Slide 1
Names describe direction
Arteries carry blood away from the heart; veins carry it back. These names describe direction, rather than oxygen content.
More to Read
Pulmonary arteries carry oxygen-poor blood toward the lungs, whereas pulmonary veins return oxygen-rich blood. This exception to a familiar colour diagram is a useful test of the actual definition.
Slide 2
Elastic walls soften a pulse
Large arteries stretch as the heart ejects blood and recoil afterward. Their elastic walls help sustain flow between heartbeats.
More to Read
Smaller arteries contain relatively more smooth muscle. Changing their diameter alters resistance, helping distribute blood among organs whose demands change with activity.
Slide 3
Exchange needs a thin barrier
Capillaries connect the outward and return routes. Their thin walls let substances pass between blood and surrounding tissues.
More to Read
Different organs have differently permeable capillaries. The tight barrier in the brain and the more open filtration surfaces of the kidney illustrate why one wall design would not serve every tissue.
Slide 4
Returning at lower pressure
Veins have thinner walls and can hold considerable blood at low pressure. Many limb veins contain valves that limit backflow.
More to Read
Contracting skeletal muscles squeeze nearby veins. With functioning valves, this intermittent compression assists movement toward the heart instead of simply shifting blood equally in both directions.
Slide 5
Pressure is a difference
Blood moves along a pressure gradient. A useful circulation requires both a driving pressure and a route with manageable resistance.
More to Read
Breathing also changes pressures in the chest and abdomen, assisting venous return. The circulation therefore depends on cooperation among vessel walls, heart action and movements elsewhere in the body.
Sources
- Structure and Function of Blood Vessels
OpenStax, Rice University
- Blood Flow, Blood Pressure, and Resistance
OpenStax, Rice University