GASEOUS EXCHANGE.

REGULATION OF RESPIRATION

Lecturer MD, Ph.D. Angelika V. Ivasenko

The minute volume of respiration

• its value during quiet breathing is 6-9 1/min.

• The V is equal to the product of tidal volume and frequency of respiration per minute.

Alveolar ventilation • is smaller than pulmonary one by the magnitude

of dead space ventilation. For example, at V equal to 8000 ml/min and frequency of respiration 16 per minute, dead space ventilation will be 150 ml x 16 = 2400 ml/min. Alveolar ventilation will be 8000 - 2400 = 5600 ml.

• With V of 8000 ml/min and respiratory frequency of 32 per minute, dead space ventilation will be 150x32 = 4800 ml/min, while alveolar ventilation will be 8000 - 4800 = 3200 ml

Alveolar air is the internal gas environment of the body

• The magnitude of pulmonary ventilation is regulated so as to ensure constant gas content in the alveolar air.

• Thus, at high CO2 concentration in the alveolar air V is increased and at its low concentration is decreased.

Composition of Dry Air (in per cent)

air O2 CO2 N2 and inert gases

INSPIRED 20,93 0,04 79,03

EXPIRED 16,0 4,5 79,5

ALVEOLAR 14,0 5,5 80,5

Diffusion of Respiratory Gases• The alveoli diameter is 150-300 nm. • about 400 mln of alveoli in one human lung• The total area of alveolar-capillary contacts is

nearly 60-90 m2. • lung membrane is made up of endothelial cells,

the membrane proper, squamous alveolar epithelium, and surfactant layer.

• The thickness of the lung membrane is 0.4-1.5 nm.

Partial pressure• of each gas in a gas mixture is proportional to

the percentage gas content and the total pressure of a mixture.

• In determining the alveolar partial pressure it should be kept in mind that here the water vapour pressure is about 47 mm Hg at 37°C. Therefore, it is 760 - 47 = 713 mm Hg. At the alveolar air O2 content of 14 per cent, O2 partial pressure (Po2 ) will be

Hg.mm1008.99100

14x)47760(P2O

gaseous tension

• Blood contains gases in dissolved (free) and in chemically bound state. Only molecules of the dissolved gas participate in diffusion.

• The higher the pressure of the given gas and the lower the temperature, the greater is the amount of gas which is dissolved in the liquid.

• Gas is dissolved in the liquid until a dynamic equilibrium has been reached between the amount of gas molecules dissolved and released into the gas medium.

• The force with which molecules of the dissolved gas are driven into the gas medium is known as the gaseous tension (or partial pressure) in a liquid

DRIVING FORCE OF DIFFUSION

• partial pressure difference of gases in the alveolar air and their blood tension.

diffusing capacity of the lungs

• Permeability of the lung membrane to gas • for O2 is about 25 ml/min x mm Hg.• For CO2 it is 24 times as much due to

high solubility of this gas in the lung membrane

OXYGEN TRANSPORT BY THE BLOOD

• Oxygen is transported in the form of oxyhaemoglobin.

• Only 0,3 ml of oxygen is dissolved per 100 ml of blood at 37°C.

Oxyhaemoglobin dissociation curve

• Conversion of haemoglobin to oxyhaemoglobin is conditioned by the tension of dissolved O2.

• This dependence is expressed graphically by the oxyhaemoglobin dissociation curve

O2 tension, mm Hg

20

20

40

40

60

60

80

80

100

100

РО2

%НвО2

рН t0 2.3ДФГ

РО2 РСО2

рН t0 2.3ДФГ

РО2 РСО2

The sloping part of the curve

• corresponds to high O2 tension (more than 60 mm Hg)

• provides evidence that the level of oxyhaemoglobin under these conditions does not depend much on the O2 tension and partial pressure in inspired and alveolar air.

A steep part of the curve

• corresponds to O2 tension which is inherent in body tissues (35 mm Hg and lower).

• Dissociation or oxyhaemoglobin is higher and sometimes almost complete in tissues that consume much oxy gen, e.g. working muscles, liver, kidneys.

Haemoglobin affinity for oxygen

• 2,3-disphosphoglycerate (2,3-DPG) content in erytrocytes

• the CO2 and H+ concentration (Bohr effect

• Temperature

Shift of the curve to the right

• oxygen enters tissues with more ease • Increase in 2,3-disphosphoglycerate (2,3-

DPG) content ( when the blood O2 tension is reduced).

