Anatomy and Physiology Study Guide for the HESI Exam

Respiratory

The respiratory system consists of the lungs, trachea, bronchi, and diaphragm. Its function is to receive and deliver oxygen and remove carbon dioxide. The diaphragm is a sheet of muscle that enables breathing.

During inhalation, the diaphragm contracts pulling downwards and expanding the lungs. This increase in volume causes the pressure in the lungs to decrease with respect to the air outside the body, which results in oxygen-rich air being sucked through the trachea and bronchi. The intercostal muscles and accessory muscles can also be used to enable greater expansion, i.e., when deep breaths are needed.

Inside the lungs, oxygen and carbon dioxide are exchanged between the air and blood via diffusion. This process occurs through the alveoli. These are small hollow structures, around 200 micrometers in diameter, with a large surface area for efficient gas exchange.

Upon exhalation, the diaphragm relaxes and the volume of the lungs decreases, and the carbon dioxide-rich air is forced out.

The rate at which gas enters or leaves the lungs is called ventilation and is controlled by the autonomic nervous system, specifically by the medulla oblongata and pons. If the level of carbon dioxide in the blood increases, this is detected by receptors in the aorta, carotid artery, and medulla. A signal is then sent to increase the rate of breathing to remove excess CO2.

Digestive

The digestive system is a large system of organs, composed of the mouth, esophagus, stomach, small intestine, large intestine, pancreas, liver, gallbladder, rectum, and anus. Its function is to break down food and absorb nutrients.

Food is passed from the mouth and digestion begins with chewing and the secretion of enzymes in saliva. The chewed food, or bolus, travels down the esophagus to the stomach via a series of muscular contractions known as peristalsis. Inside the stomach, gastric acid and more enzymes are secreted to further break down the stomach contents.

After the food has been sufficiently broken down in the stomach, the contents are passed into the small intestine. Peristalsis again moves food through the small intestine, firstly through the duodenum where it continues to be broken down by bile created by the liver and more enzymes secreted by the pancreas. Nutrients are then absorbed into the bloodstream through the jejunum and ileum, leaving a liquid residue.

After nutrient extraction, the waste contents, or stool, are passed to the large intestine or colon, which is composed of five sections―the cecum, the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. As the stool passes through the colon, water is extracted and it solidifies. When the descending colon is full, the contents are transferred to the rectum and then the anus for evacuation.

During digestion, the body secretes several enzymes that help to break down food. These include pepsin, which acts on proteins; lipase, which acts on fat; and amylase, which acts on carbohydrates.

As well as secreting enzymes, the pancreas also produces insulin. The liver processes the nutrients absorbed by the small intestine and uses them to produce chemicals required by the body. It also breaks down harmful chemicals and creates bile, which aids the digestion of fat. The bile is stored in the gallbladder and then released into the small intestine.