Chapter 21 Hemodynamics outline

1. Aorta and Arteries

• Aorta:

  • Receives blood under high pressure.

• Arteries:

  • Function: Carry blood away from the heart to tissues.

  • Elasticity: Arteries stretch and accommodate the high pressure of ventricular ejection, thanks to elastic tissue in tunica interna and media.

  • Contractility:

    • Controlled by smooth muscle in tunica media.

    • Regulates lumen size for blood flow speed (vasodilation and vasoconstriction).

    • Vasospasm: Protective response following injury, helps limit blood loss.

  • Types of Arteries:

    • Muscular Arteries: Medium diameter, more smooth muscle, fewer elastic fibers. Regulate blood flow via vasoconstriction/dilation.

    • Elastic Arteries: Large diameter, more elastic fibers, function as pressure reservoirs.

2. Arterioles and Capillaries

• Arterioles:

  • Small resistance vessels that regulate blood flow and arterial blood pressure.

  • Control capillary blood flow using precapillary sphincters.

• Capillaries:

  • Microscopic vessels between arterioles and venules.

  • Primary function: Nutrient and waste exchange between blood and tissue.

  • Types of Capillaries:

    • Continuous Capillaries: Most common, with gaps between endothelial cells.

    • Fenestrated Capillaries: Found in kidneys, small intestine, and eye, with pores for efficient fluid exchange.

    • Sinusoids: Large openings in liver and spleen, allowing large particles (including cells) to pass.

3. Post-Capillary Venules and Muscular Venules

• Post-Capillary Venules:

  • Collect blood from capillaries, no tunica media, and porous intercellular junctions for nutrient and waste exchange.

  • Main site for white blood cell emigration.

• Muscular Venules:

  • Have smooth muscle for blood storage, too thick for nutrient exchange.

4. Veins and Venules

• Veins:

  • Return blood to the heart, contain valves to prevent backflow (due to lower pressure).

  • Structure: Thinner tunica media, thicker tunica externa with collagen.

  • Varicose Veins: Caused by weak valves, blood pools or flows backward.

  • Edema: Abnormal fluid accumulation due to filtration/reabsorption imbalance.

• Venous Return:

  • Achieved through skeletal muscle and respiratory pumps, aided by vein valves.

5. Blood Distribution and Pressure

• Blood Distribution:

  • Venous reservoirs: 64% of total blood volume in systemic veins and venules.

  • During exercise or hemorrhage, venoconstriction directs blood to critical areas like muscles or vital organs.

• Blood Pressure (BP):

  • Influenced by cardiac output (CO), blood volume, and vascular resistance.

  • Systolic BP: Highest pressure during heart contraction.

  • Diastolic BP: Lowest pressure during heart relaxation.

• Vascular Resistance:

  • Factors Affecting Resistance: Blood vessel diameter, viscosity, and total vessel length.

6. Capillary Exchange

• Capillary Exchange Mechanisms:

  • Diffusion: Movement of solutes based on concentration gradients.

  • Transcytosis: Transport of large molecules across endothelial cells.

  • Bulk Flow: Movement of large volumes of fluid, regulates blood and tissue fluid volumes.

• Hydrostatic and Osmotic Pressure:

  • Filtration: Fluid movement from blood to interstitial space.

  • Reabsorption: Fluid movement from interstitial space back into blood.

7. Hemodynamics

• Blood Flow: Volume of blood circulating through a tissue per unit time (mL/min). Determined by pressure differences.

• Factors Affecting Blood Pressure (BP):

  • BP = CO × Vascular Resistance.

  • Systolic/Diastolic Pressure: BP peaks during systole and drops during diastole.

• Venous Return: Blood returning to the heart from veins, influenced by pressure gradients and assisted by skeletal muscle and respiratory pumps.

• Velocity of Blood Flow:

  • Inversely proportional to the total cross-sectional area of vessels (slows in capillaries for exchange).

• Factors that Increase BP: Increased blood volume, cardiac output, or vascular resistance.

8. Circulatory Routes

• Systemic Circulation:

  • Delivers oxygenated blood from the left ventricle to tissues.

  • Includes the aorta and its branches (iliac arteries).

• Pulmonary Circulation:

  • Carries deoxygenated blood to the lungs for gas exchange.

• Hepatic Portal Circulation:

  • Blood from the digestive organs passes through the liver before returning to the heart for detoxification and nutrient processing.

• Fetal Circulation:

  • Features unique structures like the ductus venosus, ductus arteriosus, and foramen ovale for nutrient and waste exchange between mother and fetus.

9. Control and Regulation

• Cardiovascular Center (Medulla Oblongata):

  • Regulates heart rate, contractility, and vessel diameter to maintain blood pressure and flow homeostasis.

Baroreceptors:

• Definition: Specialized pressure-sensitive neurons that detect stretching in blood vessels and the atria.

• Function:

  • Send impulses to the cardiovascular (CV) center to help regulate blood pressure (BP).

  • Key Baroreceptors:

    1. Carotid Baroreceptors:

       • Sensory axons transmit signals to the CV center in the medulla oblongata.

    2. Aortic Baroreceptors:

       • Located in the ascending aorta, they trigger the aortic reflex to regulate systemic BP.

       • Sensory axons transmit via the vagus nerve.

