Wk 7 - Respiration during exercise

Question 1

What is the function of the respiratory system and what is the 4 step process?

Answer 1

The primary purpose of the respiratory system is to maintain arterial blood-gas homeostasis. This is accomplished via a 4-step process:
1. Pulmonary ventilation
2. Alveolar gas exchange
3. Gas transport
4. Systemic gas exchange

Question 2

What is the structural and functional organisation of the respiratory system?

Answer 2

The epiglottis separates the upper and lower respiratory tracts. The lungs are enclosed within membranes called pleura. Intrapleural pressure < atmospheric pressure, which prevents the alveoli from collapsing. The pressure in the pleura is negative which forces the lungs to not collapse or close. Conducting zone is movement of oxygen in and out the body. All gas exchange takes place in the respiratory zone.

Question 3

Describe the airways:

Answer 3

There are ~23 airway generations. The main bronchi is Z1; the conducting zone extends to the terminal bronchioles (Z16). Gas exchange occurs in the respiratory zone (Z17-23). The human long contains ~300-500 million alveoli, each ~1/3mm in diameter. The trachea is generation 0. There are bunches of alveoli in the alveolar sacs. Collagen and elastin surrounded the lungs which allow them to expand.

Question 4

What is alveolar (pulmonary) gas exchange?

Answer 4

-> Pulmonary gas exchange takes place along the pulmonary capillary. Oxygen and carbon dioxide move between the air and blood by simple diffusion (from high to low partial pressure).

Question 5

What are the 2 types of alveolar cell (pneumocytes)?

Answer 5

Type 1 cells -> Cover ~95% of the internal surface of the alveolus and are critical for gas exchange
2. Type 2 cells -> Release surfactant – a molecule that lowers the surface tension (like soap). It reduces the surface tension of the water and allows the lungs to fill up easily.

Question 6

Describe the mechanisms of breathing:

Answer 6

• The mechanics of breathing is concerned with the movement of air into and out of the lungs by changing in pressure, flow and volume
• Contraction length = ∆volume
• Contraction velocity = ∆flow
• Contraction force = ∆pressure

Question 7

Describe the mechanics of breathing with regards to the thoracic cavity during inspiration?

Answer 7

• Diaphragm contraction increases vertical diameter
• Bucket handle motion of the ribs increases the transverse (lateral) diameter of the thorax during inspiration
• Pump handle motion of the ribs increases the anteroposterior diameter of the thorax during inspiration
  -During inspiration, the lateral changes are the ribs elevating and the thoracic cavity widening, while during expiration, the ribs become depressed and the thoracic cavity narrows.

Question 8

What are the muscles of respiration?

Answer 8

• At rest, diaphragm contraction is responsible for the majority of pulmonary ventilation (expiration is passive)
• During exercise, the diaphragm is assisted by the external intercostal muscles, scalenes, sternocleidomastoid and many others in order to increase pulmonary ventilation 10-20 fold above resting levels. Expiration becomes active by contraction or the rectus abdominus, internal intercostal muscles and external oblique (among others). Progressively during exercise, more muscles are recruited.
• Under appreciated, there are >60 respiratory muscles, including those of the upper airway that minimise resistance.
• The upper airway muscles keep the upper respiratory system open in the body to allow air to move in and out

Question 9

Describe respiratory muscles during fatigue:

Answer 9

The pump muscles come from the diaphragm. Respiratory muscles do fatigue eventually. Bilateral phrenic nerve stimulation is a non-volitional measure of diaphragmatic fatigue. 16% decrease in Pdi, twpost exercise > 85% VO2 max to exhaustion (exercise has to be very intense for it to exhaust/ they do not exhaust easily)

Question 10

What is Fick’s law of diffusion equation and what is it dependent upon?

Answer 10

Explain how gases are transported across the blood-gas barrier in the lung:
-Fick’s law of diffusion -> The volume of gas passing through a sheet is dependent upon:
1. Surface area (A) (the bigger the surface area the greater the surface for diffusion to occur across)
2. Thickness (T)
3. Diffusion coefficient (D)
4. Pressure gradient (alveolar to arterial)

Question 11

What is the blood-gas barrier/ interface?

