Diffusion Through the Capillary Membrane

By far the most important means by which substances are transferred between the plasma and the interstitial fluid is diffusion. Figure 16-3 demonstrates this process, showing that as the blood flows along the lumen of the capillary, tremendous numbers of water molecules and dissolved particles diffuse back and forth through the capillary wall, providing continual mixing between the interstitial fluid and the plasma. Diffusion results from thermal motion of the water molecules and dissolved substances in the fluid, the different molecules and ions moving first in one direction and then another, bouncing randomly in every direction.

Lipid-Soluble Substances Can Diffuse Directly Through the Cell Membranes of the Capillary Endothelium. if a substance is lipid soluble, it can diffuse directly through the cell membranes of the capillary without having to go through the pores. Such substances include oxygen and carbon dioxide. Because these substances can permeate all areas of the capillary membrane, their rates of transport through the capillary membrane are many times faster than the rates for lipid-insoluble

Arterial end Blood capillary Venous end

Arterial end Blood capillary Venous end

Figure 16-3

Diffusion of fluid molecules and dissolved substances between the capillary and interstitial fluid spaces.

substances, such as sodium ions and glucose that can go only through the pores.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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