respiratory lesson 2 physiology lecture

PHYSIOLOGY OF RESPIRATORY SYSTEM

1. FACTORS INFLUENCING PULMONARY
VENTILATION
These are grouped into
a)Physical factors
b)Neural factors
c)Chemical factors
Chapter 22, Respiratory System
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2. Physical factors
These include airway resistance, alveolar surface
tension and lung compliance.
Chapter 22, Respiratory System
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3. Chapter 22, Respiratory System
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Friction is the major nonelastic source of resistance
to airflow
The relationship between flow (F), pressure (P), and
resistance (R) is:

Airway Resistance
P
R
F =

4. Chapter 22, Respiratory System
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The amount of gas flowing into and out of the
alveoli is directly proportional to P, the pressure
gradient between the atmosphere and the alveoli
Gas flow is inversely proportional to resistance with
the greatest resistance being in the medium-sized
bronchi

5. Chapter 22, Respiratory System
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As airway resistance rises, breathing movements
become more strenuous
Severely constricted or obstructed bronchioles:
Can prevent life-sustaining ventilation
Can occur during acute asthma attacks which stops
ventilation
Epinephrine release via the sympathetic nervous
system dilates bronchioles and reduces air resistance

6. Chapter 22, Respiratory System
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Alveolar Surface Tension
Surface tension – the attraction of liquid molecules
to one another at a liquid-gas interface
The liquid coating the alveolar surface is always
acting to reduce the alveoli to the smallest possible
size
Surfactant, a detergent-like complex, reduces surface
tension and helps keep the alveoli from collapsing

7. Chapter 22, Respiratory System
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Lung Compliance
The ease with which lungs can be expanded
Specifically, the measure of the change in lung
volume that occurs with a given change in
transpulmonary pressure
Determined by two main factors
Distensibility of the lung tissue and surrounding
thoracic cage
Surface tension of the alveoli

8. NEURAL FACTORS
They are divided into hypothalamic control,
cortical control, pulmonary irritant reflexes and
inflation reflex.
Chapter 22, Respiratory System
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9. Hypothalamic control
Strong emotions, rise or fall in temperature and pain
send signals to the respiratory center through the
hypothalamus and the limbic system, modifying
respiratory depth and rate
For instance, when one is angry he holds breath and
when one is excited he breaths faster.
Chapter 22, Respiratory System
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10. Cortical controls
Although breathing is under involuntary control by
the respiratory centers in the medulla, we can have
conscious control of breathing.
This conscious control is when the cerebral cortex
send signals through motor neurons supplying the
respiratory muscles directly bypassing the medulla.
However, this control is limited
Chapter 22, Respiratory System
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11. Pulmonary irritant reflexes
The lung contains receptors that respond to a large
variety of irritants.
Irritants include accumulated mucus, inhaled dust
etc
These irritants stimulate the receptors which
communicate with the medulla via vagus nerve
causing reflex constriction of the airways.
When those irritants are present in the trachea and
bronchi they cause a reflex cough and when present
in the nasal cavity they cause a reflex sneeze.
Chapter 22, Respiratory System
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12. Inflation reflex
The visceral pleurae and the conducting
passageways in the lungs contain numerous stretch
receptors which are stimulated when the lungs are
inflated.
Once stimulated, impulses are sent to the respiratory
centers which send inhibitory impulses back to the
lungs to end inspiration and allow expiration to
occur.
This protective reflex is called inflation
reflex/Hering-Breuer reflex.
Chapter 22, Respiratory System
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13. CHEMICAL FACTORS
These chemicals are the arterial levels of co2, o2 and
arterial H+ concentration
The receptors for these are called chemoreceptors.
Chemoreptors are of two types i.e. central
chemoreptors and peripheral chemoreceptors
Central chemoreceptors are found in the medulla
while peripheral chemoreceptors are found in the
carotid and aortic bodies
Chapter 22, Respiratory System
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14. Chapter 22, Respiratory System
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Peripheral Chemoreceptors
Figure 22.27

