Oxygen is taken up from the blood by all cells to be used in ATP production. The process of cellular respiration (energy production) creates carbon dioxide as a waste product, which, if accumulated, can cause the blood to become dangerously acidic. Gases in the lungs diffuse passively into or out of the air entirely based on where the concentration is lowest. Thus, oxygen levels in the blood must be lower in concentration than those in the lungs in order for oxygen to enter the blood, and carbon dioxide levels in the blood must be higher than those in the lungs order for carbon dioxide to exit.

During inspiration, the lungs expand as the diaphragm contracts and internal intercostal muscles relax. As the volume of the thoracic cavity increases, its pressure decreases. This creates a pressure gradient, driving air from an area of high pressure (the environment) into the area of low pressure (the lungs).

When you are not breathing, your respiratory system cannot perform its function. Unwanted gases, such as carbon dioxide, cannot be removed from the system and necessary gases, such as oxygen, cannot enter the system. This causes a buildup of carbon dioxide in the body, which leads to acidosis. Carbon dioxide is converted to carbonic acid via carbonic anhydrase. This carbonic acid builds in the blood, lowering its pH.

The majority of chemical (enzymatic) digestion occurs in the duodenum, the first segment of the small intestine. Proteases, amylase, lipases, and other enzymes are secreted into the duodenum from the pancreas. These enzymes are used to digest proteins, carbohydrates, and lipids, respectively.

The liver produces bile, which is stored in the gall bladder. During digestion, bile is released to help emulsify fats for digestion. The spleen is part of the lymphatic system, and is not involved in digestion.

The small intestine is the primary site of digestion and absorption in the body. This chemical digestion is possible thanks to the digestive enymes secreted by the pancreas.

While it is true that the liver creates and secretes bile, storage of bile is accomplished by the gall bladder.

The correct answer is carbohydrates.

Salivary amylase can only digest carbohydrates. Proteases further along in the digestive pathway breakdown proteins, while lipases digest fats. Amino acids are the product of digested proteins.

Pepsinogen is released by chief cells into the stomach lumen. In the presence of hydrochloric acid (secreted by parietal cells), this inactive enzyme will be cleaved, creating pepsin. Pepsin is a protease responsible for breaking down proteins that enter the stomach.

Carbohydrates are digested by amylase enzymes, lipids are digested by lipases, and nucleic acids are digested by nucleases.

Bile is a salt solution used to emulsify fats in the small intestine in order to facilitate absorption. It is created by the liver, but stored in the gall bladder. During digestion, the bile is released from the gall bladder into the duodenum.

The stomach is mainly responsible for mechanical digestion, caused by smooth muscle contractions, and a small amount of protease function. The pancreas secretes a number of digestive enzymes into the duodenum to facilitate digestion of macromolecules. Neither organ is involved in the production of bile.

The islets of Langerhans contain the endocrine pancreatic cells. 3 cell types within are the , , and  cells.  cells secrete glucagon,  cells secrete insulin, and  cells secrete somatostatin.