Capillary Exchange and Function Quiz

Exercise, Capillary Exchange, and the Lymphatic System

• Exercise increases venous return and respiratory pump activity.
• Increased sympathetic activity and decreased parasympathetic activity affect heart rate, contractility, and resistance arterioles in metabolically inactive tissues.
• Local metabolites cause vasodilation in active skeletal muscle (reduced total peripheral resistance).
• Blood flow is differently distributed during movement compared to rest.
• This distribution shunts blood toward working muscles.

Cardiovascular Responses to Exercise

• Venous return and respiratory pump increase.
• Sympathetic activity increases, and parasympathetic activity decreases.
• Neuromuscular junctions in skeletal muscle send signals to the cardiovascular control center (CCC), affecting heart rate and contractility.
• Resistance arterioles in inactive tissues contract.
• Local metabolites cause vasodilation in active skeletal muscle, reducing total peripheral resistance.

Cardiovascular Responses to Exercise (Quantified)

• At rest, cardiac output is 5.8 L/min.
  • Brain receives 13%
  • Kidney receives 19%
  • Gastrointestinal (GI) tract receives 24%
  • Skin receives 9%
  • Other tissues receive 10%
  • Skeletal muscle receives 21%
• During vigorous exercise, cardiac output is 25.6 L/min.
  • Brain receives 3%
  • Kidney receives 1%
  • GI tract receives 1%
  • Skin receives 2.5%
  • Other tissues receive 0.5%
  • Skeletal muscle receives 88%
• Blood flow is differently distributed at rest than during vigorous exercise.

Review: Blood Flow Through the Cardiovascular System

• Blood flow is directed from veins, through capillaries, and to arteries.
• Pulmonary veins carry blood from the lungs, to the heart.
• Ascending arteries supply blood to the upper body, while the abdominal aorta supplies the lower body.
• Blood flows through the heart, lungs, body, and heart again, repeatedly.

Capillaries: Velocity of Blood Flow

• Capillaries have the slowest blood velocity.
• The velocity is determined by the total cross-sectional area of all vessels at the same level in the circulatory system.
• As the total cross-sectional area increases, the velocity decreases.

Practical Example: Cross-Sectional Area and Velocity

• Blood flow rate remains constant, but velocity changes based on the cross-sectional area of the vessel.
• In a narrower vessel, the velocity increases.
• If cross-sectional area is 3 cm², the velocity is 4 cm/min.

Capillaries: Exchange Site

• Exchange of material happens across the thin capillary wall.
• Capillary density is associated with metabolic activity.
• Over 10 billion capillaries provide a large surface area (500 to 700 m²) for solute and fluid exchange.

Two Types of Capillaries

• Continuous capillaries: Found in muscle and brain.
• Fenestrated capillaries: Found in high volume exchange areas like the kidney and intestines.

Capillary Exchange: Mechanisms

• Paracellular pathway: Substances pass through cell junctions.
• Transcellular pathway: Substances pass via vesicles through apical and basolateral membranes.
• Transcytosis: Large proteins transport through capillaries.
• Diffusion: Small solutes, water, and gases move through diffusion.

Effect of Molecular Size on Permeability

• Capillary intercellular slit pores have a width of 6 to 7 nanometers, influencing substance permeability.
• Permeability varies based on molecular size.

Relative Permeability of Muscle Capillary Pores

• Different substances have varying degrees of permeability.
• Water passes most easily, while larger molecules like albumin have low permeability.

Capillary Exchange: Forces

• Bulk flow: Movement of fluid due to hydrostatic or osmotic pressure gradients.
• Absorption: Fluid movement into capillaries.
• Filtration: Fluid movement out of capillaries.
• Hydrostatic pressure drives filtration.

Determinants of Net Fluid Movement

• Capillary hydrostatic pressure (P_c): Forces fluid out of capillaries.
• Interstitial fluid pressure (P_if): Opposes filtration.
• Plasma colloid osmotic pressure (π_p): Draws fluid into capillaries.
• Interstitial fluid colloid osmotic pressure (π_i): Prompts fluid movement out.

Fluid Exchange at a Capillary

• Hydrostatic and osmotic pressure regulate the net flow.
• Filtration is greater than absorption in systemic capillaries, with a net outward flow.

Example - Typical Net Forces in Capillaries

• Net filtration pressure is 0.3 mmHg.
• The entire body has a net filtration rate of 3ml/min to 4.32 L/day.

Review Question: Left-Side Heart Failure

• If pulmonary capillary pressure is raised, there is a net outward filtration of fluid.

Lymphatic System

• Returns filtered fluids and proteins to circulation.
• Carries absorbed fats.
• Acts as a filter for pathogens.

Edema - Fluid Buildup

• Inadequate lymphatic drainage can cause edema.
• Increased peripheral vein pressure can promote edema formation.

Elephantiasis

• Obstruction of lymphatic channels causes abnormal tissue swelling (edema).
• Parasitic roundworms reside in lymph channels and cause the obstruction.
• Mosquitoes transmit the infection.

Ascites: Fluid in Abdominal Cavity

• Various medical conditions (like liver cirrhosis) contribute to this fluid accumulation.

Liver Cirrhosis

• Liver cirrhosis causes decreased liver function, leading to fluid buildup in the abdomen (ascites).
• The liver's reduced function affects protein production, contributing to ascites.

Capillary Exchange Summary

• Two main capillary types exist (including sinusoids).
• Pressure gradients drive filtration and absorption.
• Protein concentrations are critical in calculating fluid movement.
• The lymphatic system reclaims fluids.
• Pathology can alter these processes.