Human Physiology Lecture 14: Cardiovascular System

Human Physiology Lecture 14: Cardiovascular System - Blood Flow and Blood Pressure

• Required Reading: Section 14.3 - Physical Laws Describing Blood Flow pp. 463-464 excluding System Interactions

• Objectives:

  • Arteries and Arterial Blood Pressure
  • Overview of Arterioles and Resistance to Flow
  • Factors Affecting Arterial Blood Pressure
  • Veins

Overview of the Vasculature

• Aorta: ID: 12.5 mm, 2 mm thick wall
• Arteries: ID: 2-6 mm, 1 mm thick walls
• Vena cava: ID: 30 mm, 1.5 mm thick wall
• Veins: ID: 5 mm, 0.5 mm thick wall

Arteries: A Pressure Reservoir

• Elastic and fibrous tissue in walls of arteries confers stiffness and flexibility
• Act as pressure reservoirs, ensuring smooth blood flow, even when the heart is in diastole
• Elastin fibres act as springs, storing elastic force and passively recoiling

Arteries: A Pressure Reservoir (Compliance)

• To act as pressure reservoirs, arteries must have low compliance
• Compliance measures how pressure in the vessel changes with a change in volume.
  • Low compliance (arteries): A large increase in pressure causes only a small degree of expansion of the vessel wall
  • High compliance (veins): A large increase in pressure causes a large increase in expansion of the vessel wall

Arterial Blood Pressure

• Arterial blood pressure = pressure in the aorta
• Pressure in the aorta varies with cardiac cycle:
  • Systolic blood pressure = maximum pressure (due to ejection of blood into aorta); pressure in aorta = arterial blood pressure
  • Diastolic blood pressure = minimum pressure (due to elastic recoil)
• Mean arterial pressure (MAP) = average pressure in arteries during one cardiac cycle

Measuring Blood Pressure

• Estimate of arterial pressure is determined using the brachial artery, a pressure cuff, and a sphygmomanometer
• Korotkoff sounds are used to measure blood pressure.
• Blood pressure is indicated by systolic pressure / diastolic pressure (e.g. 110/70)

Arterial Blood Pressure (Pulse Pressure & Mean Arterial Pressure)

• Pulse pressure (PP) = Systolic pressure (SP) – Diastolic pressure (DP) – Useful for evaluating heart function
• Mean arterial pressure (MAP) = SP + (2 × DP)/3

Arterioles

• Arterioles are resistance vessels, part of microcirculation, connecting arteries to capillaries/metarterioles
• Contain rings of smooth muscle to regulate radius, hence resistance
• They are the best site for regulating resistance to blood flow

Arterioles and Resistance to Blood Flow

• Arterioles provide the greatest resistance to blood flow (over 60% of TPR (total peripheral resistance).
• TPR = the combined resistance of all blood vessels in the systemic circuit
• Collectively, capillaries have less resistance
• Largest pressure drop in the vasculature occurs along arterioles

Arterioles and Resistance to Blood Flow (Pressures)

• Average pressure entering arterioles: 75-80 mm Hg
• Average pressure leaving arterioles: 35-40 mm Hg

Arterioles and Resistance to Blood Flow (Regulation)

• Resistance in arterioles is regulated by contraction/relaxation of circular smooth muscles.
• Major function of arterioles is as control points for blood flow regulation
• Arterioles function to control blood flow to individual capillary beds and regulate mean arterial pressure (MAP)

Arterioles and Resistance to Blood Flow (Changes in Radius)

• Arteriole radius changes depend on the contraction state of smooth muscle within the arteriole wall.
  • Arteriolar tone is contraction level (radius) independent of extrinsic influences
  • Increased contraction = decreased radius (vasoconstriction)
  • Decreased contraction = increased radius (vasodilation)
• Both intrinsic and extrinsic control mechanisms alter contraction

Intrinsic Control of Arteriole Smooth Muscle

• Intrinsic control happens via changes in metabolic activity, active hyperemia, and reactive hyperemia
  • Changes in metabolic activity are sensitive to Extracellular Fluid (ECF) conditions like carbon dioxide, potassium, and hydrogen ions.
  • Reactive hyperemia is a response to changes in blood flow rather than a change in metabolic activity due to oxygen and carbon dioxide in arterioles.
• Stretch of smooth muscles in arterioles
• Local chemical messengers

Intrinsic Control of Arteriole Smooth Muscle (Changes in Metabolic Activity)

• Vascular smooth muscle responds to changes in ECF conditions.
• Changes in metabolic activity generally cause vasodilation (increased metabolic activity) or vasoconstriction (decreased metabolic activity).

