Gas exchange Flashcards
What is the thoracic pressure like compared to atmospheric pressure?
- thoracic cavity pressure is negative pressure compared with thoracic pressure
Due to lung tissue being elastic - what do they naturally want to do?
- lung tissue wants to collapse and deflate
How is the thoracic cavity bounded and what does it naturally want to do?
- thoracic cavity bounded by ribs, sternum and diaphragm under tension
- this means it wants to spring out
What does negative pressure aid in?
- negative pressure helps keep lungs semi-inflated even when exhaling
Lung tissue has a high compliance - what does this mean?
- A small change in pressure leads to a large change in volume
- this is why its difficult to reinflate a collapsed lung
What is the equation for compliance?
- compliance = change in volume / change in pressure
Surfactant increases lung compliance.
What is it?
What does it do?
- polar phospholipid
- type 2 pneumocytes (alveolar epithelial cells)
- reduces energy needed to inflate lungs
- also prevents transudate (fluid) coming out of capillaries
Disease states can alter lung compliance - what can a lack of surfactant leas to?
- a decrease in compliance
- e.g., respiratory distress syndrome
Disease states can alter lung compliance - what does an increase in connective tissue lead to?
- decreases compliance
- e.g., fibrosis/scarring
Disease states can alter lung compliance - what does reduced lung elastic tissue cause?
- increases compliance
-e.g., emphysema (makes deflation difficult)
What type of process is inspiration?
- its an active process
What does inspiration involve?
- involves increasing the volume of the thorax to lower pressure, causing expansion of the lungs
Inspiration is achieved by pulling the ribs outward and displacing the diaphragm caudally into the abdomen - Describe the steps of this process:
- diaphragm contracts and flattens
- external intercostal muscles contract to aid ribs opening
- increased thoracic volume lowers pressure (more negative)
- this causes lungs to expand
- increased lung volume lowers pressure
- air is drawn into the alveoli where gas exchange occurs
What type of process is expiration?
- this is a passive process at rest
- except in the horse where there is an activate phase even at rest
Expiration involves the opposite to inspiration - what is the process?
- diaphragm relaxes, reducing volume of thorax
- thoracic wall and ribs return to resting position
- elastic lungs recoils, expelling air
Further reduction of thoracic volume can be achieved by active muscle contraction - what does this process involve?
- internal intercostal muscles
- also accessory muscles of respiration e.g., abdominal muscles contract to increase abdominal pressure to force diaphragm more cranially
Why does the horse show biphasic inspiration and expiration (passive and active phases)?
- because the horse has a firm thoracic wall
Describe the process of expiration in a horse:
- relaxation, relying on elastic recoil of lungs and chest
- chest reaches resting volume
- further decrease by active contraction of internal intercostal and abdominal muscles
Describe inspiration in a horse:
- relaxation, relying on elastic recoil of chest to open up
- chest reaches resting volume
- active inspiration phase by contraction of diaphragm and external intercostal muscles
What nerve innervates the diaphragm?
- innervated by phrenic nerve
What do gas molecules do to the walls they contact?
- they exert pressure on the walls they contact
What is the total pressure equal to?
- equal to all gas molecules
What is partial pressure?
- partial pressure (px) is that of a single gas
What is atmospheric pressure of air at sea level?
- 760mmHg
Calculate the partial pressure of oxygen at 20.8% if the atmospheric is 760mmHg
- 20.8% x 760 = 158 mmHg
Why is understanding partial pressure important?
- important in predicting movement of gases
How will a gas move during partial pressures?
- a gas will move from area of high px to lower px
What is the ultimate goal of respiration?
- to keep correct concentrations of certain molecules in tissues for correct function
What gases can be found in tissues?
- oxygen
- carbon dioxide
- H+ (acid base balance)
What type of diffusion is gas exchange?
- simple diffusion
Gas exchanges can be from … to …
- from alveolus to blood (external respiration)
- from blood to tissues (internal respiration)
Fricks law described factors influencing the rate of diffusion of a gas across a membrane - what are these factors?
- surface area of the membrane
- thickness of the membrane
- partial pressure difference of gas across the membrane
- diffusion coefficient (properties of gas and membrane)
The rate of diffusion across a membrane can be affected by disease - what are these?
- alveolar destruction (surface area)
- fibrosis or oedema (thickness)
- ventilation disturbances, bronchoconstriction, atelectasis (Px)
- perfusion disturbances, emboli (Px)
Diffusion of O2 from alveolus into blood does what?
- follows pressure gradient
Internal respiration - describe the process of diffusion down a pressure gradient of O2:
- blood to interstitial fluid
- interstitial fluid to cell
- vice vera for CO2
What type of solubility does oxygen have in the blood
- a low solubility in the blood
What is most oxygen transported by?
- most oxygen is transported by a protein within RBCs called haemoglobin
What groups does haemoglobin contain?
- contains 4 iron-based haem groups
What does Haem bind to?
- molecular oxygen
What can the 1st O2 binding to 1st haem group cause?
- conformational change in haem
What can the 2nd and 3rd O2 molecules binding to haem cause?
- easier binding (increased affinity)
What happens when PO2 is low to Haemoglobin affinity for oxygen?
- Hb affinity for O2 is low
- in tissues encourages Hb to drop O2
When po2 is high what happens to Hb affinity for O2?
- Hb affinity for O2 is high
- in lungs encourages Hb to pick up O2
The oxygen dissociation has an S-shaped curve - what does this show?
- as more O2 binds to Hb, its affinity changes (increases at first, then decreases)
The dissociation curve can be shifted due to other factors - what happens during a left shift?
- left shift = generally increased affinity
- same Po2 - higher Hb saturation
- easier to pick up O2
What causes a right shift of the oxygen dissociation curve?
- right shift = generally decreased affinity
- same po2, lower Hb saturation
- easier to drop off O2
What can a left shift cause?
- higher pH (alkalosis)
- lower Co2
- lower temperature
What does a right shit cause?
- lower pH (acidosis)
- higher CO2
- higher temperature
Hoe does Co2 diffuse?
- diffuses down its conc gradient into bloodstream
What can Co2 be transported as?
- dissolved in the plasma - 7%
- bound to amine groups of Hb (carbaminohaemoglobin) - 23%
- as bicarbonate ions HCO3 - 70%
Transport as HCO3- contributes to the bicarbonate buffer system how?
- Bicarbonate (HCO3-) and carbonic acid (H2CO3) together ‘buffer’ dramatic changes to pH and so contribute to acid-base balance
Why is carbon monoxide gas so poisonous?
- Binds to Hb but with a 200-300 x greater affinity than O2
- even at low PCO it doesn’t want to dissociate = irreversible binding to Hb and so displaces O2
