Blood vessels, blood pressure and lymph
Arteries carry blood away from the heart, veins bring it back and capillaries exchange materials. Normal blood pressure is about 120/80 mm Hg; lymph returns tissue fluid to the blood.
Arteries, veins and capillaries
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction | Away from the heart | Towards the heart | Links arteries to veins in the tissues |
| Blood | Oxygenated (except the pulmonary artery) | Deoxygenated (except the pulmonary veins) | Gives O₂ to the tissues and takes CO₂ |
| Wall | Thick, muscular and elastic | Thin, with less muscle | One cell thick (endothelium only) |
| Valves | Absent | Present: stop blood flowing back | Absent |
| Lumen (space inside) | Narrow | Wide | Very narrow |
| Position | Mostly deep in the body | Many near the surface | Throughout the tissues |
| Pressure | High | Low | Lower than in arteries |
Walls of arteries and veins
- Tunica externa (outer layer): connective tissue with collagen fibres.
- Tunica media (middle layer): smooth muscle and elastic fibres; thick in arteries, thin in veins.
- Tunica interna (inner layer): a smooth lining of flat cells, the endothelium.
- Capillaries have only the endothelium, so food, gases and wastes pass through easily.
Blood pressure
Blood pressure is the pressure that blood puts on the walls of the arteries. It is highest when the ventricles contract: the systolic pressure, about 120 mm Hg. It is lowest when they relax: the diastolic pressure, about 80 mm Hg. It is written as 120/80 mm Hg and measured with a sphygmomanometer. Blood pressure that stays high is hypertension; it strains the heart and can damage the brain and kidneys. Blood pressure that is too low is hypotension, which can cause dizziness.
Lymph and the lymphatic system
- Some plasma leaks out of the capillaries and bathes the cells as tissue fluid. When it enters thin lymph vessels, it is called lymph.
- Lymph is a colourless fluid like plasma but with fewer proteins. It has lymphocytes but no red cells.
- Lymph vessels join into larger ducts that empty into veins near the neck, so lymph always flows one way, towards the heart.
- Lymph nodes along the vessels filter germs and make lymphocytes. They swell during an infection, for example in the neck during a sore throat.
- Lacteals, the lymph vessels in the villi of the small intestine, absorb fats.
Blood vs lymph
Muscle movements squeeze the lymph vessels and push lymph along; valves stop it flowing back.
Eating less salt (in pickles, papads and chips), staying active and keeping a healthy weight help prevent high blood pressure. The swollen, tender 'glands' in your neck during a throat infection are lymph nodes fighting germs.
1 marks: “Name the instrument used to measure blood pressure.” Sphygmomanometer.
2 marks: “What is lymph? Give one function.” A colourless tissue fluid in the lymph vessels, with lymphocytes. It returns tissue fluid to the blood (it also absorbs fats and fights germs).
4 marks: “Give four differences between arteries and veins.” Direction of flow; kind of blood (with the pulmonary exceptions); thick vs thin wall; no valves vs valves; narrow vs wide lumen; deep vs surface position.
Arteries go Away; Veins have Valves. Blood pressure '120 over 80': the top number when the heart squeezes, the bottom when it rests.