Respiratory tract secretions aid ventilation

Mammalian lungs have two other important adaptations that do not directly influence their gas exchange properties, but do affect the process of ventilation: the production of mucus and the production of surfactant.

Many cells lining the airways produce a sticky mucus that captures bits of dirt and microorganisms that are inhaled. Other cells lining the airways have cilia whose beating continually sweeps the mucus, with its trapped debris, up toward the pharynx, where it can be swallowed or spit out. This phenomenon, called the mucus escalator, can be adversely affected by inhaled pollutants. Smoking one cigarette can immobilize the cilia of the airways for hours. A smoker's cough results from the need to clear the obstructing mucus from the airways when the mucus escalator is out of order. The genetic disease cystic fibrosis causes respiratory problems by affecting the respiratory mucus (see Figure 17.3). Due to a faulty chloride channel, the mucus that is produced is dehydrated, thick, and sticky. This mucus is difficult to clear, so debris and bacteria remain in the airways, resulting in blockage and infections.

A surfactant is a chemical substance that reduces the surface tension of a liquid by interfering with the cohesive forces that create it (see Chapter 2). Surface tension gives the surface of a liquid the properties of an elastic membrane. The thin film of fluid covering the air-facing surfaces of the alveoli has surface tension, which contributes to the elasticity of the lungs. This elasticity must be overcome to inflate the lungs.

Surface tension normally is reduced by certain cells in the alveoli, which are stimulated to produce surfactant molecules when they are stretched. If a baby is born more than a month prematurely, however, these cells may not yet be producing surfactant. Such a premature baby has great difficulty breathing because an enormous effort is required to stretch the alve-

The bronchi are the major air passageways of the lungs. They lead to the bronchioles, which are finely branched.

The bronchi are the major air passageways of the lungs. They lead to the bronchioles, which are finely branched.

Human Respiratory System With Lungs
Smallest blood vessels (capillaries)

48.10 The Human Respiratory System The diagrams trace the hierarchy of human respiratory structures, from the lungs to the minuscule alveoli.

oli. A baby with this condition, known as respiratory distress syndrome, may die from exhaustion and suffocation. Common treatments have been to put the baby on a respirator to assist its breathing and to give it hormones to speed its lung development. A new approach, however, is to apply surfactant to the lungs via an aerosol.

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Essentials of Human Physiology

Essentials of Human Physiology

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