HSE208 - The Vascular System: Arterioles and Capillaries

The Vascular System: Arterioles and Capillaries

Pressures in the Vascular System

• There is a substantial difference in pressure between the pulmonary circulation and the systemic circulation.

• Blood pressure progressively decreases as blood flows through either the pulmonary or systemic systems.

Arterial Blood Pressure

• During the systolic phase of the cardiac cycle, a volume of blood equivalent to approximately $1/3$ of the stroke volume exits the arteries.

• The increasing arterial blood volume causes the elastic arterial walls to distend, which in turn raises arterial blood pressure.

• During diastole, the elastic arterial walls recoil, propelling blood forward into the arterioles.

Graphical Representation of Arterial Blood Pressure (Example Values)

• Systolic Pressure: Typically around $120 \text{ mmHg}$.

• Diastolic Pressure: Typically around $80 \text{ mmHg}$.

• Aortic Valve Closure: Occurs during the diastolic phase.

• Mean Pressure: Represents the average pressure throughout the cardiac cycle.

Pulse Pressure (PP)

• Definition: The difference between systolic blood pressure (SBP) and diastolic blood pressure (DBP).

• Formula: $PP = SBP - DBP$

• Key Factors Determining Pulse Pressure:

  1. Stroke volume: The amount of blood ejected by the ventricle in one beat.

  2. Speed of ejection of stroke volume: How quickly the blood is ejected into the arteries.

  3. Arterial compliance: The stretchability or distensibility of the artery walls.

Mean Arterial Pressure (MAP)

• Definition: The average arterial pressure sustained throughout the entire cardiac cycle.

• Formulas for Calculation:

  • $MAP = DBP + 0.33 \times PP$

  • $MAP = CO \times TPR$ (where CO is Cardiac Output and TPR is Total Peripheral Resistance)

• Significance: MAP is observed to be relatively consistent in all large arteries because these vessels offer negligible resistance to blood flow.

Arterioles

• Primary Function: Arterioles are crucial in determining the distribution of blood flow to specific organs through processes of vasoconstriction (narrowing) and vasodilation (widening).

• Role in MAP: They play a pivotal role in establishing Mean Arterial Pressure (MAP) by regulating the overall resistance to blood flow throughout the systemic circulation, earning them the designation of "resistance vessels."

Regulation of Arteriolar Resistance

• Intrinsic Tone: The smooth muscle within arteriolar walls exhibits spontaneous activity, maintaining a baseline level of contraction known as intrinsic tone.

• Adjustment by External Signals: This baseline level of contraction can be modulated by various external signals:

  • Local Control: Mechanisms that operate independently of neural or hormonal influences, often involving local metabolic byproducts.

  • Extrinsic Control: Regulatory mechanisms involving nerves and hormones circulating in the bloodstream.

Local Control Mechanisms

• Active Hyperemia (Increased Metabolic Activity):

  • Initiation: An increase in the metabolic activity of an organ.

  • Process: Leads to a decrease in oxygen ($O<em>2$) levels and an increase in various metabolites (e.g., $CO</em>2$, $H^+$, adenosine, $K^+$) within the interstitial fluid of the organ.

  • Response: Triggers arteriolar dilation in the affected organ.

  • Outcome: Results in an increased blood flow to the organ, matching its higher metabolic demand.

• Flow Autoregulation (Response to Decreased Arterial Pressure):

  • Initiation: A reduction in arterial pressure within an organ, leading to decreased blood flow.

  • Process: Causes a decrease in $O_2$, an increase in metabolites, and a decrease in vessel-wall stretch (due to lower pressure) in the organ.

  • Response: Induces arteriolar dilation in the organ.

  • Outcome: Helps restore blood flow towards normal levels in the organ, maintaining consistent perfusion despite pressure changes.

Extrinsic Control Mechanisms

• Sympathetic Stimulation: Generally causes vasoconstriction in most vascular beds, but can cause vasodilation in some (e.g., skeletal muscle during exercise via $\beta_2$ receptors).

• Autonomic Neurons Releasing Nitric Oxide: Certain autonomic neurons release nitric oxide, a potent vasodilator, particularly in blood vessels of the gastrointestinal tract.

• Hormonal Influences:

  • Adrenaline (Epinephrine): Can be either a vasoconstrictor or vasodilator depending on the receptor type (alpha $\alpha$ or beta $\beta_2$ adrenergic receptors) and its concentration.

  • Angiotensin II: A powerful vasoconstrictor.

  • Vasopressin (Antidiuretic Hormone - ADH): A powerful vasoconstrictor.

  • Atrial Natriuretic Peptide (ANP): A vasodilator.

Summary of Factors Regulating Arteriolar Radius

• Neural Controls:

  • Vasoconstrictors: Sympathetic nerves releasing norepinephrine.

