Ventilation and compliance

Question 1

How does the pressure vary between the base and the apex of the lung?

Answer 1

Alveolar ventilation declines with height from base to apex.
Compliance declines with height from base to apex due to the alveoli being more inflated at FRC. At the base the lungs are slightly compressed by the diaphragm hence more compliant on inspiration.

Question 2

How are the lungs supplied with blood?

Answer 2

Bronchial circulation - arises from systemic circulation, bronchial arteries supply airway smooth muscle, nerves, and lung tissue.
Pulmonary circulation - supplies capillary beds surrounding alveoli and returns oxygenated blood to left atrium via pulmonary vein.

Question 3

What is the partial pressure gradient of PO2 in circulation?

Answer 3

PO2 = 100mmhg and 40 mmhg

Question 4

What is the partial pressure gradient between PCO2 in circulation?

Answer 4

PCO2= 46mmhg and 40mmhg

Question 5

What are some properties of the rate of diffusion of gases across the membrane?

Answer 5

Rate is directly proportional to the partial pressure gradient.
Rate is directly proportional to gas solubility.
Rate is directly proportional to the available surface area.
Rate is inversely proportional to the thickness of the membrane.
Rate is most rapid over short distances.

Question 6

What is the relationship between ventilation and perfusion?

Answer 6

Ideally they match each other. Want the air getting to the alveoli to be the same as air getting into local blood flow.

Question 7

Describe the distribution of blood flow across the lungs?

Answer 7

Blood flow is inversely proportional to vascular resistance and declines with height across the lung.

At base blood flow is high because arterial pressure exceeds alveolar pressure so VR is low.

At apex blood flow is low because arterial pressure is less than alveolar pressure. This compresses arterioles and VR is high.

Question 8

What is Alveolar dead space?

Answer 8

When ventilation is greater than blood flow.

Question 9

What is Shunt?

Answer 9

The passage of blood through areas of the lung that are poorly ventilated.

Question 10

What is the function of haemoglobin?

Answer 10

Increase the oxygen carrying capacity of red blood cells.

Question 11

What is the major determinant of the degree to which haemoglobin is saturated with oxygen?

Answer 11

Partial pressure of oxygen in arterial blood.

Question 12

Why does foetal haemoglobin and myoglobin have a higher affinity for oxygen than regular HbA?

Answer 12

Higher affinity is necessary for extracting O2 from maternal/arterial blood.

Question 13

What is anaemia?

Answer 13

Any condition where the oxygen carrying capacity of the blood is compromised (e.g. iron deficiency, haemorrhage, vit B12 deficiency).

Question 14

What are some chemical factors that can alter the affinity of haemoglobin?

Answer 14

pH
Temperature
PCO2
DPG (diphosphoglycerate)

Question 15

What happens to haemoglobin when ph, temperature and PCO2 increase/decrease?

Answer 15

Increase - ph increased affinity, PCO2, temperature decreased affinity.

Decrease - ph decreased affinity, PCO2, temperature increased affinity.

Question 16

What is the function of DPG?

Answer 16

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 17

What makes carbon monoxide very problematic?

Answer 17

CO binds to haemoglobin to form carboxyhaemoglobin with an affinity 250 times greater than O2 - binds readily and dissociates very slowly.

Question 18

What are the symptoms of carbon monoxide poisoning?

Answer 18

Hypoxia and anaemia, nausea and headaches, cherry red skin and mucous membranes. Potential brain damage and death. Respiration rate unaffected.

Question 19

What are the 5 main types of hypoxia?

Answer 19

Hypoxaemic Hypoxia: most common. Reduction in O2 diffusion at lungs either due to decreased PO2atmos or tissue pathology.

Anaemic Hypoxia: Reduction in O2 carrying capacity of blood due to anaemia (red blood cell loss/iron deficiency).

Stagnant Hypoxia: Heart disease results in inefficient pumping of blood to lungs/around the body

Histotoxic Hypoxia: poisoning prevents cells utilising oxygen delivered to them e.g. carbon monoxide/cyanide

Metabolic Hypoxia: oxygen delivery to the tissues does not meet increased oxygen demand by cells.

Question 20

How does hyper/hypoventilation alter plasma PCO2?

Answer 20

Hyperventilation - blowing off more CO2, lead to decreased [H+] bringing about respiratory alkalosis.

Hypoventilation - causing CO2 retention, leads to increased [H+] bringing about respiratory acidosis.

Question 21

How are the skeletal muscle of inspiration stimulated so they can control ventilation?

Answer 21

Via the phrenic (to diaphrgam) and intercostal nerves (to external intercostal muscles).

Question 22

Where do the centres of these respiratory nerves reside?

Answer 22

Located in the pons and medulla of the brain stem.

Question 23

What happens if the spinal cord id severed above C3,4,5?

Answer 23

Breathing ceases as phrenic nerve signalling can no longer be produced.

Question 24

What are functions of the respiratory centres?

Answer 24

Set an automatic rhythm of breathing through co-ordinating the firing of smooth and repetitive bursts of action potentials in DRG – travel to inspiratory muscles.
Adjust this rhythm in response to stimuli.

Question 25

What factors modulate the rhythm of respiratory centres?

Answer 25

Emotion (via limbic system in the brain)
Voluntary over-ride (via higher centres in the brain)
Mechano-sensory input from the thorax (e.g. stretch reflex).
Chemical composition of the blood (PCO2, PO2 and pH) – detected by chemoreceptors.

Question 26

What are some properties of central chemoreceptors?

Answer 26

Located in the medulla.
Responds directly to H+ conc. which is a direct reflection of PCO2.
Involved in the primary ventilatory drive.
detect changes in H+ conc. in CSF of brain.
Rise in H+ causes reflex stimulation of ventilation.

Question 27

What are some properties of peripheral chemoreceptors?

Answer 27

Located in the carotid and aortic bodies.
Respond to plasma H+ conc. and PO2.
Involved in secondary ventilatory drive.
Cause reflex stimulation of ventilation following significant fall in arterial PO2 (consider haemoglobin dissociation) or a rise in [H+] .

Question 28

What allows for a large degree of voluntary control over breathing?

Answer 28

Descending neural pathways from cerebral cortex to respiratory motor neurons. However cannot over ride involuntary stimuli such as arterial PCO2 or [H+]

Question 29

Why is respiration halted during swallowing?

Answer 29

To avoid aspiration of food or fluids into the airways. Swallowing is followed by an expiration in order that any particles are dislodged outwards from the region of the glottis.

Question 30

How does nitrous oxide effect chemoreceptors?

Answer 30

Blunts peripheral chemoreceptor response to falling PaO2.
Problematic in chronic lung disease cases where individual often on “hypoxic drive”. Administering O2 to these patients aggravates situation.