Biophysics of Breathing

Jan Jakuš

Breathing is a vital function of the body, a periodic and rhythmic process of inspiration and expiration that co-vers the metabolic demands of body for O2 and CO2.

- must assure the intake of O2 250 ml / min, and the expenditure of CO2 200 ml / min. - can be interrupted or increased volun-tarily (from the cortex) - is governed by “respiratory centre’’, localized within the brainstem

Anatomy of BreathingUpper Airways - nose, nasopharynx, larynx

Lower Airways - trachea, bronchial „tree“,

Lungs (right + left) - alveoli

Respiratory muscles

Breathing (Respiration) :

- External (the air exchange at the level of lungs)

- Internal (the O2 and CO2

exchange at the tissue level)

External and Internal Breathing

Atmosphere Lung Blood - Heart Extracel. liquid Cells Oxygen CO2

Respiratory and Cardiovascular Relationships

External Breathing :

1. VENTILATION - cyclic air exchange during breathing caused by the respiratory pump muscles - diaphragm, external and internal intercostals, abdominal, and auxiliary muscles.

2. DISTRIBUTION - mixing of inhaled air with an air that remains within the airways after expiration (150 ml-death volume).

3. DIFFUSION - transfer of O2 and CO2 through the alveol-ar-capilĺary membrane along the partial pressure gradients (Ficks Law)

4. PERFUSION- gas transport in blood between lungs and tissues by heart and vessels

Ventilation - the role of respiratory muscles

Diaphragm – moves downward at inspiration and

upward during expiration (60% of volume changes in thorax)

Intercostal muscles - external (inspiratory), and inte-rnal (expiratory) muscles

Auxiliary musles (of neck, thorax, abdomen)- help to main respiratory muscles (Paralelogram- see

Practicals)

Ventilation - types (in adults)

Minute ventilation (MV) =VT .fb =0.5.12= 6 (l/min)

(VT – tidal volume (0.5 l), fb – breathing rate)

Alveolar ventilation (AV)= 0.35.12 = 4.2 (l/min)

Comparing to Minute ventilation, the value of Alveolar ventilation is reduced, because the death volume (0.15 l ) must be substracted from

VT

Origine of Breathing.

Action potentials from respiratory centre drive respiratory muscles. These, in turn are contracted and create pressure changes. Pressure changes enable pressure gradient and this leads to a flow of air. Then lungs are filled (or emptied) with air volumes.(Hering´s model of breathing - see practicals)

Remember these changes: A / AT REST:

QUIET INSPIRATION (active process): contraction of diaphragm + external intercostals fall of pleural pressure (PPl = - 0.8 kPa) fall of intrapulmonary (Pp = - 0.1 kPa ) Pressure gradient inspiratory airflow (VI = + 0.4 l/s) inspiratory tidal volume (VT = 0.5 l)

OUIET EXPIRATION (mostly passive process) : recoil forces (i.e elasticity of the thoracic wall and lung tissue + passive movement of the diaphragm

upward slightly negative Ppl = - 0.1kP, and to

slightly positive intrapulmonay pressure PP = + 0.5

kPa pressure gradient expiratory airflow

(VE= - 0.4 l/s) expiratory volume ( VT = 0.5 l ) empties the lungs B/ AT WORK: FORCEFUL INSPIRATION consists of the same processes as shown above + contraction of external intercostals + auxiliary muscles result in higher pressure gradients, and to higher values of Ppl, PP, VE and VT

FORCEFUL EXPIRATION (e.g. in cough, sneeze, strong voluntary expiration)

It starts sudenly with contraction of abdominal muscles (expiratory), creating high abdominal pressure (PAB), very high PPl and PP pressures, also very high pressure gradient, and thus extremely strong expiratory airflow (velocity like tornado) and very high expiratory volume

Mechanics of breathing - means concomitant changes of respiratory muscles (diaphragm, intercostal and auxiliary muscles) creating particular Ppl and PP, pres-sures, inspiratory and expiratory airflows (V), and tidal volumes (VT), resulting in some Work of breathing (during inspiration and expiration).

