Respiration During Exercise I
The Respiratory System
z Provides
a means of gas exchange between
the environment and the body
– Alternatative to pulmonary ventilation?
– Pulmonary vs. cellular respiration
– Diffusion is the primary mechanism of gas
exchange
z Plays
a role in the regulation of acid-base
balance during exercise
Anatomy of
the
Respiratory
System
1
Position of
the Lungs,
Diaphragm
and Pleura
Conducting and Respiratory
Zones
Conducting zone
Respiratory zone
z Conducts air to respiratory z Where gas exchange
zone by bulk flow
occurs
z Humidifies, warms, and
z Airway generations 17filters air
23
– Respiratory bronchioles =
z Airway generations 0-16
– Trachea = generation 0
– L & R main bronchi = gen. 1
through
– Terminal bronchiole = gen.
16
gen. 17-19
– Alveolar ducts = gen. 20-
22
– Alveolar sac = gen. 23
Lung Cross-Sectional Area
z Total
cross-sectional area increases slowly
from generation 1-15 (conducting zone)
z Total cross-sectional area increases
exponentially from about generation 15
through 23
z Total surface area is around 50 to 100
square meters (tennis court)
z There are ~300,000,000 alveoli
2
Blood Gas Barrier
z
z
The BGB separates the alveolar air from the blood
in the pulmonary capillaries
It consists of:
– Alveolar endothelium
– Basement membrane (type IV collagen)
– Capillary endothelium (endothelial cell membrane)
z
Total thickness is 0.2 to 0.4 micrometers, thinner
than a red blood cell
Pathway of Air to Alveoli
Conducting
Zone
Respiratory
Zone
Muscles of Respiration
Inspiration
Expiration
3
Mechanics of Breathing
z
Ventilation
z
Inspiration - Active
– Movement of air into and out of the lungs via bulk flow
– Diaphragm contracts, pushing down on the abdomen
– External intercostals contract, push ribs and sternum up
and out
– Movements increases volume, lower thoracic pressure
– Pressure gradient creates flow
z
Expiration – Passive (at rest)
– Diaphragm & ext. intercostals relax, thoracic pressure
returns to atmospheric pressure
– During exercise, expiration can become active
with involvement of internal intercostals et al.
The Mechanics of Inspiration
and Expiration
Mechanics of Breathing:
Resistance to flow
z
z
z
Flow = potential/resistance (like I = V/R)
Airflow = delta Pressure/resistance
Primary determinant of resistance is airway diameter
(bronchodilation/bronchoconstriction – beta2 recept)
– Majority of airway resistance comes from resp. generations
~2-9 (upper conducting zone)
– Physical obstruction (choking) = resistance to inspiration
and expiration
– Asthma – bronchospasm – resistance to inflow and outflow
– Emphasema/COPD – resistance primarily to expiration,
dynamic compression of conducting airways, diagnosis
4
Law of Laplace and the role of
Surfactant
z
LaPlace: in a sphere, P = 4T/r
– P = pressure, T=surface (wall) tension, r=radius
– As r increases P decreases and vice versa
z
In a system of alveoli of different radii
– Pressure in small alveoli will exceed that in large alveoli
– A pressure gradient stimulates FLOW
– Small alveoli will collapse, larger alveoli will expand
z
Surfactant (a detergent) reduces the wall tension and
helps keep the alveoli inflated
– Surfactant is not produced until late gestation (type II cells)
– No surfactant at birth = acute infant distress syndrome
– Artificial surfactant
Mechanics: Elasticity, FRC, and
Pleural Pressure
z
z
z
z
z
Both the chest wall (ribs etc.) and the lung have
elastic properties
Lung recoil would tend to collapse the lung
The chest wall recoil would tend to expand the chest
The lung and chest wall forces reach equilibrium at
the functional residual capacity (FRC) at the lung.
This is the volume of the lung at the end of a normal,
passive expiration.
