Physiology ✅ Flashcards
What must be overcome for air to flow?
Airway resistance
What is airway resistance the result of?
Frictional force which opposes the flow of air
Of what type of flow is most airflow in airways under normal conditions?
Laminar
What equation determines the resistance for laminar flow?
Poiseuille’s equation:
Resistance = (8 x length x viscosity of gas) / (π x (radius)^4)
What does Poiseuille’s equation predict?
That resistance increases dramatically as diameter decreases
What structures produce most resistance to airflow?
Trachea and larger bronchi
Why do the trachea and larger bronchi produce the most resistance to airflow?
Because the branching of the tracheobronchial tree, the combined cross-sectional area is sufficiently large enough to provide little resistance to flow
What is the cross sectional area of the trachea compared to the total cross sectional area at the 23rd branching?
3cm^2 at trachea, 4m^2 at 23rd branching
Why do young children have an increased resistance to airflow?
Smaller size of airways
Why can the chest wall of young children become drawn inwards with each breath in young children, even in health?
- Increased resistance to airflow
- Reduction in chest wall compliance
When might the chest wall being drawn inwards with each breath get worse?
If there are any additional factors that increase airway resistance, e.g. bronchiolitis
What is the compliance of the lung a measure of?
How easily it can be distended
What formula calculates lung resistance?
Change in volume / change in pressure
What is required to generate airflow during the first part of inspiration?
A relatively greater pressure
What does compliance vary depending on?
The exact lung volume
How does compliance vary with age?
A newborn child has very low compliance compared to a young adult
What is the typical lung compliance of an adult male?
0.09 - 0.26 L/cmH2O
What is the typical lung compliance for a newborn infant?
0.0005 L/cmH2O
What is the primary control of breathing via?
The autonomic nervous system
What stimuli can increase the rate of breathing?
- Emotional stimuli
- Peripheral chemoreceptors
- Central chemoreceptors
- Receptors in muscles and joints
- Receptors for touch, temperature, and pain stimuli
- Cerebral cortex
What stimuli can reduce the rate of breathing?
- Emotional stimuli
- Stretch receptors in the lungs
- Cerebral cortex
Through what system can emotional stimuli change the rate of breathing?
Limbic systemic
What changes detected by peripheral chemoreceptors may trigger an increased rate of breathing?
- Decreased oxygen
- Increased CO2
- Increased hydrogen
What changes detected by central chemoreceptors may trigger an increased rate of breathing?
- Increased CO2
- Increased hydrogen
Where are the respiratory centres located?
In the medulla and pons
What co-ordinates voluntary control over breathing?
Cerebral cortex
Where is control of breathing mediated?
Mainly (but not exclusively) through neural centres in the brainstem
What is responsible for the rapid respiratory response to changes in arterial carbon dioxide?
Carbon dioxide in the blood diffuses rapidly into the CSF, where it reacts with water to release hydrogen ions
Where are peripheral chemoreceptors located?
In the carotid and aortic bodies
What are peripheral chemoreceptors sensitive to?
Oxygen and carbon dioxide
Where are irritant receptors located?
In the upper and lower airways
Where are mechanical receptors located?
In the lungs and chest wall
What do the peripheral chemoreceptors, irritant receptors, and mechanical receptors facilitate?
The response to hypoxia
How does the speed of response to hypoxia compare to that of hypercapnia?
It is much less rapid
Why does metabolic acidosis tend to be incompletely and slowly corrected?
As excess H+ will only cross the blood-brain barrier slowly
How can autonomic control of breathing be overridden?
Conscious control enabling, e.g. speech and breath holding
Where are voluntary conscious signals for breathing generated?
In the cortex
How are voluntary conscious signals for breathing conducted to the muscles of breathing?
Via the corticospinal tract
What is normal, quiet breathing termed?
Tidal
What is the rate and depth of quiet breathing controlled by?
The brainstem
What determines the distribution of air within the lungs during tidal breathing?
The regional variation in airway resistance and lung compliance
What happens to the volumes of the lungs in an upright child?
There is relative over-distention of the apices and relatively reduced volume at the bases
Why is their relative over-distention of the apices and reduced volume at the bases in an upright child?
Because the wright of the lungs ensures that the pleural pressure is more negative at the apex
At what age is the relative over-distention at the apices and reduced volume at bases in an upright child most prominent?
Infants and early childhood
Why is the relative over-distention at the apices and reduced volume at the bases in an upright child most prominent in infants and early childhood?
It is opposed by the rigid chest wall and stronger respiratory muscles in adult life
What is an increase in airway resistance usually due to in an infant?
Airway narrowing, e.g. smooth muscle thickening and/or airway cell oedema
What will any increase in airway resistance in an infant lead to?
Greater effort of breathing
What signs of respiratory distress may be found in infants?
- Abdominal breathing
- Sternal/intercostal recession
Why do infants in respiratory distress have abdominal breathing and sternal/intercostal recession?
The relatively weak intercostal muscles and non-compliant chest wall are unable to oppose the stronger contraction of the diaphragm
What is the major muscle of respiration?
The diaphragm
How does the muscle fibre content of the diaphragm differ in at birth compared to in adulthood?
At birth, fatigue-resistant striated muscle fibres account for only 10% of its muscle mass, which increases to 50% by early adulthood
What is the result of the diaphragm having. lower content of fatigue-resistant striated muscle at birth?
Infants will tire more quickly, and are at increased risk of apnoea and respiratory failure if their work of breathing needs to increase for more than a few minutes for any reasons
Give 2 reasons why work of breathing may increase in an infant?