Work of breathing is affected by:Lung compliance, Airway resistance

1/ Lung compliance – distensibility (C) - is the

ratio between Volume of air (VT) / Pressure (P). N = 2 (l . kPa-1) Lung fibrosis C - the lung tissue is thicker and thus its compliance (distensibility) is lowerLung emphysema C- lung tissue is thinner, and thus

compliance (distensibility) of the lungs is higher. LOOP OF LUNG COMPLIANCE

2/ Airway resistance Raw - is the relationship between PRESSURE (P) / AIR FLOW (V) (Unit is kPa / l / s)In a disease like bronchial asthma the airway resistance is high, because the contraction of smooth muscles within the lower airways decreases the diameter of airways. Thus, the airflow is low, but work of muscles and breathing is high. LOOP OF AIRWAY RESISTANCE

The Lung Volumes(Remember 4 main breathing volumes and 4 capacities).Tidal volume VT = 0.5 lInspiratory reserve volume IRV = 2.5 lExpiratory reserve volume ERV = 1.5 lResidual volume RV = 1.2 l (consists of collapse volume = 0.4 l + minimal volume = 0.8 l)

TC

FR CERV

IRV

V

RV

IC

VCT

m axim á lnyvdych

m axim á lnyvýdych

The Lung Capacities

Vital capacity VC= VT + IRV + ERV Functional residual capacity FRC = ERV + RVInspiratory capacity IC= VT + IRVTotal capacity TC= VT + IRV + ERV + RV (See practicals)

TC

FR CERV

IRV

V

RV

IC

VCT

m axim á lnyvdych

m axim á lnyvýdych

Morphology of Alveoli and Capillaries(Coupling of Respiratory and Cardiovascular Systems)

Partial Pressure of Gases - a drive for diffusion

ATHMOSPHERIC AIR is a mixture of 21% of O2 + 0.04 % CO2 + 78% of N2 ,and other residual gases (e.g. Hellium, Neon, Argon)

Partial pressures of particular gases depend on their % concentration within the air. (DALTON´S LAW). The higher is % of a gas within a gas mix-ture, the higher is its partial pressure (and vice versa).

At normal value of barometric pressure = 101.3 kPa (760 torr,1atm) the partial pressure of P02 is approx. 21 kPa and PCO2 is 0.04 kPa

Remember: Using DALTON’S LAW one can count partial pre-ssure of a gas according to formula: PO2 = V% O2 x ( PB - PH2O ) / 100

PO2 = 20.93 x (101.3 – 0.8) / 100 = 21.03 (kPa)

PCO2 = 0.04 (kPa)

PN2 = 79 (kPa)

This formula can be used for calculations of parti-cular pressures of gases in the air, in the airways or within the arterial and venous blood. See next table.

The values of GAS Volumes (in %) and their Partial pressures (kPa), in the Atmospheric air, Alveolar air, in the arterial and venous blood

O2

(%)CO2

(%)PH2O (kPa)

PN2

(kPa) PaO2

(kPa) PCO2

(kPa) Atmospher.air (dry)

20.93 0.03 0.8 79.04 21.06 0.04

Expiratory air

15.1 4.3 6.3 75.3 15.3 5.73

Alveolarair

13.2 5.1 6.2 76.4 13.4 5.33

Arterialblood

19.8 50 6.3 76.4 8 12.7

5.2 0,8

Venousblood

14 -15 55 6.3 76.4 5.2 6.13

DIFFUSION – is a transfer of gases (O2...) through the Alveolar - capillary membrane along the partial pressure gradients of O2

and CO2 or N2 being governed by : FICK’S LAW Diffusion rate: V = (P1 – P2) . A . k sP1 P2 partial pressuresA = diffusion surface (70 m2) s = thickness of Alv. - capillary membrane (0.8 um)k = diffusive constant -depends on a membrane and gas propertiesDiffusion rate : VO2 = 15 – 20 ml / min. Diffusion rate for CO2 is 20 - times higher than for O2

Diffusion through the alveolo - capillary membrane

Dynamics of Diffusion

PHYSICAL SOLUBILITY of O2 and CO 2 within the blood plasma is under HENRY’S LAW:

VO2 = x PO2 x 1000 = 3 ml O2 /1l arterial blood PB

VCO2 = x PCO2 x 1000 = 27 ml CO2 / 1l arter. blood 101 , - coefficients for O2, and CO2 (respectively) PB - atmospheric (barometric) pressure

Solubility of gases in liquids depends on their partial pressures. Gases in liquids are in two forms: physically disolved in blood plasma, and chemically bounded on Hemoglobine of the red blood cells.1 l of arterial blood takes 200 ml of O2. From this only 3 ml of O2 is physically dissolved in plasma, and 197 ml O2 binds chemically on Hemo-globine.

Wishing You Pleasant Day