Arteries carry blood away from the heart. Elastic arteries (like the aorta) expand and recoil to maintain continuous flow, while muscular arteries distribute blood to specific organs.
Arterioles are the smallest arteries and serve as the primary site of resistance, controlling blood flow into capillary beds via vasoconstriction.
Blood Flow (): The volume of blood flowing through a vessel per unit of time, determined by the pressure gradient () and resistance ().
Resistance (): The opposition to flow, primarily influenced by blood viscosity, vessel length, and most critically, vessel radius. According to Poiseuille's Law, resistance is inversely proportional to the fourth power of the radius ().
Pressure Gradients: Blood moves from areas of high pressure (arteries) to low pressure (veins). The steepest drop in pressure occurs in the arterioles, which protects the fragile capillaries from bursting.
| Feature | Arteries | Veins | Capillaries |
|---|---|---|---|
| Direction | Away from heart | Toward heart | Connection point |
| Pressure | High | Low | Very Low |
| Wall Thickness | Thick (mostly muscle/elastic) | Thin (mostly collagen) | Extremely thin (single layer) |
| Lumen Size | Small | Large | Microscopic |
| Valves | Absent (except semilunar) | Present in limbs | Absent |
Arteries are characterized by their ability to maintain shape even when empty, whereas veins often collapse due to their thinner walls.
Capillaries are unique because they lack the tunica media and externa, prioritizing permeability over structural strength.
The Oxygenation Trap: Never define arteries as 'carrying oxygenated blood.' Always define them by the direction of flow (away from the heart). The pulmonary artery is a classic exam example of an artery carrying deoxygenated blood.
Resistance Calculations: Remember that a small change in vessel diameter has a massive impact on blood flow. If a vessel radius doubles, the resistance decreases by a factor of 16 ().
Valve Function: In questions about venous return, always mention the skeletal muscle pump and valves. Valves ensure one-way flow, preventing blood from pooling in the lower extremities due to gravity.
Velocity vs. Area: Note that blood flow velocity is slowest in the capillaries. This is because the total cross-sectional area of all capillaries combined is much larger than that of the aorta, allowing time for nutrient exchange.