• increase in the CO2 and H+ concentration• Temperature rise

Oxygen capacity of the blood• The maximal amount of O2 that can be bound by blood

in full saturation of haemoglobin with oxygen • The oxygen capacity of the blood depends on its

haemoglobin content.• The volume of one mole of oxygen is 22.41. One Hb

gram-molecule is capable of binding 22 400 x 4 = 89 600 ml of O2

• The molecular mass of Hb is 66 800, hence, 1 g of Hb is capable of binding 89600 : 66800 = 1.34 ml of O2.

• With 140 g/1 of blood Hb, the O2 blood capacity is 1.34 x 140 = 187.6 ml or about 19 volume per cent

Forms of chemically bound СО2

• Carbonic acid Н2СО3 – 7%

• Bicarbonate - НСО3- - 70%

• Carbaminohaemoglobin ННвСО2 – 23%

РСО2=70 мм рт ст

СО2

СО2

СО2

НвНвСО2

Н2СО3

карбоангидразаН2О

НСО3- Н+

НСО3-

NaCl

Na

Cl Н2О

NaНСО3

НвННв

КНвК+

КНСО3

++ +

++

РСО2=40 мм рт ст

СО2

СО2

СО2

НвНвСО2

Н2СО3

карбоангидразаН2О

НСО3- Н+

НСО3-

NaClNa

Cl Н2О

NaНСО3Нв

ННв

КНвК+

КНСО3

+

++

+Cl

РО2= 100ммРО2= 100ммРО2= 100ммРО2= 100мм

alveoli tissue

РО2= 100mm

РСО2 =40 mm

РО2= 100mm РО2= 100mm

РО2= 40mm

РО2= 40mm

СО2 =46mm

СО2 =46 mm

РСО2 =40 mm

РСО2 =40 mm

РО2= 40mm

СО2 =46 mm

AP of inspiratory & exspiratory neurons

Respiratory nuclei of the medulla

1. Dorsal respiratory nucleus2. Ventral respiratory nucleus

Respiratory nuclei of the medulla

• The dorsal respiratory nucleus is located near the solitary tract and consists mainly of inspiratory neurons whose axons pass to the phrenic nuclei of the cervical segment of the spinal cord.

• The ventral respiratory nucleus lies just lateral of the nucleus amblguus and contains inspiratory and expiratory neurons sending their axons to phrenic, intercostal and laryngeal motor neurons.

• The reticular formation of the medulla and pons also has a small number of respiratory neurons.

Dependence of Activity of Respiratory Centre on Blood Gas Content

• Fredericq (1890) experiments with cross-circulation.

• The carotid arteries of anaesthetized dogs were cut and then connected cross-wise and the jugular veins were connected separately

• If occlusion of the trachea of the first dog, for example, caused asphyxia, then hyperpnoea developed in the second dog.

• The first dog experienced apnoea some time later, despite increase in the arterial CO2 and decrease in O2 tension.

• This is explained by the fact that the carotid artery of the first dog received blood of the second dog in which hyperventilation caused decrease in the arterial CO2 tension.

TYPES OF RECEPTORS involved in regulation of respiration

• Central chemoreceptors

Н+Extracellular fluid

Spino-cerebral fluid

СО2

TYPES OF RECEPTORS involved in regulation of respiration

• PERIPHERAL CHEMORECEPTORSCAROTID SYNUS AORTHIC

ARC

DECREASE 0F О2 PARTIAL PRESSURE

RC СC

• Decrease of РН=7,32 by 0,01 in the cerebro-spinal fluid causes the 4 l increase in the minute volume of respiration

• Decrease of РО2 below 100mm Hg increases impulsation from peripheral receptors

MECHANORECEPTORS of lungs & respiratory pathways (1000 receptors in each lung)

• HIGHTHRESHOLD – EXCITED IN DEEP INHALATION

• LOWTHRESHOLD –– • Increased impulsation

on inhalation• Decreased

impulsation on exhalation

INHALATION

INSPIRATORY MUSCLES

MOTONEURONS OF INSPIRATORY MUSCLES

Iα

LSR

Iβ

Inh

PTC

N.vagus

Centr. chem.

Periph. chem

exp

EXSPIRATORY MUSCLES

MOTONEURONS OF EXSPIRATORY MUSCLES

ACTIVE EXHALATION

CortexLymbic system

hypothalamus

PTC

IαCentralChem.

exp

+ -

Motoneurons of inspiratory muscles

Iα

CENTRAL CHEMORECEPTORS

RFPERIPHERAL CHEMORECEPTORS

PROPRIORECEPTORS OF WORKING MUSCLES

SKIN RECEPTORS

Vagus role in respiration

Respiratory changes after vagotomy and destruction of the

pneumotaxic centre.

a—normal breathing, eupnoea; b—appearance of apneusis after vagotomy anddestruction of the pneumotaxic centre; c—time, 10 sec.