Chapter 21: Blood Vessels and Circulation

Chapter 21: Blood Vessels and Circulation

Learning Outcomes

• 21-1 Distinguish among the types of blood vessels based on their structure and function.

• 21-2 Describe the factors that influence blood pressure and explain how and where fluid and dissolved materials enter and leave the cardiovascular system.

• 21-3 Describe the control mechanisms that regulate blood flow and pressure in tissues, and explain how the cardiac, vasomotor, and respiratory centers coordinate to control blood flow through the tissues.

• 21-4 Identify the principal blood vessels and functions of special circulation to the brain, heart, and lungs, and explain the cardiovascular system’s homeostatic response to exercise and hemorrhaging.

• 21-5 Describe the pulmonary and systemic circuits of the cardiovascular system.

• 21-6 Identify the major arteries and veins of the pulmonary circuit.

• 21-7 Identify the major arteries and veins of the systemic circuit.

• 21-8 Identify the differences between fetal and adult circulation patterns, and describe the changes in patterns of blood flow that occur at birth.

• 21-9 Discuss the effects of aging on the cardiovascular system, and give examples of interactions between the cardiovascular system and other organ systems.

Introduction to Blood Vessels and Circulation

• Blood Vessels: Classified by size and histological organization; instrumental in overall cardiovascular regulation.

• The largest blood vessels attach to the heart:

  • Pulmonary trunk: Carries blood from the right ventricle to pulmonary circulation.

  • Aorta: Carries blood from the left ventricle to systemic circulation.

Structure and Function of Blood Vessels

General Structure of Blood Vessels

• Histological Layers:

  • Tunica externa: Outer layer of connective tissue.

  • Tunica media: Middle layer composed of smooth muscle; allows for contraction and dilation.

  • Tunica intima: Inner layer consisting of endothelial cells.

• Comparison:

  • Arteries: Thicker walls, smaller and round lumen, higher blood pressure.

  • Veins: Thinner walls, larger and irregular lumen, contain valves to prevent backflow.

Types of Blood Vessels

• Arteries:

  • Elastic arteries:

  • Large vessels (e.g., pulmonary trunk, aorta)

  • Many elastic fibers and few muscle cells in the tunica media

  • Muscular arteries:

  • Medium-sized arteries with a higher proportion of muscle cells in the tunica media

  • Arterioles:

  • Small vessels

  • Little or no tunica externa and thin tunica media

• Veins:

  • Venules: Very small veins that collect blood from capillaries

  • Medium-sized veins: Thin media and few muscle cells

  • Large veins: Have all three tunica layers, thick tunica externa, thin media

  • Valves: Folds of tunica intima that prevent backflow; when weakened, may lead to varicose veins or hemorrhoids.

Capillaries

• Capillaries: Smallest vessels with thin walls that form microscopic networks permeating active tissues.

  • Structure: Endothelial tube inside a thin basement membrane; no tunica media or externa; diameter similar to a red blood cell.

  • Functions:

  • Materials diffuse between blood and interstitial fluid.

• Types of Capillaries:

  • Continuous capillaries:

  • Found in all tissues except epithelia and cartilage; complete endothelial lining allowing diffusion of water, small solutes, and lipid-soluble materials; block blood cells and plasma proteins.

  • Fenestrated capillaries:

  • Have pores permitting rapid exchange of water and larger solutes; found in the choroid plexus, endocrine organs, kidneys, and intestinal tract.

  • Sinusoids (sinusoidal capillaries):

  • Gaps between adjacent endothelial cells for free exchange of water and large plasma proteins; found in liver, spleen, bone marrow, and endocrine organs.

• Capillary beds (capillary plexus): Connect one arteriole and one venule.

  • Precapillary sphincter: Guards entrance to each capillary, opens and closes to regulate flow.

  • Thoroughfare channels: Direct connections between arterioles and venules.

• Angiogenesis: Formation of new blood vessels stimulated by vascular endothelial growth factor (VEGF); occurs in the embryo and response to hypoxia.

Pressure and Resistance in Blood Vessels

• Total Capillary Blood Flow: Equals cardiac output; determined by pressure (P) and resistance (R).

  • Pressure (P): Generated by the heart to overcome resistance.

  • Pressure Gradient (ΔP): Difference in pressure from one end of a vessel to another.

  • Flow (F): Proportional to pressure gradient divided by resistance: F = rac{ΔP}{R}.

• Circulatory Pressure: Must overcome total peripheral resistance.

  • Total Peripheral Resistance: Affected by vascular resistance, blood viscosity, turbulence.

• Vascular Resistance: Due to friction between blood and vessel walls; depends on vessel length (constant for adults) and vessel diameter (varies by vasodilation and vasoconstriction).

  • Resistance increases exponentially as vessel diameter decreases.

• Blood Viscosity: Resistance caused by molecules and suspended materials; whole blood viscosity is about four times that of water.

• Turbulence: Swirling action disturbing smooth flow; occurs in heart chambers and great vessels; atherosclerotic plaques increase turbulence.

• Arterial Blood Pressure:

  • Systolic Pressure: Peak arterial pressure during ventricular systole.

  • Diastolic Pressure: Minimum arterial pressure at end of ventricular diastole.

  • Normal blood pressure: 120/80 mmHg.

• Hypertension: Abnormally high blood pressure (≥ 140/90 mmHg).

• Hypotension: Abnormally low blood pressure (< 90/60 mmHg).

• Venous Pressure: Determines blood arriving at right atrium; low effective pressure and low resistance.

