Final answer:
Fenestrated capillaries have endothelial cells with filtration pores that allow the passage of larger molecules, crucial for organs that require rapid exchange of substances such as small intestine and kidneys.
Step-by-step explanation:
Fenestrated capillaries have endothelial cells which have many holes, known as filtration pores or fenestrations, in the plasma membrane. These capillaries facilitate the passage of larger molecules and are crucial in areas of active filtration and absorption. The fenestrations allow most substances to diffuse from the blood, primarily based on size, while preventing the filtration of blood cells or large proteins. For instance, substances less than 4 nm in size can readily cross, and most pass freely up to 8 nm in size.
Fenestrated capillaries are abundant in organs that require rapid exchange of substances, such as the small intestine for nutrient absorption and the kidneys for filtering the blood. They are also present in the choroid plexus of the brain and various endocrine organs, including the hypothalamus, pituitary, thyroid, and pineal glands. This type of capillary is well-suited for their functions in these areas because of the necessity to allow certain molecules to pass through without hindrance.