Chapter 15

15.1 The Blood Vessels

Structure and Function of Blood Vessels

The cardiovascular system consists of various types of blood vessels that transport blood throughout the body. Each vessel type has unique structural and functional characteristics.

• Arteries and Arterioles: Carry blood away from the heart. Arteries have thick, elastic walls to withstand high pressure; arterioles regulate blood flow to tissues.

• Veins and Venules: Return blood to the heart. Veins have thinner walls and valves to prevent backflow.

• Capillaries: Sites of exchange between blood and tissues. Their thin walls facilitate diffusion.

Blood Vessel Wall Layers:

• Tunica intima: Inner endothelial layer.

• Tunica media: Middle layer of smooth muscle and elastic fibers.

• Tunica externa (adventitia): Outer connective tissue layer.

Vascular Smooth Muscle: Contraction causes vasoconstriction (narrowing of vessel diameter); relaxation causes vasodilation (increase in diameter). Influenced by neural, hormonal, and local factors.

Metarterioles: Vessels between arterioles and capillaries; regulate blood flow into capillary beds.

Exchange in Capillaries

Capillaries are specialized for exchange of gases, nutrients, and waste products. Their structure (thin walls, small diameter) supports this function. Pericytes are contractile cells that regulate capillary permeability.

Comparison of Arteries and Veins

Feature	Arteries	Veins
Wall Thickness	Thick	Thin
Pressure	High	Low
Valves	Absent	Present
Direction of Flow	Away from heart	Toward heart

Angiogenesis

Angiogenesis is the formation of new blood vessels. It is crucial during growth, wound healing, and in response to tissue demands. Excessive angiogenesis can contribute to cancer progression; inhibition may be therapeutic.

• In children: supports growth and development.

• In adults: occurs in response to injury or hypoxia.

In coronary artery disease, arteries may become blocked; drugs that stimulate angiogenesis can help restore blood flow.

15.2 Blood Pressure

Blood Pressure Fundamentals

Blood pressure is the force exerted by blood on vessel walls. It is highest in arteries and lowest in veins.

• Systolic pressure: Pressure during heart contraction.

• Diastolic pressure: Pressure during heart relaxation.

• Pulse pressure: Difference between systolic and diastolic pressures.

Pulse: A pressure wave created by ventricular contraction; measured at arteries.

Venous return: Low-pressure venous blood returns to the heart aided by skeletal muscle contraction and valves.

Mean Arterial Pressure (MAP)

MAP is the average pressure in arteries during one cardiac cycle. It drives blood flow.

• Formula: $MAP = ext{Diastolic Pressure} + rac{1}{3}( ext{Systolic Pressure} - ext{Diastolic Pressure})$

• MAP is affected by cardiac output and peripheral resistance.

• If blood pressure is too low, tissues may not receive enough oxygen; if too high, risk of vessel damage increases.

Measuring Blood Pressure

Blood pressure is measured using a sphygmomanometer and by listening for Korotkoff sounds at the brachial artery.

• Normal reading: 120/80 mm Hg (systolic/diastolic).

Cardiac Output and Resistance

• Cardiac Output (CO): Volume of blood pumped by the heart per minute.

• Peripheral Resistance (PR): Resistance to blood flow in vessels.

• Relationship: $MAP = CO imes PR$

If peripheral resistance increases, MAP increases.

Blood Volume and Pressure

• Decreased blood volume leads to decreased blood pressure.

• Kidneys regulate blood volume by reabsorbing water.

• Cardiovascular system compensates for low blood volume by vasoconstriction and increased heart rate.

15.3 Resistance in the Arterioles

Factors Affecting Resistance

Arterioles control resistance and regulate blood flow to tissues.

• Relationship: $R ext{ (resistance)} hicksim rac{1}{r^4}$ (where $r$ is vessel radius)

• Local, autonomic, and hormonal mechanisms adjust arteriolar resistance.

Myogenic Autoregulation

When blood pressure increases, arterioles constrict to maintain constant flow (myogenic response).