- Bronchiolitis
- Heart failure due to VSD
What does the less compliant chest wall of an infant predispose the airway too?
Predisposes the immature airway to partial closure, particularly at the lung bases
What does the partial closure of the airways due to reduced compliance in an infant lead to?
Intrapulmonary shunting of blood through non-ventilated areas
What happens to diaphragm function during sleep?
It is largely preserved
Why is diaphragm function during sleep preserved?
Because it is essential for maintenance of adequate ventilation
What happens to accessory muscle function during sleep?
It is reduced, particularly during REM sleep
What is the result of accessory muscle function being reduced during sleep?
This may contribute to hypoventilation and ventilation-perfusion mismatching, resulting in oxygen desaturation
Why shouldn’t you place a child in respiratory distress in a horizontal position?
It can lead to ventilation perfusion mismatch
Where does the matching of ventilation with perfusion take place?
Within the airways and alveoli
Why might placing a child in respiratory distress lead to ventilation-perfusion mismatch?
Placing a child with respiratory distress in a horizontal position leads to an immediate reduction of ventilation in the dependent lungs. Sudden changes cannot be instantly compensated for, and so the change in ventilation is not immediately accompanied by a change in lung perfusion, causing mismatch and potentially decreasing sats
What is minute volume?
The volume of inspired air and expired gases that move in and out of the lungs each minute
What is the minute volume a product of in normal breathing?
Tidal volume and respiratory rate
What is tidal volume?
The volume of each breath
What does spirometry measure?
Lung volume changes
From what age can spirometry be used?
5
What values can be measured using a spirometer?
- Vital capacity
- FEV1
What values require different techniques to measure?
- Total lung capacity
- Functional residual capacity
- Residual volume
What is exchange of gas across a surface dependent on?
- The gradient of partial pressure across it
- The surface area
- The magnitude of the diffusion distance
How does gas transfer occur?
By diffusion
Is carbon dioxide water soluble?
Yes, very
What is the result of carbon dioxide being very water soluble?
It diffuses much more readily than oxygen, and is less affected bye increases in diffusion distance
Give an example of when there might be an increase in diffusion distance in the lungs?
Pulmonary oedema
What do the differential changes seen in blood carbon dioxide and oxygen content in respiratory failure allow us to understand?
Where the problem is occurring
What happens when there is an increase in FiO2 in alveolar hypoventilation?
Correction of hypoxia
What happens to arterial CO2 in alveolar hypoventilation?
Increased
What happens when there is an increase in FiO2 in impaired diffusion?
Correction of hypoxia
What happens to arterial CO2 in alveolar hypoventilation?
Normal or decreased
What happens when there is an increase in right-left shunt or ventilation-perfusion imbalance?
Little change or no change
What happens to arterial CO2 in right-left shunt or ventilation-perfusion imbalance?
Decreased
How much oxygen is dissolved in plasma?
3ml/L of arterial blood
How is the vast majority of oxygen transported?
Bound to haemoglobin
What does oxygen saturation describe?
The amount of haemoglobin molecules that are bound to oxygen
What is meant by 100% saturations?
All the possible sites for oxygen binding within haemoglobin are occupied
What shows the relationship between oxygen saturation and oxygen content of blood?
The oxygen dissociation curve
Which type of Hb has the steepest oxygen dissociation curve?
Fetal Hb
At what kPa (partial pressure of oxygen) does adult Hb unload most of its oxygen?
2.5-6.5kPa
At what kPa does fetal Hb unload most of its oxygen?
1.3-5.2kPa (so tissue has to be lower in oxygen for fetal Hb to give it up)
Why does fetal Hb unload most of its oxygen at a lower kPa?
Because fetal Hb does not bind 2,3-DPG efficiently, and thus will tend to ‘hold on’ to oxygen at the expensive of maternal Hb during pregnancy
Why is both fetal and adult Hb able to unload its oxygen?
Because the tissue store for oxygen is myoglobin, whose dissociation curve is even further to the left (meaning it binds oxygen more strongly)
What happens if Hb binds oxygen more strongly?
The dissociation curve is shifted to the left, and oxygen is less readily delivered to the tissues
What alters the oxygen dissociation curve?
- The presence of hydrogen ions (the Bohr effect)
- 2,3-DPG
- Carbon dioxide
What happens to the oxygen dissociation curve when there is an increase in hydrogen irons, 2,3-DPG, or CO2?
It is shifted to the right
What is the result of the oxygen dissociation curve being shifted to the right?
Less oxygen remains bound to Hb molecule at the same partial pressure of oxygen
What is the result of the oxygen dissociation curve being shifted to the right in the presence of hydrogen ions or CO2?
Oxygen tends to be better delivered to areas with higher concentrations of hydrogen ions and carbon dioxide
Why is newly transfused blood less efficient at delivering oxygen?
It is low in 2,3-DPG
What is meant by total lung capacity?
The maximum amount of air the lungs can accommodate
What is meant by tidal volume?
The amount of air inspired and expired at each breath
What is the usual tidal volume?
6-8ml/kg
What is meant by expiratory reserve volume?
The amount of air that can be exhaled after normal, quiet expiration
What is meant by residual volume?
The amount of air that remains in the lungs after the end of maximal voluntary expiration
What is meant by inspiratory reserve volume?
The amount of air that can be inhaled at the end of normal tidal inspiration
What is meant by vital capacity?
The sum of inspiratory reserve volume, tidal volume, and expiratory reserve volume
