Blood vessels, blood pressure, and the lymphatic system
Blood vessels would stretch about 100,000 km if laid end to end. Their structure controls where blood goes and how much pressure it has.
Vessel structure
Arteries and veins have three layers: an inner endothelium, a middle layer of smooth muscle and elastic tissue, and an outer connective tissue layer. Arteries have thicker muscular walls to withstand high pressure; veins have thinner walls, larger spaces inside, and valves to stop backflow. Capillaries are just a single layer of endothelium.
Capillary exchange
In capillaries, oxygen, nutrients, and wastes diffuse between blood and tissues. At the arterial end, blood pressure pushes fluid out; at the venous end, osmotic pull from plasma proteins draws most of it back. The roughly 10-15% that remains in the tissues is collected by the lymphatic system.
Blood pressure
Blood pressure is the force blood exerts on vessel walls, recorded as systolic (during ventricular contraction) over diastolic (during relaxation), such as 120/80 mmHg. Pressure depends on cardiac output, the resistance of vessels (mainly arterioles narrowing or widening), and blood volume.
Regulating blood pressure
Baroreceptors in the aorta and carotid arteries detect pressure changes; the brainstem responds within seconds by adjusting heart rate and vessel diameter. Longer-term control involves the kidneys and hormones: the renin-angiotensin-aldosterone system raises pressure by narrowing vessels and retaining sodium and water, while ADH also retains water.
Hypertension
Hypertension is persistently high blood pressure, defined by many guidelines as about 130/80 mmHg or higher. It often has no symptoms but strains the heart and damages vessels, raising risks of heart attack, stroke, and kidney disease. Lifestyle changes (less salt, more exercise, weight control) and medicines lower it.
Atherosclerosis
Atherosclerosis is the build-up of fatty plaques in artery walls, narrowing them and making them stiff. If a plaque ruptures, a clot can form and block the artery, causing a heart attack or stroke. Risk factors include smoking, high LDL cholesterol, high blood pressure, diabetes, and inactivity.
The lymphatic system
Lymphatic vessels collect excess tissue fluid (lymph) and return it to the blood near the heart. Lymph passes through lymph nodes, which filter it and house lymphocytes; nodes swell during infections. The spleen filters blood and removes old red cells; tonsils and the thymus are also lymphatic organs. Lymphatics also absorb dietary fats from the intestine.
Worked example
A person stands up quickly and briefly feels dizzy. Blood pools in the legs, so pressure in the head drops. Baroreceptors detect this, and within seconds the brainstem raises heart rate and constricts vessels, restoring pressure. If this reflex is slow, as it can be in older people or with some medicines, fainting can occur.
Key takeaways
Arteries have thick muscular walls; veins have valves; capillaries are one cell thick for exchange. Blood pressure is written systolic over diastolic and depends on output, resistance, and volume. Baroreceptors give fast control; kidneys and hormones give long-term control. Hypertension and atherosclerosis raise heart attack and stroke risk. Lymphatics return excess fluid and filter it through nodes.
Flashcard
Small artery controlling resistance
Check your understanding
Why do veins, but not arteries, need valves?