15. Chapter 22, Respiratory System
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This is the most potent chemical that influences respiration
Changing P
CO2 levels are monitored by chemoreceptors of
the brain stem
Carbon dioxide in the blood diffuses into the cerebrospinal
fluid where it is hydrated
Resulting carbonic acid dissociates, releasing hydrogen
ions Leading to a fall of CSF PH which stimulates the
central chemoreceptors.
P
CO2 levels rise (hypercapnia) resulting in increased depth
and rate of breathing
P
CO2

16. Chapter 22, Respiratory System
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Hyperventilation – increased depth and rate of
breathing that:
Quickly flushes carbon dioxide from the blood
Occurs in response to hypercapnia
Though a rise CO
2 acts as the original stimulus,
control of breathing at rest is regulated by the
hydrogen ion concentration in the brain

17. Chapter 22, Respiratory System
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Hypoventilation – slow and shallow breathing due
to abnormally low P
CO2 levels
Apnea (breathing cessation) may occur until P
CO2

levels rise

18. Chapter 22, Respiratory System
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Arterial oxygen levels are monitored by the aortic
and carotid bodies
Substantial drops in arterial P
O2 (to 60 mm Hg) are
needed before oxygen levels become a major
stimulus for increased ventilation
If carbon dioxide is not removed (e.g., as in
emphysema and chronic bronchitis), chemoreceptors
become unresponsive to P
CO2 chemical stimuli
In such cases, P
O2 levels become the principal
respiratory stimulus (hypoxic drive)
P
O2

19. Chapter 22, Respiratory System
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Changes in arterial pH can modify respiratory rate
even if carbon dioxide and oxygen levels are normal
Increased ventilation in response to falling pH is
mediated by peripheral chemoreceptors
Arterial pH

20. Chapter 22, Respiratory System
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Acidosis may reflect:
Carbon dioxide retention
Accumulation of lactic acid
Excess fatty acids in patients with diabetes mellitus
Respiratory system controls will attempt to raise the
pH by increasing respiratory rate and depth
Arterial ph Fall is mediated by the peripheral
chemoreceptors

21. EXTERNAL RESPIRATION
This is the gaseous exchange between the alveoli
and blood
To understand external respiration, it is important to
examine blood supply to the lungs, alveoli,
respiratory membrane, the composition of air and
the properties of gases.
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22. Chapter 22, Respiratory System
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Blood Supply to Lungs
Lungs are perfused by two circulations: pulmonary
and bronchial
Pulmonary arteries – supply systemic venous blood
to be oxygenated
Branch profusely, along with bronchi
Ultimately feed into the pulmonary capillary
network surrounding the alveoli
Pulmonary veins – carry oxygenated blood from
respiratory zones to the heart

23. Chapter 22, Respiratory System
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Blood Supply to Lungs
Bronchial arteries – provide systemic blood to the
lung tissue
Arise from aorta and enter the lungs at the hilus
Supply all lung tissue except the alveoli
Bronchial veins anastomose with pulmonary veins
Pulmonary veins carry most venous blood back to
the heart

24. Chapter 22, Respiratory System
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Alveoli
Surrounded by fine elastic fibers
Contain open pores that:
Connect adjacent alveoli
Allow air pressure throughout the lung to be
equalized
House macrophages that keep alveolar surfaces
sterile
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Respiratory System: Anatomy Review: Respiratory Structures
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25. Chapter 22, Respiratory System
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Respiratory Membrane
This air-blood barrier is composed of:
Alveolar and capillary walls
Their fused basal laminas
Alveolar walls:
Are a single layer of type I epithelial cells
Permit gas exchange by simple diffusion
Secrete angiotensin converting enzyme (ACE)
Type II cuboidal cells secrete surfactant

