Why do we breathe?

•Take in O2 (which we need to make ATP)

•Get rid of CO2 (which is a waste product of ATP synthesis)

Thought questions

•What happens to our cells and our bodies if we don’t get enough O2, or if CO2 is made but never removed from the body?

•Is the amount we breathe always the same?

•What kinds of things change it?

Partial pressure of a gas (P)

P = F x Patm

F = fraction of the gas in the atmosphere that is that gasPatm: atmospheric pressure

Partial pressure of a gas (P)

P = F x Patm

F = fraction of the gas in the atmosphere that is that gasPatm: atmospheric pressure

Example: partial pressure of O2 in this room•21% of the atmosphere is O2

•Atmospheric pressure in Boston (see level) is 760 mmHg

PO2 = 0.21 x 760 = 160 mmHg

Hypercarbia: PaCO2 greater than set point of 40 mmHg

What determines the PaCO2?

What determines the PaCO2?

Answer: The PACO2

What determines the PACO2?

What determines the PaCO2?

Answer: The PACO2

What determines the PACO2?

Answer: Alveolar ventilation

Time (sec)

0 1 2 3 4 5 6 7

Minute Ventilation = tidal volume (VT) x frequency

Breathing frequency: 1 breath/ 4 sec = 15 breaths/minVT = 0.6 liters

Ch

ange

in v

olu

me

( lit

ers)

0

.2

.4

.6

.8

VT

Is all the air that we take in useful?

A useful model of the lung – balloon on a tube

Tube: airways

Balloon: alveoli

VD = volume

of cylinder

End Expiration Begin Inspiration Mid Inspiration

End Inspiration

VT = total volume of white area

Mid Expiration End Expiration

                                                                                          

                   

PANTING: high frequency but low volume breathing

(assume a dead space of 0.1 L)

Breathing at rest:VT = 0.5 Lf = 10 breaths/minVE=5 L/minVA = 4 L/min

Panting:VT = 0.14 Lf = 100 breaths/minVE = 14 L/minVA = 4 L/min

Examples of things that can cause hypoventilation

•drugs (i.e. morphine) that suppress respiratory drive•diseases of the muscles (like myasthenia gravis) •diseases of the nerves•diseases that affect central respiratory drive:

- CCHS (Ondine’s curse).

CCHS

What causes changes in dead space?

1) Breathing through a piece of equipment• Gas mask• Snorkel• Ventilator circuit

2) Loss of blood flow to a ventilated region of lung• pulmonary embolism• Certain lung diseases (emphysema)

What causes changes in dead space?

1) Breathing through a piece of equipment• Gas mask• Snorkel• Ventilator circuit

2) Loss of blood flow to a ventilated region of lung• pulmonary embolism• Certain lung diseases (emphysema)

Jeffrey et al, Am. J. Respir. Crit. Care Med. 2001 164: 28S-38S

EMPHYSEMA

Hypoxemia: PaO2 less than the set point of about 80 mmHg

CAUSES OF HYPOXEMIA

•Hypoventilation•Diffusion Impairment•Shunt•V/Q abnormalities

What determines how fast O2 (or CO2) diffusesacross the alveolar wall?

Diffusion Impairment

.VO2 = K x A x (PAO2 – PaO2)/T

.VO2 = flux of O2 across the lungK = a constantA = surface area of the lungT = thickness of the lung (distance between air and red blood cell)PAO2 and PaO2 = partial pressure for O2 in alveolus and in capillary blood

Jeffrey et al, Am. J. Respir. Crit. Care Med. 2001 164: 28S-38S

EMPHYSEMA

Shunt: blood passes from the right heart to the leftheart without becoming oxygenated

1. Blood goes from the right to the left heart without goingthrough the lungs

• Bronchial circulation• Foramen ovale, ductus arteriosus

2. Blood goes through the lungs but never comes into closecontact with alveolar gas

• Foreign object lodged in an airway• Pneumonia (accumulation of fluid and pus in the alveoli)

How much does a shunt affect PaO2?

Pus and fluid

Air

“Good” lung“Bad” lung

60% of C.O.40% of C.O.

How much does a shunt affect PaO2?

Pus and fluid

Air

“Good” lung“Bad” lung

60% of C.O.40% of C.O.

PO2 = 40 mmHg PO2 = 100 mmHg

How much does a shunt affect PaO2?

Pus and fluid

Air

“Good” lung“Bad” lung

60% of C.O.40% of C.O.

PO2 = 40 mmHgPO2 = 100 mmHg

PO2

In Out40 mmHg 40 mmHg

In Out40 mmHg 100 mmHg

O2 carrying capacity of hemoglobin

1 g Hemoglobin (Hg) can carry 1.34 ml O2 (when fully saturated)

Normal: 15 g Hg/ 100 ml blood x 1.34 = 20 ml O2/100 ml blood = 200 ml O2/L blood

100% x 200 ml O2/L blood x 3L

“Good” lung “Bad” lung

75% x 200 ml O2/L blood x 2L

Assume the cardiac output is 5 L/min: 60% to good lung and 40% to bad lung3 L/min 2 L/min

600 ml O2300 ml O2

Blood mixes inleft atrium

900 ml O2 in 5L blood 180 ml O2/L

(Fully saturated 200 mlO2/L) 90% saturated PaO2 of 60 mmHg

Pressure (cm H2O)

Volume(L)