Respiratory Physiology 3 and 4

Question 1

Describe anatomical dead space volume.

Answer 1

Anatomical dead space volume is ~150 mL and is the volume of gas occupied by the conducting airways and this gas is not available for exchange.
Anatomical dead space volume is relatively fixed for any one individual.

Question 2

Describe the functional difference between pulmonary and alveolar ventilation.

Answer 2

Pulmonary (Minute) ventilation = total air movement into/out of lungs (relatively insignificant in functional terms)
Alveolar ventilation = fresh air getting to alveoli and therefore available for gas exchange (functionally much more significant!)

Both are measured in L/min

Question 3

Define the various lung volumes and capacities and provide approximate normal values for them.

Answer 3

Tidal volume - how much air is breathed in - 500ml
Respiratory rate - breaths/min - 12
Total pulmonary ventilation - 6000ml/min
Air to alveoli - tidal-dead space - 350ml
Alveolar ventilation - 4200ml/min
Dead space is usually 150ml.

Question 4

Be able to describe the impact dead space has on alveolar ventilation.

Answer 4

Dead space means that alveolar ventilation is less than pulmonary ventilation due to not the entire tidal volume being absorbed into the alveoli.

Question 5

Define partial pressure

Answer 5

Definition:The pressure of a gas in a mixture of gases is equivalent to the percentage of that particular gas in the entire mixture multiplied by the pressure of the whole gaseous mixture. All gas molecules exert the same pressure so partial pressure increases with increasing gas conc in mixture.
Example:
Atmospheric Pressure = 760mmHg
Pressure of air we breathe therefore = 760mmHg
21% of air we breath = O2
Partial pressure of O2 in air we breath = 21% x 760mmHg
= 160mmHg

Question 6

State the normal values for alveolar and systemic arterial gas partial pressures in different units

Answer 6

Normal alveolar partial pressure (and therefore systemic arterial PP) of O2 is 100mmHg (13.3 kPa).
Normal alveolar partial pressure (and therefore systemic arterial PP) of CO2 is 40mmHg (5.3kPa).

Normal venous partial pressure of O2 is 40mmHg (5.3kPa)
Normal venous partial pressure of CO2 is 46mmHg (6.2kPa)

Question 7

Describe the effect of hypo and hyperventilation on systemic arterial oxygen and carbon dioxide partial pressures.

Answer 7

During hyper-ventilation (increased alveolar ventilation) PO2 rises to about 120 mm Hg and PCO2 falls to about 20 mmHg.
During hypo-ventilation (decreased alveolar ventilation) PO2 falls to 30 mmHg and PCO2 rises to 100 mmHg.

Question 8

Describe pulmonary circulation and its role in the blood supply to lungs.

Answer 8

Pulmonary artery carries deoxygenated blood AWAY from the heart to the lungs.

Pulmonary vein carries oxygenated blood TOWARDS the heart from the lungs.

Pulmonary circulation is opposite from systemic circulation in function!
It delivers CO2 to the lungs and picks up O2.

Question 9

Describe the function of bronchial circulation and pulmonary circulation and relate to their role in the blood supply to the lungs.

Answer 9

Bronchial circulation (nutritive) supplied via the bronchial arteries arising from systemic circulation to supply oxygenated blood to lung tissues. Comprises 2% of left heart output. Blood drains to left atrium via pulmonary veins.

Pulmonary circulation (gas exchange) consists of L & R pulmonary arteries originating from the right ventricle. Entire cardiac output from RV. Supplies the dense capillary network surrounding the alveoli and returns oxygenated blood to the left atrium via the pulmonary vein. High flow, low pressure system: (25/10mmHg vs 120/80mmHg).

Question 10

Describe the factors that influence diffusion of gases across the alveoli.

Answer 10

Gas exchange occurs in the alveoli and tissues as gases diffuse across membranes down the partial pressure gradient. The rate of diffusion across the membrane is:
1 -directly proportional to the partial pressure gradient.
2 -directly proportional to gas solubility
3 -directly proportional to the available surface area
4 -inversely proportional to the thickness of the membrane
5 -most rapid over short distances.
3, 4 and 5 can be affected by certain pathologies thus altering rate of diffusion.

