Respiratory System
Part II
Chapter 22
Gas Exchange (Blood, Lungs, Tissues)
• As blood enters the lungs via pulmonary circuit, oxygen
levels are low and carbon dioxide levels are high
• In the alveoli oxygen is taken up and carbon dioxide is
unloaded
• Oxygen is delivered to body tissues and cells
• Three factors influence the movement of these gases
– (1). Partial pressure and gas solubilities
– (2). Alveolar ventilation and pulmonary blood perfusion
– (3). Structural aspects of respiratory membrane
Gas Laws and Properties of Gases
• Gas exchange in the lungs and tissues takes place via simple diffusion
• We measure gases in partial pressures
– Dalton’s Law of Partial Pressure states that the total pressure exerted by a
mixture of gases is the sum of the pressures exerted independently by each
gas (it’s partial pressure) in the mixture.
• N and O (21%) account for 99% of the gas in the atmosphere; 0.5%
carbon dioxide
• Total partial pressure of gases in atmosphere = 760 mmHg
• PO2 = 159 mmHg (decreases at higher altitude = 110 mmHg)
– Henry’s Law states that when a mixture of gases is in contact with a liquid,
each gas will dissolve in the liquid in proportion to it’s partial pressure
• The amount of gas that dissolves in a liquid is dependent upon the
solubility of the gas and the temperature of the liquid.
Gas Solubility
• Solubility of gases depends on:
– 1. Solubility of a particular gas in a liquid
• Carbon dioxide most soluble in air than O2
– 2. Temperature of the liquid/Pressure
• As Temperature rises, gas solubility decreases
• As Pressure increases, gas solubility increases
• The composition of gases in the alveoli differs from that of the
air (table 22.4)
– Atm (mostly O2 and N2)
– Alveoli – more CO2 and water vapor and less % of oxygen
• Oxygen moves into the blood (lower in alveoli); mixture of new gases
in the conducting tubes
• P02 = 104; PCO2 = 40 mm Hg
• The differences in PP of the gases allows for gas exchange via
diffusion
• Alveoli and gas pressure
Factors Influencing Movement of Gases
• 1. Partial Pressures and Gas Solubility's
– Pressure difference across membranes allow movement
• 2. Ventilation-Perfusion Coupling
– Ventilation (gas reaching the alveoli) must be almost at an equal
rate to perfusion (blood flow in the capillaries)
• 3. Membrane Thickness
– 0.5 – 1.0 micron is optimal and efficient
Partial Pressure Gradients
• Top: gradients promoting gas
exchange in the lungs
• Bottom: gas exchange at the
tissue level
• Gases measured in partial
pressures
– PO2
– Movement is ‘down’ the
concentration pressure
gradient
• Gas Exchange
• Gas Exchange II
Ventilation-Perfusion Coupling in the Lungs
• Definitions
– Ventilation = gas that reaches the alveoli
– Perfusion = blood flow in the pulmonary capillaries
• Autoregulatory controls (affecting the alveoli)
– 1. If ventilation is poor (low PP of oxygen); terminal arteries contract
shunting blood to areas where PP oxygen is high
– 2. If ventilation is high (high PP of oxygen); arterioles dilate which
increases blood flow in the pulmonary capillaries
• PP of carbon dioxide affect bronchiole diameter
– 1. Areas where PP CO2 is high, bronchioles dilate so that it can be
eliminated
• Results
– 1. This balances ventilation with perfusion
• Poorly ventilated alveoli (low oxygen and high carbon dioxide),
bronchioles dilate, and more air is brought in (more oxygen).
Ventilation and Perfusion Coupling
Oxygen Transport
Most oxygen is transported via (1) hemoglobin and by (2) dissolved in plasma (1.5%)- low
solubility in water
– Hemoglobin is very important in that it transports around 98% of the oxygen to the
tissues
HbO2 (oxyhemoglobin) ------- HHb + O2 (deoxyhemoglobin)
– Oxygen gas is ‘loosely’ attached to hemoglobin
HHb + O2 ↔ HbO2 + H+ (oxyhemoglobin)
– Loading and unloading of hemoglobin
– Right: lungs; Left: tissues
Rate at which Hb binds or releases oxygen depends on temperature, blood pH, PCO2, BPG
(organic chemical)
Influence of PO2 on hemoglobin saturation
– Oxygen-hemoglobin dissociation curve (22.20)
Oxygen-hemoglobin Saturation Curve
Influence of PO2 on
hemoglobin loading
and unloading
Influence of PO2 on Hemoglobin Saturation
• During resting conditions, PO2 = 100 mm Hg (98%
saturation of Hb in arterial blood)
– 100 ml of systemic blood contains 20 ml of oxygen (20 vol %)
• Blood flows into the systemic circuit (capillaries) - Hb
saturation is 75%
– 5ml of oxygen is released / 100 ml of blood
– Oxygen content is now 15% volume
• Important points
– 1. At PO2 mm Hg, Hb almost fully saturated with oxygen.
Increase in pressure causes very little increase in saturation.
