Medical School · Year 1 · Cardiovascular · includes a quiz and discussion video

Lecture 7: Vascular Physiology

Unit 1.7: Cardiovascular System

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the structure and function of different vessel types
2. Explain the mechanisms regulating vascular smooth muscle tone
3. Describe local, neural, and humoral control of blood flow
4. Explain autoregulation and reactive hyperemia
5. Describe the hemodynamics of blood flow and resistance
6. Apply vascular physiology to clinical conditions

Vascular Structure

The blood vessels form a continuous circuit that distributes blood from the heart to the tissues and returns it for reoxygenation. Each vessel type has structural characteristics adapted to its specific function.

The vessel wall consists of three layers. The tunica intima is the innermost layer, comprising the endothelium (a single layer of endothelial cells) and its underlying basement membrane. The endothelium is far more than a passive lining—it actively regulates vascular tone, coagulation, inflammation, and vessel wall remodeling. The tunica media is the middle layer, containing smooth muscle cells and elastic fibers in proportions that vary by vessel type. This layer determines the vessel's contractile and elastic properties. The tunica adventitia is the outermost layer, consisting of collagen and elastic fibers that anchor the vessel to surrounding tissues. In large vessels, the adventitia contains the vasa vasorum—small blood vessels that supply the outer layers of the vessel wall.

Vessel types differ in structure according to their function. The aorta and large arteries have thick walls rich in elastic tissue, allowing them to expand during systole and recoil during diastole. This elastic reservoir function converts the pulsatile output of the heart into more continuous flow to the periphery. Medium-sized arteries (muscular arteries) have walls dominated by smooth muscle, allowing them to distribute blood and regulate regional flow. Arterioles are small resistance vessels with a wall thickness that is large relative to their lumen diameter. Their smooth muscle can dramatically alter diameter, making them the primary determinants of vascular resistance. Capillaries have walls consisting of only a single layer of endothelial cells on a basement membrane, facilitating exchange of gases, nutrients, and wastes. Venules and veins have thin walls with less smooth muscle than corresponding arteries, allowing them to serve as capacitance vessels that can accommodate large changes in blood volume.

The functional classification emphasizes physiological roles. Elastic (conducting) arteries absorb and release pressure energy. Muscular (distributing) arteries direct blood to specific vascular beds. Resistance vessels (arterioles) control flow and determine total peripheral resistance. Exchange vessels (capillaries) permit transcapillary movement of substances. Capacitance vessels (veins) serve as a blood reservoir, containing 60 to 70 percent of total blood volume.

<image>Panel A: Cross-sectional view of aorta with thick wall rich in elastic laminae (wavy yellow lines) and medium artery with wall dominated by circular smooth muscle layers for contractile control. Panel B: Arteriole with thick media relative to small lumen emphasizing resistance function and capillary showing only endothelial cells on basement membrane with red blood cell for scale (~8 micrometers). Panel C: Vein with thin wall, fewer smooth muscle layers than corresponding artery, and venous valves depicted, with table summarizing wall thickness, diameter, and function for each vessel type. Panel D: Bar graph showing blood volume distribution: arteries 15%, capillaries 5%, veins 60-70%, heart and lungs remainder.</image>

Arterial System and the Windkessel Effect

The large elastic arteries serve a critical function in converting the intermittent output of the heart into more continuous peripheral flow.

During ventricular systole, only about one-third of the stroke volume flows immediately into the peripheral circulation. The remaining two-thirds stretches the elastic walls of the aorta and large arteries, storing energy. During diastole, when the aortic valve is closed and no blood is being ejected, the elastic recoil of these vessels propels blood forward, maintaining flow even when the heart is relaxing. This is the Windkessel effect (from the German word for "air chamber," referring to old fire engine pumps that used a compressible air chamber to smooth pulsatile pump output).

Pulse pressure—the difference between systolic and diastolic pressure—is determined by the relationship between stroke volume and arterial compliance. Compliance is the change in volume per change in pressure, reflecting the distensibility of the arterial tree. The relationship can be expressed as: pulse pressure equals stroke volume divided by arterial compliance. Increased stroke volume (as in exercise or aortic regurgitation) increases pulse pressure. Decreased arterial compliance (as in aging or atherosclerosis) also increases pulse pressure—stiff arteries cannot stretch as much during systole, so pressure rises more for the same stroke volume.

