Partial pressure in the respiratory system

What is a partial pressure?

The individual pressure of each gas

What is Dalton’s Law?

In a mixture of different gases, each gas creates its own pressure that is the same pressure if it was by itself
→ The total pressure can be calculated by adding together the pressures of each gas

What is the atmospheric pressure at sea level?

760mmHg / 101 kPa

What is the air made up of? (percentages)

N2 (78.6%)

O2 (20.9%)

CO2 (0.04%)

What is Henry’s law?

At equilibrium, the amount of gas dissolved in a liquid (that it does not combine chemically) is proportional to its partial pressure (above the liquid) and its solubility coefficient

Basically: Amount of gas dissolved = solubility coefficient of the gas x partial pressure of the gas

What is saturated vapour pressure (SVP)?

The pressure a gas mixture that is saturated with water vapour exerts

What is patrial pressure of O2 in alveoli?

13.3kPa

Why is the partial pressure of oxygen lower in the lower respiratory tract than the upper?

Inhaled air mixes with residual volume in alveoli

O2 diffuse across the membrane + CO2 diffuses in → reaches equilibrium 

** due to Henry’s law (air comes into contact with fluid)?

What is Fick’s law?

(area x driving force x diffusion coefficient)/ Thickness of membrane barrier

**Driving force (P1-P2) = Partial pressure difference across the membrane

diffusion coefficient =

solubility /molecular weight

What layers does oxygen have to pass to reach the red blood cells?

Fluid film lining alveolus→ epithelial cell of alveolus→ interstitial space→
endothelial cell of capillary→ plasma→ red cell membrane

How does pulmonary oedema affect diffusion?

Fluid in the interstitium and the alveolus → increases diffusion distance

How does emphysema affect diffusion/ gas exhange?

Destruction of alveolar walls → reduces total surface area available for gas exchange

How is Carbon monoxide transfer factor found in a person?

Calculated by measuring the CO uptake in a single breath
→ After a breath-holding time of 10 s at total lung capacity (TLC) the subject exhales, and an alveolar gas sample is collected

** CO fraction or pressure should decrease exponentially with time during breath- holding as CO diffuses into the blood

Amount of o2 dissolved In plasma =

solubility coefficient of O2 x partial pressure of O2

Total O2 content in blood =

O2 dissolved in plasma + bound O2 content

how does increasing partial pressure of oxygen affect the total oxygen content in blood?

increases dissolved o2 in plasma but not bound o2 (haemoglobin can only bind to 4 o2 so eventually increasing the pp will have no effect)

how does anaemia affect the total oxygen content in blood?

decreases bound o2 (less haemoglobin) but pp of O2 is the same so oxygen content in plasma is the same 

what is the difference between myoglobin and haemoglobin?

myoglobin:

• monomeric (binds to 1 O2)

• found in muscles and stores O2

Haemoglobin

• tetromere (binds to 4 O2)

• found in erythrocytes and transports O2

• has a tense (T) or relaxed (R) state

what is the heamoglobin and myoglobin curve?

haemoglobin: sigmoid due to cooperative binding

myoglobin: hyperbolic

explain the bohr effect

increase in CO2 in blood lowers pH (more acidic) → haemoglobin reduces affinity of O2 → oxygen is released

** graph shifts to the right

What does 2,3-Bisphosphoglycerate do to the dissociation curve and when is it in high amounts in the body?

Increased 2,3-BPG shifts Hb dissociation curve for O2 to right → more O2 is released

→ 2,3-BPG levels increase with anaemia or at altitude

Why does mitral valve stenosis lead to pulmonary oedema?

Harder for blood to flow from L atrium to L ventricle

Increases pressure in the atrium → pulmonary hypertension → R ventricular hypertrophy

Increases pressure in the atrium → blood pools in the pulmonary artery → oedema, pleural effusion

Atrial dilation→ atrial fibrillation → thrombus forming + oesophagus compression → dysphagia

What is the difference between hypoxia and hypoxaemia?

Hypoxia - Lack of O2 in tissues

Hypoxaemia- lack of O2 in arterial blood

How can you test for low oxygen in arterial blood?

Pulse oximetry:
Detects level of Hb saturation but cannot tell you amount of haemoglobin

Define ventilation and perfusion

Ventilation (V) - Amount of gas present in alveoli that participate in gas exchange

Perfusion (Q) - Amount of blood that pass through the alveoli that participate in gas exchange

** want V/Q to be close to 1 

What is a normal Ventilation-perfusion ratio?

0.8-1

Is Ventilation-perfusion ratio the same across the lungs?

No

Due to the effect of gravity, areas of the lower part of lung are better perfused

→ V/Q is lower ( a lot of air but the alveoli are already working at max capacity → ratio is smaller)

→ V/Q is higher at the apes (less air so ratio is bigger)

What conditions cause inadequate ventilation?

• Asthma early stages (airway narrowing)

• COPD early stages (structural airway damage due to inflammatory changes)

• Pneumonia (acute inflammatory exudate in affected alveoli)

• NRDS (some alveoli not expanded)

• Pulmonary oedema

What can cause inadequate perfusion?

pulmonary embolism

What happens if there is inadequate ventilation?

capillary pO2 will decrease, and pCO2 will increase → hypoxic vasoconstriction causes diversion of blood to better ventilated parts (however, haemoglobin is already fully saturated)

→ pO2 levels of blood will remain low→ hypoxia triggers respiratory centres in brain→ Increased respiratory rate→ hyperventilation → normal/low pCO2

What happens if there is inadequate perfusion?

Obstruction will lead to redirection of blood to other well ventilated areas of the lungs → V/Q <1

Hypoxia will occur because the vast majority of lungs is still working with V/Q <1 → hyperventilation → normal or low pCO2

What is the difference between type 1 and type 2 respiratory failure?

Type 1: Not enough oxygen enters the blood (Low pO2) but Carbon dioxide removal is not compromised (normal or low pCO2)

Type 2: low pO2 and high pCO2 (hypercapnia)

How does left sided heart failure lead to pulmonary oedema?

Blood accumulates in the left atrium and pulmonary veins → pulmonary venous pressure increases

→ Fluid leaks interstitial fluid forming a oedema

- decreased ventilation across alveoli → hypoxemia