Download doc Detailed Respiration Notes

SUMMARY: RESPIRATION
Functions of the respiratory system:
- Function off the respiratory zone: gas exchange:
o In the lungs, we add oxygen to the blood and release carbon dioxide in the
environment.
o In working cells, oxygen diffuses from blood to ISF to cells, while metabolic
waste products such as carbon dioxide move in the opposite direction.
o Therefore, both respiratory system and CVS work in collaboration to allow
appropriate gas exchange.
- Functions of the conducting airways:
1. Defence against bacterial infection and foreign particles:
 The epithelial cells possess cilia and secrete mucous
 Foreign particles stick to the mucous and the cilia constantly sweeps the
mucous up into the pharynx.
 Tobacco smoking paralyzes the cilia, thus inhibiting this defence
mechanism.
2. Warm and moisten inhaled air
3. Produce sound and speech with the vocal cords
4. Regulation of air flow: smooth muscle around the airways may contract or
relax to alter resistance to air flow.
Anatomy of the respiratory system:
- The respiratory tract:
o Air can enter either by the mouth or the nose
o If it enters by the nose, it passes through the
nasal septum and the nasal turbinates, which
clean the air of big dust particles.
o Air then passes into the pharynx (common to
air and food), the larynx, and the trachea.
o The trachea divides into two bronchi, each of
which divides into lobar and segmental
bronchi.
o Right main bronchus = 3 lobar bronchi
o Left main bronchus = 2 lobar bronchi
o The segmental
bronchi divides
further into smaller branches
o The smallest airways without alveoli are the
terminal bronchioles
o Surface of the lungs = visceral pleura
- Subdivisions of the conducting airways and terminal
respiratory units:
o The airways consist of a series of tubes that
branch and become narrower, shorter and more
numerous as they penetrate into the lungs.
- Conducting and respiratory zones:
o Conducting zone:
 Airways from the mouth and nose
1
openings, all the way down to the terminal bronchioles.
They conduct air from environment to the respiratory zone.
Since they do not participate in gas exchange, they are said to form the
anatomical dead space.
o Respiratory zone:
 Characterized by the presence of alveoli in the walls of the airways.
 Begins where the terminal bronchioles divide into the respiratory
bronchioles.
 Site for gas exchange
 Makes most of the lungs
 Smallest physiological unit of lungs = acinus (1 respiratory bronchiole)
Blood supply:
o Pulmonary circulation:
 Brings mixed
venous blood to
the lungs,
allowing for the
blood to get
oxygenated, and
then back to the
heart, where it
enters the
systemic
circulation.
 Branches from the
pulmonary artery
run with the
airways.
 When the alveoli
are reached,
arterioles divide
into a capillary
bed.
o Bronchial (systemic)
circulation:
 Supplies oxygenated blood to the tracheobronchial tree
 Bronchial arteries from the aorta supply the airway walls.
Alveolar cell types:
o Epithelial cells:
 Type I:
 Little is known about their specific metabolic activity
 Type II:
 Produce pulmonary surfactant, a substance that decreases the
surface tension of the alveoli
o Endothelial cells:
 Constitute the walls of the pulmonary capillaries
 May be as thin as 0,1 micron
o Alveolar macrophages:
 Remove foreign particles that may have escaped the mucocilliary
defence system of the airways.


-
-
2
-
Respiratory muscles:
o Inspiratory muscles:
 Principal:
 External intercostals: elevate ribs
 Parasternal intercartilaginous: elevate ribs
 Diaphragm: dome descend, increasing longitudinal dimension
of chest and elevating lower ribs
 Accessory:
 Sternocleidomastoid: elevates sternum
 Scalenus: elevate and fix upper ribs:
o Anterior
o Middle
o Posterior
o Expiratory muscles:
 Quiet breathing:
 Expiration results from passive recoil of lungs
 Active breathing:
 Internal intercostals, except parasternal intercartilaginous
muscles: depress ribs
 Abdominal muscles : depress lower ribs, compress abdominal
contents
o Rectus abdominus
o External oblique
o Internal oblique
o Transversus abdominus
Summary of events during respiration:
- Inspiration:
o Diaphragm and intercostal muscles contract
3
-
o Thoracic cage expands
o Intrapleural pressure becomes more subatmospheric
o Transpulmonary pressure increase
o Lungs expand
o Alveolar pressure becomes subtamospheric
o Air flows into alveoli
Expiration:
o Diaphragm and intercostal muscles stop contracting
o Chest wall moves inwards
o Intrapleural pressure goes back towards preinspiratory value
o Transpulmonary pressure goes back towards preinspiratpry value
o Lung recoil towards preinspiratory volume
o Air in lungs is compressed
o Alveolar pressure becomes greater than atmospheric pressure
o Air flows out of the lungs
Spirometry: measuring lung volumes
A spirometer is a machine that
measures amounts of inhaled or
exhaled air.