The arterial pressure waveform has characteristic features. The anacrotic limb is the rapid upstroke during ventricular ejection. The peak systolic pressure is the maximum pressure reached. The dicrotic notch (incisura) marks aortic valve closure and the end of systole. The dicrotic wave is a small secondary peak following the notch, representing a reflected pressure wave. The diastolic decay is the gradual decline in pressure as blood flows to the periphery and the elastic arteries recoil.

Pulse wave velocity is the speed at which the pressure wave travels through the arterial system. It increases with arterial stiffness and is used clinically as a measure of vascular health. Increased pulse wave velocity with age reflects arterial stiffening and is associated with cardiovascular risk.

<image>Panel A: Windkessel effect physical analogy showing pump (heart) connected to elastic reservoir (aorta) expanding during systole and propelling fluid forward via elastic recoil during diastole. Panel B: Arterial pressure waveform with labeled features: anacrotic limb, peak systolic pressure, dicrotic notch at aortic valve closure, dicrotic wave, and diastolic decay with equation PP = SV/C. Panel C: Pressure waveform comparison between young person (compliant arteries, modest pulse pressure, prominent dicrotic notch) and elderly person (stiff arteries, wide pulse pressure, elevated systolic). Panel D: Pulse wave velocity illustrated as pressure wave traveling from aorta to peripheral arteries with faster velocity in stiffer vessels.</image>

Vascular Smooth Muscle Contraction

Vascular smooth muscle maintains the tone of blood vessels and can alter vessel diameter over a wide range in response to numerous stimuli. Understanding its contraction mechanism is essential for understanding vascular regulation.

The contraction mechanism in vascular smooth muscle differs significantly from skeletal and cardiac muscle. Rather than troponin, vascular smooth muscle uses a calmodulin-based regulatory system. When intracellular calcium concentration rises (from approximately 100 nanomolar to approximately 1 micromolar), calcium binds to calmodulin, forming a calcium-calmodulin complex. This complex activates myosin light chain kinase (MLCK), which phosphorylates the regulatory light chain of myosin. Phosphorylated myosin can then interact with actin, initiating cross-bridge cycling and generating force. The result is contraction and vasoconstriction.

Relaxation occurs when calcium levels fall and myosin light chain phosphatase (MLCP) dephosphorylates myosin. Without phosphorylation, myosin cannot interact productively with actin, and the muscle relaxes, causing vasodilation.

Intracellular calcium can rise through multiple pathways. Voltage-gated calcium channels open when the membrane depolarizes, allowing extracellular calcium entry. Receptor-operated calcium channels open in response to ligand binding. IP3 (inositol trisphosphate), generated by receptor activation, triggers calcium release from the sarcoplasmic reticulum.

Vascular smooth muscle has unique properties. Basal tone refers to the partial contraction present at rest in most vascular beds, resulting from a balance of vasoconstricting and vasodilating influences. This tone can be modulated in either direction. The latch state allows vascular smooth muscle to maintain tension with low ATP consumption—cross-bridges cycle slowly but remain attached, reducing energy expenditure during sustained contraction. The myogenic response is the intrinsic property by which vascular smooth muscle contracts in response to stretch—this is fundamental to autoregulation. Multiple regulatory inputs (neural, hormonal, local) converge on this contractile apparatus, allowing fine control of vessel tone.

<image>Panel A: Vascular smooth muscle contraction cascade showing increased intracellular Ca2+ from voltage-gated channels, receptor-operated channels, or SR via IP3 receptors binding to calmodulin with 4 binding sites. Panel B: Ca2+-calmodulin complex activating MLCK which phosphorylates myosin light chain enabling actin binding and cross-bridge cycling for vasoconstriction, with relaxation pathway showing MLCP dephosphorylating myosin. Panel C: Unique properties including basal tone (partial contraction), latch state (slowly cycling cross-bridges maintaining tension with low ATP), and myogenic response (stretch causing depolarization via stretch-activated channels). Panel D: Multiple regulatory inputs (sympathetic, NO, angiotensin II) converging on the central contraction mechanism.</image>

Local Regulation of Blood Flow

Local mechanisms match blood flow to tissue metabolic demands without requiring neural or hormonal input. This intrinsic regulation ensures that active tissues receive adequate perfusion.

