Respiration, Ventilation, & Perfusion Flashcards
At end of normal inspiration, what pressure is most negative?
-intrapleural
What is transmural/transpulmonary pressure?
- the pressure that is overall acting on the alveoli
- relationship between the pressure of air in the alveoli and the intrapleural pressure
What is the equation for transmural pressure?
-PTM = PA – PIP
= Alveolar pressure - intrapleural pressure
What is alveolar interdependence?
-the pressure at the pleural surface is transmitted through the alveolar walls to more centrally located alveoli and small airways
If alveolar pressure is + when compared to atmospheric pressure, what is most likely occurring?
-expiration
What must respiratory muscles overcome for ventilation to occur?
- pressure gradient
- mechanical properties
- Compliance
- Resistance of airways
What is compliance?
- the distensibility of the lung and chest wall
- kind of like elasticity
What is the equation for compliance?
-CL = V / PTP
= change in volume over change in pressure
What is the normal compliance of the human lung?
-200mL/cm H2O
How is compliance related to the elastance of the lungs?
- elastance is the reciprocal of compliance
- high compliance means low elastance
What determines pulmonary compliance?
- compliance of lung
- compliance of chest wall
What determines compliance of the lung?
- elastic recoil
- surface tension
What are the physiologic advantages of surfactant?
- it increases the compliance thus decreasing the work of breathing
- stabilizes alveoli
- keeps alveoli “dry”
What is hysteresis and why does it occur?
- the difference in the pressure/volume curve during inspiration and expiration
- caused by difference in compliance in inspiration vs expiration
What is functional residual capacity (FRC)?
- the volume of the lung when the opposing forces of the chest wall wanting to expand and the lung wanting to collapse are equal and opposite
- airway pressure = zero
How does emphysema affect compliance?
- increases compliance and decreases elastance
- less pressure needed to increase volume of lungs
How does fibrosis affect compliance?
-decreases compliance
What pathologies increase compliance?
- emphysema
- aging
What pathologies decrease compliance?
- pulmonary fibrosis
- pulmonary edema
- increased pulmonary venous pressure
- Acute Respiratory Distress Syndrome (ARDS_
What is the equation for pulmonary resistance?
-Q = change in P / R
Airflow = pressure gradient / resistance
What influences resistance in the lungs?
- Airway radius**
- muscle tone
- lung volume
- Viscosity of gas inhaled
What is Poiseuille’s Law?
- a law for the resistance of an airway that takes into account the:
- viscosity of the gas
- the length of the tube
- and the radius of the tube
What prevents airway collapse in normal forced expiration?
-the positive transmural pressure
Where is the site of greatest resistance in the airway?
-medium sized bronchi
What is minute ventilation? What is its formula?
-The total air moved in and out of lungs per minute
Ve = Tidal volume X frequency
What is alveolar dead space?
- ventilated areas that are not well perfused
- wasted ventilation
- no gas exchange can occur
What makes up physiologic dead space?
- the combination of anatomic dead space and alveolar dead space
- in healthy individual, physiologic dead space is equal to anatomic dead space.
How do you calculate Alveolar Ventilation?
-the difference between the tidal volume and dead space volume, multiplied by the breathing frequency
How do we estimate physiologic dead space?
- based on the measurement of CO2 in mixed expired air
- there’s pretty much no CO2 in inspired air
How do we know that the Partial Pressure of alveolar CO2 is?
-measure in arterial blood, assuming alveolar CO2 is roughly equal to arterail CO2 (40mmHg)
What is the alveolar ventilation equation?
-PACO2= 0.863 x VCO2 / VA
What is the normal VCO2?
-200 ml/min
What is the body’s ideal PaCO2? (Arterial)
-40 mmHg
What does the alveolar gas equation describe?
-it is used to predict the alveolar PO2
What is the alveolar gas equation?
-PA O2 = (Pb – PH20) (F02) – PaC02 /R
R = 0.8
What is the respiratory exchange ratio?
- reflects the metabolism of body tissues in a steady state
- R = Vco2/Vo2
What does A and a stand for?
A = alveoli
a = arterial
What are the differences between pulmonary and systemic circulation?
- Pulmonary circulation has higher capacity
- Pulmonary circulation has lower resistance and pressure than systemic circulation
What is the equation for pulmonary vascular resistance?
- R = (delta)P/Q
- Resistance equals pressure gradient between pulmonary artery and left atrium, divided by the cardiac output
What happens to pulmonary vascular resistance as cardiac output increases, such as during exercise?
-it decreases so that the mean pulmonary pressure increases only slightly
How is pulmonary vascular resistance decreased?
–recruitment of previously closed pulmonary vessels
-distension of existing open vessels
What happens to total pulmonary resistance during inspiration?
- it increases
- remember that the total pulmonary resistance is the resistance of alveolar blood vessels plus resistance in the extra alveolar blood vessels. The resistance in the extra alveolar blood vessels will decrease, but the resistance in the alveolar blood vessels will increase proportionately more. Thus the overall increase in total pulmonary vessel resistance will increase.
What happens to resistance in the extra-alveolar blood vessels during inspiration?
- it decreases
- more negative intrapleural pressure pulls open extra-alveolar vessels
What happens when alveolar O2 decreases?
-hypoxic vasoconstrictiction
What is the purpose of hypoxic vasoconstriction?
- to reduce blood flow to poorly ventilated alveoli
- this increases the perfusion to well-ventilated areas that allow for greater gas exchange
What is the mechanism of hypoxic vasoconstriction?
- poorly understood
- hypothesis is that lower levels of alveolar O2 are sensed, triggering Ca entrance into muscle cells leading to vasoconsriction
- also possible inhibition of nitric oxide synthase
What influences the distribution of blood flow in the lung?
- gravitational effects
- hydrostatic pressures within the blood vessels
- regional ventilation-perfusion matching
Where is perfusion of the lung higher?
- base of lung (while standing)
- Posterior lung (while supine)
What is P-out in the lung in relation to blood flow?
-alveolar pressure
Why is zone 1 of the lung considered dead space?
- Alveolar pressure is very high, causing compression of the pulmonary capilliaries
- though, the arterial pressure is still just high enough to perfuse, just at a lower rate
What is the normal V/Q ratio in an adult lung?
- 0.8
- this means we breath 20% less air by volume than blod flows through the lungs
Where in the lung is matching of perfusion and ventilation the best?
-Zone2 (middle lung)
What is an area of the lung that is not ventilated called, but has good perfusion?
-Alveolar Shunt
What is the body’s compensation for a shunt?
-Hypoxic vasoconstriction
What is the A-a gradient?
-the difference between alveolar oxygen and arterial oxygen
What determines alveolar PO2 and PCO2?
-the relationship between alveolar ventilation and pulmonary capillary perfusion
what is an anatomic shunt?
-when cardiac output bypasses the pulmonary circulation
