Blood Vessels and Circulation: Structure, Function, and Regulation

Chapter 21: Blood Vessels and Circulation

Learning Objectives

• Understand the classes of blood vessels and their locations in the body.

• Describe the structure of blood vessels and how structure relates to function.

• Explain the physiology of circulation, including the effects of peripheral resistance, cardiac output, and blood volume on blood pressure and flow.

• Understand how the CNS, hormones, and other chemicals regulate blood flow and pressure.

• Describe the systemic circulatory routes for both arteries and veins.

Key Terms

• Artery

• Arteriole

• Capillary

• Venule

• Vein

• Fenestrated capillary

• Continuous capillary

• Precapillary sphincter

• Tunica intima

• Tunica media

• Tunica externa

• Valve

• Blood pressure

• Peripheral resistance

• Cardiac Center

• Vasomotor Center

• Vasoconstrictors

• Vasodilators

• Baroreceptor

• Chemoreceptor

• Pulmonary Circuit

• Systemic Circuit

• Superficial veins

• Deep veins

Overview of Blood Vessels

General Classes of Blood Vessels

The circulatory system contains five main types of blood vessels, each with distinct structure and function:

1. Arteries – Carry blood away from the heart under high pressure.

2. Arterioles – Small branches of arteries that regulate blood flow into capillaries.

3. Capillaries – Microscopic vessels where exchange of gases, nutrients, and wastes occurs.

4. Venules – Collect blood from capillaries and channel it into veins.

5. Veins – Return blood to the heart under lower pressure; often contain valves to prevent backflow.

Distribution of Blood in the Body

Blood is distributed unevenly among the different types of vessels:

• Large veins and venous networks contain the majority of blood volume (~64%).

• Arteries and arterioles contain about 13% of blood volume.

• Capillaries contain about 7% of blood volume.

• The heart contains about 7% of blood volume.

Types and Functions of Blood Vessels

Structural and Functional Differences

• Elastic Arteries – Large, conducting arteries with abundant elastic fibers; help maintain blood pressure during ventricular diastole.

• Muscular Arteries – Medium-sized arteries with more smooth muscle; distribute blood to specific organs.

• Arterioles – Smallest arteries; regulate blood flow into capillary beds via vasoconstriction and vasodilation.

• Capillaries – Continuous (complete endothelium) and Fenestrated (pores for increased permeability); site of exchange.

• Venules – Collect blood from capillaries; thin walls.

• Medium and Large Veins – Return blood to the heart; contain valves to prevent backflow.

Capillary Function

Precapillary Sphincters

Precapillary sphincters regulate blood flow into capillary beds:

• High tissue demand for oxygen: Sphincters open, allowing increased blood flow.

• Low tissue demand for oxygen: Sphincters close, reducing blood flow.

Structure of Blood Vessels

Three Distinct Layers

1. Tunica intima – Innermost layer; consists of endothelium and subendothelial connective tissue.

2. Tunica media – Middle layer; composed of smooth muscle and elastic fibers, responsible for vasoconstriction and vasodilation.

3. Tunica externa – Outermost layer; connective tissue providing support and protection.

Special Feature of Veins: Valves

Veins, especially in the limbs, contain valves that prevent the backflow of blood and aid venous return to the heart.

• Valve closed: Prevents backflow.

• Valve opened: Allows blood to move toward the heart.

Physiology of Circulation

Blood Flow

Blood flows due to pressure gradients created by the pumping action of the heart.

• Flow is directly proportional to the pressure gradient: $F \propto P$

• Flow is inversely proportional to resistance: $F \propto \frac{1}{R}$

• Relationship: $F = \frac{\Delta P}{R}$

Blood Pressure

• Arterial blood pressure: The force exerted by blood on arterial walls; measured as systolic/diastolic pressure.

• Venous blood pressure: Much lower than arterial pressure; aided by valves and skeletal muscle contraction.

Measuring Arterial Blood Pressure

• Systolic pressure: Pressure during heart contraction.

• Diastolic pressure: Pressure during heart relaxation.

• Pulse pressure: Difference between systolic and diastolic pressure.

What is Pulse?

The pulse is the rhythmic expansion and recoil of an artery as blood is ejected from the heart.

Factors Affecting Blood Pressure

Peripheral Resistance (PR)

Opposition to blood flow caused by friction of blood against vessel walls.

Factor	Effect on PR	Effect on BP
Vasoconstriction (diameter decreases)	PR increases	BP increases
Vasodilation (diameter increases)	PR decreases	BP decreases
Vessel length increases	PR increases	BP increases
Vessel length decreases	PR decreases	BP decreases
Viscosity increases	PR increases	BP increases
Viscosity decreases	PR decreases	BP decreases
Cardiac Output (CO) increases	-	BP increases
Cardiac Output (CO) decreases	-	BP decreases
Blood volume increases	-	BP increases
Blood volume decreases	-	BP decreases
Blood flow increases	-	BP increases
Blood flow decreases	-	BP decreases

Vessel Properties in Circulation

• Vessel diameter: Largest in aorta and veins, smallest in capillaries.

• Total cross-sectional area: Greatest in capillaries, facilitating exchange.

• Average blood pressure: Highest in arteries, lowest in veins.

• Velocity of blood flow: Fastest in arteries, slowest in capillaries.

Other Factors Regulating Blood Pressure

Autoregulation

Local regulation within capillary beds in response to tissue needs.

• Local Vasoconstrictors: Prostaglandins, thromboxanes, activated platelets, WBCs, vessel damage.

• Local Vasodilators: Increased CO2, decreased O2, increased K+ or H+, increased temperature.

Central Nervous System Regulation

• Cardiac Center (Medulla Oblongata):

  • Cardioacceleratory center: Increases cardiac output (CO).

  • Cardioinhibitory center: Decreases cardiac output (CO).

• Vasomotor Center:

  • Vasoconstriction: Increases PR.

  • Vasodilation: Decreases PR.

Chemical and Hormonal Regulation

• Vasoconstrictors: Norepinephrine, Angiotensin II, Antidiuretic hormone.

• Vasodilators: Nitric oxide (NO), Atrial natriuretic peptide.

Regulation of Blood Pressure: Baroreceptors and Chemoreceptors

Baroreceptor Reflex

Baroreceptors detect changes in blood pressure and initiate reflex responses to restore homeostasis.

• Increased BP stimulates baroreceptors, leading to decreased heart rate and vasodilation.

• Decreased BP reduces baroreceptor activity, leading to increased heart rate and vasoconstriction.

Chemoreceptor Reflex

Chemoreceptors respond to changes in blood chemistry (O2, CO2, pH) and adjust cardiovascular and respiratory activity to maintain homeostasis.

Circulatory Pathways

Pulmonary Circuit

• Pulmonary arteries: Carry deoxygenated blood from the right ventricle to the lungs.

• Pulmonary veins: Return oxygenated blood from the lungs to the left atrium.

Systemic Circuit

• Arteries: Carry oxygenated blood from the left ventricle to body tissues.

• Veins: Return deoxygenated blood from tissues to the right atrium.

Assignments and Review Questions

• Compare the structural differences among arteries, arterioles, capillaries, venules, and veins.

• Explain the physiological significance of capillary exchange.

• Describe the mechanisms that regulate blood pressure and cardiac output.

• Discuss the role of baroreceptors and chemoreceptors in cardiovascular regulation.

• Summarize the differences between pulmonary and systemic circulation.

Additional info:

• For more detailed study, refer to textbook pages 724-728, 742-758.

• Special circulations (brain, hepatic portal, fetal) are covered in laboratory sessions.