B.Sc. (H) Biochemistry

IInd Year, IVth Sem

Human Physiology

Respiratory System Lecture-3 & 4

Mechanism of respiration, pulmonary

ventilation and

related volumes, pulmonary circulation

Dr. Prabha Arya

Alveolar ventilation

The total ventilation per minute, termed the minute ventilation (VE), is equal to the tidal volume multiplied by the respiratory

rate: Minute ventilation = Tidal volume × Respiratory rate

(ml/min) (ml/breath) (breaths/min)

VE = Vt · f

For example, at rest, a normal person moves approximately

500 ml of air in and out of the lungs with each breath and

takes 12 breaths each minute. The minute ventilation is therefore

500 ml/breath × 12 breaths/minute = 6000 ml of air per minute.

Dead Space

Anatomic dead space

Tidal volume (Vt) = 500 ml

Anatomic dead space (VD) = 150 ml

Fresh air entering alveoli in one inspiration (VA) =

500 ml – 150 ml = 350 ml

The total volume of fresh air entering the alveoli per minute is called the

alveolar ventilation (V˙A):

Alveolar ventilation = (Tidal volume –Dead Space)X Respiratory rate

VA = (Vt – VD) X f

These airways do not permit

gas exchange with the blood, the

space within them is termed

the anatomic dead space (VD).

Vander’s human physiology (2019) 15th ed

Alveolar dead space and Physiologic

dead space Some fresh inspired air is not used for gas exchange

with the blood even though it reaches the alveoli

because some alveoli may, for various reasons, have

little or no blood supply. This volume of air is known as

alveolar dead space.

It is quite small in normal persons but may be very large

in persons with several kinds of lung disease.

The sum of the anatomic and alveolar dead spaces is

known as the physiologic dead space. This is also

known as wasted ventilation because it is air that is

inspired but does not participate in gas exchange with

blood fl owing through the lungs.

Exchange of gases in alveoli

Respiratory quotient (RQ)

The amount of oxygen the cells consume and the amount of carbon dioxide they produce are not necessarily identical. The balance depends primarily upon which nutrients are used for energy. The ratio of CO2 produced to O2 consumed is known as the respiratory quotient (RQ).

On a mixed diet, the RQ is approximately 0.8; that is, 8 molecules of CO2 are produced for every 10 molecules of O2 consumed. The RQ is 1 for carbohydrate, 0.7 for fat, and 0.8 for protein.

Summary of typical oxygen and carbon dioxide exchanges between

atmosphere, lungs, blood, and tissues during 1 min in a resting individual.

The volume of oxygen in 1 L of arterial blood is 200 ml O2/L of blood—

that is, 1000 ml O2/5 L of blood.

Vander’s human physiology (2019) 15th ed

4000 mL of air 840 mL O2 21% is oxygen

250 mL Crosses

alveoli into the

pulmonary capillaries

Rest is

subsequently

exhaled

Blood

contains

large amount

of oxygen

already

The blood then flows from the lungs to

the left side of the heart and is pumped by

the left ventricle through the aorta,

arteries, and arterioles into the tissue

capillaries, where 250 ml of oxygen leaves

the blood per minute for cells

to take up and utilize.

Quantities of oxygen added to the blood in the lungs and removed in the tissues are

the same.

The story reads in reverse for carbon dioxide.

Partial Pressure of gases,

Dalton’s law In a mixture of gases, the pressure each gas exerts is independent of the pressure the others exert.

This is because gas molecules are normally so far apart that they do not affect each other. Each gas in a mixture behaves as though no other gases are present.

These individual pressures, termed partial pressures. (for example: PO2

)

Diffusion of gases in liquid, Henry’s

law

Alveolar gas

pressure

Partial

pressure of

gas

Alveolar Gas

Pressure (mmHg)

Pressure in air

(mmHg)

PO2 105 160

PCO2 40 .3

The factors that determine the

precise value of alveolar

PO2 are

(1) the PO2 of atmospheric

air,

(2) the rate of alveolar

ventilation, and

(3) the rate of total-body

oxygen consumption.

Vander’s human physiology (2019) 15th ed

Redrawn from: Vander’s human physiology (2019) 15th ed

Hypoventilation and hyperventilation

Hypoventilation

It exists when there is an increase in the ratio of carbon dioxide production to alveolar ventilation. In other words, a person is hypoventilating if the alveolar ventilation cannot keep pace with the carbon dioxide production. The result is that alveolar PCO2 rises above the normal value.

Hyperventilation exists when there is a decrease in the ratio of carbon dioxide production to alveolar ventilation—that is, when alveolar ventilation is actually too great for the amount of carbon dioxide being produced. The result is that alveolar PCO2 decreases below the normal value.

Ventilation-perfusion inequality

The lungs are composed of approximately 300

million alveoli, each capable of receiving

carbon dioxide from, and supplying oxygen to,

the pulmonary capillary blood.

To be most efficient, the correct proportion of

alveolar air flow (ventilation) and capillary

blood flow (perfusion) should be available to

each alveolus. Any mismatching is termed

ventilation-perfusion inequality.

Ventilation-perfusion inequality

One effect of upright posture is to increase the filling of

blood vessels at the bottom of the lung due to gravity,

which contributes to a difference in blood flow

distribution in the lung.

Local control of ventilation-

perfusion matching.

Vander’s human physiology (2019) 15th ed

References

1. Vander’s human physiology (2019) 15th ed.,

Widmaier, E.P., Raff, H. And strang, K.T.,

Mcgraw hill international publications (new

york), ISBN: 978-1-259-90388-5.

2. Few Pictures from internet.