  • Vasodilators: Neurons primarily releasing nitric oxide.

• Hormonal Controls:

  • Vasoconstrictors: Epinephrine (acting on $\alpha\text{-receptors}$), Angiotensin II, Vasopressin.

  • Vasodilators: Epinephrine (acting on $\beta_2\text{-receptors}$), Atrial Natriuretic Peptide.

• Local Controls:

  • Vasoconstrictors: Internal blood pressure (initiating myogenic response), Endothelin-1.

  • Vasodilators: Reduced oxygen ($O<em>2$), increased potassium ($K^+$), increased carbon dioxide ($CO</em>2$), increased hydrogen ions ($H^+$), altered osmolarity, Adenosine, Eicosanoids, Bradykinin, Nitric Oxide, and various substances released during tissue injury.

Capillaries

• Function: Capillaries are the microscopic vessels responsible for facilitating the exchange of nutrients and waste products between the blood and the surrounding tissue cells.

• Diffusion Distance: The distance for diffusion between tissue cells and nearby capillaries is extremely short, typically around $0.1 \text{ mm}$.

• Structure: Capillaries are generally composed of a single layer of endothelial cells, forming a tube, which rests upon a basement membrane.

Diffusion from Capillaries

• Lipid-Soluble Substances: Substances that are lipid-soluble (e.g., oxygen ($O<em>2$) and carbon dioxide ($CO</em>2$)) can readily pass directly through the endothelial cell walls of the capillary.

• Ions and Polar Molecules: These substances, being water-soluble, traverse the capillary wall through specialized $H_2O$-filled channels. These include:

  • Intercellular clefts: Small gaps between adjacent endothelial cells.

  • Fused-vesicle channels: Formed by the fusion of pinocytotic vesicles.

• Capillary Leakiness: The permeability or "leakiness" of capillaries varies significantly between different organs. For instance, capillaries in the brain are very "tight," limiting substance exchange, while those in the liver are notably "leaky," allowing greater passage of molecules.

Fluid Movement Across Capillaries (Starling Forces)

• Fluid movement across capillary walls is governed by a balance of four primary Starling forces:

  • Capillary Hydrostatic Pressure ($P_c$): The pressure exerted by blood within the capillary, tending to push fluid out of the capillary (favoring filtration).

  • Interstitial Fluid Hydrostatic Pressure ($P_{IF}$): The pressure exerted by fluid in the interstitial space, tending to push fluid into the capillary (opposing filtration).

  • Osmotic Force Due to Plasma Protein Concentration ($oldsymbol{\pi}_c$): Also known as plasma colloid osmotic pressure or oncotic pressure. This force is exerted by plasma proteins within the capillary, tending to draw fluid into the capillary (favoring absorption).

  • Osmotic Force Due to Interstitial Fluid Protein Concentration ($oldsymbol{\pi}_{IF}$): The colloid osmotic pressure exerted by proteins in the interstitial fluid, tending to draw fluid out of the capillary (favoring filtration).

• Net Filtration Pressure (NFP) Formula:
  
  

  $NFP = P<em>c + oldsymbol{\pi}</em>{IF} - P<em>{IF} - oldsymbol{\pi}</em>c$

• Example Calculations:

  • At the Arterial End of the Capillary:

    • Given values: $P<em>c = 35 \text{ mmHg}$, $oldsymbol{\pi}</em>c = 28 \text{ mmHg}$, $P<em>{IF} = 0 \text{ mmHg}$, $oldsymbol{\pi}</em>{IF} = 3 \text{ mmHg}$.

    • Calculation: $NFP = 35 + 3 - 0 - 28 = 10 \text{ mmHg}$.

    • Result: This indicates a net filtration pressure of $10 \text{ mmHg}$ favoring filtration of fluid out of the capillary.

  • At the Venous End of the Capillary:

    • Given values: $P<em>c = 15 \text{ mmHg}$, $oldsymbol{\pi}</em>c = 28 \text{ mmHg}$, $P<em>{IF} = 0 \text{ mmHg}$, $oldsymbol{\pi}</em>{IF} = 3 \text{ mmHg}$.

    • Calculation: $NFP = 15 + 3 - 0 - 28 = -10 \text{ mmHg}$.

    • Result: This negative value indicates a net filtration pressure of $10 \text{ mmHg}$ favoring absorption of fluid into the capillary.

Factors Altering Capillary Pressure

• Capillary pressure can be significantly influenced by several factors, including:

  • Posture: Changes in body position can alter hydrostatic pressure throughout the vascular system, affecting capillary pressure.

  • Arteriole Dilation and Constriction: The upstream resistance provided by arterioles directly impacts the pressure transmitted into the capillaries. Dilation increases capillary pressure, while constriction decreases it.