Biology of Disease

X]VeiZg-/ DISORDERS OF WATER, ELECTROLYTES AND URATE BALANCES

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Figure 8.2 (A) Schematic showing the structure

of the kidney. (B) The structural relationships

between the glomerulus, proximal tubule, loop of

Henle and distal tubule (nephron).

Within the body, the kidneys regulate water, electrolyte concentrations,

including acid–base balance (Chapter 9), and excrete nitrogenous wastes, for

example urate. Disorders of water, electrolyte and acid–base homeostasis and

urate excretion account for a large number of investigations that are carried

out in hospital laboratories.

8.2 Kidneys

The functions of the kidney include maintaining the volume of the plasma

and its concentrations of electrolytes, such as Na+ and K+, and keeping its pH

within normal physiological ranges. This is largely achieved by varying the

amounts of water and salts excreted, the removal of excess H+(Chapter 9)

and the regeneration of HCO 3 –. Kidneys also excrete waste products, such as

urea, urate and creatinine, and produce the enzyme rennin and the hormones

erythropoietin and calcitriol (also called 1A,25-dihydroxycholecalciferol,

1 A,25DHCC). These control blood pressure, stimulate the production of

erythrocytes by the bone marrow (Chapter 13) and regulate the absorption of

Ca2+ by the gastrointestinal tract (GIT) (Chapter 11) respectively. Kidneys also

synthesize prostaglandins and degrade hormones, such as insulin.

Each kidney is composed of an outer fibrous capsule, a cortex, a middle medulla

and an inner pelvis region (Figure 8.2A). The tough capsule surrounding each

kidney offers protection against trauma and pr events the entry of bacteria

(Chapter 3). Kidneys are composed of about a million functional units called

nephrons each composed of a tuft of capillaries called a glomerulus and a

tubule. Tubules have four different regions: the proximal tubule, the loop

of Henle, the distal tubule and the collecting duct (Figure 8.2B). The cortex

Urine to pelvis

of kidney

Glomerulus

Bowman's

capsule

Proximal tubule

Distal tubule

Cortex

Loop of Henle

Pyramid of

medulla

Pelvis

Capsule

A)

B)

Collecting duct