Metabolic regulation is the most important local mechanism. When tissue metabolism increases, the resulting changes in local chemical environment cause vasodilation and increased blood flow. Decreased oxygen tension causes vasodilation in most vascular beds (except the pulmonary circulation, where hypoxia causes vasoconstriction). Increased carbon dioxide and decreased pH cause vasodilation by directly relaxing smooth muscle. Increased extracellular potassium (released from active cells) causes hyperpolarization of smooth muscle, reducing calcium entry and causing vasodilation. Adenosine, released when ATP consumption exceeds production, is a powerful vasodilator acting through A2 receptors to increase cAMP in smooth muscle. Increased lactate and other metabolic byproducts similarly promote vasodilation.

The endothelium produces several vasoactive substances. Nitric oxide (NO) is the most important endothelium-derived vasodilator. It is synthesized by endothelial nitric oxide synthase (eNOS) from L-arginine in response to various stimuli, particularly shear stress (the frictional force of blood flow on the endothelium). NO diffuses to adjacent smooth muscle cells, where it activates soluble guanylyl cyclase, increasing cyclic GMP (cGMP). Elevated cGMP reduces intracellular calcium and causes relaxation. Acetylcholine, bradykinin, and other substances also stimulate NO release through receptor-mediated mechanisms. Nitroglycerin and other nitrovasodilators work by providing exogenous NO.

Prostacyclin (PGI2) is another endothelium-derived vasodilator. Synthesized from arachidonic acid through the cyclooxygenase pathway, it acts on smooth muscle to increase cyclic AMP (cAMP) and cause relaxation. It also inhibits platelet aggregation.

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide produced by endothelial cells. Acting through ETA receptors on smooth muscle, it increases intracellular calcium and causes contraction. Under normal conditions, the vasodilating effects of NO predominate, but in pathological states (endothelial dysfunction, heart failure), increased ET-1 activity contributes to inappropriate vasoconstriction.

The myogenic response (Bayliss effect) is the intrinsic tendency of vascular smooth muscle to contract when stretched. When pressure in a vessel increases, the wall stretches, and the smooth muscle responds by contracting. The mechanism involves stretch-activated cation channels that depolarize the membrane, open voltage-gated calcium channels, and trigger contraction. This response is fundamental to autoregulation.

<image>Panel A: Metabolic regulation showing active tissue generating decreased O2, increased CO2, decreased pH, increased K+, adenosine, and lactate acting on arteriole smooth muscle causing vasodilation. Panel B: Endothelial NO pathway showing shear stress activating eNOS converting L-arginine to NO which diffuses to smooth muscle activating guanylyl cyclase and cGMP for relaxation, and PGI2 pathway via COX increasing cAMP. Panel C: Endothelin-1 pathway showing ET-1 activating ETA receptors on smooth muscle increasing Ca2+ causing contraction. Panel D: Myogenic response showing increased intravascular pressure stretching vessel wall, opening stretch-activated channels, depolarizing smooth muscle, and causing contraction to maintain constant flow.</image>

Autoregulation

Autoregulation is the intrinsic ability of an organ to maintain relatively constant blood flow despite changes in perfusion pressure. This remarkable property ensures that vital organs receive consistent perfusion even when blood pressure fluctuates.

Within the autoregulatory range, blood flow remains nearly constant despite substantial changes in arterial pressure. Below this range, flow falls passively with pressure. Above this range, flow rises passively with pressure. The brain, kidney, and heart demonstrate particularly robust autoregulation.

Two mechanisms contribute to autoregulation. The myogenic mechanism responds to changes in transmural pressure. When arterial pressure increases, arteriolar walls are stretched, and the smooth muscle responds by contracting (the Bayliss effect). This vasoconstriction increases resistance and prevents the increase in flow that would otherwise occur. Conversely, when pressure falls, reduced stretch allows relaxation and vasodilation, reducing resistance and maintaining flow. The metabolic mechanism responds to changes in tissue perfusion. If pressure increases and initially increases flow, the improved perfusion washes away vasodilating metabolites, allowing vasoconstriction and reducing flow back toward normal. If pressure falls and initially decreases flow, metabolite accumulation causes vasodilation and partially restores flow.

