Final answer:
Epithelial cells exchange gases and nutrients with the blood primarily through diffusion. This exchange is facilitated by the respiratory membrane in the lungs and transport molecules in the intestines for water-soluble nutrients.
Step-by-step explanation:
Epithelial cells exchange oxygen, carbon dioxide, and other materials with the blood primarily through the process of diffusion. These cells form tissues that are mostly avascular; meaning blood vessels do not directly enter the epithelial tissue. Instead, nutrients and gases must pass by diffusion or absorption from the underlying tissues or the surface. This is because many epithelial cells are tightly bound by tight junctions, preventing substances from passing between them.
For example, in the lungs, the simple squamous epithelium of type I alveolar cells is attached to a thin elastic basement membrane and is extremely thin, bordering the endothelial membrane of capillaries. The alveoli and capillary membranes create a respiratory membrane that is only about 0.5 μm thick, optimizing the exchange of gases through diffusion. Oxygen is picked up by the blood and CO2 is released into the alveoli air by moving down their respective concentration gradients.
In the intestines, water-soluble nutrients must use transport molecules to enter cells due to the hydrophobic nature of the cell membranes. These nutrients then pass into the interstitial fluid and enter the capillary blood to be transported to various parts of the body, such as the liver via the hepatic portal vein.