Question 11

Describe how the anatomy of the lung is optimised for gas exchange.

Answer 11

The anatomy of the lung is optimally adapted to maximise gas exchange – large surface area, minimum diffusion distance, thin cell membranes (type 1 alveolar cell, capillary cell).

Question 12

Why is the rate of diffusion of CO2 so rapid for such a small partial pressure gradient and compare it to the rate of diffusion of O2.

Answer 12

CO2 diffuses more rapidly because of its greater solubility. Nevertheless the overall rates of equilibrium between O2 & CO2 are similar because of the greater pressure gradient for O2.
Rate of diffusion of CO2 - 200ml/min
Rate of diffusion of O2 - 250ml/min

Question 13

Describe how certain pathologies impact on gas exchange in the lung

Answer 13

Gas Exchange: Normal lung - PO2 normal in alveoli and in the blood.
Emphysema - destruction of alveoli reduces surface area for gas exchange - PO2 normal or low in alveoli and low in blood. Fibrotic lung disease - thickened alveolar membrane slows gas exchange, loss of lung compliance may decrease alveolar ventilation - PO2 normal or low in alveoli and low in blood.
Pulmonary edema - fluid in interstitial space increases diffusion distance, arterial PCO2 may be normal due to higher CO2 solubility in water - Exchange surface normal, PO2 in alveoli normal but increased diffusion distance means that blood PO2 is low.
Asthma - increased airway resistance decreases airway ventilation - bronchioles constricted causing low PO2 in alveoli and in blood.

Question 14

Outline the basic characteristics of obstructive and restrictive lung diseases

Answer 14

Obstructive - Obstruction of air flow, especially on expiration
Restrictive - Restriction of lung expansion and loss of lung compliance.

Question 15

Describe examples of obstructive lung disorders

Answer 15

Asthma
COPD (Chronic Obstructive Pulmonary Disease)
Chronic bronchitis - Inflammation of the bronchi
Emphysema -Destruction of the alveoli, loss of elasticity

Question 16

Describe examples of restrictive lung disorders

Answer 16

Fibrosis: formation of excess fibrous connective tissue creates a “stiff” lung.
Idiopathic (cause unknown); 50:100,000 new cases per year UK
Asbestosis (and other occupational origins e.g. coal dust)

Infant Respiratory Distress Syndrome: (insufficient surfactant production)

Oedema

Pneumothorax

Question 17

Define spirometry

Answer 17

Technique commonly used to measure lung function
Measurements can be classed as static or dynamic

Static – where the only consideration made is the volume exhaled

Dynamic – where the time taken to exhale a certain volume is what is being measured

Volumes measured directly by spirometry include: Tidal volume, inspiratory reserve volume, expiratory reserve volume, inspiratory capacity and vital capacity.

Question 18

Outline how spirometry can be used to identify abnormal lung function.

Answer 18

FEV1/FVC is a common spirometry measurement used to identify abnormal lung function where:
Forced expiratory volume in 1 second (FEV1)
fit, healthy, young adult males: 4.0L
Forced vital capacity (FVC)
fit, healthy, young adult males: 5.0L
FEV1/FVC = 80%
Alteration of the value of this equation indicates abnormal lung function.

Question 19

Describe and explain the characteristic results you would observe following lung function tests in patients with obstructive lung diseases

Answer 19

The impact on air flow is greater than the impact on lung capacity so FEV1 is greatly reduced but FVC is only slightly reduced leading to a greatly reduced FEV1/FVC ratio.

eg. COPD
Rate at which air is exhaled is much slower
Total expired volume (FVC) is also reduced (FRC may be increased)
Major effect is on airways and so FEV1 is reduced to a greater extent than FVC
Ratio also reduced

Question 20

Describe and explain the characteristic results you would observe following lung function tests in patients with restrictive lung diseases

Answer 20

The impact on lung capacity is huge in restrictive lung diseases meaning less air to flow but the air that does flow is not obstructed so FEV1 and FVC are suffer the same level of reduction so the ratio remains unchanged or may increase.

eg. Pulmonary fibrosis
Absolute rate of airflow is reduced (but only because total lung volume is reduced)
Total volume is reduced due to limitations to lung expansion
Ratio remains constant or can increase as a large proportion of volume can be exhaled in the first second

Question 21

Describe the limitations of the FEV1/FVC ratio

Answer 21

Obstructive: both FEV and FVC fall but FEV more so, so ratio is reduced.
Restrictive: both FEV and FVC fall so ratio remains normal, or may even increase, despite severe compromise of function.
Therefore normal FEV1/FVC ratio not always indicative of health!

