Respiratory Physiology

Question 1

Describe the location(s) and the role of the neural control centres in initiating and automatically controlling ventilation.

Answer 1

Breathing is controlled by elements from the medulla oblongata and the pons, both located in the brain stem
Medulla oblongata: Houses the DRG and the VRG, which control inspiration and expiration respectively.
Pons: Houses the apneustic and pneumotaxic centres, which modulate and smooth breathing respectively

Question 2

Function of the DRG of the medulla

Answer 2

DRG - CONTROLS INSPIRATION

Receives input from the 9th and 10th nerves and the apneustic centre.

Question 3

Function of the VRG of the medulla

Answer 3

VRG - CONTROLS EXPIRATION
The VRG is not usually active, as expiration is passive in normal quiet breathing.
During times of laboured breathing, e.g. exercise, lung disease, the VRG is active.

Question 4

Functions of the apneustic and pneumotaxic centres of the pons

Answer 4

Apneustic centre - Modulates the breathing cycle (e.g. gasps)
Pneumotaxic centre - Smooths the breathing cycle

Question 5

Describe influences on respiratory rhythm.

Answer 5

1. Higher centres:
   Cerebral cortex (VOLUNTARILY CHANGES BREATHING PATTERN),
   Hypothalamus/limbic system (emotional)
2. Baro/thermos/mechano receptors (e.g. in muscles)
3. Chemoreceptors: Central and peripheral chemoreceptors. Sensitive to changes in PaO2, PaCO2 and pH.

Question 6

Describe why breathing can be voluntarily controlled.

Answer 6

By the higher centres of the brain (cerebral cortex)HOWEVER, these changes are overridden detection of changes in CO2 and H+ in arterial blood
e.g. holding breath until fainting and then breathing will resume

Question 7

State the location of the peripheral chemoreceptors within the body

Answer 7

The carotid bodies (predominate, immediate effect on ventilation) and the aortic bodies (greater effect on CV system)

Question 8

Describe the sensitivity of peripheral chemoreceptors

Answer 8

1. PaO2: Primarily respond to decreased PaO2 (not O2 content) but must be dramatic.
   CAROTIDS ARE THE ONLY CHEMORECEPTOR TO RESPOND TO HYPOXIA
2. pH: Sensitive to changes in arterial pH
3. PaCO2: Provide the initial response to changes in CO2 (20%)

Question 9

Which chemoreceptors are sensitive to hypoxia?

Answer 9

Carotid bodies

*Hypercapnia makes increases their sensitivity to hypoxia

Question 10

State the location of the central chemoreceptors within the body

Answer 10

Located in the medulla oblongata, close to the respiratory centres
*Main source of tonic drive to breathe

Question 11

Describe the sensitivity of central chemoreceptors

DIAGRAM

Answer 11

1. PaCO2: Very sensitive to PaCO2 changes, which manifest as pH changes in the CSF (80%)
   INSENSITIVE TO HYPOXIA

Question 12

Describe why O2 should not be administered to those with chronic hypercapnia

Answer 12

In chronic hypercapnia compensation returns the brain back to physiological pH, depressing the drive of the central chemoreceptors (as pH is reduced).
Respiration is now dependent upon carotid body peripheral chemoreceptors to drive respiration in response to hypoxia.
If oxygen is administered then this hypoxic drive is lost and ventilation decreased, PaCO2 increased and may induce a coma.

Question 13

Deduce the effect of altered pulmonary ventilation (e.g. hyperventilation, hypoventilation, breath-holding) on PaO2 and PaCO2.

Answer 13

Hyperventilation: Decreased CO2, increased O2
Hypoventiliation: Increased CO2
Breath holding: Increased CO2

Question 14

Describe the functions of the conducting and respiratory zones and relate these to their anatomical and histological features

Answer 14

Conducting: Filters, warms and moistens the air. Allows air to be transported to the respiratory zone.
Respiratory: Site of gas exchange between the air and blood.

