BIO 3520 Notes, 11/18/05
RESPIRATORY PHYSIOLOGY
I. Introduction. [Widmaier, pg. 477]
A. Joseph Priestley demonstrated the need for fresh air to support both
combustion and animal life.
1. Discovered oxygen in 1772.
B. Why do we need oxygen?
C. Respiration includes:
1. Cellular respiration (fig. 13-6).
a. Consumption of O2 and production of CO2 by cells.
b. At rest, O2 consumption = 200 ml/min.
c. May increase up to 30x during heavy exercise.
2. Pulmonary ventilation.
a. Movement of air into and out of lungs.
3. Gas exchange.
a. Between lungs and blood.
b. Between blood and cells.
D. Function of respiratory system is to deliver O2 to the blood and eliminate
CO2 from the blood.
1. O2 is delivered to tissues by cardiovascular system.
II. Comparative Respiratory Structures.
A. Importance of large surface area for gas exchange.
B. Aquatic vertebrates have gills (figure).
1. Adapted for gas exchange in water.
2. Respiratory surface is external.
3. Water must be actively passed across the surface at a very high rate.
C. Terrestrial vertebrates have lungs.
1. Adapted for gas exchange in air.
2. Respiratory surface is internal.
3. Air must be moved into and out of lungs.
D. Why can't an aquatic animal survive out of water?
E. Why can't a terrestrial animal survive underwater?
III. Anatomy of the Respiratory System. [pp. 478-480]
A. Two lungs are located within the thoracic cage (chest) (fig. 13-1).
1. Bordered by ribs, diaphragm, and heart.
B. Conducting zone -- no gas exchange.
1. Structures.
a. Nose and mouth.
b. Pharynx.
c. Larynx -- contains vocal cords.
d. Trachea.
e. Two main bronchi enter lungs.
f. Smaller bronchi.
1. Bronchi undergo about 25 divisions within the lungs (fig. 13-2).
2. Trachea and bronchi have cartilaginous rings to help maintain
an open airway.
g. Bronchioles.
1. About 1 mm or less in diameter.
2. Smooth muscle in walls.
3. No cartilage.
2. Functions of the conducting zone.
a. Conduction of air to and from lungs.
b. Warms and moistens air.
c. Protection from harmful chemicals and particles.
1. Mucus -- traps inhaled particles.
2. Cilia -- sweep mucus up to throat (mucus escalator).3. Cough -- violent expulsion of air.
4. Cystic fibrosis -- secretion of very thick mucus.
a. Bacteria are trapped ----> pneumonia.
d. Vocalization.
C. Respiratory zone.
1. Function -- Exchange of O2 and CO2 between inspired air and
capillary blood.
2. Structures (fig. 13-3).
a. Respiratory bronchioles.
1. Thin walled.
2. Very little smooth muscle.
b. Alveoli -- 300 million tiny air sacs.
3. Large surface area for diffusion.
4. Close contact with pulmonary capillaries.
IV. Pulmonary Ventilation. [pp. 478, 480-491]
A. The respiratory system is designed to draw air into the alveoli for the
purpose of gas exchange with capillary blood.
B. Air flow.
1. Air moves by bulk flow from an area of high pressure to an area of
low pressure.
2. Flow of air in lungs depends on pressure gradient between alveoli
and outside air.
Patm - Palv
Flow =
Resistance
3. When Palv = Patm, there is no movement of air (ex. at the end of each
expiration).
C. Respiratory cycle (figurea).
1. Inspiration (fig. 13-12).
a. Contraction of two sets of respiratory muscles causes thoracic
cage to expand.
1. Diaphragm moves downward.
2. Intercostal muscles move ribs upward and outward.
3. These are skeletal muscles.
b. Alveolar air pressure drops (fig. 13-8) ----> Palv < Patm ---->
air flows into lungs ----> alveoli fill with air (fig. 13-7).
2. Expiration (fig. 13-15).
a. Relaxation of diaphragm and intercostal muscles.
b. Elastic forces cause lungs and thoracic cage to decrease in volume.
c. Alveolar air pressure increases (fig. 13-8) ----> Palv > Patm ---->
air flows out of lungs (fig. 13-7).