Answer 11

• Electron micrograph showing a pulmonary capillary (C) in the alveolar wall
• The large arrow indicates the diffusion path from alveolar gas to the erythrocyte (EC) and includes 5 layers:
  1. Surfactant
  2. Alveolar epithelium (EP)
  3. Interstitium (IN)
  4. Capillary endothelium (EN)
  5. Plasma
• The blood-gas barrier is very thin (0.3 um) and has a vast surface area (50-100m2) making it ideal for gas exchange/ diffusion (Fick’s law)

Question 12

What its Ohm’s law and equation?

Answer 12

• Current = voltage/ resistance, applied to breathing
• Airflow is dependent upon a pressure gradient and airway resistance
  -In notes 1

Question 13

What is Poiseuille’s law and equation?

Answer 13

• Resistance is dependent upon viscosity, length and radius
• Radius is raised to the 4th power, thus the major determinant of airway resistance

Question 14

What is the equation if you expand both ohm and poiseuilles law?

Answer 14

-In notes 1

Question 15

What is exercise-induced asthma?

Answer 15

During exercise in healthy, young individuals, the SNS is activated and releases adrenaline causing dilation. In individuals with asthma, there is nor broncho-dilation, there is broncho-constriction. This means there is less ability to create flow in the body and less ability to generate a breath (they are flow limited). There is greater flow limitation early in exercise for asthma than healthy individuals.

Question 16

What is the pulmonary equation?

Answer 16

-In notes 1

Question 17

What is alveolar ventilation and equation?

Answer 17

Not all air breathed reaches the alveoli; the volume of air not participating in gas exchange is called dead space (VD). VD = 150mL in healthy individuals and does not change during exercise.
-Pulmonary volumes and capacities -> A volume is one segment. A capacity is two or more segments.
-In notes 1

Question 18

What are obstructive airway diseases?

Answer 18

• Spirometry (measures how much you can breathe and how fast) can be used to diagnose pulmonary disease, such as COPD
• Forced vital capacity (FVC) is the maximum volume air that can be forcefully expired after a maximum inspiration. COPD is characterised by increased airway resistance and a reduced FEV/FVC.
• The graph shows a normal volume-time plot for a healthy individual and an individual with COPD.

Question 19

What are the rest-to-work transitions?

Answer 19

-> The ventilatory response to constant load steady-state exercise occurs in 3 phases:
* Phase 1: immediate exercise in Ve
* Phase 2: exponential increase in Ve
* Phase 3: plateau
-The near precise constancy of arterial PO2 and PCO2, demonstrating an error-free exercise hyperpnoea.

Question 20

What is the ventilatory response to incremental exercise?

Answer 20

• Ventilation increases in an approximately linear fashion with exercise workload until a point commonly referred to as the ‘ventilatory threshold’. Sometimes referred to as the lactate or anaerobic threshold. Ventilatory threshold occurs ~50-75% of peak workload (i.e. VO2 max) (start to hyperventilate at this point)
• After the ventilatory threshold, Ve increases disproportionately to metabolic rate, resulting in a decrease in PaCO2 (hyperventilation)
• Some highly trained endurance athletes develop exercise-induced arterial hypoxaemia (EIAH)

Question 21

What is exercise-induced arterial hypoxaemia?

Answer 21

• EIAH is defined as a reduction in PaO2 of > 10mmHg from rest. Occurs in highly trained males (50%) during heavy exercise and the majority of females regardless of fitness or exercise intensity
• Originally theorised to occur because ventilatory demand > capacity (demand versus capacity theory)
• Causes are not fully understood but are generally believed to be due to
  1. Diffusion limitation
  2. Relative hypoventilation
  3. Ventilation-perfusion (V/Q) mismatch (air coming in has to be met by the same blood flow in the body) (ideally 1:1 ratio)

Question 22

Describe the changes in breathing patterns during exercise?

Answer 22

• At the onset of exercise, changes in Ve are largely achieved by increasing Vt.
• During heavy exercise, Vt plateaus and further increases in Ve are achieved by increasing fb
• -Vt does not exceed 60% of vital capacity
• Arterial PO2, PCO2 and pH are well maintained until heavy exercise

Question 23

What is the arterial ventilation equation?

Answer 23

-Alveolar ventilation equation = PaCO2 = CO2 production/ total-dead space ventilation

Question 24

What is the neural control of respiration during exercise?

Answer 24

-Control of ventilation -> 3- compartmental model
-The respiratory control centre ->
* The neural control of breathing is complex. Respiratory central pattern generators are located within the brainstem (pons and medulla)
-Central controller, sensors and effectors

Question 25

What are the 3 main groups of neurones in the neural control of respiration during exercise?

