Hemodynamics II: Microcirculation and Veins
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Questions and Answers

What is the primary function of arterioles in the circulatory system?

  • Connect veins to arteries
  • Facilitate gas exchange
  • Regulate regional blood flow (correct)
  • Store blood

Which blood vessels are primarily involved in the exchange of gases and nutrients?

  • Capillaries (correct)
  • Metarterioles
  • Veins
  • Arteries

What structure regulates blood flow into the capillaries?

  • Thoroughfare channel
  • Endothelial lining
  • Precapillary sphincter (correct)
  • Adventitial layer

What characterizes the structure of capillaries?

<p>Single layer of endothelium (C)</p> Signup and view all the answers

What occurs if all precapillary sphincters contract?

<p>Direct blood flow from metarteriole to venule (A)</p> Signup and view all the answers

What is the primary composition of arterioles?

<p>Thick smooth muscle layer (B)</p> Signup and view all the answers

Which vessels serve as collecting and storage vessels in the circulatory system?

<p>Veins (C)</p> Signup and view all the answers

What determines the delivery of blood to a particular tissue in the microcirculation?

<p>Metabolic activity of the tissue (D)</p> Signup and view all the answers

What happens to the shape of veins at high transmural pressures?

<p>They become circular. (B)</p> Signup and view all the answers

Which of the following factors does NOT affect venous return to the heart?

<p>Vascular elasticity (B)</p> Signup and view all the answers

What is the relationship between venous return and cardiac output?

<p>They are equal except for brief moments. (B)</p> Signup and view all the answers

Which mechanism is described as the reflex increase in heart rate with increased venous return?

<p>Bainbridge reflex (B)</p> Signup and view all the answers

What occurs to venous compliance at low pressures?

<p>Veins change shape easily. (D)</p> Signup and view all the answers

Which factor acts as a backward force on veins and impedes blood flow into the right atrium?

<p>Right atrial pressure (B)</p> Signup and view all the answers

If smooth muscle contraction in the great veins is activated, what happens to venous return?

<p>Venous return is shunted to the right atrium. (C)</p> Signup and view all the answers

Which of the following is a key factor in the peripheral circulation that affects venous return?

<p>Peripheral resistance (A)</p> Signup and view all the answers

What happens to plasma volume without the continuous return of filtered proteins and fluid?

<p>Plasma volume would be rapidly depleted. (C)</p> Signup and view all the answers

How does interstitial fluid enter lymphatic vessels?

<p>By pushing on the one-way valves. (B)</p> Signup and view all the answers

Which factor can increase lymph flow?

<p>Increased capillary hydrostatic pressure. (C)</p> Signup and view all the answers

What role do the one-way valves in lymphatic vessels serve?

<p>They ensure fluid flows in one direction only. (B)</p> Signup and view all the answers

What is the primary factor affecting venous flow in the upright position?

<p>Hydrostatic pressure (D)</p> Signup and view all the answers

Veins are known to have which of the following properties?

<p>They can contain up to 70% of the blood in circulation. (A)</p> Signup and view all the answers

Which measurement most accurately represents the pressure at the venous end of the capillary bed?

<p>12 to 18 mm Hg (B)</p> Signup and view all the answers

Which mechanism does not contribute to venous return?

<p>Capillary filtration. (C)</p> Signup and view all the answers

What is capacitance in the venous system primarily influenced by?

<p>Volume of blood carried (B)</p> Signup and view all the answers

What is a potential consequence of lymphatic vessel blockage?

<p>Accumulation of interstitial fluid leading to edema. (D)</p> Signup and view all the answers

What indicates a dysfunction of venous valves?

<p>Varicose veins (D)</p> Signup and view all the answers

What primarily influences the volume of blood returning to the heart?

<p>The reservoir function of veins and factors like preload. (B)</p> Signup and view all the answers

How does the shape of a vein change with an increase in venous transmural pressure?

