Veins and Capillaries
Veins have a structure that differs from arteries to suit their function of returning blood to the heart. They have an outer layer containing elastic fibers but have thinner muscular walls and a wider lumen compared to arteries.
Vocabulary: Vasoconstriction - The narrowing of blood vessels, which increases blood pressure. Vocabulary: Vasodilation - The widening of blood vessels, which decreases blood pressure.
Veins contain valves to prevent the backflow of blood, which is essential as blood pressure is lower in veins compared to arteries.
Highlight: The presence of valves in veins is a key characteristic often tested in Higher Human Biology Past Papers.
Capillaries are the smallest blood vessels and allow for the exchange of materials between blood and tissues. Their thin walls facilitate this exchange through pressure filtration and diffusion.
Definition: Pressure filtration is the process by which fluid is forced out of capillaries due to blood pressure.
Tissue fluid, formed from plasma that leaves the capillaries, supplies cells with glucose, oxygen, and other substances. Waste products like CO₂ and other metabolic substances diffuse from cells into the tissue fluid and then back into the blood.
Example: In a Structure and function of blood vessels in higher human biology quiz, you might be asked to name the type of blood vessel that directly exchanges substances with body tissues (answer: capillaries).
The lymphatic system plays a crucial role in returning excess tissue fluid to the circulatory system. Lymphatic vessels absorb this fluid, now called lymph, and return it to the blood.
Highlight: Understanding the relationship between capillaries, tissue fluid, and the lymphatic system is essential for success in SQA Higher Human Biology blood circulation and vessel structure questions.