Answer 25

- Ventral respiratory group (inspiratory and expiratory) - Dorsal respiratory group (inspiratory) - Pontine respiratory group (modulatory)

Question 26

What are peripheral chemoreceptors?

Answer 26

-They are chemoreceptors that contribute to the control of breathing during exercise * Peripheral chemoreceptors are located at the aortic arch and carotid body. Detects changes in PO2 of blood perfusing systemic and cerebral circulation * Relays sensory information to the medulla (NTS) via vagus (CN X) and glossopharyngeal (CN IX) nerves * Decreased PaO2 = Increased Ve * Other stimuli also activate peripheral chemoreceptors, such as temperature, adrenaline and CO2.

Question 27

What are central chemoreceptors?

Answer 27

-They contribute to the control of breathing during exercise * Central chemoreceptors are located primarily in the ventral surface of the medulla, known as the retrotrapezoid nucleus (RTN). Originally referred to as ‘area M’ after Robert Mithcell in 1963 * The RTN is sensitive to change in PaCO2 and H+ of the cerebrospinal fluid. Many other brain sites are also sensitive to CO2, such as the NTS, locus coeruleus, raphe and cerebellum * Increased PaCO2 = Increased Ve

Question 28

What is chemoreceptor feedback?

Answer 28

1. Chemoreceptors detect error signals (disturbances to blood-gas homeostasis) 2. Central and peripheral chemoreceptors increase afferent input to the brainstem in response to increasing PaCO2 or decreasing PaO2 or pH 3. Respiratory premotor neurons in the dorsal respiratory group (and ventral respiratory column) are activated 4. Inspiratory muscles contract, increasing Ve 5. Changes in Ve elicit changes in PaO2, PaCO2 and pH, thus restoring blood-gas balance

Question 29

What are ventilatory responses to O2 and CO2?

Answer 29

The ventilatory response to O2 is curvilinear (below ~65mmHg), whereas the ventilatory response to CO2 is linear – small changes in PaCO2 elicit much greater changes in VE versus PaO2.

Question 30

Describe the ventilatory control mechanisms for mild to moderate exercise?

Answer 30

* Mild to moderate exercise (below Tvent) -> Primary drive must be feedforward with respect to PaCO2 (and pH). Central and peripheral neurogenic stimuli from higher brain centres and skeletal muscle, respectively. Fine tuned by peripheral chemoreceptor feedback

Question 31

Describe the ventilatory control mechanisms for heavy to severe exercise:

Answer 31

Metabolite accumulation (H+) causes acidosis, thus activating chemoreceptor feedback, along with increases in body temperature, K+, and adrenaline. Central and peripheral neurogenic stimuli continue as with moderate exercise.

Question 32

What are the effects of endurance training?

Answer 32

* Ve is 20-30% lower during submaximal exercise in trained versus untrained individuals * Chronic training adaptations that improve aerobic capacity - Decreased metabolite accumulation - Decreased afferent feedback - Decreased ventilatory drive

Question 33

Do the lungs adapt to exercise training?

Answer 33

With few exceptions, the lungs and airways do not adapt to physical training. Airways and lungs do not get bigger and diffusing capacities are unchanged. Respiratory muscles may become stronger and more fatigue resistant. Some adaptations may be maladaptive e.g. airway hyperresponsiveness in skiers and swimmers.

Question 34

What is the 'overbuilt' lung?

Answer 34

In healthy untrained individuals, the pulmonary system is not considered a limiting factor to exercise. Possible exceptions included exercise-induced arterial hypoxaemia (EIAH), exercise induced laryngeal obstruction (EILO) or asthma (EIA), expiratory flow limitation and dyspnoea, respiratory muscle fatigue, and intrathoracic pressure effects on cardiac output

Question 35

What is Dalton's law?

Answer 35

-Dalton's law states that the total pressure of a gas mixture is equal to the sum of the pressure that each gas would exert independently

Question 36

What is the partial pressure of a gas given by?

Answer 36

-Pgas = Ggas . Pbar

Question 37

Describe partial pressure of gases?

