| Figure 22.6 Daily water balance in an adult person. In a steady state, intake of water equals output: water is obtained from the diet and from oxidative metabolism, and is lost through the kidneys, skin, lungs, and intestine. Note how much water enters and leaves the gastrointestinal tract daily; this explains why severe diarrhea leads to dehydration (see above). |
The main functions of the kidneys are to maintain the composition, osmolality, and volume of the ECF and also to control the acid-base balance (see Chapter 23). Kidneys remove products of metabolism such as urea , uric acid, and creatinine, and retain valuable substances such as glucose, amino acids, and proteins. They also metabolize and remove drugs and toxins. Kidney function is regulated hormonally by vasopressin (antidiuretic hormone; ADH) and renin-angiotensin system. The kidneys themselves produce renin involved in the regulation of blood pressure, calcitriol (also known as 1α-25-dihydroxycholecalciferol; 1,25(OH)2D3) involved in calcium homeostasis, (see Chapter 24), and erythropoetin
which controls the production of erythrocytes (see Chapter 3).
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| The nephron is the functional unit of the kidney
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| Each kidney consists of approximately 1 million tiny structures called nephrons (Fig. 22.7). Glomeruli located in the kidney cortex connect the cardiovascular system to the excretory tubules. An incoming small artery (afferent arteriole) branches out inside the glomerular capsule (Bowman's capsule) into bundles of capillary vessels that combine again into the outgoing (efferent) arteriole. The glomerular capsule itself transforms into a long tube, the renal tubule; the first part of this is the proximal tubule, which becomes the thin loop of Henle; further along its length it thickens again to form the distal tubule and the collecting duct.
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| DIARRHEA MAY LEAD TO SEVERE FLUID LOSS |
| A 4-year-old child was admitted to the Pediatric Unit after 2 days of severe diarrhea. The child had tachycardia and low blood pressure. Her serum sodium concentration was 145 mmol/L, creatinine concentration 50 mmol/L (0.57 mg/dL), and urea concentration 5.2 mmol/L (31 mg/dL). Treatment with an intravenous infusion of sodium chloride and antibiotics was started, and the girl recovered within 2 days. |
| Comment. This child was quite severely hypovolemic, but there was little evidence of abnormality in her biochemistry tests. This was because she lost fluid having an electrolyte concentration similar to that of plasma. Therefore, in spite of volume loss, there was no change in electrolyte concentrations. |
Children may have slightly different reference values than the adult values given here:
- sodium: 133-145 mmol/L
- creatinine: 20-80 mmol/L (0.23-0.90mg/dL)
- urea: 2.5-6.5 mmol/L (16.2-39 mg/dL)
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| Normal kidney function depends on the integrity of the glomeruli and the tubular cells
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| The nephron twists around, so that the distal tubule ends up close to the glomerulus. This results in an important functional junction formed by a group of cells in the distal tubule, known as the macula densa, and the so-called juxtaglomerular cells of the afferent arteriole. Together they form the juxtaglomerular apparatus, which secretes a proteolytic enzyme, renin (see below) in response to a decrease in blood pressure in the afferent arteriole. The macula densa also senses sodium chloride concentration in the distal tubule, participating in the so-called tubulo-glomerular feedback mechanism (TGF). Macula densa sends vasoconstriction signals to the afferent arteriole in response to an increase in the solute concentration in the distal tubule; this way TGF modulates glomerular perfusion and filtration rate in each nephron.
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