Chapter 10: Circulation Flashcards

Learning Objectives for Circular Physiology

Describe the general functions of the major components of the circulatory system.
Describe the structures and pathways of the pulmonary and systemic circulations.
Compare the structure of an artery and vein, and explain how the structure of each type of vessel relates to its function.
Describe the structure of capillaries and explain the physiological significance of this structure.
Explain how atherosclerosis may develop and comment on the significance of this condition.
Describe the components and functions of the lymphatic system.
Define cardiac output and describe how it is affected by cardiac rate and stroke volume.
Explain how autonomic nerves regulate the cardiac rate and the strength of ventricular contraction.
Explain the intrinsic regulation of stroke volume (the Frank-Starling law of the heart).
List the factors that affect the venous return of blood to the heart.
Explain how tissue fluid is formed and how it is returned to the capillary blood.
Explain how edema may be produced.
Define total peripheral resistance and explain how vascular resistance is regulated by extrinsic control mechanisms.
Describe the intrinsic mechanisms involved in the autoregulation of blood flow.
Explain the mechanisms by which blood flow to the heart and skeletal muscles is regulated.
Describe the changes that occur in the cardiac output and in the distribution of blood flow in the body during exercise.
Describe the factors that regulate the arterial blood pressure.
Describe the baroreceptor reflex and explain its significance in blood pressure regulation.
Explain how the sounds of Korotkoff are produced and how these sounds are used to measure blood pressure.
Describe how the pulse pressure and mean arterial pressure are calculated and explain the significance of these measurements.

Hemodynamics of Circulatory Flow

Flow Rate Principles: Use Poiseuille’s Equation to calculate the dynamics of flow.
- Flow is directly proportional to the radius of the vessel raised to the 4th power ( $r^4$ ).
- Flow is inversely proportional to viscosity ( $\text{n}$ ) and the length ( $l$ ) of the vessel.
- Equation: $\text{Flow} = \frac{3.14 \times \Delta P \times r^4}{8 \times \text{n} \times l}$
- Simplified Relationship: $\text{Flow} = \Delta P \times r^4$
Determinants of Flow and Pressure Gradient (\Delta P):
- Flow is determined by the pressure difference ( $\Delta P$ ) between the start and end of a vessel, not the absolute pressure values.
- Example 1 (Vessel 1): Start pressure $90\,mmHg$ , end pressure $50\,mmHg$ . $\Delta P = 40\,mmHg$ .
- Example 2 (Vessel 2): Start pressure $90\,mmHg$ , end pressure $10\,mmHg$ . $\Delta P = 80\,mmHg$ . Since $\Delta P$ is twice that of Vessel 1, flow is twice that of Vessel 1.
- Example 3 (Vessel 3): Start pressure $180\,mmHg$ , end pressure $100\,mmHg$ . $\Delta P = 80\,mmHg$ . Flow is identical to Vessel 2 despite higher absolute pressures.
The Impact of Radius (Resistance Control):
- Resistance is proportional to $\frac{1}{r^4}$ .
- If the radius of Vessel 2 is twice ( $2 \times$ ) that of Vessel 1:
  - Resistance in Vessel 2 is $\frac{1}{16}$ that of Vessel 1.
  - Flow in Vessel 2 is $16 \times$ that of Vessel 1.

Arterioles and Vascular Regulation

Primary Regulation Sites: Arterioles are the primary sites for regulating vascular flow.
Functional Analogy: Arterioles act as "valves" in a pipe system.
- High resistance (narrow radius) results in no flow.
- Moderate resistance results in moderate flow.
- Low resistance (wide radius) results in large flow.
Clinical Significance: Drugs and therapies targeting arterioles have the most direct effects on Blood Pressure (BP) and systemic blood flow.

Determinants of Blood Pressure (BP)

Pressure Pulses:
- Systolic BP (SBP): The result of heart ejection into arteries and the relative lack of compliance (stiffness) of those vessels. Represents pressure during heart contraction.
- Diastolic BP (DBP): The result of the elastic recoil of the aorta pushing blood to distal arteries. Represents pressure during heart relaxation.
- Pulse Pressure (PP): Calculated as SBP minus DBP ( $PP = SBP - DBP$ ).
Mean Arterial Pressure (MAP):
- MAP is the average blood pressure throughout the cardiac cycle.
- The heart spends approximately twice as much time in diastole than in systole.
- Calculation: $MAP = \frac{SBP + (2 \times DBP)}{3}$

Cardiac Output (CO) and Stroke Volume (SV)

Core Formulas:
- $\text{Cardiac Output (CO)} = \text{Heart Rate (HR)} \times \text{Stroke Volume (SV)}$
- $\text{Blood Pressure (BP)} = CO \times \text{Systemic Vascular Resistance (SVR)}$
Example Calculations:
- If $HR = 70\,b/m$ and $SV = 70\,mL/b$ , then $CO = 4,900\,mL/m$ .
- If $CO = 4,900\,mL/m$ and $SV = 100\,mL/b$ , then $HR = 4,900 / 100 = 49\,b/m$ .
Determinants of HR and SV:
- Heart Rate: Determined by the balance of Sympathetic Nervous System (SNS) and Parasympathetic Nervous System (PNS) influence, largely regulated by baroreceptors.
- Stroke Volume:
  - Preload: The amount of blood filling the heart before contraction (End-Diastolic Volume or EDV).
  - Contractility: The amount of force the myocardium generates to eject blood, independent of the volume (influenced by sympathetic activity and $Ca^{2+}$ ).

