MBBS 1st Year Medical Physiology-1(102)-Respiratory Physiology

The goals of respiration are to provide oxygen to
the tissues and to remove carbon dioxide. To
achieve these goals, respiration can be divided into
four major functions: (1) pulmonary ventilation,
which means the inflow and outflow of air between
the atmosphere and the lung alveoli; (2) diffusion
of oxygen and carbon dioxide between the alveoli
and the blood; (3) transport of oxygen and carbon
dioxide in the blood and body fluids to and from the body’s tissue cells; and
(4) regulation of ventilation and other facets of respiration. This chapter is a
discussion of pulmonary ventilation, and the subsequent five chapters
cover other respiratory functions plus the physiology of special respiratory
abnormalities.

Mechanics of Pulmonary Ventilation:
Muscles That Cause Lung Expansion and Contraction-
The lungs can be expanded and contracted in two ways: (1) by downward and
upward movement of the diaphragm to lengthen or shorten the chest cavity,
and (2) by elevation and depression of the ribs to increase and decrease the
anteroposterior diameter of the chest cavity.
Normal quiet breathing is accomplished almost entirely by the first method,
that is, by movement of the diaphragm. During inspiration, contraction of the
diaphragm pulls the lower surfaces of the lungs downward. Then, during expiration,
the diaphragm simply relaxes, and the elastic recoil of the lungs, chest
wall, and abdominal structures compresses the lungs and expels the air. During
heavy breathing, however, the elastic forces are not powerful enough to cause
the necessary rapid expiration, so that extra force is achieved mainly by contraction
of the abdominal muscles, which pushes the abdominal contents
upward against the bottom of the diaphragm, thereby compressing the lungs.
The second method for expanding the lungs is to raise the rib cage. This
expands the lungs because, in the natural resting position, the ribs slant downward thus allowing the sternum to fall
backward toward the vertebral column. But when the rib cage is elevated, the
ribs project almost directly forward, so that the sternum also moves forward,
away from the spine, making the anteroposterior thickness of the chest about
20 per cent greater during maximum inspiration than during expiration.Therefore,
all the muscles that elevate the chest cage are classified as muscles of inspiration,
and those muscles that depress the chest cage are classified as muscles
of expiration. The most important muscles that raise the rib cage are the external
intercostals, but others that help are the (1) sternocleidomastoid muscles,
which lift upward on the sternum; (2) anterior serrati, which lift many of the
ribs; and (3) scaleni, which lift the first two ribs.
The muscles that pull the rib cage downward during expiration are mainly
the (1) abdominal recti, which have the powerful effect of pulling downward on
the lower ribs at the same time that they and other abdominal muscles also compress
the abdominal contents upward against the diaphragm, and (2) internal
intercostals.

Pleural Pressure and Its Changes
During Respiration:
Pleural pressure is the pressure of the fluid in the thin
space between the lung pleura and the chest wall
pleura.

Alveolar Pressure:
Alveolar pressure is the pressure of the air inside the
lung alveoli.

Transpulmonary Pressure: The
difference between the alveolar pressure and the
pleural pressure is called the transpulmonary
pressure.

Compliance of the Lungs:
The extent to which the lungs will expand for each unit
increase in transpulmonary pressure (if enough time is
allowed to reach equilibrium) is called the lung compliance.

Pulmonary Volumes
and Capacities:

1. The tidal volume is the volume of air inspired or
expired with each normal breath; it amounts to
about 500 milliliters in the adult male.
2. The inspiratory reserve volume is the extra volume
of air that can be inspired over and above the
normal tidal volume when the person inspires
with full force; it is usually equal to about
3000 milliliters.
3. The expiratory reserve volume is the maximum
extra volume of air that can be expired by
forceful expiration after the end of a normal
tidal expiration; this normally amounts to about
1100 milliliters.
4. The residual volume is the volume of air
remaining in the lungs after the most forceful
expiration; this volume averages about
1200 milliliters.

Pulmonary Capacities:
In describing events in the pulmonary cycle, it is
sometimes desirable to consider two or more of the
volumes together. Such combinations are called pulmonary
capacities.
1. The inspiratory capacity equals the tidal volume
plus the inspiratory reserve volume. This is the
amount of air (about 3500 milliliters) a person can
breathe in, beginning at the normal expiratory
level and distending the lungs to the maximum
amount.
2. The functional residual capacity equals the
expiratory reserve volume plus the residual
volume. This is the amount of air that remains in
the lungs at the end of normal expiration (about
2300 milliliters).
3. The vital capacity equals the inspiratory reserve
volume plus the tidal volume plus the expiratory
reserve volume. This is the maximum amount of
air a person can expel from the lungs after first
filling the lungs to their maximum extent and then expiring to the maximum extent (about
4600 milliliters).
4. The total lung capacity is the maximum volume to
which the lungs can be expanded with the greatest
possible effort (about 5800 milliliters); it is equal
to the vital capacity plus the residual volume.
All pulmonary volumes and capacities are about 20
to 25 per cent less in women than in men, and they are
greater in large and athletic people than in small and
asthenic people.