Different organs have different autoregulatory ranges. The brain autoregulates over a pressure range of approximately 60 to 150 mmHg, using both myogenic and metabolic mechanisms. Carbon dioxide is particularly potent in the cerebral circulation—increased CO2 causes marked vasodilation, which is why hypercapnia raises intracranial pressure. The kidney autoregulates over approximately 80 to 180 mmHg, using myogenic mechanisms and tubuloglomerular feedback (a unique renal mechanism involving the macula densa). The heart autoregulates over approximately 60 to 140 mmHg, primarily through metabolic mechanisms (adenosine is particularly important).

Conditions that impair autoregulation include chronic hypertension (which shifts the autoregulatory curve rightward, so the brain becomes vulnerable to ischemia at pressures that would be tolerated by normotensive individuals), diabetes (which damages small vessels), trauma, and ischemia itself.

<image>Panel A: Autoregulation curve with arterial pressure on x-axis and blood flow on y-axis showing flat plateau within autoregulatory range (60-150 mmHg) with flow falling passively below and rising passively above, with superimposed curves for brain (blue), kidney (green), and heart (red). Panel B: Myogenic mechanism showing increased pressure causing stretch, contraction, increased resistance, and maintained flow; metabolic mechanism showing washout of vasodilators with initial increased flow. Panel C: Clinical panel showing chronic hypertension shifting autoregulation curve rightward so 80 mmHg may be below adapted range causing ischemia. Panel D: Treatment implications noting rapid pressure lowering in chronic hypertension can cause cerebral hypoperfusion.</image>

Reactive and Active Hyperemia

The vasculature responds dynamically to changes in metabolic demand and periods of interrupted flow.

Reactive hyperemia is the transient increase in blood flow that occurs after a period of arterial occlusion. When blood flow is interrupted, oxygen delivery ceases, metabolites accumulate, and tissue oxygen tension falls. These changes cause progressive vasodilation of the resistance vessels. When the occlusion is released, blood flows through the dilated bed with greatly reduced resistance, producing flow rates that substantially exceed baseline—the hyperemic response. As oxygen is delivered and metabolites are washed away, vascular resistance gradually returns to normal, and flow decreases back to baseline.

The magnitude and duration of reactive hyperemia depend on the duration of occlusion and the metabolic rate of the tissue. Longer occlusion periods cause greater metabolite accumulation and more prolonged hyperemia. Tissues with high metabolic rates generate more vasodilating metabolites during occlusion. Reactive hyperemia represents repayment of an oxygen debt—the excess flow delivers oxygen to meet the accumulated deficit.

Active (functional) hyperemia is the increase in blood flow that accompanies increased tissue activity. When skeletal muscle contracts during exercise, its metabolic rate may increase twentyfold. The local accumulation of metabolites (decreased oxygen, increased CO2, increased adenosine, decreased pH, increased potassium) causes vasodilation and dramatically increased blood flow. This response is precisely matched to metabolic demand—more active tissues receive more blood. Active hyperemia in skeletal muscle is the primary mechanism by which blood flow to exercising muscle increases from approximately 20 percent of cardiac output at rest to as much as 80 percent during maximal exercise.

Both forms of hyperemia illustrate the metabolic theory of blood flow control: blood flow is locally regulated to match the metabolic requirements of the tissue.

<image>Panel A: Reactive hyperemia timeline showing artery occlusion with pressure cuff, accumulation of metabolites in ischemic tissue, and progressive arteriolar dilation. Panel B: Upon occlusion release, blood flow spike above baseline (hyperemic response) with area labeled "oxygen debt repayment" and gradual return to normal as metabolites clear. Panel C: Active hyperemia showing skeletal muscle at rest (low metabolism, modest flow) versus exercise (high metabolism producing CO2, adenosine, K+, lactate with intense vasodilation). Panel D: Graph showing blood flow increasing rapidly at exercise onset, plateauing matched to metabolic demand, then declining to baseline after exercise ceases.</image>

Neural Control of the Vasculature

The sympathetic nervous system provides tonic vasomotor control, allowing centrally mediated adjustment of vascular resistance and blood flow distribution.

Sympathetic innervation to blood vessels is extensive but varies by vascular bed. Sympathetic postganglionic neurons release norepinephrine, which acts predominantly on alpha-1 adrenergic receptors on vascular smooth muscle to cause vasoconstriction. Alpha-2 receptors, also present on some vessels and on sympathetic nerve terminals (where they inhibit further norepinephrine release), contribute to vasoconstriction when activated. In some vascular beds, particularly skeletal muscle, beta-2 adrenergic receptors mediate vasodilation; these receptors respond primarily to circulating epinephrine rather than neurally released norepinephrine.

