Chapter 19: Blood Vessels – Structure, Function, and Physiology

Blood Vessels: Structure and Function

Overview of the Vascular System

The vascular system is a closed network of blood vessels that begins and ends at the heart. It is responsible for transporting blood throughout the body, delivering oxygen and nutrients, and removing waste products. Blood vessels are dynamic, capable of pulsation, constriction, relaxation, and proliferation.

• Arteries: Carry blood away from the heart.

• Capillaries: Facilitate exchange between blood and tissues.

• Veins: Return blood to the heart.

Only capillaries directly serve cellular needs by allowing exchange of materials.

Structure of Vessel Walls

General Structure

Except for capillaries, all blood vessels have three layers (tunics) surrounding a central lumen:

• Tunica intima: Innermost layer; consists of endothelium providing a smooth, friction-reducing lining.

• Tunica media: Middle layer; mainly smooth muscle and elastic fibers. Regulated by sympathetic vasomotor fibers, it controls vasoconstriction and vasodilation, thus influencing blood flow and pressure.

• Tunica adventitia (externa): Outermost layer; composed of collagen fibers that protect and anchor the vessel.

Capillaries have only a thin tunica intima, ideal for exchange.

Arterial System

Types of Arteries

• Elastic arteries: Largest, thick-walled arteries near the heart (e.g., aorta). High elastin content allows them to withstand pressure fluctuations and maintain continuous blood flow. Their expansion and recoil create the arterial pulse.

• Muscular arteries: Distribute blood to specific organs. They have the thickest tunica media, more smooth muscle, and are more active in vasoconstriction.

• Arterioles: Smallest arteries, primarily smooth muscle. Their diameter regulates blood flow into capillary beds in response to neural and chemical stimuli.

Capillaries: Structure and Function

Types of Capillaries

• Continuous capillaries: Uninterrupted endothelial lining; abundant in skin and muscles.

• Fenestrated capillaries: Endothelial cells have pores (fenestrations) for increased permeability; found in areas of active absorption (e.g., intestines, endocrine organs, kidneys).

• Sinusoids: Highly modified, leaky capillaries with large lumens; found in liver, bone marrow, lymphoid tissues, and some endocrine glands. Allow passage of large molecules and cells.

Capillaries form networks called capillary beds, consisting of:

• Vascular shunt (metarteriole-thoroughfare channel): Directly connects arteriole and venule.

• True capillaries: Branch off the shunt; site of exchange with tissues.

Precapillary sphincters regulate blood flow into true capillaries.

Venous System

Structure and Function

• Venules: Formed by the union of capillaries; very porous, allowing fluid and WBC movement.

• Veins: Formed by merging venules; have thinner walls and larger lumens than arteries. Serve as blood reservoirs, containing up to 65% of blood volume.

• Valves: Prevent backflow, especially in limb veins.

Vascular Anastomoses

Anastomoses are interconnections between blood vessels, providing alternate pathways for blood flow. They are common in joints and abdominal organs. Venous anastomoses are more common than arterial, making venous occlusion less dangerous.

Physiology of Circulation

Key Concepts

• Blood flow: Volume of blood moving through a vessel or organ per unit time; for the entire system, it equals cardiac output.

• Blood pressure: Force per unit area exerted by blood on vessel walls; measured in mm Hg.

• Peripheral resistance: Opposition to flow due to friction; determined by blood viscosity, vessel length, and vessel diameter.

Vessel diameter is the most important factor affecting resistance and blood flow.

Maintenance of Systemic Blood Pressure

Blood Pressure Regulation

• Blood flows from high to low pressure areas, driven by the heart's pumping action.

• Blood pressure is highest in the aorta and declines to zero in the right atrium.

• Systolic pressure: Peak pressure during ventricular contraction (about 120 mm Hg).

• Diastolic pressure: Lowest pressure during ventricular relaxation.

• Pulse pressure: Difference between systolic and diastolic pressures.

Key Equations:

• Blood pressure: $\text{Blood pressure} = \text{Cardiac output} \times \text{Peripheral resistance}$

• Cardiac output: $\text{Cardiac output (ml/min)} = \text{Stroke volume (ml/beat)} \times \text{Heart rate (beats/min)}$

• Resistance and radius: $\text{Resistance} \propto \frac{1}{r^4}$

Small changes in vessel radius have large effects on resistance and pressure.

