MEDI7111 Lecture Notes - Lecture 1: Smooth Muscle Tissue, Hemothorax, The Fluid
Respiratory 1
Normal physiology and biochemistry of breathing
In order to fully understand the physiology and mechanics of breathing, there are some terms that
need to be defined.
Ventilation is the action of moving air in and out of the alveoli.
Perfusion describes the exchange of gases between alveoli and the pulmonary capillaries.
Respiration is the delivery of oxygen to the tissues, use of that oxygen by the tissues and delivery
carbon dioxide away from the tissues. It involves the lungs, the circulatory system and the end
tissue.
Respiratory Mechanics
In the healthy lung, there are ~300 million alveoli which form a surface area for gas exchange of ~80-
100m2.
Micromechanics
The basic structure of an alveolus is given below. Type 1 pneumocytes form the simple squamous
epithelium of the alveolus and allow for minimal diffusion distance between the alveolus and the
pulmonary capillary. Type two pneumocytes are much larger cells which secrete a substance called
surfactant to reduce the surface tension within the alveolus, thus preventing its collapse on
expiration.
The Law of LaPlace states that the magnitude of the inward pressure of a bubble is:
2 x Surface Tension
Radius
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In an alveolus, surface tension is created by the fluid that is necessary for inhaled gases to be
dissolved. If only this fluid was in the alveoli, each would collapse after exhalation and the work
required to expand them with each breath would be enormous.
Surfactant is a lipid and protein mixture which reduces the surface tension of the lungs by reducing
hydrogen bonding opportunities for water. This reduces the surface tension within the alveoli, thus
reducing their inward pressure. It is important to note that type 2 pneumocytes secrete the same
amount of surfactant regardless of the size of the alveolus, as smaller alveoli require a greater
concentration of surfactant to maintain patency.
This constant inward pressure of the alveoli is termed the elastic recoil of the lungs and is very
important when understanding lung pathology and the macromechanics of ventilation.
Macromechanics
An important concept to understand is the function of the pleura in ventilation. The fluid between
the parietal and visceral pleural membranes adhere the lungs to the chest wall whilst allowing for
friction free movement. The elastic recoil of the alveoli creates an inward pressure on the visceral
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Document Summary
In order to fully understand the physiology and mechanics of breathing, there are some terms that need to be defined. Ventilation is the action of moving air in and out of the alveoli. Perfusion describes the exchange of gases between alveoli and the pulmonary capillaries. Respiration is the delivery of oxygen to the tissues, use of that oxygen by the tissues and delivery carbon dioxide away from the tissues. It involves the lungs, the circulatory system and the end tissue. In the healthy lung, there are ~300 million alveoli which form a surface area for gas exchange of ~80- The basic structure of an alveolus is given below. Type 1 pneumocytes form the simple squamous epithelium of the alveolus and allow for minimal diffusion distance between the alveolus and the pulmonary capillary. Type two pneumocytes are much larger cells which secrete a substance called surfactant to reduce the surface tension within the alveolus, thus preventing its collapse on expiration.