Answer 37

-The partial pressure of inspired O2 and CO2 is 159 and 0.3mmHg, respectively -Oxygenated inspired air is mixed in the lungs with deoxygenated air returning from the venous blood -A very small gas exchange impairment means the arterial PO2 (~100mmHg) is slightly less than alveolar PO2 (~105mmHg) -O2 is consumed and CO2 is produced by tissues via cellular respiration, consequently venous PO2 is decreased to 40mmHg and venous PCO2 is increased to 46mmHg

Question 38

List the 4 steps in pulmonary circulation:

Answer 38

1. Pulmonary artery carries deoxygenated blood from the right ventricle to the lungs 2. Gas exchange between the alveoli and pulmonary capillaries occurs 3. Oxygenated blood is returned to the left atrium via the pulmonary vein 4. Oxygenated blood is pumped around the systemic circulation to systemic cells

Question 39

Describe the pressure in the pulmonary circuit:

Answer 39

-Low pressure, low resistant circuit -Mean pulmonary artery pressure = 15mmHg -Systemic arterial pressure = 100mHg

Question 40

Describe the wall in the pulmonary circuit:

Answer 40

-Thin walled, little smooth muscle -Accepts entire cardiac output -No redistribution of blood flow -Pulmonary vascular resistance decreases during exercise due to recruitment and distension of pulmonary capillaries

Question 41

What are ventilation-perfusion relationships?

Answer 41

-Gas exchange requires a matching of ventilation to blood flow (V/Q) -The ideal V/Q is 1 -V/Q > 1 = underperfused (apex of lung) -V/Q < 1 = overperfused (base of lung) -In the upright lung, blood flow increases disproportionately more than ventilation from the top to bottom of the lung due to the effects of gravity

Question 42

Why does V/Q (ventilation to blood flow) improve upon mild exercise?

Answer 42

-Increased tidal volume -Increased pulmonary artery pressures -In some cases, V/Q worsens, especially during heavy exercise

Question 43

Which 2 forms is O2 carried in the blood by?

Answer 43

1. Dissolved (2%) 2. Combined with haemoglobin (98%)

Question 44

Describe how oxygen is transported in the blood when dissolved:

Answer 44

-Dissolved O2 follows Henry's law (amount dissolved is proportional to partial pressure) -The amount of O2 dissolved is 0.003mL O2/ 100mL blood/ mmHg -At an arterial pressure PO2 of 100mHg, this results in 0.3mL O2/100mL blood

Question 45

How is oxygen transported in the blood through haemoglobin?

Answer 45

-The majority if O2 transported in the blood is chemically bound to haemoglobin (Hb) -Each Hb molecule can transport four molecules of O2 -The amount of O2 transported as oxyhaemoglobin is dependent upon Hb mass -1.34 mL O2/g Hb; 15g/ 100mL blood

Question 46

What is the equation for the total content of O2 carried by blood

Answer 46

-Found in notes 3 -Therefore, Ca02 = 20.4 mL 02/ 100mL blood

Question 47

Describe the effects of exercise and the Bohr effect:

Answer 47

-Exercise causes an increase in H+, CO2 and core body temperature, causing a rightward shift in the ODC, known as the Bohr effect -The Bohr effect facilitates unloading of O2 to active tissues (less saturated for any given PO2)

Question 48

Describe oxygen transport in muscle:

Answer 48

-Myoglobin is an O2 binding protein found in skeletal muscle -High O2 affinity; unloads at very low PO2 -Shuttles O2 from muscle cell membrane to mitochondria where PO2 is 1-2mmHg for aerobic respiration -Provides intramuscular O2 storage (reserve)

Question 49

How is carbon dioxides transported in blood?

Answer 49

-Co2 is carried in the blood in 3 forms: dissolved (10%), bound to haemoglobin (20%), bicarbonate (70%) -CO2 is 20x more soluble than O2 -HCO3 leaves the cell and Cl- moves into the cell to maintain neutrality (chloride shift) -H+ binds to Hb to form HHb which binds to CO2 to create carboamino Hb -Most of the CO2 forms a reversable reaction when bound with water

Question 50

What is the CO2 dissociation curve and the Haldane effect?

Answer 50

-Unlike the ODC, the CO2 dissociation curve is linear -Deoxygenated Hb increases affinity for CO2 (left shift) and vice versa -Facilitates carboamino CO2 carriage in venous blood and unloading at the lungs

Question 51

Describe ventilation and acid-base balance:

Answer 51

-CO2 transport in blood equation: C02 + H20 <--> H2CO3 <--> H+ + HCO3- -The increase in CO2 production during exercise causes an increase in H+ and thus a decrease in arterial pH, which stimulated breathing via a feedback loop