D. Maintaining lung inflation.
1. What causes the lungs to expand when the chest wall expands?
2. Narrow, fluid-filled space between wall of thoracic cage and lungs --
intrapleural space (fig. 13-5).
a. Lungs have a tendency to collapse.
b. Creates a slight vacuum inside intrapleural space which prevents
lungs from collapsing.
c. When thoracic cage expands, vacuum forces lungs to expand with it.
3. Pneumothorax (figureb).
a. Puncture intrapleural space ----> allows lung to collapse.
b. Respiratory efforts cause air to move in and out of wound, without
inflating lung.
c. Insertion of chest tube to reinflate lung.
E. Lung compliance.
1. Normal breathing requires little muscular effort.
2. Compliance is the expandability of the lungs.
a. A 2 mmHg change in intrapleural pressure is all that is needed to
inhale 500 ml of air (fig. 13-12a).
Compliance = D lung volume
D intrapleural pressure
3. Low compliance ----> lungs are less elastic ----> requires more work
to produce same volume change (fig. 13-16).
a. Example: Pulmonary fibrosis.
4. Lung compliance is monitored by measuring the vital capacity using
a spirometer (figureb).
5. Most important determinant of lung compliance is surface tension
within alveoli.
a. Alveoli have a tendency to collapse due to attractive forces between
water molecules (figureb).
b. Surface tension is reduced by pulmonary surfactant produced by
alveoli.
c. Surfactant production begins late in gestation.
d. Lung immaturity in premature infants ----> respiratory distress
syndrome.
1. Treated with artificial surfactant.
F. Airway resistance.
1. The most important determinant of airway resistance is the diameter
of the bronchioles.
2. Decrease diameter ----> increase resistance to flow.
3. Diameter of the bronchioles is controlled by airway smooth muscle.
a. Parasympathetic stimulation causes contraction of airway smooth
muscle ----> bronchoconstriction.
b. Sympathetic stimulation causes relaxation of airway smooth muscle
----> bronchodilation.
4. Asthma.
a. Characteristics (figure).
1. Bronchoconstriction.
2. Excessive mucus secretion.
3. Inflammation.
4. All serve to decrease diameter of airways ----> increase resistance
----> increase work of breathing.
b. Treatment.
1. Bronchodilators.
2. Anti-inflammatory drugs.
5. Measurement of airway resistance.
a. Forced expired volume in one second (FEV1) is measured
using a spirometer.
b. FEV1 is below normal in asthma.
a Johnson, M.D. Human Biology: Concepts and Current Issues, 2nd ed., 2003.
b Sherwood, L. Human Physiology: From Cells to Systems, 5th ed., 2004.
V. Pulmonary Function Testing. [pp. 491-494]
A. Spirometer = Instrument used to measure lung volumes (figureb).
B. Normal breathing.
1. Tidal volume (VT) = Volume of air entering or leaving the lungs in a
single breath.
a. Normal VT = 500 ml.
2. Respiratory rate (f) = Number of breaths per minute.
a. Normal f = 12 - 20 breaths/min.
3. Minute ventilation (VE) = Total volume of air expired per minute.
VE = VT x f
4. Dead space (VD) = Volume of air in each breath that is not available
for gas exchange.
a. A portion of inspired air never reaches alveoli.
b. Stays in conducting system until expired (fig. 13-20).
c. VD (in ml) is about twice the body weight (in kg).
5. Alveolar ventilation (VA) = Volume of air that reaches the alveoli
per minute.
VA = (VT - VD) x f
a. Importance of breathing pattern (table 13-5).
1. Rapid, shallow breathing ----> inefficient.
2. Slow, deep breathing ----> more efficient.
6. Efficiency of different breathing patterns.
Pattern
VT
(ml)
VD
(ml)
f
(br/min)
VE
(ml/min)
VA
(ml/min)
Normal
500
150
12
Rapid, shallow
200
150
30
Slow, deep
1000
150
6
a. Most efficient pattern --
C. Total lung capacity can be divided into four volumes (fig. 13-19).
1. Tidal volume (defined above).
2. Inspiratory reserve volume (IRV) = volume of air that can be
inspired over and above the resting tidal volume.
3. Expiratory reserve volume (ERV) = Volume of air that can be
expired beyond a normal expiration.
