Endothelial cells line the insides of blood vessels and lymphatic vessels and have many important functions, including but not limited to those described in the question. One additional function of endothelial cells is involvement in blood clotting.

Endothelium generally lines fully internal pathways (such as the vascular system), while epithelium generally lines pathways that are open to the external environment (such as the respiratory and digestive systems). Nerve cells are specialized for signaling, and red blood cells are specialized for oxygen transport.

Epithelial cells compose the outside of the body, namely skin and the lining of systems that connect to the outside of the body, such as the respiratory, excretory, and digestive tracts. The stomach is part of the digestive tract, as are the intestines. Knowing that the digestive tract is lined with epithelial cells allows us to eliminate the lining of the intestine from the answer choices. Similarly, the alveoli are a part of the respiratory system and nephrons are part of the excretory system.

The lining of the heart's chambers is part of the vascular lining, which is made of endothelial cells and is not exposed to the outside environment.

Epithelial tissue acts as a barrier in the human body. The functions of epithelial tissue are broad and include selective absorption, secretion, transport, and protection.

Squamous cells are flat and wide. Both stratified and simple squamous epithelia will form smooth and flat surfaces that are specialized for the simple diffusion across the layer. Squamous cells are found in lung epithelia, the lining of blood vessels, and in the lining of body cavities.

The main storage form of lipids is in the form of triglycerides. Phospholipids are found in the cell membrane.

The endothelium is a specialized type of epithelium that lines the lymphatic and circulatory system. The endothelium originates from the mesoderm during embryogenesis and develops into squamous cells. Within the lymphatic and circulatory systems, endothelial cells function as barriers, in blood clotting, and in inflammation.

Light enters the eye through the pupil and is focused at the back of the eye to form an image on the retina. The retina contains rods and cones that can convert the image to nerual signals for the brain to interpret.

The iris is the muscle around the pupil that allows it to dilate or constrict. Changing the size of the pupil will alter the amount of light entering the eye.

The ciliary muscles attach the lens to the scelera (the outer white part of the eye). Contracting or relaxing these muscles will change the focal point of the lens, allowing the eye to properly focus the image on the retina. The ciliary muscles and lens do not affect the amount of light to enter the eye.

The ciliary body is adjacent to the ciliary muscles and produces aqueous humor, a liquid that fills the space between the cornea and the lens.

Epithelial cells can be categorized by morphology, or shape. Epithelial cells exist in the following morphologies: squamous, columnar, or cuboidal.

Stratified epithelia are composed of two or more layers of epithelial cells. The increased number of cells associated with stratified epithelia creates a more complex function; stratified epithelia are involved in protection from mechanical and chemical forces.

Stratified epithelia consist of two or more layers of epithelial cells. They plan an important role in protection from chemical and mechanical forces. Stratified epithelia are specialized into keratinized and transitional layers.