Relationship Between NaK and H

The plasma K+ concentration indirectly affects the plasma H+ concentration (pH). Changes in plasma pH likewise affect the K+ concentration of the blood. When the extracellular H+ concentration increases, for example, some of the H+ moves into the cells and causes cellular K+ to diffuse outward into the extracellular fluid. The plasma concentration of H+ is thus decreased while the K+ increases, helping to reestablish the proper ratio of these ions in the extracellular fluid. A similar effect occurs in the cells of the distal region of the nephron.

In the cells of the late distal tubule and cortical collecting duct, positively charged ions (K+ and H+) are secreted in

Table l7.6

Regulation of Renin and Aldosterone Secretion

Stimulus

Effect on Renin Secretion

Angiotensin II Production

Aldosterone Secretion

Mechanisms

iBlood volume

Increased

Increased

Increased

Low blood volume stimulates renal baroreceptors; granular cells release renin.

ÎBlood volume

Decreased

Decreased

Decreased

Increased blood volume inhibits baroreceptors; increased Na+ in distal tubule acts via macula densa to inhibit release of renin from granular cells.

TSympathetic nerve activity

None Increased

Not changed Increased

Increased Increased

Direct stimulation of adrenal cortex a-adrenergic effect stimulates constriction of afferent arterioles; ^-adrenergic effect stimulates renin secretion directly.

Peritubular capillaries

Low Na+ intake

Low Na+ intake

Low plasma Na+ concentration

t Na+ reabsorption in cortical collecting duct

Peritubular capillaries

Low plasma Na+ concentration

Hypothalamus

Posterior pituitary

L ADH

J Water reabsorption in collecting ducts t Na+ reabsorption in cortical collecting duct t Aldosterone Adrenal cortex t Angiotensin II f Renin t Urine volume J Blood volume-Juxtaglomerular apparatus

' Sympathetic -nerve activity

■ Figure 17.26 Homeostasis of plasma Na+. This is the sequence of events by which a low sodium (salt) intake leads to increased sodium reabsorption by the kidneys. The dashed arrow and negative sign indicate the completion of the negative feedback loop.

response to the negative polarity produced by reabsorption of Na+ (fig. 17.27). When a person has severe acidosis, there is an increased amount of H+ secretion at the expense of a decrease in the amount of K+ secreted. Acidosis may thus be accompanied by a rise in blood K+. If, on the other hand, hyperkalemia is the primary problem, there is an increased secretion of K+ and thus a decreased secretion of H+. Hyper-kalemia can thus cause an increase in the blood concentration of H+ and acidosis.

Reabsorption And Secretion

Distal tubule Cortical collecting duct

Distal tubule Cortical collecting duct

- Ascending limb of Henle's loop

Medullary collecting duct

■ Figure 17.27 The reabsorption of Na+ and secretion of K+. In the distal tubule, K+ and H+ are secreted in response to the potential difference produced by the reabsorption of Na+. High concentrations of H+ may therefore decrease K+ secretion, and vice versa.

Aldosterone indirectly stimulates the secretion of H+, as well as K+, into the cortical collecting ducts. Therefore, abnormally high aldosterone secretion, as occurs in primary aldosteronism, or Conn's syndrome, results in both hypokalemia and metabolic alkalosis. Conversely, abnormally low aldosterone secretion, as occurs in Addison's disease, can produce hyperkalemia accompanied by metabolic acidosis.

If a person is suffering from potassium deprivation, according to recent evidence, the collecting duct may be able to partially compensate by reabsorbing some K+. This occurs in the outer medulla, and results in the reabsorption of some of the K+ that was secreted into the cortical collecting duct.

Fox: Human Physiology, 17. Physiology of the Text © The McGraw-Hill

Eighth Edition Kidneys Companies, 2003

Chapter Seventeen

ATPase ak+

Mcgraw Hill Acidosis

H2CO3

ATPase ak+

hpo4-2-

■ Figure 17.28 Acidification of the urine. This diagram summarizes how the urine becomes acidified and how bicarbonate is reabsorbed from the filtrate. It also depicts the buffering of the urine by phosphate and ammonium buffers. (CA = Carbonic anhydrase.) The inset depicts an expanded view of proximal tubule cells.

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