<p>From elliptical to circular (C)</p> Signup and view all the answers

Which statement about dynamic pressure is correct?

<p>It dissipates in the arterial system before reaching the capillary bed. (A)</p> Signup and view all the answers

What is a major contributor to maintaining cardiovascular stability?

<p>Volume fluctuations in veins (A)</p> Signup and view all the answers

Which factor plays a minor role in venous flow compared to hydrostatic pressure in the upright position?

<p>Dynamic pressure (B)</p> Signup and view all the answers

What happens to precapillary vessels during increased metabolic activity in tissues?

<p>They open to increase capillary perfusion. (A)</p> Signup and view all the answers

Which of the following organs typically has a high capillary density?

<p>Heart (A)</p> Signup and view all the answers

What type of blood flow bypasses the capillaries and is known as nonnutritional flow?

<p>Shunt flow (D)</p> Signup and view all the answers

Which chemical is a known vasodilator, stimulating cGMP production?

<p>Nitric oxide (C)</p> Signup and view all the answers

What is the primary mechanism for transcapillary exchange?

<p>Diffusion (C)</p> Signup and view all the answers

What fluid do lymphatic vessels collect for recirculation into blood?

<p>Fluid and proteins escaping from blood (A)</p> Signup and view all the answers

Which of the following is NOT a component of the lymphatic system?

<p>Blood vessels (B)</p> Signup and view all the answers

Which tissue type lacks lymphatic vessels?

<p>Cartilage (D)</p> Signup and view all the answers

Flashcards

Microcirculation

The flow of blood through the smallest blood vessels in the body: arterioles, capillaries, and venules.

Arterioles

Small branches of arteries that regulate blood flow to specific tissues by controlling the resistance of blood vessels.

Capillaries

Tiny blood vessels where the exchange of gases, nutrients, and waste products between blood and tissues occurs.

Metarterioles

Small vessels that connect arterioles to capillaries and can bypass the capillaries if needed.

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Precapillary sphincter

A single smooth muscle cell that forms a ring around metarterioles-capillary junctions, controlling blood flow into capillaries.

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Thoroughfare channel

A direct pathway from metarterioles to venules that bypasses the capillary bed.

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Blood vessel structure

Blood vessels get thinner and more muscular as they get further from the aorta.

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Blood flow regulation

Metabolic activity in tissues determines the amount of blood delivered.

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Capillary Perfusion

Blood flow through capillaries to tissues.

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Capillary Density

The amount of capillaries in an area. Higher in active tissues.

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Transcapillary Exchange

Movement of substances across capillary walls (diffusion, filtration, pinocytosis).

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Diffusion (in Capillaries)

Important for nutrient, waste, and gas exchange across capillaries.

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Lymphatic System

System that collects leaked fluid and returns it to the bloodstream.

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Lymphatic Vessels

Part of the lymphatic system, transporting fluid (lymph).

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AV Shunts

Blood vessels that bypass capillaries. Non-nutritional blood flow.

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Vasoactive Capillary Role

Capillaries can change their diameter due to chemical signals.

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Venous return

The amount of blood flowing from veins into the right atrium each minute.

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Cardiac output

The amount of blood pumped into the aorta by the heart each minute.

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Venous volume and pressure

Venous volume increases with pressure, but at low pressures, veins are compliant; at high pressures they become stiff.

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Frank-Starling mechanism

Heart's automatic mechanism to adjust to venous return.

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Bainbridge reflex

A reflex that increases heart rate in response to increased venous return.

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Right atrial pressure

Pressure exerted on veins that backs up blood flow from the veins to the right atrium.

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Systemic circulation filling

The filling of the blood vessels in the body affects how much blood moves toward the heart.

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Peripheral Resistance

Resistance to blood flow between periphery and right atrium affecting venous filling.

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Lymphatic Circulation Importance

Essential for returning high-molecular-weight substances (like proteins) and fluid to the bloodstream, preventing fluid buildup (edema) and maintaining blood volume.