The balance of the inward pull of the lungs and the
outward pull of the chest creates the intrapleural
pressure of about –5 mmH2O
A key to some symbols
zA
= alveolar
z a = arterial
z c= capillary
z v = venous
z I = inspired
z E = expired
5
Pulmonary or Minute
Ventilation (V)
z
The amount of air moved in or out of the lungs per
minute (Liters/min)
– Product of tidal volume (VT) and breathing frequency (f)
V = VT x f
z
terms:
– VT
– f
– VA
– VD
Tidal volume – amount of air breathed in and
out in a single, normal breath (ml or L)
Respiratory rate – breaths/min
Alveolar ventilation – L/min
Deadspace ventilation – L/min
Alveolar vs. Dead Space
Ventilation
Dead Space – airway space where gas
exchange cannot take place – nose/mouth
through end of the conducting zone
- VD is the portion of the VE that
ventilates the dead space
- VA is the portion of the VE that
ventilates the respiratory zone
VE = VA + VD
Panting is primarily dead space ventilation
Pulmonary Volumes and Capacities
z
z
Measured by spirometry
Tidal Volume (VT)
z
Functional Residual Volume (FRC)
z
Vital capacity (VC)
– Volume of air inspired and expired in a a normal breath
– Volume of air remaining in the lungs after a normal expiration
– Maximum amount of air that can be expired following a
maximum inspiration
z
Residual volume (RV)
– Volume of air remaining in the lungs after a maximum
expiration
– Impossible to expire the RV
– Difficult to directly measure the RV
6
A Spirogram Showing
Pulmonary Volumes and
Capacities
Diffusion – Fick Principle
z
Gases diffuse from high → low partial pressure (as
ions diffuse from high to low concentration)
– Between lung and blood
– Between blood and tissue
z Fick’s
V gas =
law of diffusion
A
T
x D x (P1-P2)
V gas = rate of diffusion
A = tissue area
T = tissue thickness
D = diffusion coefficient of gas
P1-P2 = difference in partial pressure
Diffusion - Fick Principle
V gas =
A
T
x D x (P1-P2)
V gas = rate of diffusion
A = tissue area
T = tissue thickness
D = diffusion coefficient of gas
P1-P2 = difference in p pressure
A = dependent on alveolar surface area
T = thickness of the blood gas barrier – can be thickened by disease
or edema
D is proportional to solubility over the squ root of molecular wt
In general, bigger molecules have lower diffusion coefficients
CO2 is about 20X more soluble than O2 – much higher D
P1-P2 is affected by the PAO2 (P1) and the mixed capillary PO2 (P2)
*or CO2 or other gas*
7
Partial Pressure of Gases –
Computing the PIO2
z
z
z
Each gas in a mixture exerts a portion of the total
pressure of the gas (Dalton’s Law)
The partial pressure of oxygen (PO )
2
– Air is 20.93% oxygen
– Total pressure of air at sea level = 760 mmHg
– PO2 = 760*0.2093 = 159 mmHg
Water vapor: 100% saturation at body temp = 47
mmHg
PIO = 0.2093 x (760-47) = 150 mmHg
2
Got PIO2, Now computing the PAO2
z In healthy mammals, Alveolar PO2
– Atmospheric Pressure (sets PIO2)
– PACO2 – very similar to PaCO2
- RER
depends on:
“alveolar gas equation”
PAO2 = PIO2 – PaCO2/RER
Example:
PAO2 = 150 – 40/.8 = 150 – 50 = 100 mmHg
Computing PaCO2 (~PACO2)
z
In healthy mammals, PaCO2 depends on:
– Metabolic rate – VCO2
– Alveolar ventilation rate – VAO2
PACO2 = VCO2/VA * K
(K = constant)
So, PaCO2 is proportional to the metabolic rate
divided by the alveolar ventilation
- hyperventilation - high altitude - hypoventilation
8
Partial Pressure and Gas
Exchange
Why is PaO2 slightly less than
PAO2?
z PAO2
is always greater than PaO2
normal conditions in a healthy
mammal, this difference is < or = ~10 mmHg
z Contributors:
z Under
– Shunt – thebian circulation from lungs
– Incomplete equilibrium (diffusion limitation)
– Mismatch of ventilation and perfusion (more
later)
EXAMPLE: Flagstaff
What is the atmospheric pressure in Flagstaff
(~7,000 ft)?
9
O2 Transport in the Blood
z
z
O2 is not very soluble in water at body temp (CO2 is
~20X more soluble)
O2 is bound to hemoglobin (Hb) for transport in the
blood
– Oxyhemoglobin: O2 bound to Hb
– Deoxyhemoglobin: O2 not bound to Hb
z
Carrying capacity:
–
–
–
–
4 O2 per Hb = 100% saturated (3/Hb = 75% etc.)
1.39 ml O2/g Hb
Human males have about 15 g Hb/100ml blood
Human females tend to have lower [Hb]
Oxyhemoglobin Dissociation Curve
Flagstaff?
O2-Hb Dissociation Curve:
Effect of pH
z Blood
pH declines during heavy exercise in
humans (not necessarily in dogs etc.)
z Muscle pH may decline more (source of the
acid)
z Results in a “rightward” shift of the curve
– “Bohr effect”
– Favors “offloading” of O2 to the tissues
– Of course, increased pH leads to left shift
10
O2-Hb Dissociation Curve: Effect of pH
-Exercise
humans,
dogs
-High
altitude
short term,
llamas
-High
altitude
long term
O2-Hb Dissociation Curve:
Effect of Temperature
z Increased
blood temperature results in a
weaker Hb-O2 bond
z Decreased
temperature leads to left shift
z Increased temperature leads to right shift of
curve
– Easier “offloading” of O2 at tissues
O2-Hb Dissociation Curve: Effect of Tº
Tº
11
O2 Transport in Muscle
z Myoglobin
(Mb) shuttles O2 from the cell
membrane to the mitochondria
z Higher affinity for O2 than hemoglobin
– Even at low PO2
– Allows Mb to store O2
Dissociation Curves for
Myoglobin and Hemoglobin
CO2 Transport in Blood
z
Dissolved in plasma (5-10%)
– Solubility in H2O at body temperature is 0.067 ml
CO2/100ml H20/mmHg – about 3 ml/100ml blood if
PaCO2 is 40 mmHg
z
Bound to Hb (5-20%)
z
Bicarbonate (70-80%)
– “carbaminohemoglobin”
– CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3– Enzyme: carbonic anhydrase – found in RBC’s, not
free in plasma
– Also important for buffering H+
12
CO2 Transport in Blood
Release of CO2 From Blood
Ventilation and AcidAcid-Base Balance
z
z
Blood pH is regulated in part by ventilation
H+ + HCO3- --- CO2 + H2O
A decrease in ventilation causes a build up of CO2
– Hypoventilation if less than VCO2
– Increases blood PaCO2
– Raises H+ concentration – respiratory acidosis
z
An increase in ventilation causes exhalation of
additional CO2
– Hyperventilation if exceeds VCO2
– Reduces blood PaCO2
– Lowers H+ concentration – respiratory alkalosis
13