• Mechanism: Stretch of smooth muscle triggers contraction.

Paracrine and Sympathetic Control

• Precapillary sphincters: Regulate blood flow into capillaries.

• Reactive hyperemia: Increased blood flow after a period of occlusion.

• Sympathetic control: Norepinephrine acts on alpha-receptors (vasoconstriction); epinephrine acts on beta-receptors (vasodilation).

Receptor Type	Effect
Alpha-adrenergic	Vasoconstriction
Beta-adrenergic	Vasodilation

15.4 Distribution of Blood to the Tissues

Blood Flow Regulation

Not all tissues receive equal blood flow at all times. Distribution depends on metabolic needs and is regulated by arterioles.

• At rest, brain and heart receive priority.

• Total flow through all arterioles equals cardiac output.

• Redistribution occurs when flow decreases through one set of arterioles.

15.5 Regulation of Cardiovascular Function

Neural Control of Blood Pressure

The cardiovascular control center in the brainstem regulates heart rate and vessel diameter.

• Baroreceptors: Located in carotid arteries and aortic arch; sense changes in blood pressure.

• Baroreceptor reflex restores homeostasis after pressure changes.

• Decrease in blood pressure increases sympathetic activity and decreases parasympathetic activity.

• Sympathetic activity increases heart rate, contractility, and vasoconstriction; parasympathetic activity decreases heart rate.

Orthostatic Hypotension

Orthostatic hypotension is a sudden drop in blood pressure upon standing, triggering the baroreceptor reflex to restore normal pressure.

15.6 Exchange at the Capillaries

Capillary Density and Structure

Capillary density varies by tissue type; high in muscles and glands, low in connective tissue.

• Continuous capillaries: Tight junctions, found in muscle and brain.

• Fenestrated capillaries: Pores for rapid exchange, found in kidneys and intestines.

• Some tissues have modified vessels (sinusoids) for specialized exchange.

Velocity of Blood Flow

Velocity is inversely related to total cross-sectional area. Capillaries have the largest area and slowest flow, facilitating exchange.

Types of Capillary Exchange

• Diffusion: Movement of small molecules down concentration gradients.

• Transcytosis: Transport of larger molecules via vesicles.

• Bulk flow: Movement of fluid driven by pressure differences.

Filtration and Reabsorption

• Filtration: Fluid moves out of capillaries due to hydrostatic pressure.

• Reabsorption: Fluid moves into capillaries due to osmotic pressure.

• Colloid osmotic pressure: Created by plasma proteins; draws water into capillaries.

• Hydrostatic pressure decreases along capillary length.

15.7 The Lymphatic System

Structure and Function

The lymphatic system returns excess interstitial fluid to the bloodstream and is involved in immune defense.

• Lymph: Fluid collected from tissues.

• Lymph nodes: Filter lymph and house immune cells.

• Lymph rejoins blood at the subclavian veins.

• Bulk flow moves fluid, proteins, and bacteria into lymph capillaries.

• Lymph flow is aided by muscle contraction; no central pump.

Edema

Edema is swelling caused by excess fluid in tissues, resulting from increased capillary filtration or decreased lymphatic drainage.

15.8 Cardiovascular Disease

Risk Factors

• Uncontrollable: Genetics, age, gender.

• Controllable: Diet, exercise, smoking, obesity.

• Elevated blood lipids and diabetes increase risk.

Atherosclerosis

Atherosclerosis is an inflammatory process leading to plaque formation in arteries. Plaques can be stable or vulnerable; rupture may cause myocardial infarction or stroke.

• Process: LDL cholesterol accumulates, immune cells infiltrate, plaque forms.

Hypertension

Hypertension is chronically elevated blood pressure (systolic > 140 mm Hg or diastolic > 90 mm Hg). It increases risk for cardiovascular disease.

• Essential (primary) hypertension: No identifiable cause.

• Secondary hypertension: Due to underlying condition.

• High blood pressure strains the heart and vessels.

• Common treatments: lifestyle changes, antihypertensive drugs.