Regional variation in sympathetic innervation reflects the different regulatory priorities of various vascular beds. The skin has dense sympathetic innervation for thermoregulatory control—vasoconstriction conserves heat, while vasodilation dissipates heat. The splanchnic circulation (gut, liver, spleen) has extensive innervation and serves as a blood reservoir that can be mobilized through sympathetic vasoconstriction during stress or exercise. The renal circulation has substantial sympathetic innervation affecting both vascular resistance and renin release. Skeletal muscle has moderate sympathetic innervation with both alpha-1 (vasoconstriction) and beta-2 (vasodilation) receptors. The brain has minimal sympathetic innervation—cerebral blood flow is primarily controlled by local metabolic factors and autoregulation. The coronary circulation similarly has minimal sympathetic neural control, with metabolic regulation predominating.

Sympathetic tone refers to the continuous baseline level of sympathetic discharge to the vasculature. This tone partially constricts blood vessels, creating a baseline level of vascular resistance. Increases in sympathetic activity increase vasoconstriction and resistance. Decreases in sympathetic activity allow vasodilation—this is the primary mechanism of neurally mediated vasodilation in most vascular beds, rather than a dedicated vasodilator nerve supply.

Parasympathetic vasodilation is limited to a few specialized beds. The salivary glands receive parasympathetic vasodilator fibers that release vasoactive intestinal peptide and stimulate local nitric oxide release, producing vasodilation during salivation. The external genitalia receive parasympathetic innervation mediated by nitric oxide, essential for erectile function. Most vascular beds have no parasympathetic innervation.

<image>Panel A: Sympathetic nervous system showing preganglionic fibers from spinal cord synapsing in paravertebral ganglia, postganglionic fibers to blood vessels releasing norepinephrine binding alpha-1 receptors causing vasoconstriction with alpha-2 feedback and beta-2 vasodilation. Panel B: Body diagram showing regional sympathetic innervation variation: dense in skin and splanchnic bed, moderate in kidney and skeletal muscle, minimal in brain and coronary circulation. Panel C: Sympathetic tone concept showing vessel at baseline tone (partially constricted), increased tone (more constricted), and decreased tone (dilated). Panel D: Parasympathetic vasodilation in specific beds: salivary glands (ACh, VIP, NO) and external genitalia (NO-mediated erection).</image>

Humoral Control of the Vasculature

Circulating hormones contribute to vascular regulation, particularly during stress and in the maintenance of long-term vascular tone.

Vasoconstrictor hormones include catecholamines, angiotensin II, vasopressin, and endothelin. Epinephrine from the adrenal medulla acts on alpha-1 receptors to cause vasoconstriction at high concentrations, while at lower concentrations it may cause vasodilation in skeletal muscle through beta-2 receptors. Norepinephrine, also released from the adrenal medulla and from sympathetic nerve terminals, acts primarily on alpha-1 and alpha-2 receptors to cause vasoconstriction. Angiotensin II, generated by the renin-angiotensin system, acts on AT1 receptors on vascular smooth muscle to cause potent vasoconstriction. It also potentiates sympathetic neurotransmission and promotes vascular remodeling. Vasopressin (ADH) acts on V1 receptors on vascular smooth muscle to cause vasoconstriction, particularly important in hemorrhage and other hypovolemic states. Endothelin-1, produced by endothelial cells and acting locally on ETA receptors, is one of the most potent vasoconstrictors known; its role is increased in pathological states such as heart failure and pulmonary hypertension.

Vasodilator hormones include natriuretic peptides, kinins, and histamine. Atrial and brain natriuretic peptides (ANP, BNP) act through guanylyl cyclase-linked receptors to increase cGMP and cause vasodilation, in addition to their renal effects. Bradykinin and other kinins act on B2 receptors on endothelial cells to stimulate NO and prostacyclin release, causing vasodilation. ACE (kininase II) degrades bradykinin, so ACE inhibitors enhance bradykinin's vasodilating effect. Histamine acts on H1 receptors on endothelial cells to stimulate NO release and on H2 receptors on smooth muscle to directly cause relaxation; the net effect is vasodilation, which contributes to the hypotension of anaphylaxis.