The Frank-Starling Law and Venous Return

Intrinsic Regulation of SV: The heart normally pumps out during systole the volume of blood returned to it during diastole.
Correlation: There is a direct correlation between End-Diastolic Volume (EDV) and Stroke Volume.
Preload Chain: Increased venous return $\rightarrow$ increased EDV (preload) $\rightarrow$ increased stroke volume.
Skeletal Muscle Pump: Contraction of skeletal muscles surrounding veins, combined with one-way venous valves, prevents backflow and aids in pushing blood toward the heart (increasing venous return).

Autonomic Nervous System (ANS) Effects on the Heart

Table: Sympathetic vs. Parasympathetic Stimulation:

SA Node:
- Parasympathetic: Decreases rate of depolarization to threshold; decreases HR.
- Sympathetic: Increases rate of depolarization to threshold; increases HR.
AV Node:
- Parasympathetic: Decreases excitability; increases AV nodal delay.
- Sympathetic: Increases excitability; decreases AV nodal delay.
Ventricular Conduction Pathway:
- Parasympathetic: No effect.
- Sympathetic: Increases excitability; hastens conduction through the Bundle of His and Purkinje cells.
Atrial Muscle:
- Parasympathetic: Decreases contractility; weakens contraction.
- Sympathetic: Increases contractility; strengthens contraction.
Ventricular Muscle:
- Parasympathetic: No effect.
- Sympathetic: Increases contractility; strengthens contraction.
Adrenal Medulla:
- Parasympathetic: No effect.
- Sympathetic: Promotes secretion of epinephrine, augmenting sympathetic actions.
Veins:
- Parasympathetic: No effect.
- Sympathetic: Increases venous return via venoconstriction.

Control Mechanisms for Stroke Volume and Heart Rate

Extrinsic Control of SV: Sympathetic stimulation increases contractility (strength of contraction at a given EDV) by increasing $Ca^{2+}$ influx.
Parasympathetic Specifics: Mediated by the Vagus nerve, which primarily supplies the atrium (SA and AV nodes) but notably NOT the ventricles. It releases Acetylcholine, which increases $K^+$ permeability, causing hyperpolarization.
Sympathetic Specifics: Releases Norepinephrine, which decreases $K^+$ permeability, leading to depolarization.
Athletic Training: Physical conditioning results in a greater SV. Consequently, HR is lower for any given CO. Slower HR increases the time spent in diastole, which is the only time the heart muscle itself receives nourishment via coronary circulation.

The Baroreceptor Reflex

Locations: Found in the Carotid sinus (monitoring blood to the brain) and the Aortic arch (monitoring blood to the rest of the body).
Control Center: Neural signals are sent to the cardiovascular control center in the medulla oblongata via the Vagus and Glossopharyngeal nerves.
Reflex Mechanics:
- Increase in BP: Stimulates Baroreceptors $\rightarrow$ Inhibits SNS + Stimulates PNS $\rightarrow$ Decreases HR and SV $\rightarrow$ Decreases BP.
- Decrease in BP: Detected by Baroreceptors $\rightarrow$ Increased SNS activity $\rightarrow$ Vasoconstriction (arterioles), Venoconstriction (veins), increased HR, and increased Contractility $\rightarrow$ Increases BP.

Blood Vessel Structure

Arteries: Consist of three layers (tunics):
- Tunica interna: Endothelial layer.
- Tunica media: Smooth muscle.
- Tunica externa: Outermost structural layer.
- Large arteries contain elastin for recoil; smaller arteries/arterioles provide higher resistance.
Capillaries: Consist of a single layer of endothelial cells supported by a basement membrane. They are the sites of molecular exchange. Blood flow into them is regulated by precapillary sphincters.
Veins: Same three tunics as arteries but with a thinner muscular layer. They are more distensible (act as capacitance vessels) and contain valves to ensure one-way flow.

Microcirculation and Starling Forces

Capillary Dynamics: Fluid movement across capillary membranes is dependent on pressure gradients and osmotic gradients.
Starling Equation: $Jv = Kf [(Pc - Pi) - (\Pi c - \Pi i)]$
- $Jv$ : Net fluid movement.
- $Kf$ : Capillary filtration coefficient (permeability).
- $Pc$ : Capillary hydrostatic pressure (outward force).
- $Pi$ : Interstitial fluid hydrostatic pressure (inward force).
- $\Pi c$ : Plasma oncotic (osmotic) pressure (inward force).
- $\Pi i$ : Interstitial fluid oncotic pressure (outward force).
Filtration and Reabsorption Values:
- Arteriolar End: Outward pressure ( $Pc = 37\,mmHg$ , $\Pi i = 0\,mmHg$ ) vs Inward pressure ( $\Pi c = 25\,mmHg$ , $Pi = 1\,mmHg$ ). Net outward pressure (Ultrafiltration) = $11\,mmHg$ .
- Venular End: Outward pressure ( $Pc = 17\,mmHg$ , $\Pi i = 0\,mmHg$ ) vs Inward pressure ( $\Pi c = 25\,mmHg$ , $Pi = 1\,mmHg$ ). Net inward pressure (Reabsorption) = $9\,mmHg$ .
Net Filtration: Normally, net filtration is slightly positive; the excess fluid is collected by the lymphatic system. Failure results in edema.

Lymphatic System and Coronary Circulation

Lymphatics: Provides a one-way pathway from tissues back to the venous system. Key structures include initial lymphatics, lymph nodes, and ducts (Thoracic and Right Lymphatic Ducts).
Coronary Circulation: The heart muscle receives its own blood supply via coronary arteries during diastole.
- During systole, vessels are compressed by the myocardium and blocked by the open aortic valve.
- Adenosine: Released by heart muscle during increased metabolic demand to cause vasodilation of coronary vessels.
- Anatomy: Includes Left Main Coronary Artery, Right Coronary Artery, Circumflex Branch, and Anterior Interventricular Branch. Bypass surgery (CABG) often uses vein grafts or the internal mammary artery.