, valves, low pressure, towards the heart.
Capillary: wall one cell thick, tiny, links arteries to veins, exchange happens here.
Rate of blood flow
The rate of blood flow tells you how much blood passes a point in a given time. It is a simple compound measure, because it combines a volume with a time.
rate of blood flow = volume of blood ÷ time
If the volume is in cm3 and the time is in minutes, the rate is in cm3 per minute (cm3/min).
Worked example 1
An artery carries 300 cm3 of blood in 2 minutes. What is the rate of blood flow?
rate = 300 ÷ 2 = 150 cm3/min
Worked example 2
Blood flows through a vessel at 40 cm3/min. How much blood passes in 6 minutes?
Rearrange the equation: volume = rate × time = 40 × 6 = 240 cm3
Check the units before you start. If the time is given in seconds and the answer needs to be per minute, change the time first.
The lungs
The lungs are in the chest. They are the organ where gas exchange takes place: oxygen goes into the blood and carbon dioxide comes out. Your spec needs you to know four parts.
Key terms:
- Trachea: the windpipe, the tube that carries air from the mouth and nose down towards the lungs.
- Bronchi: the two tubes that the trachea splits into, one going to each lung (one of them is a bronchus).
- Gaseous exchange: the swapping of gases, with oxygen entering the blood and carbon dioxide leaving it.
The path of air is: trachea → bronchi → alveoli. The alveoli are tiny air sacs at the ends of the airways. Each one is wrapped in a capillary network, a mesh of capillaries that carries blood past the air.
How the lungs are adapted for gaseous exchange
Gas exchange happens by diffusion between the air in the alveoli and the blood in the capillaries around them. Oxygen diffuses from the air into the blood. Carbon dioxide diffuses from the blood into the air to be breathed out. The lungs have four adaptations that make this fast.
🌀 Large surface area
There are millions of alveoli. Together they give a huge surface for gases to cross.
📏 Thin walls
The wall of an alveolus is one cell thick, and so is the wall of a capillary. The gases have only a very short distance to diffuse.
❤ Good blood supply
The capillary network around each alveolus brings blood with plenty of carbon dioxide and takes oxygenated blood away. This keeps a steep concentration gradient.
🌬 Ventilation
Breathing in and out brings fresh air with lots of oxygen and removes air with more carbon dioxide. This also keeps the concentration gradient steep.
Common mistakes
1. Saying arteries carry oxygenated blood. Most do, but the pulmonary artery carries deoxygenated blood. Learn them as vessels away from the heart.
2. Saying capillaries have valves or thick walls. Capillary walls are one cell thick.
3. Writing that the alveoli are thin. It is their walls that are thin.
4. Saying that the lungs "breathe in oxygen" only. Both oxygen and carbon dioxide are exchanged.
Exam-style question
(a) Describe two ways in which the structure of an artery is different from the structure of a vein. [2 marks]
(b) A vein carries 450 cm3 of blood in 3 minutes. Calculate the rate of blood flow. [2 marks]
(c) Explain how the structure of the alveoli helps oxygen to get into the blood quickly. [3 marks]
Model answer
(a) Any two from: the artery has a thicker wall (1); the artery has a narrower lumen (1); the vein has valves, the artery does not (1).
(b) rate = volume ÷ time = 450 ÷ 3 (1) = 150 cm3/min (1).
(c) Any three from: the alveoli have thin walls, one cell thick, so the diffusion distance is short (1); there are millions of alveoli, giving a large surface area (1); they are surrounded by a network of capillaries, giving a good blood supply (1); the blood carries oxygen away, which keeps a steep concentration gradient (1).
Exam tip
On a "how does the structure help" question, always link each feature to its effect, for example "thin wall, so a short diffusion distance". Naming the feature alone does not get the mark.