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Lymphatic Action Mechanism

Driven by tissue pressure, muscle contractions, lymphatic vessel contractions, and one-way valves that prevent backflow.

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Lymphatic Capillary Structure

Endothelial cells overlap to form valves allowing fluid to enter but not exit, crucial for high-molecular-weight substance absorption.

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Lymph Flow Variation

Lymph flow increases with exercise and is minimal during rest, reflecting the body's need to manage fluid.

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Edema Cause

Occurs when interstitial fluid builds up due to more fluid filtered from capillaries than drainage by lymphatics, or lymphatic blockage.

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Vein Function (Reservoir)

Veins store a significant portion of blood, acting as a reservoir. This allows for adjustments to blood flow to the heart based on body needs.

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Venous Return Factors

Venous return relies on pressure differences, skeletal muscle pumping action, and one-way valves for efficient blood flow back to the heart.

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Venous Capacitance

This means veins can expand and contract to effectively store and release blood as needed, matching to blood flow requirements. This accommodates changes in cardiac output.

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Venous Return in Supine Position

Blood flow is driven by dynamic pressure gradients, where arterial pressure is higher than venous pressure. This pressure difference pushes blood through the circulatory system toward the heart.

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Venous Return in Upright Position

In the upright posture, hydrostatic pressure significantly influences blood flow in the lower extremities. Muscle pumps and valves assist overcoming this pressure to return blood to the heart.

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Hydrostatic Pressure

Pressure exerted by fluid due to gravity. In the body, it's crucial for venous return in the upright position, influencing venous blood flow against gravity.

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Venous Valves

One-way valves in veins that prevent backflow of blood, aiding in venous return against gravity in the upright position.

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Muscle Pumps

Muscles that squeeze veins during movement, assisting in propelling blood towards the heart, especially in the limbs.

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Varicose Veins

Condition caused by dysfunctional venous valves, leading to swollen, twisted, and often discolored veins, usually in the legs.

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Venous Compliance

Indicates how much a vein's volume changes for a given change in pressure. High compliance means the vein distends easily.

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Study Notes

Hemodynamics II

  • Lecture topic: Microcirculation, veins, lymphatics, and venous return
  • Lecturer: Ian Dixon
  • Location: Molecular Cardiology Lab, R3010, St. Boniface Hospital Research Centre, Institute of Cardiovascular Sciences
  • Note: Overlap with other lectures exists.

The Circulatory System

  • Function: Supplies blood and nutrients to tissues
  • Arterioles: Regulate regional blood flow (resistance vessels)
  • Capillaries: Exchange gases, water, and solutes with interstitial fluid
  • Venules and veins: Collect and store blood

Blood Vessels

  • Aorta: Predominantly elastic, large diameter
  • Peripheral arteries: Become thinner and more muscular as branches narrow
  • Arterioles: Primarily muscular (small diameter)
  • Capillaries: Single layer of endothelium, crucial for exchange
  • Dimensions: Various vessel sizes given (e.g., Aorta diameter, artery diameter etc.)

Microcirculation

  • Circulation through the smallest vessels (arterioles, capillaries, and venules)
  • Arterioles: Thin adventitial layer, thick smooth muscle layer, endothelial lining (5-100 µm diameter)
  • Metarterioles/Capillaries: 5-10 µm (or 10-20 µm) in diameter
  • Precapillary sphincters: Smooth muscle cells that regulate blood flow into capillaries
  • Thoroughfare channels: Direct flow from metarteriole to venule if precapillary sphincters close
  • Metabolic activity influences capillary perfusion

Capillary Exchange

  • High density found in metabolically active tissues (heart, muscle, glands)
  • Diffusion: Main method for exchange of fluids, gases, nutrients, waste products between capillaries and tissues. Capillaries have water filled channels/pores. Fluid moves across capillary wall at a rate of 300 mL per minute per 100g of tissue.
  • Lipid-insoluble molecules restricted to water filled channels in capillaries

