Ross & Wilson Anatomy and Physiology in Health and Illness (58 page)

Blood vessels

High blood pressure damages blood vessels. The walls of small arteries become hardened, and in larger arteries, atheroma is accelerated. If other risk factors for vascular disease are present, such as diabetes or smoking, damage is more extensive. The vessel wall may become so badly weakened by these changes that an aneurysm develops, and as the blood vessels become progressively damaged and less elastic, hypertension worsens.

The capillaries of the retina and the kidneys are particularly susceptible to the effects of chronic hypertension, leading to retinal bleeding and reduced renal function.

Pulmonary hypertension

Normally, the pulmonary circulation is a low-pressure system, to prevent fluid being forced out of the pulmonary capillaries into the alveoli. When blood pressure rises, alveoli begin to fill with fluid, which blocks gas exchange. Rising pulmonary blood pressure may result from left-sided heart failure (
p. 120
), or other problems with left ventricular function, when blood accumulates in the pulmonary circulation because the left ventricle is not pumping efficiently. Lung disease can also increase in pulmonary blood pressure because of destruction of lung capillaries, e.g. in emphysema. Primary pulmonary hypertension, where there is no identifiable cause, is rare.

Hypotension

This usually occurs as a complication of other conditions, such as shock (
p. 111
) or Addison’s disease (
p. 226
). Low blood pressure leads to inadequate blood supply to the brain. Depending on the cause, unconsciousness may be brief (fainting) or more prolonged, possibly causing death.

Postural hypotension is an abrupt fall in blood pressure on standing up suddenly from a sitting or lying position. It causes dizziness and occasionally syncope (fainting).

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www.rossandwilson.com
.

CHAPTER 6

The lymphatic system

Lymph and lymph vessels
129

Lymph
129

Lymph capillaries
129

Larger lymph vessels
129

Lymphatic organs and tissues
130

Lymph nodes
130

Spleen
131

Thymus gland
132

Mucosa-associated lymphoid tissue (MALT)
133

Lymph vessel pathology
134

Spread of disease
134

Lymphatic obstruction
134

Diseases of lymph nodes
134

Lymphadenitis
134

Lymphomas
135

Malignant metastases
135

Disorders of the spleen
135

Splenomegaly
135

Diseases of the thymus gland
136

ANIMATIONS

6.1
Lymph and lymph vessels
129

6.2
Lymphatic drainage pathways
129

6.3
Lymph nodes
130

6.4
The spleen
131

6.5
The thymus
132

The cells of the body are bathed in
interstitial (tissue) fluid
, which leaks constantly out of the bloodstream through the permeable walls of blood capillaries. It is therefore very similar in composition to blood plasma. Some tissue fluid returns to the capillaries at their venous end and the remainder diffuses through the more permeable walls of the lymph capillaries, forming
lymph
.

Lymph passes through vessels of increasing size and a varying number of
lymph nodes
before returning to the blood. The lymphatic system (
Fig. 6.1
) consists of:

•
lymph

•
lymph vessels

•
lymph nodes

•
lymph organs, e.g. spleen and thymus

•
diffuse lymphoid tissue, e.g. tonsils

•
bone marrow.

Figure 6.1 
A.
Major parts of the lymphatic system.
B.
Regional drainage of lymph.

The first sections of this chapter explore the structures and functions of the organs listed above. In the final section, the consequences of disorders of the immune system are considered.

Functions of the lymphatic system include the following.

Tissue drainage

Every day, around 21 litres of fluid from plasma, carrying dissolved substances and some plasma protein, escape from the arterial end of the capillaries and into the tissues. Most of this fluid is returned directly to the bloodstream via the capillary at its venous end, but the excess, about 3–4 litres of fluid, is drained away by the lymphatic vessels. Without this, the tissues would rapidly become waterlogged, and the cardiovascular system would begin to fail as the blood volume falls.

Absorption in the small intestine

Fat and fat-soluble materials, e.g. the fat-soluble vitamins, are absorbed into the central lacteals (lymphatic vessels) of the villi.

Immunity (
Ch. 15
)

The lymphatic organs are concerned with the production and maturation of lymphocytes, the white blood cells responsible for immunity. Bone marrow is therefore considered to be lymphatic tissue, since lymphocytes are produced there.

Lymph and lymph vessels

Learning outcomes

After studying this section, you should be able to:

describe the composition and the main functions of lymph