It can directly measure tidal volume,
vital capacity, inspiratory capacity,
expiratory reserve volume and
expiratory reserve volume, but not
residual volume, functional residual
capacity or total lung capacity.
Ventilation:
- Minute ventilation versus alveolar ventilation:
o Ventilation = amount of air inspired into the lungs over some period of time.
o It is usually meausured for 1 minute: this is why we call it minute ventilation.
4
-
-
o Not all air inspired reaches the respiratory zone. Some stays in the conducting
airways, forming the anatomical dead space.
o Thus, we can define alveolar respiration as the amount of air that reaches the
respiratory zone per minute and available for gas exchange.
Physiological dead space:
o In some pathological conditions, an amount of inspired air that reaches the
respiratory zone does not take place in gas exchange. This represents the
alveolar dead space.
o Physiological dead space is the sum of anatomical and alveolar dead space.
Types of alveolar ventilation:
o Normal alveolar ventilation: alveolar ventilation matches carbon dioxide
production and keeps its partial pressure at a constant level.
o Alveolar hyperventilation: more oxygen is supplied and more carbon dioxide is
removed than metabolic rates needs.
o Alveolar hypoventilation: alveolar ventilation is below that required by the
metabolic activity of the body.
Gas diffusion:
- Diffusion rate:
o Passage of oxygen across the alveolar-capillary membrane occurs by passive
diffusion and is governed by Fick’s law.
o Diffusion rate is proportional to :
 Surface area
 Partial pressure gradient
 1/thickness of the alveolar capillary membrane.
o Diffusion of oxygen and carbon dioxide always occurs from higher to lower
partial pressure.
o In order for a gas to diffuse through a liquid, the gas must be soluble in the
liquid.
o Since carbon dioxide is considerably more soluble than oxygen, it diffuses
about 20x more rapidly.
o However, the time required for equilibrium between alveolar and capillary
blood is about the same for the two gases, since the difference of pressure
between alveolus and capillary is 10x higher for oxygen than for carbon
dioxide, thus compensating for the solubility difference.
- Transit time:
o Although the transit time of blood through the pulmonary capillaries is only
0,75 seconds at rest, diffusion is so rapid than the partial pressure of oxygen of
the air and that of the blood reach equilibrium before the blood has passed
even half way along the pulmonary capillary.
o In a normal long, diffusion of both oxygen and carbon dioxide is accomplished
within 1/3 of the transit time.
Pulmonary blood flow:
- Pulmonary circulation and blood pressure:
o Differences between systemic and pulmonary circulation:
 Blood pressure is lower in the pulmonary circulation
 The walls of the pulmonary capillaries are thinner than those of the
systemic circulation. The mean pulmonary artery pressure is about
15mmHg while the left atrial pressure is about 5mmHg.
5
-
-
-
-
-
Vascular resistance:
o Remember that flow = pressure/resistance
o The pulmonary resistance is 1/10 that of the systemic circulation.
o The low vascular resistance in the pulmonary circulation relies on the thin
walls of the vascular system
o Pulmonary circulation = low vascular resistance and high compliance. This
allows the lungs to accept the whole cardiac output at all times.
o Because of the high compliance, the vessels are affected by pressure at all
time.
o When the alveolar pressure increases above the pressure in the capillaries, they
collapse.
o On the other hand, arterioles and veins are pulled open when lungs expand
because they are subjected to intrapleural pressure.
Accommodation of pulmonary blood vessels:
o 2 methods of accommodation are used to decrease vascular resistance in the
lungs:
 Distension: already perfused blood vessels increase their diameter.
 Recruitment: previously closed vessels may open as the cardiac output
rises.
o Drugs:
 Drugs such as serotonin, histamine, NE, which cause the contraction of
smooth muscle increase pulmonary resistance in the larger pulmonary
arteries
 Drugs such as acetylcholine and isoproteranol, which can relax smooth
muscle, may decrease pulmonary vascular resistance.
Effects of gravity on pulmonary blood flow:
o In upright position, blood flow increases almost linearly from top to bottom,
because gravity distends blood vessels at the bottom of the lungs, while the
capillaries at the top may be completely compressed if pressure is greater in
alveoli than in capillaries.
Effects of gravity on ventilation:
o Ventilation occurs preferentially at the bottom than at the top of the lungs
because the alveoli are more opened at the bottom than at the top of the lungs
(think of the slinky demonstration).
Distribution of ventilation perfusion ration in the lungs in normal gravity:
o Ventilation increases from top to bottom of the lung, but blood flow increases
more rapidly.
o Therefore, the ventilation-perfusion ratio is abnormally high at the top and
much lower at the bottom.