Control of Blood Pressure

Regulatory Mechanisms

• Neural controls: Adjust blood distribution and maintain systemic pressure via sympathetic vasoconstriction and baroreceptor reflexes.

• Chemical controls: Hormones like norepinephrine, epinephrine, and ADH affect vessel tone and blood volume.

• Renal controls: Kidneys regulate blood pressure by controlling water retention and releasing renin, which leads to angiotensin formation (a vasoconstrictor).

Baroreceptors and chemoreceptors in major arteries detect pressure and chemical changes, triggering appropriate responses.

Variations from Normal Blood Pressure

Hypotension

• Defined as systolic pressure below 100 mm Hg.

• Often benign, but chronic hypotension may indicate poor nutrition or anemia.

• Orthostatic hypotension: Temporary drop in blood pressure upon standing.

Hypertension

• Defined as sustained pressure of 140/90 mm Hg or higher.

• "Silent killer" due to asymptomatic early stages; leads to heart failure, vascular disease, renal failure, and stroke.

• Primary hypertension (90%): No identifiable cause; risk factors include diet, obesity, heredity, and stress.

• Secondary hypertension (10%): Due to identifiable disorders (e.g., kidney disease, endocrine disorders).

Physiology of Blood Flow

Blood velocity is fastest in large arteries and slowest in capillaries, due to the inverse relationship with total cross-sectional area. This allows efficient exchange in capillaries.

Autoregulation of Localized Blood Flow

Autoregulation is the automatic adjustment of blood flow to tissues based on their needs, primarily by altering arteriole diameter. Chemical (e.g., low oxygen, high CO2) and physical (e.g., stretch) factors trigger vasodilation or vasoconstriction. Long-term autoregulation involves growth of new vessels (angiogenesis).

Blood Flow in Special Areas

• Skeletal muscle: Blood flow increases dramatically during activity (exercise hyperemia) due to local metabolic needs.

• Brain: Maintains constant flow; highly sensitive to pH and CO2 changes.

• Lungs: Low pressure system; low oxygen causes vasoconstriction (opposite of systemic circulation).

• Heart: Blood flow increases during relaxation; high oxygen extraction requires increased flow during activity.

Blood Flow Through Capillaries

Capillary Exchange

• Blood flow is slow and intermittent (vasomotion), controlled by precapillary sphincters.

• Fluid movement is governed by hydrostatic and osmotic pressures.

Key Concepts:

• Hydrostatic pressure: Forces fluid out at arterial end (filtration).

• Osmotic pressure: Draws fluid in at venous end (reabsorption).

• Net fluid loss to tissues is returned by the lymphatic system.

Forms of Circulatory Shock

• Hypovolemic shock: Due to large-scale blood loss; managed by fluid replacement.

• Vascular shock: Extreme vasodilation (e.g., due to septicemia) causes poor circulation.

• Cardiogenic shock: Heart pump failure, often from myocardial infarction.

Blood Flow in Circulatory Circuits

Pulmonary Circuit

• Pulmonary trunk → right/left pulmonary arteries → lobar arteries → pulmonary capillaries → pulmonary veins

Systemic Circuit

• Arterial: Aorta and its branches supply all body regions.

• Venous: Peripheral veins merge into superior/inferior vena cava, returning blood to the heart.

Hepatic Portal System

• Capillary beds of intestines → mesenteric veins → hepatic portal vein (also receives splenic vein) → liver (sinusoids) → hepatic veins → inferior vena cava

Unique Aspects of Fetal Circulation

• Foramen ovale: Connects fetal atria, bypassing pulmonary circuit.

• Ductus arteriosus: Connects pulmonary trunk and aorta.

• Ductus venosus: Bypasses fetal liver.

• Umbilical vein and arteries: Exchange blood with placenta.

Effects of Exercise on the Cardiovascular System

• Heart becomes larger and more efficient.

• Exercise helps clear fatty deposits from vessel walls, reducing risk of atherosclerosis and coronary heart disease.