Prostaglandins and thromboxane have variable vascular effects. Prostacyclin (PGI2), produced by endothelial cells, is a vasodilator and inhibits platelet aggregation. Thromboxane A2 (TXA2), produced by platelets, is a vasoconstrictor and promotes platelet aggregation. The balance between these eicosanoids influences vascular tone and hemostasis.

<image>Panel A: Vasoconstrictors showing epinephrine (adrenal medulla, alpha-1 receptors), norepinephrine (alpha-1, alpha-2), angiotensin II (RAAS, AT1 receptors), and vasopressin (posterior pituitary, V1 receptors) with sources and mechanisms. Panel B: Endothelin-1 (endothelium, ETA receptors) as potent vasoconstrictor. Panel C: Vasodilators showing ANP/BNP (heart, increasing cGMP), bradykinin (stimulating endothelial NO, degraded by ACE), and histamine (H1 releasing NO, H2 directly on smooth muscle). Panel D: Prostaglandins as variable with PGI2 (endothelium, vasodilator) versus TXA2 (platelets, vasoconstrictor), and blood vessel cross-section showing factors converging to determine vessel tone.</image>

Hemodynamics

Hemodynamics describes the physical principles governing blood flow through the cardiovascular system.

Blood flow through a vessel is driven by the pressure gradient and opposed by resistance. The fundamental relationship, analogous to Ohm's law for electrical circuits, is: flow equals pressure gradient divided by resistance. For the systemic circulation, this becomes: cardiac output equals mean arterial pressure minus right atrial pressure (approximately zero), divided by total peripheral resistance. Rearranged, this is the relationship MAP equals CO times TPR.

Poiseuille's law describes laminar flow through a cylindrical tube: flow equals pi times pressure gradient times radius to the fourth power, divided by 8 times viscosity times length. Alternatively, resistance equals 8 times viscosity times length divided by pi times radius to the fourth power. The critical insight is that resistance is inversely proportional to the fourth power of the radius. Doubling the radius decreases resistance sixteen-fold and increases flow sixteen-fold for the same pressure gradient. This explains why small changes in arteriolar diameter have large effects on blood flow.

The factors in Poiseuille's equation have different practical significance. Radius is the most important factor and is actively regulated by vascular smooth muscle. Length is anatomically fixed. Viscosity is determined primarily by hematocrit; polycythemia (increased red blood cell mass) increases viscosity and resistance, while anemia decreases them. The pressure gradient is determined by cardiac output and vascular resistance.

Resistance vessels in series add directly (total resistance equals R1 plus R2 plus R3), while vessels in parallel add as reciprocals (1/R_total equals 1/R1 plus 1/R2 plus 1/R3). Because the systemic circulation has many vascular beds in parallel, total peripheral resistance is less than the resistance of any individual bed, and closing off one bed (such as by vasoconstriction) increases total resistance only modestly.

Blood flow velocity is related to flow and cross-sectional area: velocity equals flow divided by area. Although the aorta has a smaller cross-sectional area than any of the individual smaller vessels it feeds, the total cross-sectional area increases dramatically as blood distributes into millions of capillaries. Consequently, velocity is highest in the aorta (approximately 40 centimeters per second), decreases through the arterial system, reaches its minimum in the capillaries (less than 1 millimeter per second—optimizing time for exchange), then increases again as blood collects into veins.

Laminar flow is smooth, streamlined flow with a parabolic velocity profile—fastest in the center, slowest at the walls. Turbulent flow is chaotic, with eddies and whorls, occurring when the Reynolds number exceeds approximately 2000. Turbulence occurs at high flow velocities, at bifurcations, and distal to stenoses; it produces murmurs and bruits audible with a stethoscope.

<image>Panel A: Flow equation (Q = delta P/R) and Poiseuille's law emphasizing resistance proportional to 1/r to the fourth, with illustration showing two vessels demonstrating 16-fold resistance and flow difference when radius doubles. Panel B: Resistances in series (adding directly) and parallel (adding as reciprocals) with example showing parallel vascular beds resulting in lower total resistance. Panel C: Graph of cross-sectional area and velocity through circulation showing CSA lowest in aorta, peaks in capillaries, and velocity inverse (highest aorta, lowest capillaries for exchange). Panel D: Laminar flow (smooth parabolic profile) versus turbulent flow (chaotic eddies) with Reynolds number formula and threshold ~2000, showing clinical turbulence examples (stenotic valve murmur, arterial stenosis bruit).</image>

Clinical Applications

Understanding vascular physiology illuminates the pathophysiology and treatment of common vascular disorders.

