The respiratory system allows air to travel to the lungs. Air enters the mouth or nasal passages, travels through the trachea, and then into the bronchi. The bronchi split into smaller and smaller bronchioles before terminating in small sacs called alveoli.

The aorta is a major artery that carries blood away from the heart. The urethra carries urine from the bladder. The small intestine is a portion of the digestive tract that carries nutrients from the stomach to the large intestine.

Blood is composed of multiple cell types that have specific functions in the body. The red blood cells (erythrocytes) help carry oxygen to the tissues in the body. The white blood cells (leukocytes) help defend the body from bacteria and other illnesses. Platelets are cell fragments responsible for helping blood clot, and are created by megakaryocytes.

The heart has four chambers: two atria and two ventricles. The atria are responsible for receiving blood returning from the body, while the ventricles pump blood out of the heart. The right ventricle pumps blood to the lungs, while the left ventricle pumps blood to the rest of the body.

The flow of blood through the heart is: right atrium, right ventricle, lungs, left atrium, left ventricle, body.

Arteries are the high pressure blood vessels responsible for carrying blood away from the heart. All arteries in the body travel away from the heart toward other tissues.

Most arteries carry oxygenated blood, but the pulmonary arteries carry deoxygenated blood; these vessels are responsible for transporting deoxygenated blood from the right side of the heart to the lungs. Veins are responsible for returning blood back to the heart. Arteries do not allow for diffusion of nutrients; this process primarily occurs in capillaries.

There are three primary types of hormones: peptide hormones, steroid hormones, and tyrosine derivative hormones. Of the three, peptide hormones are the only polar hormones, so they cannot pass the cell membrane. As a result, they must attach to a membrane-bound receptor in order to elicit a response in the cell.

Tyrosine derivatives and steroid hormones are smaller, nonpolar molecules. This allows them to pass directly through the membrane, rather than binding to a receptor on the surface. Steroid hormones are derived from cholesterol, but there is no such thing as "cholesterol hormones."

Steroid hormones are created in the gonads and the adrenal cortex. They include testosterone, estrogen, cortisol, aldosterone, and progesterone.

Human growth hormone, oxytocin, and insulin are all peptide hormones, meaning that they are proteins made of amino acids. In contrast, steroid hormones are derived from cholesterol and have distinct ring structures.

Managing blood sugar levels is primarily the responsibility of the pancreatic hormones glucagon and insulin. Glucagon is responsible for raising blood sugar levels while insulin helps reduce the sugar levels.

Oxytocin is a hormone secreted from the posterior pituitary gland; it induces labor and contractions during childbirth. Thyroxine (also known as thyroid hormone or T4) is secreted from the thyroid and helps regulate metabolism.

The pituitary gland is responsible for a number of peptide hormones, including follicle-stimulating hormone, thyroid-stimulating hormone, and prolactin. Aldosterone, however, is produced in the adrenal cortex and functions to raise blood pressure by preventing fluid loss via urination.

B-cell lymphocytes are responsible for the production of antibodies. These white blood cells secrete antibodies, which bind to and promote the destruction of pathogens

Macrophages attack pathogens directly. T-cell lymphocytes can help attack pathogens as well, and can also trigger B-cells to release antibodies. Red blood cells (erythrocytes) are not involved in the immune response.

When it comes to blood type, the type of blood you have is determined by the type of antigens present on your red blood cells. For example, people with type A blood have the A antigen on their red blood cells, but not the B antigen. People with O blood have no antigens on their red blood cells. The immune system of a person with A blood will respond negatively to B blood if it is introduced into the body.

A person with type A blood could receive type A blood (A antigens) or type O blood (no antigens), but could not receive type B blood (B antigens) or type AB blood (A and B antigens).