## **Core Concept**
Blood capillaries are the smallest blood vessels in the body and play a crucial role in the exchange of oxygen, nutrients, and waste products between the bloodstream and tissues. They are characterized by a unique structure that facilitates this exchange. The key feature of blood capillaries is their thin wall, which allows for the diffusion of substances across the capillary wall.

## **Why the Correct Answer is Right**
The correct answer, , is that all blood capillaries have a **continuous basal lamina** or **basement membrane**. This structural feature is common to all blood capillaries and provides a selective barrier for the exchange of molecules between the blood and tissues. The basal lamina is composed of a dense network of collagen and glycoproteins that help to regulate the passage of molecules across the capillary wall.

## **Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because not all capillaries have fenestrae; fenestrated capillaries are typically found in organs with high rates of exchange, such as the kidneys and intestines.
* **Option B:** This option is incorrect because a discontinuous endothelium is characteristic of lymphatic capillaries and some specialized blood capillaries, but not a universal feature of all blood capillaries.
* **Option C:** This option is incorrect because a thick muscular wall is not a feature of blood capillaries; instead, capillaries have a thin wall that facilitates exchange.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the **continuous basal lamina** of blood capillaries plays a critical role in maintaining the integrity of the capillary wall and regulating the exchange of molecules. Damage to the basal lamina can lead to edema and other vascular disorders.

## **Correct Answer:** . Basement membrane or basal lamina.