Lecture 2: Control of Breathing, Ventilation, Breathing Pattern, and Acid Base

Dolinay

What is ventilation?

movement of air into and out of the lungs for the purpose of gas exchange

What do inhalation and exhalation depend on?

cyclical ventilatory muscle excitation by motor nerves to diaphragm, intercostal abdominal, and upper airway muscles (ventilation largely driven by muscles)

Hypoxia is a system-wide deficiency in the __ that reach the __.

levels of oxygen; tissues

• low O2 sat (lower than 94%)

Hypoxemia is low __.

blood oxygen level

• measured invasively in an artery

Where are central chemoreceptors located? What are they sensitive to?

• located within medulla

• sensitive to pH of environment

Where are peripheral chemoreceptors located? What do they detect and monitor?

• located in aortic and carotid bodies

• act principally to detect variation of oxygen concentration in arterial blood

• monitor arterial carbon dioxide and pH

Central controller (pons, medulla, other parts of brain) → __ → __ → central controller

central controller → effectors (respiratory muscles) → sensors (chemoreceptors, lung, other receptors) → central controller

What are 2 types of ventilatory control? Which ones are under voluntary control?

1. metabolic: not under voluntary control

2. behavioral: under voluntary control

The metabolic portion of ventilatory control is geared to the __ requirements of the body in terms of __ consumption and __ production.

metabolic; oxygen; carbon dioxide

The behavioral part of ventilatory control is modulated to facilitate: (3)

phonation, breath holding, express emotion

What contains the inspiratory (drive external intercostals) and expiratory (drive the internal intercostals and abdominal muscles) neurons?

ventral respiratory group

Clusters of specific neurons called __ control breathing.

respiratory centers

The respiratory centers located in the medulla oblongata set the __ and __ of normal breathing.

rate; rhythm

The respiratory centers in the pons regulate the __ and __ of breathing.

rate; depth

The __ is the inspiratory center, and the __ is the expiratory center.

dorsal respiratory group (DRG); ventral respiratory group (VRG)

What are the 5 functions of the dorsal respiratory group?

• stimulates inhalations

• inspiratory neurons

• set by basic rhythm pacemaking

• excites inspiratory muscles and sets eupnea (12-15 breaths/min)

• cease firing during expiration

What are the 4 functions of the ventral respiratory group?

• inspiratory and expiratory neurons

• remains inactive during quiet breathing

• activity when demand is high

• involved in forced inspiration and expiration

How are the DRG and VRG controlled?

via phrenic and intercostal nerves (to diaphragm and external intercostal muscles)

• if phrenic nerve is severed, diaphragm goes up and decreases inspiratory capacity

DRG and VRG are both responsible for __ but only __ controls expiration.

inspiration; VRG

In cortical control, there is __ ventilation to a considerable degree.

voluntary

In cortical control, __ is easier than hypoventilation.

hyperventilation

In cortical control, breath holding is limited by both __ and __.

PCO2 and PO2

What can help breath holding? (2)

• preliminary hyperventilation: drives PCO2 down

• pre-oxygenation: supplemental O2

The __ and __ are involved in fear and rage responses.

limbic system; hypothalamus

The __ is involved in involuntary control of breathing.

medulla

Chemoreceptors in the __ monitor blood O2, CO2, and pH

carotid and aortic bodies

Chemoreceptors in the __ monitor blood CO2 and pH

medulla oblongata

Decreased blood O2, increased CO2, and decreased pH decrease __ stimulation of the heart, which __ the heart rate.

parasympathetic; increases

Decreased blood O2, increased CO2, and decreased pH increase __ stimulation of the heart, which __ heart rate and stroke volume.

sympathetic; increases

Decreased blood O2, increased CO2, and decreased pH increase __ stimulation of blood vessels, which increase __.

sympathetic; vasoconstriction

In the central nervous system, PCO2 in blood → __ in the CSF due to __.

H+; dissociation (which accounts for 70-80% of total drive)

In the peripheral nervous system, __ is 20-30% of the total drive, __ is triggered only when PaO2 <60 (hypoxemia), and __ is from metabolic processes.

PCO2; PO2, H+

Central chemoreceptors are triggered the most in response to what process?

changes in pH

What is the mechanism to activate central chemoreceptors?

CO2 passes to medulla and then to CSF → in CSF, it combines with water to form H2CO3, which dissociates into HCO3- + H+ → inc in H+ will activate/trigger central chemoreceptors → sends signal to inspiratory center

• blood brain barrier doesn’t let H+ pass through, only CO2

Small changes in the carbon dioxide content of the blood (PaCO2) quickly trigger changes in __.

ventilation rate

The ventilatory response to CO2 tends to be linear with an increase in ventilation of 1-2 L/min for every __ mm Hg rise in PaCO2.