• Higher altitudes there is enough oxygen (sufficient Hb loading) when
PP oxygen is lower
Influence of PO2 on Hemoglobin Saturation
• Only around 25% of oxygen unloading occurs during
one systemic circuit
– High reserve of oxygen is left in the venous blood (venous
reserve)
• Applications for exercise (more oxygen will dissociate from
hemoglobin during vigorous exercise and move into the tissues
with out increasing respiratory rate or CO
Dissociation Curve and the Bohr Shift
Other Factors Influencing Hemoglobin Saturation
Temperature, blood pH, PCO2 and BPG(2,3-bisphosphate) affect
saturation at a given PO2 in blood
Cells metabolize glucose using oxygen and produce CO2 and H+
(lowers pH) levels in the capillary blood
– Occurs during cell respiration. More oxygen is needed by the cell to
continue respiration
All factors modify hemoglobin structure-affects it’s affinity for
oxygen
– Increase in all of the factors listed above will decrease Hb affinity for oxygen
(curve to shift right)
• Declining pH and increasing PCO2 weakens the Hb bond which is called
the Bohr Effect.
• Allows for oxygen to be unloaded when really needed
Decrease in factors causes higher affinity and curve shifts left
– Increasing pH causes higher Hb affinity for oxygen
Carbon Monoxide and Hb Affinity
• Any inadequate delivery of oxygen to the tissues is
called hypoxia.
• Hb saturation falls below 70%
–
–
–
–
1. Anemic
2. Ischemic – congestive heart failure blocks blood flow
3. Histotoxic – cells cannot use oxygen due to poisons (CN)
4. Hypoxemic – reduced air flow to lungs; CO poisoning
• Carbon monoxide poisoning
– 200 times Hb affinity for CO compared to oxygen
– Takes oxygen’s place
– Cyanosis, headache and respiratory distress
Making Sense of CO2 Transport
• Transported by following means
– (1). Blood plasma (7%)
– (2). Chemically bonded to hemoglobin
(carbaminohemoblobin) (23%)
• CO2 + Hb ↔ HbCO2
– (3). HCO3- (70%)
• Bicarbonate ion in plasma
• CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3• Carbonic anhydrase in RBCs
– Carbonic acid converted to H+ and binds to Hb
increasing the RELEASE OF OXYGEN!
– Hb loading increases O2 unloading!
• HCO- is released into the plasma and onto the lungs
– Cl- moves into the cell to account for the loss of the
negative ion (chloride shift)
Gas Transport
Haldane Effect
• Lower PO2 and lower Hb saturation - the more CO2 can
be transported
• As CO2 enters systemic bloodstream, causes more
oxygen to dissociate from Hb (Bohr effect) 22.22A
– Allows for more CO2 to combine with Hb
– HbO2 --------- O2 + Hb
• Opposite in pulmonary circulation
– Hb saturated with oxygen, the H+ released combines with HCO3----------- unloads carbon dioxide
– O2 + HHb ---------- HbO2 + H+
– H+ + HCO3- --------- H2CO3 ----- CO2 + H2O unloaded in lungs
22.22b
Buffering pH Changes
• Carbonic Acid – Bicarbonate Buffer System
• CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3• Points
– Resists shifts in pH
– If concentration of H+ rises (pH falls) will be tied up by
bicarbonate to form carbonic acid (weak acid- changes pH very
little)
– Maintains pH of around 7.3
• Medullary groups (control
respiration)
– 1. DRG (dorsally near root of CN IX)
• Integrate peripheral sensory
input; modify rhythms of VGR
• Takes input from peripheral
stretch and chemoreceptors and
sends to the VGR
– VRG
• Rhythm generators ‘drive’
respiration
• VRG neurons fire which activate
respiratory motor neurons which
cause inspiratory muscles to
contract (inspiration)
• Expiratory neurons fire inhibiting
the VRG and expiration occurs
Neural Pathways
Neural Pathways
• Pons
– PGR (pontine respiratory group)
– Sends impulses to the VRG of
the medulla
– Helps to modify the rhythm of
the VRG (during sleep, voice,
exercise)
Neural and Chemical Influences on Medullary Respiratory
Centers
•
Irritant Receptors
– Mucus, dust constrict
bronchioles (-)
•
Inflation Reflex (stretch)
– Baroreceptors: lungs
over-inflate sends
signals to medulla
which causes decrease
in inspiration (-)
•
Chemical Factors
– PCO2 ; PO2; pH
Chemical Factors
• Chemoreceptors monitor gas levels and H+ levels in the blood
• 1. Effects of PCO2 (central chemoreceptors)
– As carbon dioxide rises, pH falls in the cerebrospinal fluid --- increase in
respiration (hypercapnia)
• 2. Effects of PO2 (peripheral chemoreceptors)
– Levels must drop below 60 mm Hg before it causes increased ventilation
• Will stimulate ventilation even if carbon dioxide levels are normal
• 3. Effects of pH
– Falling pH levels is monitored by the peripheral chemoreceptors
• Causes increased breathing
– Central chemoreceptors are not involved due to the lack of H+ diffusing
from the blood to the cerebrospinal fluid
Location of Chemoreceptors
Adjustments
• Exercise
– Breathing becomes hyperpnia but not hyperventilation
– Gas levels maintained (actually oxygen may increase slightly)
– Higher Exercise induced ventilation caused by
• Psychic stimuli, proprioceptors relay stimuli to respiratory centers
• High Altitude
– As oxygen levels drop above 8000 ft. carbon dioxide sensors become more sensitive
– Low oxygen levels means increased ventilation
– After 2 or 3 days you acclimatize or respiratory volume stabilizes at a level 2-3 L/min
higher than at sea level
– Hb saturation is only 67% at 19,000 feet; but Hb unloads only 25% of oxygen so tissue
oxygen needs are met
– If all out effort takes place then fatigue sets in. Kidneys will produce EPO and more
erythrocytes