I. Overview of the Respiratory System
A. Airways to the Lungs
1. The respiratory system brings in oxygen that each body cell requires and takes away carbon dioxide that
every cell generates.
2. Through nasal cavities, air enters or leaves the respiratory system; hair and ciliated epithelium filter dust
and particles. Blood vessels warm the air and mucus moistens it.
3. Air moves via this route: nasal cavities >>> pharynx >>> larynx >>> vocal cords (space between is
glottis) >>> trachea >>> bronchi >>> bronchioles respiratory bronchioles >>> alveoli.
a) The trachea leads from the larynx downward to branch into two bronchi, which are lined with
cilia and mucus to trap bacteria and particles.
b) The bronchi branch repeatedly into smaller bronchioles (including respiratory bronchioles)
which lead ultimately to the dead-end alveoli.
c) The vocal cords, which lie on either side of the larynx wall, are elastic ligaments that vibrate
when air passes out through the space between them called the glottis, which is closed off during
swallowing to prevent choking.
B. Sites of Gas Exchange in the Lungs
1. Human lungs are a pair of organs housed in the rib cage above the diaphragm.
a) Each lung lies within in a thin pleural membrane that is folded in a manner that forms a pleural
sac leaving an intrapleural space filled with a lubricating intrapleural fluid.
2. Inside the lungs, respiratory bronchioles bear outpouchings of their walls called alveoli (usually
clustered as alveolar sacs), which provide a tremendous surface area for gaseous exchange with the blood
located in the dense capillary network surrounding each alveolar sac.
II. A Closer Look at Gas Exchange
A. Pressure Gradients and Transport Pigments
1. Respiratory systems rely on the diffusion of gases down pressure gradients.
a) Air is 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, and 0.96% other gases.
(1) Partial pressures for each gas in the atmosphere can be calculated; for example,
oxygen's is 160 mm Hg.
(2) Oxygen and carbon dioxide diffuse down pressure gradients from areas of high partial
pressure to areas of low partial pressure.
(3) Diffusion will occur across a respiratory surface if it is thin, moist.
(4) The speed and extent of diffusion depends on the surface area present in the
2. Transport pigments carry most of the oxygen.
a) Hemoglobin is the main transport pigment.
b) Each molecule binds four molecules of oxygen in the lungs (high concentration) and releases
them in the tissues where oxygen is low.
B. Gas Exchange in Unusual Environments
1. In decompression sickness bubbles of nitrogen come out of solution in the blood and block blood flow to
a) Divers must take tanks of compressed air when they go below.
b) They must ascend slowly to prevent nitrogen bubbles in the blood–the "bends," and must also
guard against nitrogen narcosis.
2. Hypoxia occurs when tissues do not receive enough oxygen.
a) At high altitudes the partial pressure of oxygen is lower than at sea level, so that
hyperventilation may occur.
b) Carbon monoxide combines with hemoglobin 200 times faster than does oxygen; CO poisoning
III. Breathing–Cyclic Reversals in Air Pressure Gradients
A. The Respiratory Cycle
1. To inhale (inspiration), the diaphragm contracts and flattens, muscles lift the rib cage upward and
outward, the chest cavity volume increases, internal pressure decreases, air rushes in.
2. To exhale (expiration) the actions listed above are reversed; the elastic lung tissue recoils passively.
B. Lung Volumes
1. About 500 ml of air (tidal volume) enters and leaves with each breath .
a) A human can forcibly inhale 3,100 ml of air (inspiratory reserve volume) and forcible exhale
1,200 ml (expiratory reserve volume).
b) The maximum volume that can be moved in and out is called the vital capacity (4,800 ml for
males, 3,800 ml for females). About 1,200 ml of residual air remains in the lungs and cannot be
IV. Gas Exchange and Transport
A. Ventilation is not the same as respiration, which is the actual exchange of gases between the blood and cells.
1. In external respiration, oxygen moves from the alveoli to the blood; carbon dioxide moves in the
2. In internal respiration, oxygen moves from the blood into tissues and vice versa for carbon dioxide.
B. Gas Exchange in Alveoli
1. Each alveolus is only a single layer of epithelial cells through which gases can readily diffuse into
interstitial fluid and blood capillaries.
2. The partial pressure gradients are sufficient to move oxygen in and carbon dioxide out of the blood,
C. Gas Transport Between Blood and Metabolically Active Tissues
1. Blood cannot carry sufficient oxygen and carbon dioxide in dissolved form as the body requires;
hemoglobin helps enhance its capacity.
2. Oxygen Transport
a) Oxygen diffuses down a pressure gradient into the blood plasma >>> red blood cells >>>
hemoglobin where it binds at a rate of four oxygens to one hemoglobin to form oxyhemoglobin.
b) Hemoglobin gives up its oxygen in tissues where partial pressure of oxygen is low, blood is
warmer, and pH is lower; all three conditions occur in tissues with high metabolism.
3. Carbon Dioxide Transport
a) Because carbon dioxide is higher in the body tissues, it diffuses into the blood.
b) Seven percent is dissolved in plasma, 23% binds with hemoglobin (forming
carbaminohemoglobin) and 70% is in bicarbonate form.
c) Bicarbonate and carbonic acid formation is enhanced by carbonic anhydrase, an enzyme located
in the red blood cells.
V. Controls Over Gas Exchange
A. Matching Air Flow with Blood Flow
1. Gas exchange in the alveoli is most efficient when air flow is matched with the rate of blood flow.
2. Respiratory centers in the brain play a central role in the rhythm and magnitude of breathing events.
a) A portion of the reticular formation in the medulla coordinates the signals for the timing of
exhalation and inhalation; the pons fine tunes the rhythmic contractions.
b) When breathing is deep and rapid, stretch receptors in the alveoli send signals to the brain
control centers, which respond by inhibiting contraction of the diaphragm and rib muscles.
c) The brain monitors input from carbon dioxide sensors in the bloodstream and from receptors
sensitive to decreases in oxygen partial pressure (carotid bodies and aortic bodies).
B. Local Chemical Controls
1. When the rate of blood flow in the lungs is faster than the air flow, the bronchioles dilate to enhance the
air flow and thus the rate of diffusion of the gases.
2. When the air flow is too great relative to the blood flow, oxygen levels rise in the lungs and cause the
blood vessels to dilate increasing blood flow.
C. Disrupted Controls over Gas Exchange
1. Apnea is a brief interruption in the respiratory cycle.
2. Sleep apnea is a common problem in aging because the mechanisms for sensing changing oxygen and
carbon dioxide levels gradually become less effective over the years.