Gas Exchange and Transport Flashcards
Hypoxia
too little oxygen
- a result of impaired diffusion from alveoli to blood or impaired blood transport
Hypercapnia
excess CO2
Hypoxic Hypoxia
- low arterial PO2
- causes: high altitude; alveolar hypoventilation; decreased lung diffusion capacity; abnormal ventilation-perfusion ratio
What Three Variables does the Body Respond to to Avoid Hypoxia
- Oxygen: ATP production
- Carbon Dioxide: CNS depressant/acid precursor
- pH: denaturing of protein
How do Gases Diffuse?
- gases diffuse down partial pressure gradients
Alveolar Gas Exchange is Influenced by…
- O2 reaching the alveoli
- Gas diffusion between alveoli and blood
- Adequate perfusion of alveoli
Two Causes of Low Alveolar PO2
- inspired air has low O2 content
- > alterations in atmospheric PO2 - alveolar ventilation (hypoventilation)
- > increase airway resistance, decrease lung compliance, or CNS issue, decrease rate and/or depth of breathing
What Could be Another cause of Hypoxia
problems within gas exchange between the alveoli and blood
Diffusion
- random movement of molecules from a region of high concentration to a region of low concentration
Factors that Affect the Random Movement of Gas Molecules Between the Alveoli and Capillaries
- Concentration Gradient**
- Surface Area
- Barrier Permeability
- diffusion distance, solubility of gas
Emphysema
- decreased surface area
- causes hypoxia
- destruction of alveoli means less surface area for gas exchange
Fibrotic Lung Disease
- decreased barrier permeability
- causes hypoxia
- thickened alveolar membrane slows gas exchange
- loss of lung compliance may decrease alveolar ventilation
Pulmonary Edema
- increased diffusion distance
- causes hypoxia
- fluid interstitial space increases diffusion distance
- arterial PCO2 may be normal due to higher CO2 solubility in water
Asthma
- decreased concentration gradient
- causes hypoxia
- increased airway resistance decrease alveolar ventilation
- bronchiole constricted
Gas Solubility and Diffusion
- alveoli are lined with liquid, the small interstitial space between alveoli and capillaries contains liquid and blood itself is liquid
- respiratory gases must be soluble in liquids
Movement of Gas Molecules is Directly Proportional to…
- the pressure gradient of the gas
- solubility of gas in liquid
- temperature-relatively constant (not really relevant)
Gas Transport in Blood
- demonstrates the general principles of mass flow and mass balance
O2 consumption by _______ tissues
systemic
Fick Equation
CO x (Arterial [O2] - Venous [O2]) = QO2
Cellular Oxygen Consumption
QO2 = arterial O2 transport - venous O2 transport
Oxygen Transport
- > 98% of oxygen in blood is bound to hemoglobin in RBCs
- <2% is dissolved in plasma
Oxygen Binding Reaction Equation
Hb + O2 ⇌ HbO2
What Law does Oxygen Binding Obey?
- law of mass action
Oxygen Binding: Law of Mass Action
- as [free O2] increases, more oxygen binds to Hb –> HbO2
- free O2 is taken up until plasma and Hb reach equilibrium
- transfer of O2 happens rapidly
How long does RBC spend in pulmonary capillary?
~0.75 sec
How long does it take for RBC to become saturated?
~0.40 sec
Reverse Reaction of Oxygen Binding
- blood travels to tissues with low PO2
- O2 drawn out of plasma
- equilibrium is disrupted
- Hb releases its O2 into plasma
at rest we consume about ____ ml O2/min
250
Plasma O2 is determined by?
alveolar PO2
Alveolar PO2 depends on:
- composition of inspired air
- alveolar ventilation rate
- efficiency of gas exchange
PO2 determines what?
Oxygen-Hb binding
Plasma O2 determines
% saturation of Hb
Amount of Hb determines
Total # of Hb binding sites
How to calculate total # of Hb binding sites
Hb content per RBC x # of RBCs
Oxygen Binding is expressed as…
percentage
- percent saturation of hemoglobin
Resting Cell PO2?
40 mmHg
Alveoli PO2
100 mmHg
Can active cells have a lower PO2?
yes
- active muscle cells can have 20 mmHg
- results in larger release of O2
Physical Factors Altering Hb’s Affinity for O2
- pH
- PCO2
- temperature
- 2,3-DPG
Effect of pH
- max exertion produces excess CO2 and pushes a cell into anaerobic metabolism
- increased H+ and lactic acid in cytoplasm and extracellular space
- more oxygen delivered @ low pH
Bohr Effect
- shift in Hb saturation as a result in pH or CO2 change
Increased Aerobic Metabolism results in…
increased CO2 production
Effects of PCO2
- readily binds Hb altering conformation and decreases binding spots for O2
* *2. CO2 is readily converted to carbonic acid
Carbonic Anhydrase
- enzyme in reaction of CO2 to carbonic acid (H2CO3)
Effect of Temperature
- increased heat causes a conformational change in Hb
- decreased affinity and more O2 to be dropped at very active muscles
- increased heat = increased delivery
2,3-DPG
- diphosphoglycerate
- metabolic compound
- by-product of glycolysis in RBCs
Increase Production of 2,3-DPG
- chronic hypoxia increase 2,3-DPG production
- RBCs release ATP during hypoxia
- ascent to higher altitude and anemia can increase 2,3-DPG
Effect of 2,3-DPG
- increase 2,3-DPG = increased delivery
Fetal Hemoglobin
- two alpha, two gamma globulin subunits
- gamma increases oxygen binding
Importance of Removing CO2 from Body
- elevated PCO2 causes acidosis, low pH leads to interruptions in H bonds and denaturing of proteins
- abnormally high PCO2 depresses CNS causing confusion, coma, death
Cells produce far ___ CO2 than plasma is capable of carrying
more
How much CO2 is carried by venous blood dissolved in plasma
7%
What happens to remaining 93% of CO2
- diffuses into RBCs
____ of CO2 binds to Hb
23%
carbaminohemoglobin
- HbCO2
70% of CO2 is converted to ____
HCO3- (bicarbonate)
CO2 Transport
- dissolve in plasma (7%)
- Bound to Hb (23%)
- Converted to HCO3- (70%)
Purposes of HCO3-
- provides additional means of CO2 transport from cells to lungs
- HCO3- is available to act as a buffer for metabolic acids, stabilizing body’s pH
Carbonic Acid
- H2CO3
- intermediate step (ignored)
To Ensure Equilibrium is not Reached in CO2->HCO3-
- remove HCO3- from RBC
2. mop up excess H+
Hb acts as a ____ and binds excess H+ ions
buffer
Why does Hb act as a buffer?