Question 22

Describe why inspiration requires a greater change in pressure to reach a particular volume than it does to maintain during expiration.

Answer 22

1- Overcome lung inertia during inspiration
2- Overcome surface tension during inspiration
3- During expiration compression of the airways means more pressure is required for air to flow along them.
This means inspiration has a lower compliance naturally than expiration.

Question 23

Describe how the compliance of inspiration can be altered by lung diseases.

Answer 23

Normally effort (work) of inspiration is recovered as elastic recoil during expiration (hence expiration is passive).

(obstructive) Emphysema – loss of elastic tissue means expiration requires effort - increased compliance

(restrictive) Fibrosis – inert fibrous tissue means effort of inspiration increases - decreased compliance

Question 24

Describe the pathophysiology of asthma

Answer 24

Asthma – over-reactive constriction of bronchial smooth muscle. Increases resistance, expiration phase most affected.

Question 25

What is perfusion and ventilation?

Answer 25

Perfusion - Local blood flow L/min Ventilation - air getting out of the alveoli L/min

Question 26

Describe the relationship between ventilation and perfusion and its significance in health

Answer 26

Ventilation and perfusion should ideally match (complement) each other. Both blood flow and ventilation decrease with height across the lung. However: At the base of the lungs blood flow is higher than ventilation because arterial pressure exceeds alveolar pressure. This compresses the alveoli. At the apex of the lungs blood flow is low because arterial pressure is less than alveolar pressure. This compresses the arterioles.

Question 27

Describe the conditions under which there may be an imbalance in the ventilation- perfusion ratio in regions of the lung, and describe how this affects the O2 and CO2 content of alveolar gas and arterial blood.

Answer 27

Under optimal conditions ventilation = perfusion through the pulmonary capillaries. Various conditions in the lungs can lead to mismatch in ventilation perfusion ratio such as blood flow>ventilation at the base of the lung, and ventilation>blood flow at the apex of the lung.. This can cause an imbalance in the ventilation perfusion ratio. When perfusion > ventilation Alveolar PO2 falls, PCO2 rises causing pulmonary vasoconstriction and bronchial dilation. When ventilation > perfusion alveolar PO2 rises and PCO2 falls causing pulmonary vasodilation and bronchial constriction.

Question 28

Define the terms shunt, alveolar dead space, physiologic dead space and anatomical dead space

Answer 28

Shunt is a term used to describe the passage of blood through areas of the lung that are poorly ventilated (ventilation << perfusion). Alveolar Dead Space refers to alveoli that are ventilated but not perfused. Anatomical Dead Space refers to air in the conducting zone of the respiratory tract unable to participate in gas exchange as walls of airways in this region (nasal cavities, trachea, bronchi and upper bronchioles) are too thick. Physiologic Dead Space = Alveolar DS + Anatomical DS

Question 29

What is the role of respiratory sinus arrhythmia in the breathing cycle.

Answer 29

Respiratory sinus arrhythmia acts to minimise ventilation:perfusion mismatch during the breath cycle.

Question 30

Describe gas transport in blood.

Answer 30

Blood transports O2 from lungs to tissues to use in energy production and transports the waste product of this process, CO2 , from tissues to lungs for removal. O2 travels in two forms in the blood: in solution in plasma and bound to haemoglobin protein in red blood cells Only 3ml O2 dissolve per litre plasma 200ml O2 per litre whole blood, 197ml of which is bound to haemoglobin in red blood cells Bulk (77%) of CO2 is transported in solution in plasma, 23% is stored within haemoglobin.

Question 31

Apply the concept of partial pressure gradients to the movement of O2 and CO2 between the blood and the alveoli

Answer 31

Oxygen diffuses down the partial pressure gradient from the alveoli into the plasma until it has reached equilibrium. The partial pressure gradient of carbon dioxide also allows it's movement from tissues back into the lungs.

