I. Respiration is the word we use for gas exchange. The gasses need to be exchanged because animals use oxygen to extract energy from their food molecules and because carbon dioxide is a principle waste product of those chemical reactions (think back to cellular respiration. Glucose and oxygen were on one side of the equation, CO₂ and energy were on the other). Respiration is the process of getting the gasses (CO₂ and O₂) into and out of an organism. There are 3 phases:

A. Breathing, or Ventilation. This is the moving of air (or water) over the exchange surface.
B. Transport. Gasses dissolved in blood are moved to the cells (O2) or away from the cells (CO2).
C. Gasses move from circulatory fluid into cells and from cells into fluid.

A. Some animals use their outer surface as the exchange surface. They must live in a moist environment and, because of surface area limitations, must be either small, flat, or both.
B. Gills are exchange organs used by aquatic organisms. Their complex structure increases their surface contact with the water. They may be external (fish, salamanders) or internal (mussels, dragonfly larvae). In either case they are exposed to the water.
C. Insects have a series, or network, of internal tubes that connect to the outside of the animal. Gasses move through these tracheae and diffuse into the fluid inside the animal.
D. Most terrestrial vertebrates use lungs, moist internal sacs, for gas exchange.

III. Lungs. Most vertebrates have them. They are only in one part of the body so there must also be a circulatory system to get the gasses to and from the lungs. The following pertains to humans (and other mammals).

A. Air enters through the mouth and nose
B. Passes through the pharynx.
C. Then passes through the larynx.
D. Air continues along the trachea, a tube made rigid by cartilage.
E. The trachea has two branches called bronchi.
F. Bronchi branch into bronchioles.
E. At the ends of the bronchioles are tiny sacs called alveoli. The sacs are lined with a moist layer of epithelial tissue. Just beneath the epithelial tissue layer are many capillaries - tiny blood vessels.

A. Inhalation takes place when the diaphragm moves down and rib muscles move the ribs out. Chest cavity expands, reducing air pressure in the lungs, air moves in.
B. Exhalation happens when rib mussels and diaphragm relax. Chest cavity gets smaller, air goes out. We do A & B about 4 -10 million times a year. The maximum volume of air that a person moves when exercising is 3500 ml to 4800 ml. This is called a person's vital capacity.
C. Breathing is controlled by the brain at the level of the brain stem. We don't have to think about it. The medulla, at the base of the brain, monitors blood pH. As carbon dioxide builds up in the blood the blood pH drops. This triggers an increase in breathing rate. We don't rely on oxygen sensors to gauge oxygen levels in the blood. Because production of CO₂ at the cellular level is directly coupled to consumption of oxygen, balancing CO₂ levels takes care of oxygen levels.

A. The flow of blood is:  Blood is pumped up from the heart to the lungs where it picks up oxygen. It flows back to the heart (the other side) and is pumped out to the body where it is stripped of its oxygen and where it takes on CO2. It returns to the heart where it is pumped to the lungs.....
B. Oxygen is soluble in water, but not enough to help much in a circulatory system. Most of the O2 is carried on hemoglobin, a complex protein with a special affinity for oxygen. Oxygen is released by hemoglobin in areas where O2 concentrations are low, i.e. at the cells. Carbon Monoxide also binds to hemoglobin blocking the oxygen binding sites. This is not good.