Question 15

Define anatomical and physiological deadspace. Explain influential factors in the volume of each.

Answer 15

Deadspace = Volume of air not participating in gas exchange
Anatomical: Morphological. Volume of the conducting zones. Influenced by position, weight
Physiological: Function. The total volume of breath that does that participate in gas exchange (conducting and respiratory zones). Similar to anatomical in health. Influenced by how intact alveoli are, amount of mucus

Question 16

Explain the principles underlying gas flow and exchange across the alveolar-capillary walls. How can this be measured?

Answer 16

Fick’s Law:
Proportional to surface area and difference in partial pressure (concentration gradient)
Inversely proportional to the thickness of the diffusion surface
Can be measured using the diffusing capacity (DL gas). Compare inspired and expired CO content

Question 17

Describe the partial pressure gradients for O2 and CO2 exchange. State normal values across circulation

Answer 17

x

Question 18

Explain the clinical relevance of VQ matching and how it can be disrupted. State the normal range.

Answer 18

Normal ratio: 0.8 -1 (a range seen from the apex - base)
VQ matching maximises exchange.
Loss of VQ matching is the most common cause of decreased PaO2.
Can be disrupted by pathology (alveolar dysfunction, lack of inspired O2, decreased perfusion). Leads to an increased surface not involved in gas exchange.

Question 19

Describe the means by which oxygen is transported in the blood

Answer 19

Bound to haemoglobin - 98 %

Free in the plasma - 2 %

Question 20

Describe the means by which carbon dioxide is transported in the blood

Answer 20

As bicarbonate ion - 78 %
In plasma - 13 %
As carbaminoglobin - 9 %

Question 21

Describe the structure and function of oxygen binding proteins

Answer 21

Myoglobin: Has a single oxygen binding site. Mostly found in muscles, used during periods of intense physical activity. Acts to shuttle oxygen from capillaries to mitochondria.
Haemoglobin: Formed from 2 alpha and 2 beta chains. Has a central iron molecule. Has 4 oxygen binding sites. Acts to transport oxygen

Question 22

Explain the shapes and significance of oxygen and carbon dioxide binding curves

Answer 22

Oxygen: Sigmoidal shaped curve, resultant of allosteric shape change which allows for cooperative binding of haemoglobin. Plateau observed as haemoglobin becomes saturated.
Carbon dioxide: The curve is more linear. Plateau not observed (not limited by transport protein)

Question 23

Describe factors which affect gas transport and cause left and right hand shifts of the oxygen-haemoglobin dissociation curve.

Answer 23

Factors: Exercise, body temperature, metabolic rate, respiratory rate
Right hand shift: Reduced oxygen affinity. Increased H+ (Bohr effect), increased CO2, increased 2, 3-DPG and increased temperature
Left hand shift: Increased oxygen affinity. Decreased H+, decreased 2, 3-DPG and decreased temperature
Axis: x-axis = PO2 y-axis=Saturation

Question 24

Normal plasma pH range

Answer 24

7.35 - 7.45

Question 25

Describe factors involved in pH homeostasis

Answer 25

PaCO2, [HCO3-], respiratory rate, metabolic disturbance

Question 26

Define PaO2

Answer 26

The amount of O2 DISSOLVED in plasma

Question 27

Define O2 content

Answer 27

Amount of dissolved and bound O2

Question 28

Define O2 capacity

Answer 28

Amount of O2 bound to haemoglobin

Question 29

Define O2 saturation

Answer 29

Number of binding sites (haemoglobin) bound by oxygen

Question 30

Define haemoglobinopathy

Answer 30

Caused by a defect in the genetically determined molecular structure of haemoglobin e.g. sickle cell anaemia (becomes sticky)

Question 31

Effect of anaemia on O2 transport

Answer 31

Reduced amount of haemoglobin leads to a reduced ability to transport O2.