Atherosclerosis is the progressive disease of arterial walls characterized by lipid accumulation, inflammation, and plaque formation. It begins with endothelial injury or dysfunction, allowing low-density lipoprotein (LDL) to enter the vessel wall, where it is oxidized and triggers an inflammatory response. Macrophages ingest oxidized LDL, becoming foam cells. Smooth muscle cells migrate from the media to the intima and proliferate, contributing to plaque growth. The mature atherosclerotic plaque has a lipid-rich necrotic core covered by a fibrous cap. Plaque rupture exposes thrombogenic material to blood, triggering thrombosis that can occlude the vessel or embolize distally. Risk factors include hyperlipidemia, hypertension, smoking, diabetes, and family history.

Peripheral arterial disease (PAD) results from atherosclerosis of the limb arteries, typically affecting the lower extremities. Reduced blood flow causes intermittent claudication—pain with walking that is relieved by rest, analogous to angina in the heart. The ankle-brachial index (ABI), the ratio of ankle to arm systolic blood pressure, is normally greater than 1.0; values less than 0.9 indicate PAD. Severe PAD leads to critical limb ischemia with rest pain, ulceration, and gangrene.

Raynaud phenomenon is episodic vasospasm of digital arteries triggered by cold exposure or emotional stress. The characteristic color sequence is white (ischemia from vasospasm), blue (cyanosis from deoxygenation of static blood), and red (reactive hyperemia upon reperfusion). Primary Raynaud phenomenon is idiopathic and benign. Secondary Raynaud phenomenon is associated with connective tissue diseases (especially scleroderma), thoracic outlet syndrome, and other conditions.

Vasodilator therapy is fundamental in treating cardiovascular disease. Nitrates (nitroglycerin, isosorbide) provide exogenous NO, causing venodilation (reducing preload) and arterial dilation (reducing afterload); they are used in angina and heart failure. Calcium channel blockers (amlodipine, diltiazem) block L-type calcium channels in vascular smooth muscle, reducing calcium entry and causing vasodilation; they are used in hypertension and angina. ACE inhibitors reduce angiotensin II production, causing vasodilation and reducing aldosterone; they are first-line agents in hypertension and heart failure.

<image>Panel A: Atherosclerosis progression from normal artery to fatty streak (foam cells) to fibrous plaque (lipid core, fibrous cap) to complicated plaque (rupture, thrombosis) with risk factors listed. Panel B: Peripheral arterial disease showing leg with narrowed arteries, claudication pattern (pain with walking, relief at rest), and ABI measurement with Doppler at ankle and arm. Panel C: Raynaud phenomenon showing hands with white-blue-red color sequence and triggers (cold, stress). Panel D: Vasodilator drugs with mechanisms: nitrates (NO donors, cGMP, venous), calcium channel blockers (block L-type Ca2+ channels, arterial), and ACE inhibitors (decreased angiotensin II and aldosterone) with indications listed.</image>

Summary

Blood vessels are structurally specialized for their functions: elastic arteries serve as pressure reservoirs (Windkessel effect), muscular arteries distribute blood, arterioles are the primary resistance vessels, capillaries permit exchange, and veins provide capacitance.

Vascular smooth muscle contracts through a calcium-calmodulin-MLCK pathway. Increased intracellular calcium activates MLCK, which phosphorylates myosin light chains, enabling cross-bridge cycling and contraction.

Local regulation matches flow to metabolic demand through metabolic factors (O2, CO2, pH, adenosine) and endothelial factors (NO causes vasodilation via cGMP, endothelin causes vasoconstriction). The myogenic response causes contraction in response to stretch.

Autoregulation maintains constant blood flow over a range of perfusion pressures through myogenic and metabolic mechanisms. Reactive hyperemia is increased flow after occlusion; active hyperemia is increased flow during increased metabolic activity.

Neural control is primarily sympathetic: norepinephrine acts on alpha-1 receptors to cause vasoconstriction. Sympathetic tone provides a baseline level of vasoconstriction that can be modulated up or down. Parasympathetic vasodilation is limited to specific beds.

Hemodynamics: Flow equals pressure gradient divided by resistance. Poiseuille's law shows that resistance is inversely proportional to radius to the fourth power, making vessel diameter the critical determinant of resistance.

Key Terms

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