Lining of Lungs vs. Lining

Lining of Lungs vs. Lining of Intestines on Absorption

In the study of zootomy, epithelium is a term to describe a tissue composed of a layer of cells. Epithelium can be found lining internal (e.g. endothelium, which lines the inside of blood vessels) or external (e.g. skin) free surfaces of the body - but there are several important distinctions within, especially with regard to absorption.

The outermost layer of our human skin contains and is comprised of deceased squamous epithelial cells, as are the mucous membranes lining the inside of mouths and body cavities. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands - so both intestines and lungs have epithelial cells that perform absorption, as noted below in greater length.

Functions of epithelial cells include secretion, absorption and protection. Epithelial cells sit on a basal lamina (formerly called a basement membrane).

First, Absorption in lungs is highly integrated into the respiratory process in humans.

In air-breathing vertebrates, respiration occurs in a series of steps. Air is trained into the animal via the airways -- in reptiles, birds and mammals this often comprises the nose, the pharynx, the larynx, the trachea, the bronchi and bronchioles, and the terminal branches of the respiratory tree.

The lungs of these animals are a rich lattice of alveoli, which provide an enormous surface area for gas exchange. A network of fine capillaries take blood over the surface of alveoli. Oxygen from the air inside the alveoli diffuses into the bloodstream across the exceptionally thin alveolar membranes, and carbon dioxide moves from the blood to the alveoli via the same process.

Indeed, the drawing and expulsion of air is driven by muscular action; in early tetrapods, air was driven into the lungs by the pharyngeal muscles, whereas in reptiles, birds and mammals a more complicated musculo-skeletal system is used. In the mammal, a large muscle, known as the diaphragm, manages to drive ventilation by periodically altering the intra-thoracic volume and pressure; by increasing volume and decreasing pressure, air is sucked into the airways, and by reducing volume and increasing pressure, the reverse occurs.

Absorption in the intestines occurs mainly in the small intestine. This is evidenced in the fact that the small intestine has a particular folded texture in order to increase the surface area available for diffusion of nutrients through the intestinal wall so they can be absorbed. These microscopic folds are called microvilli. In an adult human, the small intestine is, on average, about seven meters long - which is incredible given the size of a human body.