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.

Epithilial tissue is usually classified in two ways: by the shape of the cells and by their organization. Cell shape can be flat and polygonal (squamous), elongated and rectangular (columnar), or short and rectangular (cuboidal). Cells can be found in a single layer (simple) or multiple layers (stratified). Pseudostratified epithelium is usually columnar in shape and consists of a single cell layer that has the appearance of multiple layers.

Sprilli is a classification of spiral-shaped bacteria.

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 cuboidal cells are involved in secretion and absorption in the exocrine system in the lining of glands.

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.

Epithelial tissue lines the blood vessels, organs, and cavities in the human body. The primary function of epithelial tissue is to regulate secretion, absorption, and transport across surfaces. Epithelial tissue additionally plays a role in protection and detection of signals; therefore, the role of the epithelial tissue is to act as a regulatory barrier.

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

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.

Epithelial cells have three distinct morphologies: squamous, cuboidal, and columnar. Squamous cells are wider than they are tall. In other words, they are flat.