Respiratory System Part C

OPTOMETRY · SEMESTER 1

Human Anatomy and Physiology

Functional residual capacity

Functional residual capacity – the amount of air remaining in the lungs at the end of a normal expiration, therefore it is the sum of ERV + RV = 2.2 -2.4 litres.

Total lung capacity

Total lung capacity – the sum of all four lung volumes – the total amount of air a lung can hold i.e. = TV+ IVR + EVR + RV = 5.7 – 6.2 litres.

Inspiratory capacity

Inspiratory capacity is the Tidal volume plus the inspiratory reserve volume

Dead spaces

Anatomical dead space – air in passageway that do not participate in gas exchange.

Dead space:since gaseous exchange in the respiratory system occurs only in the terminal portions of the airways, the gas that occupies the rest of the respiratory system is not available for gas exchange with pulmonary capillary blood.

This gas is known to be in dead space

Dead spaces

Dead space can be:

Anatomical dead space – anatomical dead space is the gas in the conducting areas of the respiratory system, such as the mouth, trachea and bronchi where the air doesn't come to the alveoli of the lungs, that do not participate in gas exchange Physiological dead space.

Physiological dead space

The physiological dead space is equal to the anatomical dead space plus the alveolar dead space Alveolar dead space is the area in the alveoli that does get air to be exchanged, but there is no enough blood flowing through the capillaries for exchange to be effective It is normally very small (less than 5 mL) in healthy individuals.

It can increase dramatically in some lung diseases

Physiological dead space

Physiologic dead space can be measured by Bohr's method.

An equation and example are provided below:

In physiology, dead space is air that is inhaled by the body in breathing, but does not partake in gas exchange.

In adults, it is usually in the range of 150 mL.

Alveolar ventilation

Alveolar ventilation is the volume of new air that moves into and out of the alveoli per minute.

It is equal to the tidal volume minus the anatomical dead space, multiplied by the respiratory rate:

Alveolar ventilation

Alveolar ventilation per minute is the total volume of new air entering the alveoli and adjacent gas exchange areas each minute.

Alveolar ventilation is one of the major factors determining the concentrations of oxygen and carbon dioxide in the alveoli.

FACTORS REGULATING RESPIRATION

Introduction

Control of respiration is normally involuntary. Voluntary control is exerted during activities such as speaking and singing but is overridden if blood CO2 rises (hypercapnia) 14

Factors regulating respiration

The respiratory centre: This is formed by groups of nerve cells that control the rate and depth of respiration They are situated in the brain stem, in the medulla oblongata and the pons influence respiration 15

Factors regulating respiration

Central chemoreceptors: These are on the surface of the medulla oblongata and are bathed in cerebrospinal fluid.

When the arterial PCO2 rises (hypercapnia), the central chemoreceptors respond by stimulating the respiratory centre, increasing ventilation of the lungs and reducing arterial PCO2.

The sensitivity of the central chemoreceptors to raised arterial PCO2 is the most important factor in maintaining homeostasis of blood gases in health.

Factors regulating respiration

Peripheral chemoreceptors: These are situated in the arch of the aorta and in the carotid bodies They are more sensitive to small rises in arterial PCO2 than Nerve impulses, generated in the peripheral chemoreceptors, are conveyed by the glossopharyngeal and vagus nerves to the medulla and stimulate the respiratory centre.

The rate and depth of breathing are then increased.

An increase in blood acidity stimulates the peripheral chemoreceptors, resulting in increased ventilation, increased CO2 excretion and increased blood pH.

Factors regulating respiration

Breathing may be modified by the higher centres in the brain by:
speech, singing
emotional displays, e.g. crying, laughing, fear
drugs, e.g. sedatives, alcohol

Sleep

Temperature influences breathing: In fever respiration is increased due to increased metabolic rate while in hypothermia it is depressed, as is metabolism.

Temporary changes in respiration occur in swallowing, sneezing and coughing.

Factors regulating respiration

In strenous exercise, both the rate and depth of breathing increase, increasing oxygen uptake and carbon dioxide excretion in order to meet increased needs.

Physiological variables affecting

respiration

Elasticity. Elasticity is the term used to describe the ability of the lung to return to its normal shape after each breath.

Loss of elasticity of the connective tissue in the lungs necessitates forced expiration and increased effort on inspiration 21

Physiological variables affecting

respiration

Compliance. This is a measure of the distensibility of the lungs, i.e. the effort required to inflate the alveoli.

When compliance is low the effort needed to inflate the lungs is greater than normal, e.g. in some diseases where elasticity is reduced or when insufficient surfactant is present.

It should be noted that compliance and elasticity are opposing forces.

Factors regulating respiration

Airflow resistance. When this is increased, e.g. in bronchoconstriction, more respiratory effort is required to inflate the lungs.

GAS EXCHANGE THROUGH RESPIRATORY MEMBRANES

Respiratory membrane

The respiratory membrane has the following layers :

1. A layer of fluid lining the alveolus and containing surfactant that reduces the surface tension of the alveolar fluid 2. The alveolar epithelium composed of thin epithelial cells 3. An epithelial basement membrane

Respiratory membrane

4. A thin interstitial space between the alveolar epithelium and the capillary membrane 5. A capillary basement membrane that in many places fuses with the alveolar epithelial basement membrane 6. The capillary endothelial membrane 27

Factors that affect the rate of gas diffusion

The increase of thickness of the respiratory membrane ,as a result of edema fluid in the interstitial space of the membrane and in the alveoli—so that the respiratory gases must then diffuse not only through the membrane but also through this fluid delaying the exchange The surface area of the respiratory membrane can be greatly decreased in some lung diseases(emphysema) making the exchange of gases through the membrane is also decreased 28

Factors that affect the rate of gas diffusion

The pressure difference across the respiratory membrane is the difference between the partial pressure of the gas in the alveoli and the partial pressure of the gas in the pulmonary capillary blood When the partial pressure of a gas in the alveoli is greater than the pressure of the gas in the blood, as is true for oxygen, net diffusion from the alveoli into the blood occurs When the pressure of the gas in the blood is greater than the partial pressure in the alveoli, as is true for carbon dioxide, net diffusion from the blood into the alveoli occurs.