Blood Vessels - MCAT Biology | Premier MCAT Prep

Blood Vessels

7 min read Updated Mar 26, 2026

The circulatory system uses five types of blood vessels, each engineered for a specific job. Understanding their structural differences is the key to understanding why blood flows the way it does - and why it fails in disease.

The Three Layers of Blood Vessels

Most blood vessels share a common three-layer wall structure:

Side-by-side comparison of artery, vein, and capillary showing structural differences in wall thickness and lumen size — Comparison of artery, vein, and capillary. **Focus on:** arteries have thick walls (more smooth muscle) to handle high pressure, veins have thinner walls with valves to prevent backflow, and capillaries are one cell thick for gas exchange. *Credit: Wikimedia Commons, CC BY-SA 3.0*

Layer	Composition	Function
Tunica intima (innermost)	Single layer of endothelial cells	Smooth, non-thrombogenic surface for blood flow; regulates permeability
Tunica media (middle)	Smooth muscle + elastic fibers	Vasoconstriction/vasodilation; maintains vessel tone
Tunica externa (outermost)	Connective tissue (collagen)	Anchors vessel to surrounding tissue; structural support

Arteries

Arteries carry blood away from the heart under high pressure. They have thick walls with abundant smooth muscle and elastic fibers in the tunica media.

Elastic (conducting) arteries - the largest arteries (aorta, pulmonary trunk, carotid arteries). Their walls are rich in elastic fibers that stretch during systole and recoil during diastole, smoothing out the pulsatile flow from the heart. This is why you still have blood flow during diastole - the elastic recoil of these arteries acts like a secondary pump.

Muscular (distributing) arteries - medium-sized arteries (brachial, femoral, renal arteries). More smooth muscle, less elastic tissue. They distribute blood to specific organs and can constrict or dilate to redirect flow.

Arterioles: The Resistance Vessels

Arterioles are the smallest arteries (diameter: 10-100 micrometers) and the primary regulators of blood pressure and blood flow distribution. Their smooth muscle walls can constrict (vasoconstriction) or relax (vasodilation) to change resistance dramatically.

A small change in arteriole radius produces a massive change in resistance. This relationship follows Poiseuille’s law: resistance is inversely proportional to the fourth power of the radius. Halve the radius and resistance increases 16-fold.

Capillaries: The Exchange Vessels

Capillaries are the thinnest vessels (diameter: 5-10 micrometers, just wide enough for a single red blood cell). Their walls are only one endothelial cell thick - no tunica media, no tunica externa. This thinness is essential for diffusion of gases, nutrients, and waste.

There are three types of capillaries:

Diagram comparing the three types of capillaries: continuous capillaries with tight junctions, fenestrated capillaries with pores, and sinusoidal capillaries with large gaps between endothelial cells — The three types of capillaries differ in how permeable they are. Continuous capillaries are the most restrictive; sinusoidal are the most permeable. *Credit: OpenStax Anatomy and Physiology 2e, CC BY 4.0*

Type	Structure	Where Found	What Passes Through
Continuous	Tight junctions, no pores	Muscle, skin, lungs, brain (BBB)	Small molecules, water, ions only
Fenestrated	Small pores (60-80 nm) in endothelial cells	Kidneys, intestines, endocrine glands	Small proteins, filtered fluids
Sinusoidal	Large gaps (30-40 micrometers)	Liver, spleen, bone marrow	RBCs, WBCs, large proteins - almost everything

Velocity and Cross-Sectional Area

Blood flows fastest in the aorta and slowest in the capillaries. This seems counterintuitive - the capillaries are tiny, so shouldn’t flow speed up? No, because while each individual capillary is narrow, the total cross-sectional area of all capillaries combined is enormous (about 600 times the area of the aorta).

By the continuity equation (A1v1 = A2v2), when total cross-sectional area increases, velocity must decrease. This slow flow in capillaries is essential - it gives time for gas and nutrient exchange.

Veins: The Capacitance Vessels

Veins return blood to the heart under low pressure. Their walls are thinner and more compliant (stretchable) than arteries. Veins hold about 60-65% of total blood volume at any time, which is why they are called capacitance vessels.

Because venous pressure is so low (~5-10 mmHg), veins use several mechanisms to push blood back to the heart:

Venous valves - one-way flaps that prevent backflow, especially in the legs where blood must travel against gravity
Skeletal muscle pump - contraction of surrounding muscles squeezes veins, pushing blood upward
Respiratory pump - during inhalation, decreased thoracic pressure creates a pressure gradient that pulls venous blood toward the heart

Vessel Comparison Summary

Feature	Arteries	Arterioles	Capillaries	Venules	Veins
Wall thickness	Thickest	Thick	Thinnest (1 cell)	Thin	Moderate
Lumen diameter	Large	Small	Tiny (5-10 um)	Small	Large
Blood pressure	Highest	High	Low	Very low	Lowest
Flow velocity	Fast	Moderate	Slowest	Slow	Moderate
Special features	Elastic recoil	Resistance control	Gas exchange	WBC migration	Valves, high volume

Why does blood flow slowest in the capillaries, even though they are the narrowest individual vessels?

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The total cross-sectional area of all capillaries combined is far greater than any other vessel type. By the continuity equation (A x v = constant), when total area increases, velocity must decrease. Each capillary is tiny, but there are billions of them, so their combined area is enormous. This slow flow maximizes time for gas and nutrient exchange.

A person stands motionless for a long time and starts to feel lightheaded. Which venous return mechanisms are being compromised?

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The skeletal muscle pump is inactive. Without leg muscle contractions, blood pools in the lower extremity veins. Reduced venous return decreases preload (Frank-Starling), which decreases stroke volume and cardiac output, leading to reduced cerebral perfusion and lightheadedness. This is why soldiers standing at attention sometimes faint.