Lecture 11 - Pulmonary Ventilation and Gas Diffusion Flashcards
what is airflow?
- flow of air or any other fluid, caused by a pressure differential between two points
- aka pressure gradient (high to low)
what is the pulmonary system?
- the system in which you breathe with
- includes nasal cavity, pharynx (trachea), primary bronchi, bronchioles and alveoli
what are alveoli?
- sacs of air (like grapes)
- thin-walled structures
- the site of transfer between the atmosphere and the internal environment
- lots of capillaries here (oxygen in and carbon dioxide out)
what are the mechanics of ventilation?
- *creating pressure differentials to manipulate airflow
- inhalation and exhalation
what happens in the body during inspiration?
- diaphragm contracts/flattens/moves downwards
- air in lungs expands, reducing its pressure
- caused by pressure difference between lungs and atmosphere
what happens in the body during expiration?
- sternum and ribs swing down
- diaphragm moves towards thoracic cavity
- air in lungs compresses, increasing its pressure
- caused by pressure difference between lungs and atmosphere
what is the respiratory cycle?
- a single breath
where is the intrapleural space?
- space between the lung and the thoracic cavity
where is the intrapulmonary space?
- space within the lung
how does pressure change across the respiratory cycle?
- pressure has to be lower in intrapleural space than intrapulmonary space (or lung will collapse)
- pressure decreases during inspiration and increases during expiration (greater volume in the lungs = lower pressure)
what causes the max flow-volume loop to be very high?
- elastic recoil
what is ventilation?
- the movement of air in and out of the lungs
- VE = total volume of expired gas per minute
what is the equation for VE?
- VE = respiratory rate (RR) x tidal volume (VT)
how is VE influenced by metabolic demand?
- VE rises with metabolic demand
- we need to breathe more to supply muscles with oxygen
- we also need to breathe more to get rid of carbon dioxide (so we don’t become acidotic)
what is alveolar ventilation (VA)?
- the volume of gas per minute that participates in gas exchange (only the air that reaches the alveoli)
- a large fraction of VE
what is dead space ventilation (VD)?
- the fraction of minute ventilation that does not contribute to gas exchange
- made up of respiratory passages and non-perfused alveoli passages
what is the relationship between VD and VE?
- higher VD means a higher VE is needed to maintain VA for adequate gas exchange
what is the equation for alveolar ventilation?
- VA = VE - VD
- VA = RR x (VT - VD)
how does breathing change during exercise?
- increase in inspiratory and expiratory reserve volume used during exercise (bigger pressure gradient, easier to breathe when lungs are more empty)
- breathing is more frequent
why do we breathe?
- to take up oxygen
- to get rid of carbon dioxide
- to fulfill the metabolic needs of the body
where does gas exchange occur?
- at the alveolar-pulmonary capillary interface
how does gas exchange occur?
- through the process of pulmonary diffusion
how do we know if gases are exchanging properly?
- partial pressure of oxygen and carbon dioxide in arterial blood
what determines proper gas exchange?
- VA matches Q
- diffusion capacity
What are the two laws of diffusion?
- dalton’s law of partial pressure
- henry’s law of diffusion between gases and liquids
what is dalton’s law?
- individual gases in a mixture exert pressure proportional to their abundance
- more molecules = greater partial pressure
- sum of partial pressures = total pressure
what is henry’s law?
- the amount of gas dissolved in a fluid depends on pressure differential and solubility
- without a gradient, gases are at equilibrium and therefore diffusion will not occur
what is gas concentration?
- the amount of gas in a given volume determined by product of gas partial pressure and solubility
what is gas pressure?
- force exerted by gas molecules against surfaces they encounter (mmHg)
how are gas concentration and gas pressure connected?
- partial pressure difference is what allows diffusion (and therefore allows concentration to change)
how do you calculate partial pressure?
PP = F (percentage concentration) x P (total pressure of gas mixture)
what is the alveolar-capillary interface?
- site of pulmonary diffusion (where gases are exchanged)
- contains alveolar wall, capillary wall, and basement membranes
what is the gas exchange between the alveoli and capillaries?
- air inflow = bronchial tree –> alveoli
- blood inflow = right ventricle –> pulmonary arteries –> pulmonary capillaries
- alveoli surround the capillaries
how does oxygen exchange in the alveoli?
- gases diffuse from high to low pressure
- takes about 0.75 seconds for equilibrium to be established between capillaries and vein
what is fick’s law of diffusion?
- the rate of diffusion (V gas) is directly proportional to surface area, differential partial pressure and diffusion constant
- it is inversely proportional to the thickness of the tissue
what is the relationship between VA and Q?
- VA ~ Q (in healthy individuals)
- VA/Q = > 1 –> poorly perfused alveoli
- VA/Q = < 1 –> poorly ventilated alveoli
- VA/Q = anything but 1 –> need increased VE to satisfy requirements
