Cardiovascular System: Blood Vessels & Circulation

Overview

This section covers the physiology of blood vessels and circulation, focusing on capillary exchange, filtration and reabsorption, and the dynamics of fluid movement between blood and interstitial fluid. Understanding these processes is essential for grasping how nutrients, gases, and wastes are transported throughout the body.

Capillary Exchange

Mechanisms of Capillary Exchange

Capillary exchange refers to the movement of substances between blood and interstitial fluid across capillary walls. This process is vital for tissue nourishment and waste removal.

• Diffusion: The primary method of capillary exchange. Substances move down their concentration gradients.

• Lipid-soluble substances: (e.g., O2, CO2, some hormones) diffuse directly through the plasma membrane of endothelial cells.

• Water-soluble substances: (e.g., glucose, amino acids) pass through intercellular clefts or fenestrations (pores) in the capillary wall. The route depends on the size and water solubility of the molecule.

• Transcytosis: Larger, water-soluble substances (e.g., proteins like insulin) are transported via endocytosis at the apical surface and exocytosis at the basal surface of endothelial cells.

• Limited passage: Most plasma proteins and red blood cells cannot cross continuous and fenestrated capillaries.

Key Terms:

• Intercellular clefts: Small gaps between endothelial cells allowing passage of small molecules.

• Fenestrations: Pores in endothelial cells that facilitate rapid exchange of substances.

• Transcytosis: Transport of macromolecules across the cell via vesicles.

Example: Glucose and amino acids move through intercellular clefts or fenestrations, while oxygen and carbon dioxide diffuse through the plasma membrane.

Filtration and Reabsorption

Fluid Movement Across Capillaries

Filtration and reabsorption describe the movement of fluid and solutes between blood and interstitial fluid, driven by pressure gradients. These processes maintain tissue fluid balance and are crucial for homeostasis.

• Filtration: Movement of fluid and solutes from blood capillaries into interstitial fluid, typically at the arterial end of capillaries.

• Reabsorption: Movement of fluid from interstitial fluid back into blood capillaries, usually at the venous end.

• Bulk flow: Large numbers of ions, molecules, or particles move together in one direction due to pressure gradients.

Example: At the arterial end of a capillary, filtration predominates, while at the venous end, reabsorption is favored.

Forces Affecting Filtration and Reabsorption

Fluid movement is determined by the balance of hydrostatic and osmotic pressures:

• Capillary Hydrostatic Pressure (BHP): The pressure exerted by blood against the capillary wall; promotes filtration. Typical values: 35 mmHg (arterial end), 16 mmHg (venous end).

• Interstitial Fluid Hydrostatic Pressure (IFHP): The pressure of interstitial fluid; usually close to 0 mmHg.

• Blood Colloid Osmotic Pressure (BCOP): The osmotic pressure due to plasma proteins in blood; promotes reabsorption. Typical value: ~26 mmHg.

• Interstitial Fluid Osmotic Pressure (IFOP): The osmotic pressure due to proteins in interstitial fluid; usually ~1 mmHg.

Note: Unlike hydrostatic pressure, osmotic pressure does not vary significantly from one end of the capillary to the other.

Net Filtration Pressure (NFP)

The net movement of fluid is determined by the Net Filtration Pressure (NFP), calculated as:

Formula:

• If NFP is positive (> 0 mmHg): Filtration occurs (fluid leaves capillary).

• If NFP is negative (< 0 mmHg): Reabsorption occurs (fluid enters capillary).

Example: At the arterial end, NFP is usually positive, favoring filtration; at the venous end, NFP is negative, favoring reabsorption.

Fluid Balance and Lymphatic System

Not all filtered fluid is reabsorbed; a small amount (~1.5 mL/min) is lost to the tissue spaces and is returned to the blood via the lymphatic system.

• Lymphatic vessels: Collect excess interstitial fluid and return it to venous circulation.

• Edema: Occurs when filtration exceeds reabsorption or lymphatic drainage is impaired, leading to fluid accumulation in tissues.

Example: Each day, about 20 L of fluid filters from capillaries, with most reabsorbed and the remainder returned by lymphatics.

Summary Table: Forces Affecting Capillary Exchange

Clinical Relevance

Edema and Fluid Imbalance

Disruption in the balance of filtration and reabsorption can lead to edema, which is the accumulation of excess fluid in tissues. Causes include increased capillary hydrostatic pressure, decreased plasma protein concentration, or impaired lymphatic drainage.

• Example: Heart failure can increase venous pressure, leading to increased filtration and edema.

• Example: Malnutrition or liver disease can decrease plasma proteins, reducing BCOP and causing edema.

Key Equations

Net Filtration Pressure:

Interpretation:

• NFP > 0: Filtration

• NFP < 0: Reabsorption

Summary

• Capillary exchange is essential for tissue health and involves diffusion, transcytosis, and bulk flow.

• Filtration and reabsorption are governed by hydrostatic and osmotic pressures.

• Net Filtration Pressure determines the direction of fluid movement.

• Imbalances can lead to clinical conditions such as edema.