A hormone is a secreted molecule that is carried in the bloodstream to where it acts on target cells that bear specific receptors for that hormone.

Persons with type AB blood are sometimes called universal recipients, not donors. This means they can safely receive a blood transfusion using any other blood type. Choice A is wrong because it confuses type AB blood with type O negative blood.

Type O negative blood is considered the universal donor type because it has no antigens on the surface of red blood cells and can be given to people of any blood type.

Type AB-positive blood is also considered the universal plasma donor type because it has both A and B antigens and can be given to patients with any blood type.

Plasma is the liquid part of blood that carries cells and proteins throughout the body.

The presence or absence of antigens and the Rh factor determine how a person’s immune system reacts to a blood transfusion or organ transplant.

Antigens are substances that can trigger an immune response if they are foreign to the body.

The Rh factor is a protein that can be either present (+) or absent (-) on the surface of red blood cells.

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.