CARDIOVASCULAR PHYSIOLOGY LECTURE 5: CAPILLARIES, VEINS & LYMPHATICS

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By the end of this module and after reading the relevant textbook sections, students should be able to:

List the components of microcirculation and their characteristics.
Describe the various transport processes across the capillary wall.
Explain factors influencing filtration and absorption at the arteriolar and venous ends of the capillary.
Understand the role of venous pressure and the control of cardiac output.
Explain the function of muscle and respiratory pumps.
Understand the structure and function of lymphatic vessels.

Vander’s Human Physiology by Eric Widmaier, Hershel Raff & Kevin Strange, 16th Edition, Chapter 12, Sections 12.11-12.13.

Major Site for Exchange:
- Capillaries are the primary location for the exchange of substances (e.g., water, O₂, CO₂, nutrients, and waste products) between the blood and cells.
Network of Capillaries:
- Comprise an elaborate network with an enormous degree of branching. Most cells in the body remain within approximately 10 µm of a capillary.
- Individual capillaries are about 1 mm long but collectively extend approximately 40-50,000 km in humans.

Structure:
- Composed of thin-walled tubes made up of a single layer of endothelial cells resting on a basement membrane.
- Lacks smooth muscle or elastic tissue.
- Diameter ranges from 5-8 μm; Red blood cells travel through in “single file.”
- High permeability to water and small solutes due to inter-endothelial pores; impermeable to large proteins.

Velocity of Blood Flow:
- Blood velocity is slow in capillaries due to their large total cross-sectional area which allows more time for the exchange of substances.
Concept Distinction:
- Velocity: Speed of fluid elements moving (e.g., in cm/sec).
- Flow: Volume of fluid passing through a cross-section per unit time (e.g., liters per minute).

Pathways for Transport:
- Diffusion: Movement through endothelial pores or plasma membranes.
- Bulk Flow: Movement of water and dissolved substances.
- Endocytosis and Exocytosis: Utilized for transporting certain materials.
- Driving Force: Concentration gradient drives diffusion, with substances moving from high to low concentration.
Important Fluid Distribution:
- Key in extracellular fluid distribution between plasma and interstitial fluid.

Starling’s Forces:
- Two major forces regulating bulk flow:
1. Hydrostatic Pressure: Driven by the pressure difference between the capillary and interstitial fluid.
2. Colloid Osmotic Pressure: Due to the presence of impermeable plasma proteins, affecting fluid movement.
Net Filtration Pressure:
- Calculated as the difference between hydrostatic and colloid osmotic pressures.
- At the arteriole end, filtration occurs (hydrostatic pressure > colloid osmotic pressure), while at the venule end absorption occurs (colloid osmotic pressure > hydrostatic pressure).
- Normal filtration excess is approximately 4 L/day, returned via lymphatic vessels.

Effect on Hydrostatic Pressure:
- Arteriolar state affects capillary pressures. Dilation increases pressure, promoting filtration; constriction reduces pressure, limiting filtration.
Practical Application:
- Local control of arterioles can be utilized in treatments such as applying ice packs to reduce swelling.

Function:
- Veins facilitate the return of blood to the heart, acting as a “volume reservoir” containing about 60% of total blood volume at rest.
- Structure: Thin-walled, compliant, with few smooth muscle layers, allowing for volume accommodation at low pressure.

Main Factors:
- Volume of Blood: Drives pressure within veins, similar to any elastic tube.
- Compliance of Veins: Determines how volume affects pressure.

Mechanism:
- Sympathetic stimulation releases noradrenaline, causing smooth muscle contraction in veins, increasing venous pressure and enhancing blood flow into the heart.
- Note the minimal change in resistance due to the large diameters of veins—main effect is stiffening the walls which enhances venous pressure.

Skeletal Muscle Pump:
- Intermittent contractions of surrounding skeletal muscles compress veins, assisting in blood return to the heart, particularly during exercise.
- One-way valves in veins are critical for this function.
Respiratory Pump:
- Changes in abdominal and thoracic pressures during respiration influence blood flow from abdominal veins into the thorax.

Functionality:
- One-way valves allow blood flow towards the heart while preventing backflow.
- Crucial for skeletal muscle pump operation, respiratory pump function, and counteracting gravity effects in an upright position.

Cardiac Output Dependency:
- Ensures that the heart receives adequate volume to pump, influencing stroke volume. Low venous return can result in decreased cardiac output, risking inadequate blood supply to organs.

Role:
- Provides a return route for excess interstitial fluid and plasma proteins, playing a crucial role in maintaining fluid balance.
- Lymphatic vessels start as blind-ended tubes in tissues and eventually drain into veins in the neck.
- Valves prevent backflow, with external compression and smooth muscle contractions helping to propel lymphatic fluid.

Understanding capillary function, venous return, and the lymphatic system is essential for comprehending overall cardiovascular physiology and its critical importance in maintaining homeostasis in the human body.