4. Residual volume (RV) = Volume of air remaining in the lungs after
a maximal expiration.
a. Cannot be measured -- must be estimated.
b. RV ~ 25% of vital capacity (see below).
D. Any combination of two or more voumes is called a capacity.
1. Vital capacity (VC) = Maximum volume that can be exhaled after a
maximal inspiration.
VC = VT + IRV + ERV
a. Vital capacity is reduced when lung compliance is low
(ex. pulmonary fibrosis).
2. Total lung capacity (TLC) = Volume of lungs when fully inflated.
TLC = VC + RV
VI. Gas Exchange. [pp. 494, 498-499]
A. General principles about gas molecules.
1. Able to diffuse across membranes.
2. Able to dissolve in blood.
3. Movement will be from an area of high concentration to an area of
low concentration.
4. Gas concentration is usually expressed in terms of partial pressure.
B. Site of gas exchange -- alveoli.
1. Capillary network surrounds alveoli (fig. 13-3).
2. Alveoli are lined with a thin layer of epithelial cells (fig. 13-5).
C. Gas exchange in lungs (fig. 13-22).
1. Inspired air contains about 21% O2 and 78% N2, almost no CO2.
2. Blood returning to lungs is high in CO2 and is low in O2.
3. In pulmonary capillaries, O2 diffuses into capillary blood, while CO2
diffuses into alveolar air.
4. Blood leaving lungs is enriched with O2, low in CO2.
5. No exchange of gases occurs in heart, arteries, or arterioles.
D. Gas exchange in tissues.
1. Due to cellular respiration, tissues contain more CO2 and less O2.
2. O2 diffuses into cells, CO2 diffuses out.
3. Oxygen-poor blood returns to right heart, then lungs.
VII. Transport of Gases in Blood. [pp. 500-506, 514-515]
A. Oxygen -- bound to hemoglobin in red blood cells (fig. 13-29).
1. Oxyhemoglobin dissociation curve (fig. 13-27).
a. Arterial blood ----> 97% saturated with O2.
b. Venous blood ----> 75% saturated.
2. Carbon monoxide.
a. Competes with oxygen for binding sites on hemoglobin.
b. Affinity is more than 200x greater than oxygen.
c. Carboxyhemoglobin is bright red.
3. Hypoxia = Inadequate supply of oxygen to the tissues.
a. Cyanosis -- blue color due to reduced hemoglobin.
B. Carbon dioxide -- converted to bicarbonate ion (fig. 13-31).
CO2 + H2O < > HCO3- + H+
VIII. Control of Breathing. [pp. 507-514]
A. The major controlled variables in the control of breathing are the oxygen,
carbon dioxide, and hydrogen ion concentrations in blood.
B. Sensors -- chemoreceptors.
1. Central chemoreceptors.
a. Located near surface of brainstem (figurea).
b. Respond to increased CO2 or increased H+.
2. Peripheral chemoreceptors.
a. Located in carotid bodies and aortic arch (fig. 13-33).
b. Respond to decreases in O2 (hypoxia).
c. Increase firing of neurons going to brainstem.
C. Respiratory center.
1. Located in brainstem.
2. Controls rate and depth of breathing.
3. Receives input from chemoreceptors, other sensors, and from cerebral
cortex (figurec).
4. Basic respiratory rhythm involves interplay of inspiratory and
expiratory neurons, subject to alteration by other neurons.
5. Slice of brainstem less than 1 mm thick generates a respiratory-like
rhythm.
D. Neurons from respiratory center go to respiratory muscles (figurea).
1. Contraction ----> inspiration.
2. Relaxation ----> expiration.
E. Factors that stimulate breathing.
1. Increased CO2 (fig. 13-36).
a. 5% CO2 inhalation will double VE (figurec).
2. Acidosis (increased H+, decreased pH) (fig. 13-38).
3. Hypoxia (decreased O2) (fig. 13-34).
4. Stress (activation of sympathetic n.s.).
5. Exercise (fig. 13-41).
F. Control system diagram: Regulation of arterial Pco2 (fig. 13-37).
c Moffett, Moffett, and Schauf, Human Physiology: Foundations & Frontiers, 2nd ed., 1993.
