L5. Respiratory Physiology

Question 1

What are the main functions of the respiratory system? [3]

Answer 1

1. To oxygenate pulmonary arterial blood
2. To remove carbon dioxide from the blood
3. The maintain appropriate acid-base balances

Question 2

What is meant by the lungs are ‘fractal organs’?

Answer 2

They allow efficient matching of ventilation and perfusion at the alveolar-capillary level

Question 3

What are the components and the major function of the upper airways?

Answer 3

The upper airways is made up of the nasopharynx, orophraynx and larynx. They act to humidify the air we breath and to filter out particulates. (The epiglottis also ensures no food enters the RT).

Question 4

What are the major differences between the conducting and the respiratory airways of the RT?

Answer 4

The conducting pathways are purely for air travel (ventilation) and contain ciliated columnar cells surrounded by decreasing layers of smooth muscle.
The respiratory pathways are sites for gas exchange. There are cuboidal to squamous cells with very little to no smooth muscle. It has a very large surface area.

Question 5

What are the muscles of respiration?

Answer 5

The main muscle used in INSPIRATION (quiet) is the diaphragm. The accessory muscles of inspiration are the external intercostals. In very deep and full inspiration the scalene, trapezius, pectoralis and sternomastoid muscles.

The muscles of EXPIRATION are all accessory muscles. These are the internal intercostal muscles and the abdominal muscles

Question 6

How is the respiratory system functionally organised?

Answer 6

Control: the brain respiratory centre and peripheral chemoreceptors
Pump: in the chest that gets air in and out (ventilation)
Exchange: Keeps the gases in the homeostatic ranges

Question 7

What are the normal arterial blood partial pressures?

Answer 7

PaO2 = 100 mmHg
PaCO2 = 40 mmHg
pH = 7.40

Question 8

What is oxygen consumption and carbon dioxide production of the body under normal (basal metabolic rate) conditions?

How does this compare to exercise?

Answer 8

O2 = 250 mL per minute
CO2 = 200 mL per minute
This is only a fraction of the total lung capacity.

In exercise, the body consumes 4L per minute of oxygen and produce 4L per minute of carbon dioxide.

Question 9

What is the consequence if the lungs are unable to clear t carbon dioxide?

Answer 9

Hypoxaemia - lack of oxygen in the blood and tissues and in serious states there is a hypercapnoea) and respiratory acidosis.

Question 10

How is oxygen carried in the body? Define Full Saturation and the total blood oxygen content.

Answer 10

The majority of oxygen is attached to Hb which can bind to 4 molecules of O2.
Full saturation is when 1 gram of Hb binds with 1.3mL of oxygen.

The total blood oxygen content (per 100 mL) is 1.3 x [Hb] x Hb saturation/100 + 0.003 x PaO2 = 19.8 mL of oxygen per 100 mL. Thus there is about 200 mL of oxygen per L of blood.

Question 11

What factors determine how much oxygen binding to Hb?

Answer 11

The saturation is the amount of oxygen bound to Hb. The concentration dissolved in the plasma (partial pressure of O2) is the driving force for Hb binding. (The Hb sigmoidal curve). Other factors affecting binding are: temperature, pH, 2-3-BPG.

Question 12

What evidence suggests that the oxygen delivery to the blood at rest is more than adequate to meet the body’s metabolic needs.

Answer 12

At rest Cardiac Output is 5L and thus the oxygen tissue delivery is 1000 mL/minute. This is 4 times the requirement that is needed and thus Hb leave the tissues still 75% saturated.

Question 13

What are the three forms of Carbon Dioxide Transport?

Answer 13

Dissolved (10%) in the blood
Attached to proteins: Hb and carbamino compounds (30%)
As bicarbonate (60%) formed in RBCs under the action of the enzyme carbonic anhydrase. This is a major component of the acid buffering capacity of the blood.

Question 14

What is involved in inspiration?

Answer 14

Stimulation of the diaphragm by the phrenic nerves (and the intercostal nerves stimulate the external intercostal muscles) leads to an increase in the volume of the thoracic cage, this generates a negative pressure which increases the pressure gradient and sucks air in.

Question 15

What is involved in expiration?

Answer 15

Expiration is normally a passive process where the inspiratory muscles relax and recoil generating a positive intrapulmonary pressure to push air out.
Active inspiration involves contraction of the accessory muscles pushing air out.

Question 16

How thin is the alveolar: capillary membrane? What is the surface area? The volumes of alveolar and capillary flows?

Answer 16

The membrane is 0.5um
The surface area is 50-100m2
The alveolar volume is 3-6L
the capillary volume is 80 mL - increases a LOT if CO is increased

Question 17

How does flow of the gases occur between each major compartment?

Answer 17

Flow is always passive and DOWN the partial pressure gradient for both oxygen and carbon dioxide.

Question 18

What determines the rate of diffusion?

Answer 18

The rate of diffusion is determined by Fick’s Law where diffusion rate is proportional to the surface area, a constant, the difference between partial pressures and inversely proportional to the thickness of the membrane.

Question 19

What is the difference of the rate of diffusion between CO2 and O2?

Answer 19

CO2 diffuses much faster than O2 (20x the rate of oxygen).
Oxygen is very fast though:
0.75 seconds at rest which is able to decrease and become more efficient to 0.25 during exercise.

Question 20

When is gas exchange most efficient?

Answer 20

When ventilation and perfusion (V/Q) are matched across all units.

Question 21

Draw a diagram that represents one alveolar-capillary unit and the partial pressures involved in this

Answer 21

Atmospheric:
PiO2 = 150 mmHg
PiCO2 = 0 mmHg

Alveoli: mixing
PAO2 = 100 mmHg
PACO2 = 40 mmHg

Pulmonary Artery:
PvO2 = 40 mmHg
PvCO2 = 46 mmHg

Tissue:
PtO2 = 5mmHg
PtCO2 >40 mmHg

Pulmonary Veins:
PaO2 = 100 mmHg
PaCo2 = 40 mmHg

Question 22

What contributes to the work involved in breathing?

Answer 22

1. Overcoming the friction of the air going into the airways (Resistive work)
2. Overcoming the stiffness of the lungs (elastic work) - tissue composition and surface tension in the alveoli (surfactant) and affected by the compliance on the lungs.

Question 23

Describe the control systems in the brain of respiration

Answer 23

Brainstem contains neurons in the medulla and the pons which have automatic rhythmic inspiratory stimuli.
They receive input from peripheral sensors to control the rate of firing.
The major output is to the phrenic nerves on the diaphragm.

The cortex controls voluntary hyperventilation in hypercapnoea and is able to override the brainstem controls.

Question 24

What is the distribution of chemoreceptors in the body?

Answer 24

CENTRAL chemoreceptors: on the ventral surface of the medullar and respond to pH as an indication of PCO2.

PERIPHERAL chemoreceptors: carotid bodies in the neck and aorta which are rapidly responding and respond to decreases in PO2 and pH and the increased in CO2

LUNG and OTHER: pulmonary stretch receptors and upper airway receptors that respond to triggers and irritants

Question 25

Describe the differences between the symptoms and signs of respiratory diseases

Answer 25

SYMPTOMS: What is felt by the patient
Cough, Wheeze (narrowing), Sputum, Breathlessness, Tightness and pain, Snoring

SIGNS: Cyanosis, peripheral clubbing, percussion noises and lung sounds (crepitations vs. wheezing).