Intrinsic Control of Arteriole Smooth Muscle (Active Hyperemia)

• Results from changes in O2 and CO2 in response to changes in metabolic activity. Increased metabolic activity increases O2 consumption and CO2 production, resulting in vasodilation and increased release of blood flow.

Intrinsic Control of Arteriole Smooth Muscle (Reactive Hyperemia)

• Results from changes in O2 and CO2 in response to a change in blood flow. Changes/reduced blood flow increase O2 consumption and CO2 production and results in vasodilation and increased blood flow

Intrinsic Control of Arteriole Smooth Muscle (Stretch of Smooth Muscles)

• Stretch of the smooth muscle in arterioles affects the myogenic response (the immediate response of vasoconstriction/vasodilation to changes in blood pressure)

Intrinsic Control of Arteriole Smooth Muscle (Local Chemical Messengers)

• Local chemical messengers (e.g., nitric oxide, prostacyclin, endothelin-I) secreted by endothelial cells themselves influence the radius of arterioles.
• Nitric oxide and prostacyclin cause vasodilation; endothelin-I causes vasoconstriction

Extrinsic Control of Arteriole Smooth Muscle (Summary)

• Control of arteriole radius by external factors (e.g., sympathetic nerves, epinephrine, vasopressin, angiotensin II)

Capillaries and Venules (Microcirculation)

• The simplest way to regulate exchange of material across capillary walls and regulate blood flow via arterioles, metarterioles, and precapillary sphincters.

Capillaries and Venules (Metarterioles)

• Intermediate structurally between arterioles and capillaries which have rings of smooth muscle to act as shunts in arterioles and venules.

Capillaries and Venules (Precapillary Sphincters)

• Regulate flow through the capillary bed, control flow when capillaries contract/relax resulting in decreased or increased flow through the capillary bed .

Capillaries and Venules (Exchange Across Capillary Walls)

• Transport mechanisms across capillary walls include simple diffusion, mediated transport, and transcytosis

Capillaries and Venules (Bulk Flow Across Capillary Walls)

• Capillary walls are permeable to water and small solutes, allowing fluid to move from blood to interstitial fluid (filtration) and from interstitial fluid to blood (absorption). This maintains fluid balance, and movement between interstitial fluid and plasma may cause edema (swelling).
• Starling Forces are forces that explain the movement of fluids in/out of cells. Include capillary hydrostatic pressure (PCAP), interstitial fluid hydrostatic pressure (PIF), capillary osmotic pressure (πCAP), interstitial fluid osmotic pressure (πIF)

Capillaries and Venules (Net Filtration Pressure)

• Net filtration pressure (NFP) is the difference between filtration pressure and absorption pressure.
  • NFP= (PCAP + ПIF) - (πCAP + PIF)
• Filtration occurs at arteriole end of capillary (positive NFP). Absorption at venous end of capillary (negative NFP)

Veins (Volume Reservoir)

• Veins act as volume reservoirs due to their high compliance, allowing them to expand with little pressure change compared to arteries.
  • ~60% of the total blood volume is in systemic veins at rest.

Factors Influencing Venous Pressure and Return (Skeletal Muscle Pump)

• One-way valves in veins prevent backflow.
• Skeletal muscle contractions in the limbs drive blood flow toward the heart.

Factors Influencing Venous Pressure and Return (Respiratory Pump)

• During inspiration, decreases pressure in the thoracic cavity and increases pressure in the abdominal cavity. Favoring blood movement from abdominal veins to the thoracic cavity (toward heart).
• During exhalation, reverses pressure in thoracic cavity and abdominal cavity, but this pressure change does not result in significant backward movement of blood.

Factors Influencing Venous Pressure and Return (Blood Volume)

• Increased blood volume increases venous pressure; decreased volume results in decreased venous pressure (and diminished venous return)

Factors Influencing Venous Pressure and Return (Venomotor tone)

• Venomotor tone is smooth muscle tension in veins.
• Increased venomotor tone, via constriction of veins, increases venous pressure and moves blood toward heart.
• Increased wall tension in veins reduces compliance, causing a rise in venous pressure and increased stroke volume

Explore the intricacies of the cardiovascular system in this quiz focusing on blood flow and blood pressure. Delve into key concepts such as arterial pressure, arterioles, and factors affecting blood pressure. Enhance your understanding of vascular anatomy including the aorta, arteries, and veins.