Question 32

Describe the role of haemoglobin in the transport of O2 in the blood.

Answer 32

Hb effectively sequesters O2 from the plasma, thus maintaining a partial pressure gradient that continues to suck O2 out of the alveoli, until the Hb becomes saturated with O2

Question 33

Explain why the shape of the oxyhaemoglobin dissociation curve aids O2 loading in the lungs and unloading in the tissues.

Answer 33

Haemoglobin is almost 100% saturated at the normal systemic arterial PO2 of 100 mm Hg. Even at PO2 of 60mmHg though haemoglobin is still 90% saturated with O2. This permits a relatively normal uptake of oxygen by the blood even when alveolar PO2 is moderately reduced. At normal venous PO2, there is still 75% reserve capacity

Question 34

Describe the factors that affect the oxyhaemoglobin dissociation curve.

Answer 34

The affinity of haemoglobin for oxygen is decreased by a decrease in pH, or an increase in PCO2, or temperature. These conditions exist locally in actively metabolising tissues and facilitate the dissociation of oxygen from haemoglobin. Conversely a rise in pH or a fall in PCO2, or temperature increases the affinity of haemoglobin for oxygen. These conditions make oxygen unloading more difficult but aid collection of oxygen in the pulmonary circulation. The affinity of haemoglobin for oxygen is decreased by binding 2,3-diphosphoglycerate (2,3-DPG) synthesised by the erythrocytes. 2,3- DPG increases in situations associated with inadequate oxygen supply (heart or lung disease, living at high altitude) and helps maintain oxygen release in the tissues.

Question 35

Use anaemia to demonstrate how PaO2 determines, but is independent of, total blood oxygen content

Answer 35

Anaemia causes a decrease in the total oxygen content of blood but his has no effect on PO2 as it is possible to have normal PO2 whilst having low total blood oxygen content but not the other way round. This is due to it being possible that red blood cells can still be fully saturated with oxygen as PO2 is normal. However iron deficiency may limit the binding sites the ones that are working will still be fully saturated.

Question 36

Identify the forms in which CO2 is carried in the blood.

Answer 36

When CO2 molecules diffuse from the tissues into the blood, 7% remains dissolved in plasma and erythrocytes, 23% combines in the erythrocytes with deoxyhemoglobin to form carbamino compounds, and 70% combines in the erythrocytes with water to form carbonic acid.

Question 37

Explain the action of carbonic anhydrase in CO2 transport.

Answer 37

Carbonic anhydrase allows the 70% of carbon dioxide which combines in the erythrocytes with water to form carbonic acid. The dissociation of carbonic acid drives the transport of carbon dioxide.

Question 38

Identify the factors which favour CO2 unloading to the alveoli at the lungs.

Answer 38

The concentration gradient of CO2 favours it unloading at the alveoli at the lungs.

Question 39

State the differences between partial pressure and gas content.

Answer 39

Partial pressure is just the amount of oxygen in solution in the plasma not total oxygen content of blood. Partial pressures however determine total oxygen content if the blood by determining saturation of haemoglobin.

Question 40

Compare oxyhaemoglobin dissociation for adult haemoglobin with that of foetal haemoglobin and myoglobin in relation to their physiological roles.

Answer 40

HbF and myoglobin have a higher affinity for O2 than HbA, this is necessary for extracting O2 from maternal/arterial blood.

Question 41

Define the five different types of hypoxia

Answer 41

Inadequate supply of oxygen to tissues; Various causes. 5 main types: 1. Hypoxaemic Hypoxia: most common. Reduction in O2 diffusion at lungs either due to decreased PO2atmos or tissue pathology. 2. Anaemic Hypoxia: Reduction in O2 carrying capacity of blood due to anaemia (red blood cell loss/iron deficiency). 3. Stagnant Hypoxia: Heart disease results in inefficient pumping of blood to lungs/around the body 4. Histotoxic Hypoxia: poisoning prevents cells utilising oxygen delivered to them e.g. carbon monoxide/cyanide 5. Metabolic Hypoxia: oxygen delivery to the tissues does not meet increased oxygen demand by cells.