- prevents large changes in body’s pH
2. if blood CO2 is elevated too high Hb can’t soak up al H+ which can result in acidosis
Where does CO2 bind to Hb
- at exposed amino groups (-NH2)
- forms carbaminohemoglobin
CO2 Removal at the Lungs
- plasma CO2 diffuses into alveoli –> RBC CO2 diffuses into plasma
- causes CO2 to unbind from Hb and diffuse out of RBC
- CO2 levels drop = reverse reaction
as [HCO3-] drops, ____ exchanger ______
Cl-/HCO3-
reverses
Breathing
- rhythmic process
- occurs subconsciously
Skeletal Muscles that Control Ventilation
- can’t contract spontaneously
Regulation of Ventilation
- spontaneously firing networks of neurons in the brainstem
- network influenced by sensory and chemoreceptors, high brain enters
Blackbox
- considered as the neural control of ventilation
Neurons in the Medulla
- control inspirator and expiratory muscles
Neurons in the Pons
- integrate sensory info
- interact with medullary neurons to influence ventilation
Rhythmic Pattern of Breathing comes from…
- a neural network with spontaneously discharging neurons
How is Ventilation Continuously Modulated?
- by various chemo and mechanical receptor-linked reflexes
- by higher brain centers
What Neurons are in the Medulla that Control Breathing
- nucleus tractus solitaris (NTS)
- dorsal respiratory group (DRG)
- pontine respiratory group (PRG)
- ventral respiratory group (VRG)
NTS
- contains the DRG
- receives input from the peripheral mechanical and chemoreceptors
DRG
- mainly control inspiratory muscles via phrenic nerve and intercostal nerve
PRG
- provides tonic input to DRG to help medullary network coordinate a smooth rhythm
- doesn’t create the rhythm
VRG
- has a few areas with different functions
- Pre-botzinger complex
- control muscles of active inspiration and expiration
- outputs that keep upper airways open
Pre-Botzinger Complex
- contain pacemaker neurons that may initiate respiration
Slow Output in VRG
- slows down too much while asleep
- sleep apnea
- snoring
Upper Airways
- tongue, larynx, pharynx
Neural Activity During Quiet Breathing
- believed to be initiated by a pacemaker
- positive feedback loop recruits more neurons “ramping” recruiting more outputs to inspiratory muscles
- activity shuts off abruptly after inspiration
Peripheral Chemoreceptors
- aortic and carotid bodies
- sense changes in arterial PO2, PCO2, and pH
- adjust ventilation accordingly
- structurally similar to neruons
Type I (Glomus) Cell
- sensing changes in oxygen and pH
- excitable cells
- can send vesicles
- has a variety of neurotransmitters (dopamine)
Type II (Sustentacular) cells
- like glia cells
- support cells
It takes a _____ drop in arterial PO2 to trigger peripheral chemoreceptors
large
Carotid Body
- oxygen sensor
- releases neurotransmitter when PO2 decreases
What Triggers Peripheral Chemoreceptors
- can respond to increases in H+
- primarily respond to increases in CO2
Glomus Cells Process
- Low PO2
- K+ channels close
- Cell depolarizes
- voltage-gated CA2+ channel opens
- Ca2+ enters
- exocytosis of neurotransmitters
- signal to medullary enters to increase ventilation
Central Chemoreceptors
- located in the medulla
- provide continuous input to respiratory control centre
- explained to respond mainly to changes in PCO2
- respond to changes in pH in CSF caused by CO2, but not to changes in plasma pH
- neurons in this region contain ASIC
Can H+ cross the blood brain barrier well.
NO
ASIC
- H+ sensitive channel
- become activated and transmit AP’s to the respiratory control centre
Decreased Arterial O2
d. inspired PO2
d. alveolar PO2
d. arterial PO2
peripheral chemoreceptor i.fire
respiratory muscles i.contract
i. ventilation
return of alveolar and arterial PO2 toward normal
Increased Arterial H+
- increase in H+ independent of CO2 increase
- peripheral mediated
Increased Arterial CO2
- most sensitive to changes in CO2
- mediated by both central chemoreceptors (70% primary) and peripheral chemoreceptors (30%)
Irritant Receptors
- in the lungs
- respond to inhaled particles or noxious gases
- send input to CNS, parasympathetic outputs and results in bronchoconstriction
- leads to rapid shallow breathing and turbulent airflow to deposit irritant in mucosa
- reflexes: coughing/sneezing
Stretch Receptors
- in the lung
- prevent over inflation of the lungs
- “Hering-Breuer inflation reflex”
High Brain Centers
- cerebral cortex
- voluntary control over breathing
- we can actively hold our breath until chemoreceptors take over
- can breath out for set amounts of time –> higher control