1 mmHg

When PO2 is <60, __ increases.

respiration rate

The levels of __ (3) in blood and CSF provide information that alters the rate of ventilation.

oxygen, CO2, hydrogen

Describe the mechanism that occurs when there is a decrease in arterial PO2.

dec arterial PO2 → inc firing of peripheral chemoreceptors → inc firing of medullary inspiratory neurons → inc firing of neurons to diaphragm and inspiratory intercostals → inc contractions in diaphragm and inspiratory intercostals → inc ventilation

Describe the mechanism that occurs when there is an inc production of non-CO2 acids.

inc production of non-CO2 acids → inc arterial H+ conc → inc firing of peripheral chemoreceptors → inc firing of medullary inspiratory neurons → inc firing of neurons to diaphragm and inspiratory intercostals → inc contractions in diaphragm and inspiratory intercostals → inc ventilation

Describe the mechanism that occurs when there is an inc of arterial PCO2.

inc arterial PCO2 →

• inc arterial H+ conc → inc firing of peripheral chemoreceptors

• inc brain extracellular fluid PCO2 → inc brain extracellular fluid H+ conc → inc firing of central chemoreceptors

→ inc firing of medullary inspiratory neurons → inc firing of neurons to diaphragm and inspiratory intercostals → inc contractions in diaphragm and inspiratory intercostals → inc ventilation

The ventilatory response to CO2 tends to be __ with an increase in ventilation of __ L/min for every 1 mmHg rise in PCO2. This response tends to be characterized by noticeable increases in __.

linear; 1-2; tidal volume

The ventilatory response to low PO2 is __ with minimal increase in ventilation until PO2 falls below __. This response tends to be characterized by noticeable __ in breathing frequency and a sense of __.

non-linear; 60 mmHg; increases; dyspnea

The __ chemoreceptors mechanism is responsible for 70-80% of ventilatory responsiveness to CO2.

central

The fastest way to alter pH is to __

change respiratory rate

How does the body respond to acid load?

buffering (after minutes) → respiratory compensation (after minutes) → renal excretion (after days)

What is the acid-base equilibrium equation?

CO2 + H2O ←→ H2CO3 ←→ H+ + HCO3-

What is the importance of arterial blood gas sampling? (3)

• determines pH and partial pressures of respiratory gases

• determines other serum blood levels

• assesses response to therapeutic interventions (left atrium has most oxygenated blood and should have 100% O2 Sat)

What is the normal range for arterial blood CO2 tension (PaCO2) when doing an arterial blood gas?

35-45 mmHg (higher or lower = not good)

What is the normal range for arterial bicarbonate (HCO3-) when doing an arterial blood gas?

22-26 mmol/L

The pH of arterial blood gas should be between:

7.35-7.45 (7.4 ideal)

The pCO2 of arterial blood gas should be between:

35-45 (40 ideal)

The bicarbonate of arterial blood gas should be between:

22-26 (24 ideal)

Acidosis occurs at a pH of __, and the danger zone is __.

<= 7.35; danger zone <7.1

Alkalosis occurs at a pH of __, and the danger zone is __.

>= 7.45; danger zone >7.5

What are the two types of acidosis and what compensation mechanisms occurs as a result?

1. [HCO3-] < 24 mEq/L → metabolic acidosis → PCO2 < 40 mmHg → respiratory compensation (breathing more: exhaling more CO2)

   1. (low bicarbonate/ high H+, can’t buffer anymore)

2. PCO2 > 40mmHg → respiratory acidosis → [HCO3-] > 24 mEq/L → renal compensation

What are the two types of alkalosis and what occurs as a result to compensate?

1. [HCO3-] > 24 mEq/L → metabolic alkalosis → PCO2 > 40 mmHg → respiratory compensation (breathing less)

2. PCO2 < 40mmHg → respiratory alkalosis → [HCO3-] < 24 mEq/L → renal compensation

If compensation is not appropriate, suspect __.

mixed acid-base disorder

Hyperventilation → [inc/dec] CO2 → [inc/dec] pH

dec; inc

Hypoventilation → [inc/dec] CO2 → [inc/dec] pH

inc; dec

The __ is the measurement of the difference/gap bw negatively charged and positively charged electrolytes.

anion gap

If the anion gap is too high or too low, it may be a sign of: (3)

pulmonary, renal, or other organ dysfunction

What is the formula for anion gap, and what is the normal anion gap?

anion gap = Na+ - (Cl- + HCO3-) - 12 ± 2

normal = 12

The peripheral chemoreceptors are primarily responsive to __ and __ in systemic arterial blood.

hypoxia; unbuffered H+

__ chemoreceptors account for 20-30% of the ventilatory responsiveness to CO2.

peripheral

Metabolic __ augments the ventilatory response to __, whereas metabolic __ attenuates (reduces) it.

acidosis; CO2; alkalosis

The __ is the respiratory center that sets our basis pacemaking for breathing.

DRG

The __ is involved in forced inspiration and expiration.

VRG

__ are most sensitive to pH changes.

central chemoreceptors

The __ are most sensitive to hypoxemia with PaO2 < 60 mmHg.

peripheral chemoreceptors

When the body senses that the pH has strayed from pH 7.5, the immediate changes to normalize come from __, and the delayed changes come from __.

respiratory system; renal system