  • Assisted Return:

  • Skeletal muscular compression of veins.

  • Respiratory pump: Thoracic cavity expands during inhalation, decreasing venous pressure.

Capillary Exchange

• Vital for Homeostasis: Materials move across capillary walls by filtration (fluid moves from capillary to interstitial fluid) and reabsorption (fluid moves into capillary).

  • Filtration Dominates: At arterial end, while reabsorption predominates at venous end.

• Factors Affecting Filtration and Reabsorption:

  • Net Capillary Hydrostatic Pressure (CHP): Pressure pushes water and solutes out.

  • Net Capillary Colloid Osmotic Pressure (BCOP): Pulls water and solutes in; is the difference between blood colloid osmotic pressure (BCOP) and interstitial fluid colloid osmotic pressure (ICOP).

  • Net Filtration Pressure (NFP): Difference between net hydrostatic pressure and net osmotic pressure:
    ext{NFP} = ( ext{CHP} - ext{IHP}) - ( ext{BCOP} - ext{ICOP}).

Capillary Dynamics

• Influence of Hemorrhaging: Reduces CHP and NFP, increasing reabsorption of interstitial fluid.

• Dehydration: Increases BCOP, accelerating reabsorption.

• Fluid Dynamics:

  • If CHP rises or BCOP declines, fluid moves out of blood and builds up in tissues (edema).

  • Conditions like bruising (ICOP increases), starvation (BCOP declines), and heart failure (CHP increases) can affect fluid balance.

Blood Flow and Pressure in Tissues

• Tissue Perfusion: Blood flow through tissues, delivering O2 and nutrients while removing waste products like CO2.

• Affected By: Cardiac output, peripheral resistance, and blood pressure.

Cardiovascular Regulation

• Ensures Blood Flow Changes: Occur appropriately and without altering vital organ blood flow.

• Regulatory Mechanisms:

  • Autoregulation: Immediate, localized homeostatic adjustments.

  • Neural Mechanisms: Respond quickly to regional changes.

  • Endocrine Mechanisms: Control long-term changes.

• Autoregulation Mechanisms: Adjust blood flow by altering peripheral resistance and stimulating precapillary sphincters to constrict or dilate.

  • Local Vasoconstrictors: Reduce blood flow (e.g., endothelins from damaged cells).

  • Local Vasodilators: Increase blood flow (e.g., low O2, high CO2, histamine, nitric oxide).

Neural Mechanisms

• Cardiovascular Center: Located in the medulla oblongata; has cardioacceleratory and cardioinhibitory centers, which monitor blood pressure and chemical composition through baroreceptor and chemoreceptor reflexes.

  • Baroreceptors: Located in carotid and aortic sinuses; adjust cardiac output and resistance in response to blood pressure changes.

  • Chemoreceptors: Monitor pH, O2, and CO2, and coordinate cardiovascular and respiratory activities.

Endocrine Mechanisms

• Hormonal Influence: Short-term and long-term effects on cardiovascular regulation.

  • Epinephrine and Norepinephrine: Stimulate cardiac output and peripheral vasoconstriction.

  • Angiotensin II: Helps regulate blood pressure and fluid balance.

  • ADH: Increases blood pressure and reduces water loss at kidneys.

  • Erythropoietin (EPO): Stimulates red blood cell production and vasoconstriction in response to low blood pressure or oxygen saturation.

  • Natriuretic Peptides (ANP, BNP): Reduce blood volume and pressure in response to excessive cardiac stretching.

Cardiovascular Adaptation

• Collaboration of Blood, Heart, and Circulatory System: Respond to exercise, blood loss, and maintain homeostasis.

Special Regions and Blood Flow

• Brain:

  • High priority; has significant oxygen demand; vessels can dilate in response to peripheral constriction to maintain blood flow.

  • Cerebrovascular Accident (CVA): Stroke caused by blockage or rupture of cerebral arteries.

• Heart: Blood flow through coronary arteries increases with activity; lactic acid and low O2 levels cause dilation.

  • Epinephrine: Enhances dilation and increases heart activity.

• Lungs: Blood flow regulated by oxygen levels in alveoli; high oxygen promotes dilation, while low levels cause constriction to shunt blood to better-oxygenated areas.

Exercise Response

• Light Exercise: Increases circulation, venous return enhances due to muscle contractions; cardiac output rises proportionally with venous return (Frank-Starling principle).

• Heavy Exercise: Activates the sympathetic nervous system, increasing cardiac output and redirecting blood flow away from nonessential organs.

Blood Distribution Changes

• Exercise and Cardiovascular Disease: Regular moderate exercise reduces LDLs, slows plaque formation, and lowers heart attack risk significantly.

• Hemorrhaging Response: Adjusts cardiovascular system to maintain pressure and restore volume; includes short-term reflexes and long-term hormonal responses.

  • Short-term Elevation: Carotid and aortic reflexes increase cardiac output and cause vasoconstriction.

  • Long-term Restoration: Recall of fluids, aldosterone increases fluid retention, erythropoietin stimulates red blood cell production.

Effects of Aging on cardiovascular System

• General Decline of Cardiovascular Capabilities with Age:

  • Decreased hematocrit, blockage issues (thrombus), pooling blood in legs due to valve deterioration.

• Heart Changes:

  • Reduced maximum cardiac output and changes in conducting cell function; reduced elasticity leads to higher atherosclerosis risk and scar tissue formation.

• Blood Vessel Changes:

  • Arteries become less elastic, leading to potential aneurysms; deposits can cause strokes and plaques due to arterial blockages.