Chapter 21: Blood Vessels and Hemodynamics

1. Overview

Blood vessels are dynamic and functional components of the cardiovascular system, responsible for regulating the distribution, pressure, and exchange of blood between the heart and tissues. They are not merely passive conduits but play active roles in maintaining homeostasis.

• Dynamic and functional components: Blood vessels adjust diameter and permeability in response to physiological needs.

• Key functions: Distribution, pressure regulation, and exchange of substances.

2. Vessel Structure and Function

Blood vessels are classified by their structure and function, with each type playing a distinct role in circulation.

Types of Vessels

Layers of Blood Vessels (Tunics)

1. Tunica intima (interna):

   • Endothelium (continuous with heart lining)

   • Involved in vessel tone and permeability

2. Tunica media:

   • Smooth muscle – controls vessel diameter and blood pressure

3. Tunica externa:

   • Connective tissue – anchors vessels

Arterial Types

• Elastic (Conducting) Arteries: Large (e.g., aorta); high elasticity for pressure smoothing during systole and diastole.

• Muscular (Distributing) Arteries: Medium-sized; distribute blood to organs; more smooth muscle for vasoconstriction and vasodilation.

Anastomosis

• Connection between vessels supplying the same tissue.

• Provides collateral circulation—alternate route if blockage occurs.

3. Capillaries and Exchange

Capillaries are the primary sites for exchange of gases, nutrients, and wastes between blood and tissues. Their structure allows for efficient diffusion.

• Structure: Single layer of endothelium + basement membrane; allows easy diffusion.

• Precapillary Sphincters: Control blood entry into capillary beds; adjust based on tissue demand.

4. Venous System

Veins return blood to the heart under low pressure and have structural adaptations to prevent backflow and aid venous return.

• Low pressure: Thinner walls, contain valves to prevent backflow.

• Venous return aided by:

  • Skeletal muscle pump: Contraction pushes blood.

  • Respiratory pump: Pressure changes during breathing.

• Venous reserve: Veins hold ~64% of total blood volume.

5. Fluid Exchange – Starling Forces

Fluid movement across capillary walls is governed by Starling forces, which balance filtration and reabsorption.

• Net Filtration Pressure (NFP):

  • Arterial end: mmHg (filtration)

  • Venous end: mmHg (reabsorption)

  • ~85% reabsorbed; remainder enters lymphatic system.

6. Pressure, Flow, and Resistance

Blood flow through vessels is determined by pressure and resistance, described by Ohm’s Law.

• Ohm’s Law Relationship:

• Blood Pressure (BP): Force exerted by blood on vessel wall.

• Resistance depends on:

  • Blood viscosity

  • Vessel length

  • Vessel radius (most important, adjustable)

• Halving diameter increases resistance 16-fold.

7. Autoregulation

Autoregulation maintains homeostasis in blood flow by adjusting vessel diameter and tone in response to local and systemic signals.

• Key Components:

  1. Vascular system – senses BP/flow changes.

  2. Nervous system – adjusts heart rate, vessel tone.

  3. Endocrine system – hormones (renin, ADH, ANP, etc.).

  4. Heart – modifies rate and contractility.

8. Circulatory Routes

Blood follows specific routes through the body, each with unique functions and adaptations.

• Pulmonary Circulation: Right heart → lungs → oxygenation → left heart.

• Systemic Circulation: Left heart → body → returns via veins.

• Portal Circulation: GI tract → liver (via hepatic portal vein) → systemic.

• Fetal Circulation: Unique temporary key structures:

  • Ductus venosus, foramen ovale, ductus arteriosus.

  • These close after birth, forming adult ligaments.

9. Blood Pressure Regulation and Disorders

Blood pressure is tightly regulated to ensure adequate tissue perfusion. Disorders can arise from chronic elevation or reduction in BP.

• Normal BP Regulation: Determined by cardiac output × resistance.

• Hypertension (HTN):

  • Chronic elevation of systolic and/or diastolic BP.

  • Leads to atherosclerosis, stroke, kidney disease.

• Low BP: Insufficient perfusion; can lead to shock if severe.

10. Shock and Homeostasis

Shock is a life-threatening condition where blood flow is insufficient to meet tissue needs. Compensation mechanisms attempt to restore perfusion.

• Compensation:

  • ↑ Heart rate, ↑ contractility

  • Vasoconstriction in nonessential organs

  • Hormonal responses (ADH, renin, epinephrine)

• If compensation fails → organ failure → death.

11. Key Circulatory Pathways to Know

Major arteries and veins to identify for anatomical study:

• Arteries: Aorta, carotids, subclavian, brachial, radial/ulnar, iliac, femoral, popliteal

• Veins: Vena cava, jugular, subclavian, iliac, femoral, hepatic portal

12. Review Questions

1. Describe how elastic arteries help maintain blood flow during diastole.

2. Explain Starling forces and how they regulate capillary exchange.

3. What mechanisms aid venous return to the heart?

4. Compare the actions of RAA system and ANP on blood pressure.

5. Outline the fetal circulation and the changes that occur at birth.