CardiovasLec 07

Exercise, Capillary Exchange, and the Lymphatic System

• Exercise significantly impacts cardiovascular responses, increasing venous return and respiratory pump action.
• Sympathetic activity increases, and parasympathetic activity decreases.
• Neuromuscular junctions in skeletal muscles signal the cardiovascular control center (CCC), affecting heart rate and contractility, impacting resistance arterioles in metabolically inactive areas.
• Local metabolites lead to profound vasodilation in active skeletal muscles (reducing tissue pressure).
• Blood flow distribution differs at rest and during exercise, with vasoconstriction in inactive tissues and vasodilation of exercising muscles.

Cardiovascular Responses to Exercise

• Venous return and respiratory pump actions increase.
• Sympathetic activity increases, parasympathetic activity decreases.
• Neuromuscular junctions send signals to the cardiovascular control center (CCC).
• This affects heart rate, contractility, and resistance in arterioles of non-exercising tissues.
• Local metabolites facilitate profound vasodilation in skeletal muscle, reducing tissue pressure.

Cardiovascular Responses to Exercise Charts

• Cardiac output at rest is 5.8 liters/min, with 21% (1.2L/min) directed to skeletal muscle.
• During vigorous exercise, the cardiac output increases to 25.6 liters/min, with 88% (22.5 L/min) directed to skeletal muscle.
• Other tissues receive significantly less blood flow during exercise (brain, kidneys, GI tract).
• Blood is shunted to the exercising muscles.

Capillaries and Blood Flow

• Capillary velocity is intimately linked to cross-sectional area.
• Total cross-sectional area of all the vessels at the same level in the vascular system controls the blood flow rate.
• Wider vessels mean slower blood flow, and vice versa.
• This principle is crucial for efficient nutrient and waste exchange in tissues.

Capillary Exchange

• Exchange between plasma and interstitial fluid occurs through paracellular pathways (between cells) or through cell-cell junctions.
• Larger substances move through transcellular transport (vesicular transport) through apical and basolateral membranes.
• Large proteins are transported by transcytosis in most capillaries.
• Small dissolved substances, water, and gases move through diffusion in capillaries.

Capillary Structure and Function

• Capillary walls offer a thin pathway for the most efficient and fastest exchange of water-soluble substances.
• Capillary density correlates with tissue metabolic needs.
• Over 10 billion capillaries provide a huge exchange surface area (500-700 m²).

Two Types of Capillaries

• Continuous capillaries are found in muscle and brain tissues.
• Fenestrated capillaries, more permeable, are found in areas like the kidneys and intestine requiring high volume exchange.

Capillary Exchange: Different Movement Methods

• Permeability of capillary pores varies by molecular size.
• Larger molecules have lower permeability across capillary pores compared to smaller molecules.
• Blood transports a variety of molecules, ranging greatly in molecular size.

Capillary Exchange: Determining Forces

• Bulk flow involves mass movement of fluid due to hydrostatic or osmotic pressure gradients.
• Absorption involves fluid movement into capillaries.
• Filtration involves fluid movement out of capillaries.
• Hydrostatic pressure is a significant force driving pressure out of capillaries.
• Osmotic considerations include both plasma and interstitial fluid colloid pressures.

Fluid Exchange at a Capillary

• Hydrostatic and osmotic pressures control bulk flow.
• Hydrostatic pressure pushes fluid out of the capillary.
• Osmotic pressure pulls fluid back into the capillary.
• Net filtration pressure is calculated (hydrostatic – osmotic).

Typical Capillary Net Forces

• Net filtration pressure is typically 0.3 mmHg.
• Filtration coefficient is 10 ml/min/mmHg.
• Net filtration rate ~ 3 mL/min.

Lymphatic System

• Returns fluid and proteins to the circulatory system.
• Absorbs and transfers absorbed fats to the circular system.
• Acts as a filter for pathogens (microbes).
• Lymph vessels collect interstitial fluid and return it to the bloodstream.
• Lymphatic vessels have a vital role in maintaining fluid balance and supporting the immune system.

Edema

• Accumulation of fluid in tissues can have varied causes, including lymphatic blockage or increased pressure in peripheral veins (heart failure).
• Elephantiasis is a specific edema pathology caused by parasitic roundworms blocking lymphatic channels.
• Ascites, fluid buildup in the abdomen, can relate to liver cirrhosis or other diseases impacting fluid balance.

Terminology Summary

• Kwashiorkor: inadequate protein intake, causing edema as a prominent symptom, often with visible abdominal bloating.
• Ascites: Fluid accumulation leading to noticeable abdominal swelling.