26. Chapter 22, Respiratory System
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What is Composition of Air?
Air = 21% O2, 78% N2 and .04% CO2
Alveolar air = 14% O2, 78% N2 and 5.2% CO2
Expired air = 16% O2, 78% N2 and 4.5% CO2
Observations
alveolar air has less O2 since absorbed by blood
mystery-----expired air has more O2 & less CO2 than
alveolar air?
Anatomical dead space = 150 ml of 500 ml of tidal volume

27. PROPERTIES OF RESPIRATORY GASES
The two important respiratory gases are oxygen and
carbon dioxide
Their properties are explained using the following
laws:
Dalton’s law of partial pressures
Henry’s law
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28. Chapter 22, Respiratory System
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Total pressure exerted by a mixture of gases is the
sum of the pressures exerted independently by each
gas in the mixture
Pressure exerted independently by a gas in a mixture
of gases is called partial pressure
The partial pressure of each gas is directly
proportional to its percentage in the mixture
Nitrogen has highest partial pressure, then oxygen
and finally carbon dioxide
Dalton’s Law of partial pressures

29. Chapter 22, Respiratory System
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When a mixture of gases is in contact with a liquid,
each gas will dissolve in the liquid in proportion to
its partial pressure, solubility and temperature of
the liquid
Various gases in air have different solubilities:
Carbon dioxide is the most soluble
Oxygen is 1/20
th
as soluble as carbon dioxide
Nitrogen is practically insoluble in plasma
The higher the temperature the lower the solubility
Henry’s Law

30. Chapter 22, Respiratory System
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Factors influencing the movement of oxygen and
carbon dioxide across the respiratory membrane
Partial pressure gradients and gas solubilities
Matching of alveolar ventilation and pulmonary
blood perfusion (ventilation-perfusion coupling)
Structural characteristics of the respiratory
membrane
FACTORS INFLUENCING EXTERNAL
RESPIRATION

31. Partial pressure gradients and gas solubilities
Po2 of venous blood is 40mmHg
Po2 in the alveoli is 104mmHg.
This makes oxygen diffuse down its pressure
gradient from alveoli into the blood.
Chapter 22, Respiratory System
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32. Pco2 of venous blood is 45mmHg and that in the
alveoli is 40mmHg.
This makes co2 diffuse down its pressure gradient
from blood into the alveoli and is later expelled from
the body during expiration.
Chapter 22, Respiratory System
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33. Despite a small partial pressure difference of co2
(5mmHg) equal amounts of the two gases are
exchanged
This is because co2 is 20 times more soluble in
alveolar fluid and in plasma as oxygen
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34. Chapter 22, Respiratory System
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Ventilation – the amount of gas reaching the alveoli
Perfusion – the blood flow reaching the alveoli
Ventilation and perfusion must be tightly regulated
and synchronized for efficient gas exchange
Ventilation-Perfusion Coupling

35. Ventilation-perfusion coupling
Poorly ventilated alveoli have low Po2 and the
pulmonary arterioles supplying them constrict
 Those pulmonary arterioles supplying well
ventilated alveoli tend to dilate.
Chapter 22, Respiratory System
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36. On the other hand, when Pco2 is high in the alveoli,
the bronchioles dilate to expel the co2
When the Pco2 is low the bronchioles constrict.
These two auto-regulatory mechanisms ensure that
there is efficient gaseous exchange
Chapter 22, Respiratory System
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37. Chapter 22, Respiratory System
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Respiratory membranes:
Are only 0.5 to 1 m thick, allowing for efficient
gas exchange
Have a total surface area (in males) of about 60 m
2

(40 times that of one’s skin)
Surface Area and Thickness of the Respiratory
Membrane

38. Thicken if lungs become waterlogged and
edematous, whereby gas exchange is inadequate
and oxygen deprivation results
Decrease in surface area with emphysema, when
walls of adjacent alveoli break through
Chapter 22, Respiratory System
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