Physiologic Anatomy of the Pulmonary
Circulatory System:
Pulmonary Vessels.-The pulmonary artery extends only 5 centimeters beyond the apex
of the right ventricle and then divides into right and left main branches that supply
blood to the two respective lungs.
The pulmonary artery is thin, with a wall thickness one third that of the aorta.
The pulmonary arterial branches are very short, and all the pulmonary arteries, even
the smaller arteries and arterioles, have larger diameters than their counterpart systemic
arteries.This, combined with the fact that the vessels are thin and distensible,
gives the pulmonary arterial tree a large compliance, averaging almost 7 ml/mm Hg,
which is similar to that of the entire systemic arterial tree. This large compliance
allows the pulmonary arteries to accommodate the stroke volume output of the
right ventricle.
The pulmonary veins- like the pulmonary arteries, are also short. They immediately
empty their effluent blood into the left atrium, to be pumped by the left heart
through the systemic circulation.
Bronchial Vessels- Blood also flows to the lungs through small bronchial arteries that
originate from the systemic circulation, amounting to about 1 to 2 per cent of the
total cardiac output. This bronchial arterial blood is oxygenated blood, in contrast
to the partially deoxygenated blood in the pulmonary arteries. It supplies the supporting
tissues of the lungs, including the connective tissue, septa, and large and
small bronchi. After this bronchial and arterial blood has passed through the supporting
tissues, it empties into the pulmonary veins and enters the left atrium, rather
than passing back to the right atrium. Therefore, the flow into the left atrium and
the left ventricular output are about 1 to 2 per cent greater than the right ventricular
output.
Lymphatics- Lymph vessels are present in all the supportive tissues of the lung, beginning
in the connective tissue spaces that surround the terminal bronchioles, coursing
to the hilum of the lung, and thence mainly into the right thoracic lymph duct.
Particulate matter entering the alveoli is partly removed by way of these channels,
and plasma protein leaking from the lung capillaries is also removed from the lung
tissues, thereby helping to prevent pulmonary edema.

Pressures in the Pulmonary System:

Pressures in the Pulmonary Artery- During systole, the
pressure in the pulmonary artery is essentially equal
to the pressure in the right ventricle,

Pulmonary Capillary Pressure- The mean pulmonary capillary
pressure, is about
7 mm Hg.

Left Atrial and Pulmonary Venous Pressures- The mean
pressure in the left atrium and the major pulmonary
veins averages about 2 mm Hg in the recumbent
human being, varying from as low as 1 mm Hg to as
high as 5 mm Hg.

Physics of Gas Diffusion and Gas
Partial Pressures:
Molecular Basis of Gas Diffusion-
All the gases of concern in respiratory physiology are simple molecules that are free
to move among one another, which is the process called “diffusion.”This is also true
of gases dissolved in the fluids and tissues of the body.
For diffusion to occur, there must be a source of energy. This is provided by the
kinetic motion of the molecules themselves. Except at absolute zero temperature,
all molecules of all matter are continually undergoing motion. For free molecules
that are not physically attached to others, this means linear movement at high velocity
until they strike other molecules. Then they bounce away in new directions and
continue until striking other molecules again. In this way, the molecules move
rapidly and randomly among one another.
Net Diffusion of a Gas in One Direction—Effect of a Concentration Gradient. If a gas chamber
or a solution has a high concentration of a particular gas at one end of the chamber
and a low concentration at the other end, net diffusion of
the gas will occur from the high-concentration area toward the low-concentration
area.The reason is obvious:There are far more molecules at end A of the chamber
to diffuse toward end B than there are molecules to diffuse in the opposite direction.
Therefore, the rates of diffusion in each of the two directions are proportionately
different.

Diffusion of Gases Through
Fluids—Pressure Difference Causes
Net Diffusion:
Now, let us return to the problem of diffusion. From the
preceding discussion, it is clear that when the partial
pressure of a gas is greater in one area than in another
area, there will be net diffusion from the high-pressure
area toward the low-pressure area. For instance, one can readily see that the molecules
in the area of high pressure, because of their
greater number, have a greater statistical chance of
moving randomly into the area of low pressure than
do molecules attempting to go in the other direction.
However, some molecules do bounce randomly from
the area of low pressure toward the area of high pressure.
Therefore, the net diffusion of gas from the area of
high pressure to the area of low pressure is equal to the
number of molecules bouncing in this forward direction
minus the number bouncing in the opposite direction;
this is proportional to the gas partial pressure difference
between the two areas, called simply the pressure difference
for causing diffusion.Once oxygen has diffused from the alveoli into
the pulmonary blood, it is transported to the
peripheral tissue capillaries almost entirely in combination
with hemoglobin. The presence of hemoglobin
in the red blood cells allows the blood to
transport 30 to 100 times as much oxygen as could
be transported in the form of dissolved oxygen in
the water of the blood.
In the body’s tissue cells, oxygen reacts with various foodstuffs to form large
quantities of carbon dioxide. This carbon dioxide enters the tissue capillaries
and is transported back to the lungs. Carbon dioxide, like oxygen, also combines
with chemical substances in the blood that increase carbon dioxide transport
15- to 20-fold.
The purpose of this chapter is to present both qualitatively and quantitatively
the physical and chemical principles of oxygen and carbon dioxide transport in
the blood and tissue fluids.

The nervous system normally adjusts the rate of
alveolar ventilation almost exactly to the demands
of the body so that the oxygen pressure (Po2) and
carbon dioxide pressure (Pco2) in the arterial blood
are hardly altered even during heavy exercise and
most other types of respiratory stress. This chapter
describes the function of this neurogenic system for
regulation of respiration.
Respiratory Center:
The respiratory center is composed of several groups of neurons located bilaterally
in the medulla oblongata and pons of the brain stem, It is divided into three major collections of neurons: (1) a dorsal respiratory
group, located in the dorsal portion of the medulla, which mainly causes
inspiration; (2) a ventral respiratory group, located in the ventrolateral part of
the medulla, which mainly causes expiration; and (3) the pneumotaxic center,
located dorsally in the superior portion of the pons, which mainly controls rate
and depth of breathing.The dorsal respiratory group of neurons plays the most
fundamental role in the control of respiration.