Transcript respiration - people.vcu.edu

RESPIRATION
Gas Exchange
PARTIAL PRESSURES
In a mixture of gasses, the total pressure
distributes among the constituents
proportional to their percent of the total
The concentration of a gas can therefore
be expressed as its partial pressure
Partial Pressures in air
Oxygen = 21%
Po2
Nitrogen = 79%
PN2 = 600 mm Hg
=
160 mm Hg
Total Pressure (at sea
level) = 760mm Hg
Effect of water vapor
As fresh air enters the nose and mouth it
is immediately mixed with water vapor
Since the total pressure remains constant,
the water vapor lowers the partial
pressure of all other gases
For this reason, the PO2 is lowered to
about 149 mmHg
DEAD SPACE VOLUME
At the height of expiration, about 150ml
of gas still occupies the respiratory tree
This “old gas” is necessarily mixed with
the incoming fresh air and further lowers
the PO2 to about 100 mmHg
GAS EXCHANGE ACROSS
PULMONARY CAPILLARIES
Both oxygen and
carbon dioxide diffuse
down their
concentration (partial
pressure) gradients
Inspired Air PO2 = 160mmHg
PCO2 = 0.03mmHg
LUNG
PO2 = 100mmHg
PCO2 = 40mmHg
PULMONARY CAPILLARIES
PO2 = 40mmHg
PCO2 = 46mmHg
PO2 = 100mmHg
PCO2 = 40mmHg
GAS EXCHANGE ACROSS
SYSTEMIC CAPILLARIES
Both oxygen and
carbon dioxide diffuse
down their
concentration (partial
pressure) gradients
TISSUE
PO2 < 40mmHg
PCO2 > 46mmHg
SYSTEMIC CAPILLARIES
PO2 = 40mmHg
PCO2 = 46mmHg
PO2 = 100mmHg
PCO2 = 40mmHg
Carbon dioxide/Bicarbonate
Relationship
CO2 + H2O <---> H2CO3 <---> H+ + HCO3-
Carbon dioxide dissolved in water readily combines with
water to form carbonic acid. The carbonic acid then
dissociates into the hydrogen ion and bicarbonate ion.
The former reaction is catalized by and enzyme called
Carbonic Anhydrase in many tissues.
GAS TRANSPORT IN BLOOD
Oxygen physically dissolved = 1.5%
Oxygen bound to hemoglobin = 98.5%
Carbon dioxide physically dissolved =
10%
Carbon dioxide bound to hemoglobin =
30%
Carbon dioxide as bicarbonate = 60%
HEMOGLOBIN/OXYGEN DISSOCIATION
Resting PO2
Systemic
Normal PO2
Capillaries
% Hemoglobin
Saturation
PO2 of blood (mmHg)
Agents which shift the Hb/O
Dissociation curve: The Bohr Effect
UNDERSTANDING THE HB/O
DISSOCIATION CURVE
The plateau: Provides a margin of safety
in the oxygen carrying capacity of the
blood
The steep portion: Small changes in
Oxygen levels can cause significant
changes in binding. This promotes
release to the tissues.
Agents which shift the Hb/O
Dissociation curve: The Bohr Effect
Carbon dioxide/Bicarbonate
Relationship
CO2 + H2O <---> H2CO3 <---> H+ + HCO3-
Carbon dioxide dissolved in water readily combines with
water to form carbonic acid. The carbonic acid then
dissociates into the hydrogen ion and bicarbonate ion.
The former reaction is catalized by and enzyme called
Carbonic Anhydrase in many tissues.
Carbon Dioxide Transport
in the Blood: At the tissues
Tissue Cell
Carbonic Anhydrase
CO2 + H2O ---> H2CO3 ---> H+ + HCO3
+ Hb --->HbH
+ Hb ---> HbCO
2
HbO2 -----> Hb + O2
Red Blood Cell
Carbon Dioxide Transport
in the Blood: At the lungs
Alveolus
Carbonic Anhydrase
CO2 + H2O <--- H2CO3 <--- H+ + HCO3+ Hb <---HbH
+ Hb <--- HbCO
2
HbO2 <--- Hb + O2
Red Blood Cell
The Haldane Effect
Removal of oxygen from hemoglobin
increases hemoglobin’s affinity for carbon
dioxide
This allows carbon dioxide to “ride” on the
empty hemoglobin
RESPIRATORY CONTROL
Pons: Pneumotactic center
Pons: Apneustic center
Medulla: Dorsal respiratory group
Medulla: Ventral respiratory group
Medulla: Dorsal respiratory
group
Inspiratory neurons
Pacemaker activity
Expiration occurs when these cease firing
Medulla: Ventral
respiratory group
Both inspiratory and expiratory neurons
Inactive during normal quiet breathing
Rev up inspiratory activity when demands
for ventilation are high
Pons: Pneumotactic center
Fine tuning over medullary centers
Switches off inspiration
Pons: Apneustic center
Fine tuning over medullary centers
Blocks switching off of inspiritory neurons
CARBON DIOXIDE
CONTROLLS RESPIRATION
High carbon dioxide generates acidity of blood
in brain
Acidity of blood in systemic circulation is
prevented from directly influencing the brain
due to the blood/brain barrier’s impermeability
to H+
 CO2 + H2O <---> H2CO3 <---> H+ + HCO3
OXYGEN LEVELS MUST FALL DRASTICALLY
TO AFFECT BREATHING
Receptors in carotid bodies
Below 60 mmHg for oxygen partial
pressure, breathing is stimulated
This is a last-ditch, fail-safe mechanism
only!