Vasoactive Role of Capillary Endothelium

  • Chemicals that cause vasodilation (relaxation of smooth muscle):
    • Prostacyclin (PGI2): from arachidonic acid (AA) through cyclooxygenase and prostacyclin synthase; increase in cAMP
    • Nitric oxide (NO): Stimulates guanylyl cyclase (G Cyc) to increase cyclic GMP (cGMP)
    • Nitroprusside (NP): Acts directly on vascular smooth muscle
    • Adenosine: Stimulates cAMP levels to relaxed smooth muscle

The Lymphatic System

  • Function: Collects fluid and proteins that have escaped from blood vessels, and return them to circulation
  • Components: Lymph vessels, lymph nodes, lymphatic tissue
  • Lymphatic capillaries: Intercalate with blood capillaries; One-way valves; Endothelial cells overlapped. Anchoring filaments to surrounding tissue. Interstitial fluid moves into lymphatic capillaries.
  • Importance: Returns filtered proteins and fluid to blood; Prevents edema (swelling)

Lymphatic System - Additional Points

  • Lymph flow influenced by skeletal muscle activity
  • Increased by increased capillary filtration and decreased oncotic pressure
  • Blockage can result in edema.

The Venous System

  • Function: Returns blood to the heart from tissues
  • Capacitance: Ability to hold large volumes of blood with little change in pressure
  • Resistance: Blood flow resistance determined by muscle pumps and valves, pressure gradients
  • Venous return: Quantity of blood flowing into right atrium each minute.

Pressure-Flow Relationships and Venous Return

  • Dynamic pressure: Pressure generated by cardiac contraction
  • Dynamic pressure gradients determine blood flow
  • Pressure drops from arterial side to venous side, reaching 12-18 mmHg at the venous end of capillaries
  • Atrial pressure averages 4-7 mm Hg in normal conditions; this drives blood flow from veins to the right atrium
  • Hydrostatic pressure influences venous flow in the upright position.
  • Muscle pumps and valves are crucial for venous return, working against hydrostatic pressure

Varicose Veins

  • Dysfunction of venous valves, resulting in swollen and misshapen veins

Capacitance and Compliance

  • Capacitance: Relationship between pressure and volume at a given smooth muscle tone. Veins have low resistance to big volume changes
  • Compliance: Change in blood volume per unit of transmural pressure change; slope of capacitance curve.

Venous Capacitance

  • Controlled by the collapsible nature of the venous wall
  • Elliptical shape at low blood volume; low pressure
  • High increase of volume can occur at minimal pressure increase
  • Circular shape at high pressure requiring an increase in pressure to add more volume

Cardiac output = Venous Return (CO=VR)

  • Cardiac Output: Amount of blood pumped into aorta each minute
  • Venous Return: Amount of blood flowing into right atrium each minute
  • CO and VR must equal each other, except for brief periods when blood is temporarily stored in lungs/heart

Control of Cardiac Output by Venous Return – The Frank-Starling Mechanism of the Heart

  • Various peripheral circulation factors affect venous return
  • Heart has automatic mechanisms for pumping (Frank-Starling law of the heart)
  • Bainbridge reflex – increased heart rate with increased venous return

Factors affecting Venous Return

  • Right atrial pressure: Backwards force to impede blood flow
  • Systemic circulation filling: Volume pushing blood toward the heart
  • Resistance between peripheral vessels and right atrium: Reduced volume in arterioles = Reduced blood flow back

(Note: This document does not include the references themselves, which were present in the original pages. The references contain authors, titles, and publication details which are not included in this summary)

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Explore the intricate details of hemodynamics in this quiz focusing on microcirculation, veins, and lymphatics. Learn about the function and structure of blood vessels, including arterioles, capillaries, and the venous return process. This quiz is essential for understanding the circulatory system's role in supplying nutrients and oxygen to tissues.

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