Respiratory System - Courtney Krishnan and Sam Dunkle


  • Gas exchange - synonym for respiration; uptake of oxygen from the environment and discharge of CO2 to the environment. This is necesssary to help produce ATP and help cellular respiration.
  • Respiratory medium - source of 02 (air for animals)
  • Respiratory surface - where gases are exchanged in an animal's body
  • Lungs - respiratory surface for terrestrial vertebrates that connects to the atmosphere by narrow tubes
  • Diaphragm - sheet of skeletal muscle that forms bottom wall of chest cavity
  • Tidal volume - volume of air a mammal inhales and exhales with each breath
  • Vital capacity - maximum tidal volume for a forced breath
  • Residential volume - air remaining after you exhale as much as possible
  • Epithelium lining - lining surrounding major branches of the respiratory system; covered by cilia and a thin layer of mucus; it helps cleanse respiratory system
  • Partial pressure - portion of atmospheric pressure contributed by a certain molecule

Process of Breathing

    1. Pathway starts at nostrils and leads to the nasal cavity, which is where air is filtered by hairs and warmed
    2. From the nasal cavity, air goes to the pharynx - where food and air paths cross - and then to the larynx
    3. Air then goes to the trachea, which is the wind pipe
    4. Trachea forks into two bronchi, with one going into each lung
    5. Bronchus branches into smaller tubes called bronchioles (this part of the respiratory system looks like an upside-down tree)
    6. Clusters of air sacs called alveoli form at then tips of the bronchioles

Gas exchange
Gas exchanged occurs across the epithelia of the millions of alveoli. Oxygen from the air entering the alveoli dissolves and diffuses across the epithelium into a web of capillaries, which surround each alveolus. Carbon dioxide diffuses in the opposite direction as air does (from capillaries to the alveoli and air space).

Inhalation and Exhalation
Breathing is the inhalation and exhalation of air; it is the process which ventilates the lungs. Mammals have negative pressure breathing, where air is pulled - as opposed to pushed - into the lungs. The rib cage expands and get smaller, and the lungs match this change in volume. Two specific muscles, the diaphragm and the ribs, are essential in the breathing process. The diaphragm contracts and descends like a piston. The rib muscles contract and expand the rib cage by pulling the ribs upward and the breastbone outward. Together when inhaling, they cause an increase in lung volume. The air pressure in the alveoli becomes lower than the atmospheric pressure and air goes into the alveoli (gas always moves from high to low pressure). The exhalation process is opposite to inhalation (lung volume decreases, air pressure is high in the alveoli, and air goes out). The changing lung volume depends whether a person is at rest or active. If active, other muscles including the rib and diaphragm cause the rib cage to open even more.

Control of Breathing

The body has two breathing control centers. The pons is considered the control center, and the medulla oblongata is where the basic rhythm of breathing is set. The medulla oblongata regulates in response to changes in pH of cerebrospinal tissue fluid in the brain. This fluid is determined by CO2 in the blood. Therefore, if CO2 is increased, there will be a decrease in pH, and the body will increase breathing to gather more oxygen. Secondary control factors also exist. There are sensors in the aorta and carotid arteries that monitor O2 and CO2 concentrations, as well as blood pH. When O2 is very low, an alarm is sent to the medulla, and the body will increase its breathing rate. It is important to note that breathing control is only effective if coordinated with cardiovascular control.

Respiratory Pigmentscontrol_of_breathing.JPG

Oxygen and carbon dioxide are cylced through the body via the bloodstream. Oxygen doesn't dissolve well in water, and thus blood, so it is bound to respiratory pigments to help it enter the bloodstream quickly. When oxygen is entering the bloodstream to be distributed to tissues, it has a high partial pressure of oxygen and a low carbon dioxide partial pressure. The oxygen enters the bloodstream and is distributed to tissues throughout the body. When blood has given the oxygen to tissues, it has a high partial pressure for carbon dioxide and a low partial pressure for oxygen. The CO2 then enters the alveoli, which have a low CO2 patial pressure. The CO2 is eventually released into the atmosphere. The protein hemoglobin is the primary respiratory pigment in humans. It has four subunits with iron attached, allowing it to carry four molecules of oxygen. Hemoglobin also helps transport CO2, and prevents harmful changes in blood pH.


Reece, Campbell. Biology, Seventh Edition. 2005.