Transport of oxygen and carbon dioxide:
- Oxygen physically dissolved in plasma:
o Henry’s law: the number of gas molecules dissolved in a liquid is proportional
to the partial pressure of the gas above the liquid.
o Because oxygen is relatively insoluble in water, the amount of oxygen
dissolved in blood is small and proportional to the partial pressure of oxygen
(0,3ml O2 / 100ml plasma).
o However, the metabolic need for oxygen is much greater than the amount
dissolved in blood so there must be another way of transporting oxygen in
blood.
6
-
-
-
-
Oxygen bound to hemoglobin:
o Found in red blood cells
o Increases the amount of oxygen transported by 65x at partial pressure of
oxygen of 100mmHg
o Each molecule of hemoglobin consists of a heme (iron porphyrin) joined to a
globin (protein)
o Consists of 4 polypeptide chains, each containing a Fe++ that can bind 1
molecule of oxygen.
o It is essential for the transport of oxygen because it combines rapidly and
reversibly with it.
o Note that oxygen that is bound to Hb does not contribute to the partial pressure
of oxygen in blood. However, the partial pressure of oxygen in plasma
determines the amount of oxygen that combines with Hb.
The oxygen dissociation curve:
o The HbO2 dissociation curve determines
the amount of oxygen by hemoglobin for
a given partial pressure of oxygen.
o The curve is flat at high pressures of
oxygen (in alveoli) and steep at low
pressures of oxygen (in peripheral
tissues)
o Thus, at small values of PO2, as seen in
peripheral tissues, a small drop in PO2
unloads the oxygen from hemoglobin to
the tissue.
o This provides an automatic mechanism that matches tissue oxygen supply to
tissue oxygen need.
o In the muscle cell, a similar substance to hemoglobin has been found:
myoglobin. Its major function is to act as an intracellular carrier which
facilitates the diffusion of oxygen throughout the muscle cell.
o The total amount of oxygen in the blood depends mostly on the concentration
of hemoglobin.
The Bohr effect:
o Bohr effect: shift of the HbO2 dissociation curve to the right when blood
carbon dioxide or temperature increases, or blood pH decreases.
o The curve shifting to the right means that for a given drop in oxygen pressure,
an additional amount of oxygen is released from hemoglobin to the working
tissues.
o This has almost no effect on the total amount of oxygen combined with
hemoglobin above 80mmHg.
Transport of CO2:
o Physically dissolved in blood (10%)
o Combined to Hb to form HbCO2 (11%): it combines with the globin portion of
Hb, not with the heme portion, as oxygen does. Therefore there is no
competition for binding site.
o As bicarbonate (79%): carbon dioxide combines with water to form carbonic
acid. This reaction is enhanced by carbonic anhydrase.
7
-
-
-
The Haldane effect:
o In the tissue capillaries, Hb free of oxygen may act as a buffer and combine
with H+. This occurs because reduced Hb is less acidic than HbO2.
o Therefore, the presence of reduced Hb in the tissue capillaries helps with the
blood loading of capillaries by pushing equations 1 and 2 to the right.
o Result: for a given partial pressure of carbon dioxide, more CO2 is carried in
deoxygenated blood than in oxygenated blood.
Respiratory failure:
o Occurs when the respiratory system is unable to do its job properly, due to
failure of:
 The gas exchanging capabilities of the lungs
 The neural control of ventilation
 The neuromuscular breathing apparatus (respiratory muscles and their
innervation)
Arterial hypoxia (hypoxemia):
o Deficient blood oxygenation
o There are 5 general causes of hypoxia:
 Inhalation of low PO2
 Hypoventilation
 Ventilation/perfusion imbalance in the lungs. Shunts of blood across
the lungs.
 Oxygen diffusion impairment.
Important equations:
- Measurement of FRC:
o FRC can be measured by helium dilution.
o You put a given volume of helium, whit a known concentration, in a
spirometer and you ask the patient to breath in and out from the spirometer
until equilibrium is reached.
FRC = (C1 x V1/C2) – V1
o Where:
o C1 is the concentration of helium in the spirometer at the beginning
o V1 is the volume of helium in the spirometer
o C2 is the concentration of helium in the spirometer after equilibration.
- Minute ventilation:
VE = VT x f
o Where VT is the tidal volume and f is the number of breaths per minute
o Note that there should be a dot over VE to indicate that it is a change with
respect to time.
- Alveolar ventilation
VA = VE –VD
o Where VD is the volume f the anatomical dead space
o Note that there should be a dot over each of the values in the equation
o Anatomical dead space is around 150mL in the average adult. A good
estimation is the subject’s weight in pounds.
8