Negative Feedback Regulation of BP

• Mechanism:

  1. BP drops ⇒ baroreceptor stretch decreases.

  2. Slower nerve impulses to the CV center.

  3. This causes a decrease in parasympathetic output and an increase in sympathetic nervous system (SNS) activity.

  4. Enhanced secretion of epinephrine/norepinephrine from the adrenal medulla.

  5. Increased catecholamines ⇒ increase HR, contractility, and vasoconstriction.

  6. These adjustments raise BP back to normal.

Chemoreceptors:

• Location: Near baroreceptors (carotid sinus, aortic arch). Also called carotid bodies and aortic bodies.

• Function: Monitor levels of O2, CO2, and H+ in the blood.

• Triggers:

  • Hypoxia (low O2 levels)

  • Acidosis (high H+)

  • Hypercapnia (high CO2)

• CV Center Response:

  • Increased SNS stimulation to arterioles and veins causes vasoconstriction, raising BP.

  • Respiratory output increases to enhance breathing rate and depth.

Hormonal Regulation of BP

• RAAS:

  • Decreased BP ⇒ Renin from kidneys converts angiotensinogen to Angiotensin I.

  • ACE in lungs converts Angiotensin I to Angiotensin II, which:

    • Causes vasoconstriction ⇒ increases BP.

    • Stimulates aldosterone secretion ⇒ increases Na+ and H2O reabsorption by kidneys.

    • Results in reduced urine output, helping to retain water and sodium.

• ADH:

  • Secreted by hypothalamus and posterior pituitary.

  • In response to dehydration or low blood volume, ADH increases water reabsorption, raising blood volume and BP.

  • Vasopressin causes vasoconstriction, raising BP.

• Epi/Nor:

  • SNS activation releases epinephrine/norepinephrine from the adrenal medulla.

  • Increases HR, contractility, and CO.

  • Vasoconstriction in skin and abdominal arteries.

  • Vasodilation in heart and skeletal muscles.

• ANP (Atrial Natriuretic Peptide):

  • Released by atrial cells in response to high blood pressure or volume.

  • Causes vasodilation and decreases BP.

  • Promotes Na+ and H2O excretion through urine, further lowering BP.

Autoregulation of BP

• Definition: The ability of tissues to adjust blood flow based on metabolic needs.

• Purpose: Ensures optimal oxygen/nutrient delivery and waste removal.

• Used by the brain for regional blood flow control.

• Mechanism: Arterioles dilate and precapillary sphincters open to increase blood flow.

Autoregulatory Stimuli

• Physical:

  • Temperature:

    • Warming ⇒ vasodilation (to release heat).

    • Cooling ⇒ vasoconstriction (to retain heat in core organs).

  • Myogenic Response:

    • If arteriole walls are stretched (due to high flow), smooth muscle contracts.

    • If flow decreases, the wall stretch reduces, causing relaxation and vasodilation to increase blood flow.

• Chemical:

  • Vasodilators:

    • Active tissues: K+, H+, lactic acid, adenosine.

    • Inflammatory response: NO, kinins, histamine.

  • Vasoconstrictors:

    • TXA2, superoxide, serotonin, endothelins.

Checking Circulation: Pulse

• Pulse: The alternating expansion and recoil of artery walls with each heart beat.

  • Felt in arteries near the surface or over hard tissues.

  • Commonly checked at the radial artery.

• Normal HR: 60-100 bpm (typically 70-80 bpm).

  • Tachycardia: HR > 100 bpm.

  • Bradycardia: HR < 60 bpm.

Checking Circulation: BP

• BP: Pressure of blood against artery walls.

  • Systolic: Peak pressure during ventricular contraction (first heart sound).

  • Diastolic: Pressure during ventricular relaxation (last heart sound).

• Normal BP:

  • Young male: 120/80 mm Hg.

  • Young female: 112/70 mm Hg.

Types of Shock

• Definition: Inadequate circulation to meet the body's oxygen/nutrient demands.

  • Types:

    • Hypovolemic (low blood volume)

    • Cardiogenic (heart failure)

    • Vascular (vasogenic): Septic, anaphylactic, neurogenic.

    • Obstructive (blockage in blood flow).

Homeostatic (Compensatory) Responses to Shock

• Goal: To restore cardiac output (CO) and BP.

• Mechanisms:

  • RAAS activation: Renin release leads to increased sodium reabsorption.

  • ADH release: Promotes water reabsorption.

  • SNS activation: Increases HR, releases epinephrine.

  • Local vasodilators: Released due to tissue hypoxia to increase blood flow.

Signs and Symptoms of Shock

• Clammy, cool, pale skin (SNS constriction of vessels).

• Tachycardia (SNS stimulation, epinephrine).

• Weak, rapid pulse.

• Sweating.

• Hypotension: SBP < 90 mm Hg.

• Altered mental status.

• Decreased urine output (ADH release).

• Thirst (fluid loss).

• Acidosis (buildup of lactic acid, nausea).

Aging on the CV System

• Loss of aortic compliance.

• Reduced cardiac muscle fiber size.

• Progressive reduction in cardiac muscle strength.

• Decreased cardiac output (CO).

• Decline in max HR.

• Increased systolic BP.

• Associated with:

  • Coronary artery disease (CAD).

  • Congestive heart failure.

Disorders: Hypertension

• Definition: Chronic high BP.

  • Known as the “silent killer” due to lack of symptoms until damage occurs.

• SBP > 140 mm Hg or DBP > 90 mm Hg.

• Primary Hypertension: 90-95% of cases, no identifiable cause.

• Secondary Hypertension: 5-10% of cases, identifiable causes like kidney problems or hormonal imbalances.

• Management:

  • Weight loss, limit alcohol intake, exercise, reduce sodium intake.

  • Increase intake of K+, Mg+, and Ca+ to lower the risk.