Ross & Wilson Anatomy and Physiology in Health and Illness (122 page)

Impaired surfactant function

Premature babies, born before the 34th week, may be unable to expand their lungs by their own respiratory effort because their lungs are too immature to produce surfactant (
p. 245
). Surfactant coats the inner surface of each alveolus and reduces surface tension, allowing alveolar expansion during inspiration. These babies may need to be mechanically ventilated until their lungs begin to produce surfactant. This is called
respiratory distress syndrome
.

In the condition called
adult respiratory distress syndrome
(ARDS), dilution of surfactant by fluid collecting in the alveoli (pulmonary oedema) causes surfactant efficiency to be reduced and atelectasis follows. These patients are nearly always gravely ill already, and collapse of substantial areas of lung contributes to the mortality rate of around 50%.

Pressure collapse

When air or fluid enters the pleural cavity the negative pressure becomes positive, preventing lung expansion. Fluids settle in the lung bases, whereas collections of air are usually found towards the lung apex (
Fig. 10.31B
). The collapse usually affects only one lung and may be partial or complete. There is no obstruction of the airway.

Pneumothorax

In this condition there is air in the pleural cavity. It may occur spontaneously or be the result of trauma.

Tension pneumothorax

This occurs as a complication when a flap or one-way valve develops between the lungs and the pleural cavity. Air enters the pleural cavity during inspiration but cannot escape on expiration and steadily, but sometimes rapidly, accumulates. This causes shift of the mediastinum and compression of the other lung, resulting in severe respiratory distress that is often fatal unless promptly treated.

Spontaneous pneumothorax

This may be either primary or secondary.
Primary spontaneous pneumothorax
is of unknown cause and occurs in fit and healthy people, usually males between 20 and 40 years.
Secondary spontaneous pneumothorax
occurs when air enters the pleural cavity after the visceral pleura ruptures due to lung disease, e.g. emphysema, asthma, pulmonary tuberculosis, bronchial cancer.

Traumatic pneumothorax

This is due to a penetrating injury, e.g. compound fracture of rib, stab or gunshot wound, surgery.

Haemothorax

This is blood in the pleural cavity. It may be caused by:

•
penetrating chest injury involving blood vessels

•
ruptured aortic aneurysm

•
erosion of a blood vessel by a malignant tumour.

Pleural effusion

This is excess fluid in the pleural cavity that may be caused by:

•
increased hydrostatic pressure, e.g. heart failure (
p. 119
), increased blood volume

•
increased capillary permeability due to local inflammation, e.g. lobar pneumonia, pulmonary tuberculosis, bronchial cancer, mesothelioma

•
decreased plasma osmotic pressure, e.g. nephrotic syndrome (
p. 344
), liver cirrhosis (
p. 325
)

•
impaired lymphatic drainage, e.g. malignant tumour involving the pleura.

Following haemothorax and pleural effusion, fibrous adhesions which limit reinflation may form between the layers of pleura.

Alveolar hypoventilation

In the normal individual breathing quietly at rest there are always some collapsed lobules within the lungs because of the low tidal volume. These lobules will re-expand without difficulty at the next deep inspiration. Another common cause of hypoventilation collapse occurs postoperatively, particularly after chest and upper abdominal surgery, when pain restricts thoracic expansion. Postoperative collapse (atelectasis) predisposes to chest infections, because mucus collects in the underventilated airways and is not coughed up (expectorated).

For a range of self-assessment exercises on the topics in this chapter, visit
www.rossandwilson.com
.

CHAPTER 11

Introduction to nutrition

The balanced diet
266

Carbohydrates
268

Proteins (nitrogenous foods)
268

Fats
269

Vitamins
270

Fat-soluble vitamins
270

Water-soluble vitamins
271

Minerals, trace elements and water
272

Minerals and trace elements
272

Water
273

Non-starch polysaccharide (NSP)
273

Functions of NSP (dietary fibre)
273

Disorders of nutrition
274

Malnutrition
274

Malabsorption
274

Obesity
274

Conditions with dietary implications
275

Before discussing the digestive system it is necessary to have an understanding of the nutritional needs of the body, i.e. the dietary constituents and their functions.

A
nutrient
is any substance that is digested, absorbed and used to promote body function. These substances are:

•
carbohydrates

•
proteins

•
fats

•
vitamins

•
mineral salts, trace elements and water.

Many foods contain a number of nutrients, e.g. potatoes and bread are mainly carbohydrate, but both contain protein and some vitamins. Foods are described as carbohydrate or protein because they contain a higher proportion of one or the other.
Fibre
, more correctly known as
non-starch polysaccharide
(NSP), consists of indigestible material. It is not a nutrient, as it is not digested or absorbed, but it has many beneficial effects on the digestive tract.

The
diet
is the selection of foods eaten by an individual. A
balanced diet
is essential for health. It provides the appropriate amounts of all nutrients in the correct proportions to meet body requirements. An
essential nutrient
is a substance that cannot be made by the body and must therefore be included in the diet.

Many health problems arise as the result of poor diet. In developed countries obesity is increasingly common, while in other countries malnutrition is widespread. The final section of the chapter considers some consequences of poor nutrition.

The balanced diet

Learning outcomes

After studying this section, you should be able to:

list the constituent food groups of a balanced diet

calculate body mass index from an individual’s weight and height.