Adobe Systems
Department of Physiology, Faculty of Medicine, Masaryk University
1
Determination of the sensitivity of the respiratory center to hypercapnia
Physiology I – practice
Autumn, weeks 7–9

Adobe Systems
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Regulation of breathing
Respiratory centre
Central chemoreceptors, automatic breathing
Cerebral cortex
Voluntary breathing, conditioned reflexes
Subcortical structures
Emotions, changes in central temperature, reactions of autonomic nervous system
Direct effect of environment
Hormones
 (epinephrine, steroid hormones)
Baroreceptors
Nespecific mechanoreceptors
Receptors in skin, muscles, tendons and joints
Receptors in breathing muscles
Receptors in lung and airways
Peripheral chemoreceptors

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Breathing centres in the medulla
̶Breathing is an automatic process that takes place unconsciously.
̶Automaticity of breathing comes from regular (rhythmic) activity of groups of neurons anatomically
localized in the medulla and its vicinity. They can be divided into three main groups:
̶Dorsal respiratory group – only inspiratory neurons, their axons go to the motoneurons of the
inspiratory muscles (diaphragm, external intercostal muscles; their activation = inspiration,
during their relaxation = expiration), they participate in inspiration at rest and also during
exercise
̶Ventral respiratory group – located on the ventrolateral part of the medulla, the upper part:
neurons whose axons activate the motoneurons of the primary and accessory inspiratory muscles; the
lower part: expiratory neurons with innervation of expiratory muscles. Neurons of this group are
active only during forced inhalation and exhalation
̶Pontine respiratory group (pneumotaxic centre) – participates in the control of the frequency and
depth of breathing; it affects the activity of respiratory neurons in the medulla

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Central chemoreceptors
̶on the front side of the medulla
̶CO2 can cross the hematoencephalic barrier to the cerebrospinal and intercellular fluid
̶CO2 + H2O → CHO3- + H+
̶Higher H+ concentration of H+ in cerebrospinal fluid stimulates central chemoreceptors → increased
ventilation
̶central chemoreceptor are stimulated also by other types of acidosis (lactate acidosis,
ketoacidosis)
̶a sudden change of pCO2 does not have an immediate effect, the changes in ventilation through
central chemoreceptors occur after 20-30 s

Adobe Systems
Department of Physiology, Faculty of Medicine, Masaryk University
5
Peripheral chemoreceptors
̶Located in the aortic and carotid bodies
̶glomus caroticum, glomus aorticum
̶They convey their sensory information to the medulla via the vagus nerve and glossopharyngeal
nerve.
̶Sensitive to a decrease in arterial pH and pO2 and an increase in arterial pCO2
̶The most important reaction is to decrease in arterial pO2, particularly to decrease of O2 under
10-13 kPa in the arterial blood (caused mainly by a general decrease of pO2 or impaired flow of the
blood)
̶Mechanism: decreased pO2 leads to the decrease of ATP synthesis in mitochondria resulting in
membrane depolarization
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Hypoxia, hypoxemia (norm. 11-16 kPa)
̶Hypoxia is a general name for a lack of oxygen in the body or certain tissues.
̶Hypoxemia is a lack of oxygen in arterial blood.
̶A complete lack of oxygen is known as anoxia.
̶The most common types of hypoxia:
̶Hypoxic – physiological: stay at higher altitudes, pathological: hypoventilation during lung or
neuromuscular diseases, thorax injuries, opioid overdose, …
̶Transport (anemic) – reduced transport capacity of blood for oxygen (anemia, blood loss, CO
poisoning)
̶Ischemic (stagnation) – restricted blood flow to tissues (heart failure, shock, obstruction of an
artery)
̶Histotoxic – cells are unable to utilize oxygen (cyanide poisoning – disruption of the respiratory
chain)

Adobe Systems
Department of Physiology, Faculty of Medicine, Masaryk University
7
Hypercapnia (norm. 5.3-6.65 kPa)
̶An increase in the concentration of carbon dioxide in the blood or tissues caused by the retention
of CO2 in the body
̶Possible causes: total alveolar hypoventilation – decreased respiration or extension of dead space
̶
̶
̶Mild hypercapnia (5-7 kPa) causes stimulation of the respiratory center (therapeutic use:
pneumoxid = mixture of oxygen + 2-5% CO2)
̶Hypercapnia around 10 kPa – CO2 narcosis – respiratory depression (preceded by headache,
confusion, disorientation, a feeling of breathlessness)
̶Hypercapnia over 12 kPa – significant respiratory depression – coma and death.

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The partial pressure of gases (mmHg)
̶In different parts of the respiratory and circulatory system

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Saturation of hemoglobin with oxygen
̶Percentage of hemoglobin saturated with oxygen
̶In arterial blood around 95-98%
̶Hypoxia <85%
100%
50%
¯ temperature
 pH
¯pCO2
¯ DPG
 temperature
¯ pH
 pCO2
 DPG
pO2 [mmHg]
25
50
75
100

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Pulse oximetry
̶Is a photometric method of non-invasive measurement of hemoglobin saturation with oxygen in the
arterial blood.
̶The method is based on the evaluation of the absorption of transmitted light of two different
wavelengths after passing through tissues.
̶How to get saturation only in the arterial blood: subtract the value between individual heartbeats
from the value at the peak of the pulse wave. The component calculated in this way is then equal to
the absorption of the variable component, which is the arterial blood (the representation of other
tissue is stable).
C:\Users\Johanka\Desktop\výuka\prezentace k praktikám ppt\Nové verze praktik podzim 2018\hypoxie,
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660
920
1000
800
600
Wavelenght (nm)
Red
Infrared
oxyhemoglobin
deoxyhemoglobin
Tissues
Venous blood
Arterial blood – constant
Pulse absorption in the arterial blood
Time

Adobe Systems
Department of Physiology, Faculty of Medicine, Masaryk University
11
Sensitivity of the respiratory centre to hypercapnia
̶Aim: to demonstrate the changes in ventilation during induced hypercapnia and to compare the
sensitivity of the respiratory centre to hypercapnia in several subjects
̶Method: Hypercapnia is induced by re-breathing of air expired into Krogh respirometer with a
closed circuit (Krogh respirometer with oxygen, without the soda lime – the CO2 concentration
naturally increases)
̶Interpretation: The slope of the curve shows the sensitivity of the respiratory centre to
hypercapnia

inspCO2 (%)
ΔCO2
Δ V
Clinical note: the sensitivity of the respiratory centre to change of partial pressure of CO2 is
reduced in patients with chronic end-stage lung disease, in patients with heart failure as well as
in subjects training breath holding (e.g. divers without oxygen tanks)