The chief cells are responsible for releasing pepsinogen. Pepsinogen is an inactive enzyme that will only become activated in acidic conditions, such as those found in the stomach. Acid converts pepsinogen into pepsin, which helps chemically digest protein nutrients in the stomach.

Parietal cells release hydrochloric acid into the stomach, which activates pepsinogen. Goblet cells, or mucous cells, secrete mucous to help prevent degradation of the stomach lining.

The esophagus assists moving food into the stomach using a wave-like contracting motion called peristalsis. Swallowing initiates the process of peristalsis, but unlike swallowing, peristaltic contractions are not voluntary and are generated by smooth muscle.

Mastication is another term for "chewing." Emulsification refers to the grouping together of fat molecules when in an aqueous environment, particularly during digestion in the small intestine.

Food is digested to some degree in many different parts of the digestive system. The mouth, stomach, and small intestine all contribute to food digestion. The mouth contains salivary amylase, which breaks down carbohydrates. The stomach contains pepsin, which breaks down proteins. The small intestine contains several enzymes, such as lipase and trypsin, and breaks down carbohydrates, lipids, and proteins.

The esophagus, however, does not contribute to chemical digestion in any way.

Bile is created by the liver and released into the small intestine from the gall bladder during digestion. Because fat tends to clump in the aqueous environment of the duodenum, bile helps to increase the surface area of fat, a process called emulsification. This helps lipase break down the fats adequately.

Muscles do not attach directly to bones. Instead, a tendon is used to attach the two structures. Ligaments connect a bone to another bone, while joints are the locations where bones move around one another.

There are three types of muscle in the body: skeletal, cardiac, and smooth muscle. Of the three, skeletal muscle is the only one that can be consciously controlled. Cardiac and smooth muscle are controlled involuntarily.

An example of skeletal muscle is your biceps brachii muscle; you can voluntarily control this muscle to flex your arm. An example of smooth muscle is the muscle surrounding your arteries; you cannot voluntarily contract this muscle to constrict your arteries. Cardiac muscle is found in the heart; you cannot voluntarily choose to slow your heart rate.

Red bone marrow is found in the ends (epiphyses) of long bones such as the femur and is responsible for red blood cell development. Yellow bone marrow is found in the center (diaphysis) of long bones and can store fat for energy.

Blood filtration primarily occurs in the spleen or kidney. Glycogen is primarily stored in the liver. Calcium is released from the mineral matrix of the bone, but is not actually found in the bone marrow.

Skeletal and cardiac muscle have quite a bit in common: both are examples of striated muscle that use the sarcomere as the basic functional unit. Sarcomeres are small units that are capable of contracting. When these units line up in a cell, they create a striped appearance known as "striated" or "striations." Both skeletal and cardiac muscle are capable of contracting; this is a key characteristic of any muscle cell.

A primary difference between skeletal and cardiac muscle cells is the number of nuclei each cell possesses. Skeletal muscle cells usually have several nuclei, but cardiac muscle cells usually have only one nucleus per cell.

The circulatory system is responsible for transporting blood, oxygen, nutrients, and wastes throughout the body. The vessels of the circulatory system include arteries, arterioles, capillaries, venules, and veins. The heart is responsible for pumping the fluids of the circulatory system through these vessels, and is a major component of the circulatory system.

The respiratory system is responsible for transporting air to and from the lungs, and facilitates gas exchange with the capillaries of the circulatory system. The lymphatic system helps regulate fluid balance and immune system function; it consists of lymphatic vessels, the spleen, and the thymus. The digestive system breaks down and absorbs nutrients; it consists of the digestive tract from the mouth, through the stomach and small intestine, and to the rectum.

The heart is able to pump independently of the brain thanks to a collection of cells that fire impulses automatically. These cells are located in the sinoatrial node, which leads to the node's nickname as the "pacemaker" of the heart. Even without stimulation by a nerve, the heart can continue beating.

The heart is made of cardiac muscle tissue; smooth muscle tissue lines internal organs and blood vessels. Both sides of the heart pump at the same rate, but the left side pumps with more force. Each side of the heart must pump the same volume, but the left side must pump it a greater distance since it pushes blood to the entire body. A normal resting heart rate is around 60-80 beats per minute. A heart rate of 120 would be more typical during exercise.