The respiratory zone allow inhaled oxygen to diffuse into the lung capillaries in exchange for carbon dioxide. Each lung is enveloped in a double-membrane pleural sac visceral and parietal layer.
A thin layer of watery fluid is secreted into the pleural cavity that separates the layers. Bored by anatomy?
Try this. The air space is exchanged around 10 to 15 times a minute. The air spaces are within 0. This arrangement means there is a fast efficient transfer of oxygen and carbon dioxide between the blood and the air, the major function of the respiratory portion.
The terminal bronchioles branch to give rise to respiratory bronchioles, which lead to alveolar ducts, alveolar sacs and alveoli. This diagram shows a diagram of an alveolar sac, showing how the organisation of the alveoli, and the network of blood capillaries that surround the alveoli in red. These capillaries are derived from the pulmonary arterioles. Gaseous exchange between the blood and air takes place in the alveoli, but the detailed structure of the alveolar walls cannot be resolved with the light microscope.
This shows a photograph of a section of adult lung. You should be able to recognise the terminal bronchioles, respiratory bronchioles, alveolar ducts and alveolar sacs, together with blood vessels.
The respiratory bronchioles have single alveoli off their walls. The epithelium is ciliated cuboidal epithelium and contains some secretory cells called clara cells. The large surface area makes gas exchange with the bloodstream more efficient. The alveoli are highly elastic, so the alveoli can stretch as they are filled with air during inhalation.
They then spring back during exhalation in order to expel the carbon-dioxide-rich air. The alveoli consist of an extremely thin epithelial layer and an extracellular matrix a fluid space made of collagen and elastin that contains no cells ; it is surrounded by many capillaries, the tiniest type of blood vessel.
The fluid extracellular matrix supports the structure of the alveoli and allows gases to dissolve in water, making passive diffusion of those gases with the capillaries possible. In some alveolar walls there are pores between alveoli called the pores of Kohn, that connect alveoli in order to equalize air pressure between the different sacs of an alveolus. Besides these epithelium cells, there are many macrophages found in the alveoli that provide immune system defense of the alveoli from pathogens and foreign material.
The surfactant produced by type II epithelial cells is very important for maintaining the elastic recoil of the lungs. It is a lipoprotein with hydrophilic and hydrophobic ends that reduce the amount of surface tension from water in the lungs. Without surfactant, the surface tension would cause the lungs to collapse during exhalation, making normal breathing impossible.
Surfactant is first produced by human lungs between 24 and 28 weeks in the womb, and many infants born prematurely do not have enough surfactant to breathe on their own after birth. Surfactant replacement therapy is necessary to save the lives of these premature births. The alveoli are the site of alveolar ventilation, and are not normally considered dead space. However, alveoli that are injured and can no longer contribute to gas exchange become alveolar dead space.
This is a common occurrence in people with lung diseases like COPD chronic pulmonary obstructive disorder, i. Physiological dead space is the sum of normal anatomical dead space and alveolar dead space, and can be used to determine the rate of ventilation gas exchange in the lungs.
When any type of dead space increases, the rate of ventilation in the lungs will decrease.
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