In all organisms, the basic unit of structure and function is:

Capillary walls consist of a single layer of epithelial cells, and they exchange substances in the blood for substances in the tissue fluid surrounding body cells.

This single layer of cells is called the endothelium and it forms the barrier between the blood and the interstitial fluid.

The endothelium can be either continuous or fenestrated, depending on the tissue type and function.

The capillaries are very thin and allow red blood cells to flow through them single file.

The capillaries also have a layer of a glycoprotein called the glycocalyx that covers their luminal surface.

Choice B. False is wrong because it contradicts the definition and structure of capillaries.

Capillaries are not made of multiple layers of cells, nor do they prevent the exchange of substances between the blood and the tissue fluid.

Macrophages are the main cells that clear out the cell debris by phagocytosis, a process that involves recognition, engulfment, and degradation of the disposable particles.

Macrophages are professional phagocytes that can be found in various tissues and organs, where they perform efferocytosis, the removal of dead and dying cells.

Choice A is wrong because lymphocytes are not phagocytes, but rather immune cells that mediate adaptive immunity by producing antibodies or killing infected cells.

Choice B is wrong because cytokines are not cells, but rather soluble molecules that regulate inflammation and immunity by acting as signals between cells.

Choice C is wrong because mast cells are not primarily involved in clearing cell debris, but rather in allergic reactions and innate immunity by releasing histamine and other mediators.