2.1 Structure and Function of Blood Vessels

Blood vessel types and structures

Blood vessels form the network that moves blood between the heart and every tissue in the body. Three vessel types handle this job: arteries carry blood away from the heart, veins return it, and capillaries connect the two while serving as the exchange sites where nutrients, gases, and wastes move between blood and tissues.

The three main types of blood vessels

• Arteries are thick-walled vessels that carry blood away from the heart under high pressure.
• Veins are thinner-walled vessels that return blood to the heart under lower pressure.
• Capillaries are the smallest blood vessels, made of a single layer of endothelial cells.
  • Their thin walls make them the only vessels where exchange between blood and tissues actually occurs.

Structural components of blood vessels

Most blood vessels (arteries and veins) share a three-layer wall structure:

• Tunica intima (innermost): a smooth endothelial lining that contacts the blood directly
• Tunica media (middle): smooth muscle and elastic fibers that control vessel diameter
• Tunica externa / adventitia (outermost): connective tissue that anchors the vessel to surrounding structures

The key structural difference between arteries and veins comes down to the tunica media. Arteries have a much thicker tunica media packed with smooth muscle and elastic fibers, which lets them handle the high pressure of blood pumped directly from the heart. Veins have a thinner tunica media but a larger lumen (internal diameter), and they contain valves that prevent blood from flowing backward.

Capillaries lack the tunica media and tunica externa entirely. Their walls are just a single cell layer thick, which is what makes diffusion across them so efficient.

Arteries, veins, and capillaries

Functions of arteries

Arteries transport blood from the heart to the tissues under high pressure. Their elastic walls expand when the heart contracts (systole) and recoil during relaxation (diastole). This stretch-and-recoil action smooths out the pulsatile flow from the heart and helps maintain relatively steady blood pressure between beats.

Two subtypes are worth knowing:

• Elastic (conducting) arteries like the aorta have walls dominated by elastic fibers. They handle the highest pressures closest to the heart.
• Muscular (distributing) arteries have proportionally more smooth muscle. They direct blood to specific organs and regions.

The smallest arteries, called arterioles, are the primary sites of resistance in the cardiovascular system. Small changes in arteriole diameter have a large effect on blood pressure and flow to downstream capillary beds.

Functions of veins

Veins return blood from the tissues back to the heart. Because they operate under much lower pressure, they rely on several mechanisms to keep blood moving:

• Skeletal muscle pump: contracting muscles around veins squeeze blood toward the heart
• Respiratory pump: pressure changes during breathing help draw blood into the thoracic veins
• Venous valves: one-way valves prevent backflow, especially in the limbs where blood must travel against gravity

Veins also act as a blood reservoir, holding roughly 60-70% of the body's total blood volume at any given time. During situations like hemorrhage or intense exercise, sympathetic stimulation can constrict veins (venoconstriction) to redirect this stored blood back into active circulation.

Functions of capillaries

Capillaries are where the real work of the circulatory system happens. Their walls are just one endothelial cell thick, and they form vast networks called capillary beds that bring blood within diffusion distance of nearly every cell.

Exchange across capillary walls occurs through several mechanisms:

• Diffusion: oxygen and carbon dioxide move down their concentration gradients ($O_2$ into tissues, $CO_2$ into blood)
• Filtration: hydrostatic pressure at the arterial end pushes fluid and small solutes out into the interstitial space
• Reabsorption: osmotic pressure (largely from plasma proteins like albumin) draws fluid back in at the venous end

Three capillary types exist, each suited to the tissues they serve:

• Continuous capillaries: tight junctions between cells; found in muscle, skin, and the brain (where they form the blood-brain barrier)
• Fenestrated capillaries: small pores in the endothelium; found in the kidneys, intestines, and endocrine glands where rapid exchange or filtration is needed
• Sinusoidal capillaries: large gaps and an incomplete basement membrane; found in the liver, spleen, and bone marrow where large molecules and even cells need to pass through

Smooth muscle in blood flow regulation

Role of smooth muscle cells

Smooth muscle in the tunica media is what gives blood vessels the ability to actively change their diameter. This is the body's primary short-term mechanism for controlling where blood goes and how much pressure it's under.

• Vasoconstriction: smooth muscle contracts, narrowing the lumen and reducing blood flow to that area. Blood pressure upstream increases.
• Vasodilation: smooth muscle relaxes, widening the lumen and increasing blood flow. Local blood pressure decreases.

Even small changes in vessel radius have a dramatic effect on flow. Blood flow is proportional to the fourth power of the radius ($F \propto r^4$), so halving a vessel's radius reduces flow to $\frac{1}{16}$ of the original.

Factors regulating smooth muscle tone

Several systems work together to control vascular smooth muscle:

• Autonomic nervous system
  • Sympathetic stimulation releases norepinephrine, which generally causes vasoconstriction (via alpha-adrenergic receptors). In skeletal muscle, however, sympathetic activation can cause vasodilation during exercise (via beta-2 receptors).
  • Parasympathetic influence on blood vessels is limited. It contributes to vasodilation mainly in specific areas like the salivary glands and erectile tissue.
• Hormones
  • Epinephrine and norepinephrine from the adrenal medulla reinforce sympathetic effects
  • Angiotensin II is a potent vasoconstrictor involved in blood pressure regulation
  • Atrial natriuretic peptide (ANP) promotes vasodilation
• Local (intrinsic) factors
  • Nitric oxide (NO): released by endothelial cells, causes vasodilation. This is one of the most important local regulators.
  • Endothelin: released by endothelial cells, causes vasoconstriction
  • Increased metabolic activity, decreased $O_2$, increased $CO_2$, or decreased pH in tissues all trigger local vasodilation to match blood supply to metabolic demand. This process is called metabolic autoregulation.

Blood vessel layer distinctions

Tunica intima

The tunica intima is the innermost layer, in direct contact with blood. It consists of a single layer of endothelial cells resting on a thin subendothelial layer of connective tissue. In arteries, the tunica intima is bounded by an internal elastic lamina, a sheet of elastic tissue that separates it from the tunica media.

Endothelial cells do far more than just provide a smooth surface. They actively:

• Prevent clotting by producing anticoagulant molecules on their surface
• Release nitric oxide and endothelin to regulate vessel diameter
• Mediate inflammation by controlling which immune cells can cross the vessel wall
• Contribute to new blood vessel growth (angiogenesis)

Damage to the endothelium is a key early step in the development of atherosclerosis, where plaques build up inside arterial walls.

Tunica media

The tunica media is the middle layer, composed of smooth muscle cells and elastic fibers. It's bounded by the internal elastic lamina on the inside and the external elastic lamina on the outside.

• In arteries, the tunica media is the thickest and most prominent layer. The high proportion of smooth muscle and elastic tissue lets arteries withstand and regulate high-pressure blood flow.
• In veins, the tunica media is noticeably thinner because veins operate under much lower pressures and don't need the same structural reinforcement.
• In capillaries, this layer is absent entirely.

Tunica externa (adventitia)

The tunica externa is the outermost layer, made primarily of collagen fibers with some elastic fibers mixed in. Its main jobs are anchoring the vessel to surrounding tissues and providing structural support.

In larger vessels, the tunica externa contains two notable features:

• Vasa vasorum ("vessels of the vessels"): tiny blood vessels that supply oxygen and nutrients to the outer layers of the vessel wall itself. The inner layers receive nutrients by diffusion from the blood flowing through the lumen.
• Nervi vasorum: small nerves (mainly sympathetic) that innervate the smooth muscle in the tunica media.

In veins, the tunica externa is often the thickest layer, which is the opposite of arteries where the tunica media dominates.