Medicine – Hormone Effects on the Kidney

The kidneys are both targets and producers of hormones. Hormones regulate renal handling of sodium, water, potassium, hydrogen ions, calcium and phosphate, while the kidneys themselves produce or activate substances such as renin, erythropoietin and calcitriol.

The major hormones shown in the original table are aldosterone, atrial natriuretic peptide, catecholamines, calcitriol, erythropoietin, prostaglandins, parathyroid hormone, vasopressin and renin.

1. Aldosterone

Aldosterone is a mineralocorticoid produced by the:

Zona glomerulosa of the adrenal cortex.

Its major renal actions occur mainly in the:

Late distal nephron and collecting duct.

Renal Effects of Aldosterone

Aldosterone increases:

Sodium reabsorption.

It also increases:

Potassium secretion

and

Hydrogen ion secretion.

Therefore:

ALDOSTERONE → ↑ Na⁺ REABSORPTION + ↑ K⁺ SECRETION + ↑ H⁺ SECRETION.

Water tends to follow retained sodium, helping expand:

Extracellular fluid volume.

Clinical Importance

Excess aldosterone can produce:

Hypertension.

Hypokalaemia.

Metabolic alkalosis.

Conversely, aldosterone deficiency or resistance may produce:

Hyperkalaemia

and

Metabolic acidosis.

2. Atrial Natriuretic Peptide

Atrial natriuretic peptide – ANP is released mainly from atrial cardiac myocytes in response to:

Atrial stretch and increased intravascular volume.

Its overall purpose is to reduce:

Sodium and fluid overload.

Renal Effects of ANP

ANP promotes:

Natriuresis → increased sodium excretion.

It also promotes:

Diuresis → increased water excretion.

Therefore:

ANP → ↑ Na⁺ EXCRETION + ↑ H₂O EXCRETION.

Additional Actions

ANP also opposes sodium-retaining systems by suppressing:

Renin.

Aldosterone.

It therefore acts broadly against the:

Renin–angiotensin–aldosterone system.

3. Catecholamines

Catecholamines such as:

Noradrenaline

and

Adrenaline

affect renal haemodynamics and renin release.

Sympathetic stimulation of:

β₁ receptors on juxtaglomerular cells

increases:

Renin secretion.

Therefore:

SYMPATHETIC β₁ STIMULATION → ↑ RENIN.

Additional Renal Effects

Strong sympathetic activation also causes:

Renal vasoconstriction

and can reduce:

Renal blood flow.

This becomes particularly important during:

Haemorrhage, severe hypotension and physiological stress.

4. 1,25-Dihydroxyvitamin D

1,25-Dihydroxyvitamin D, also called:

Calcitriol,

is the biologically active form of vitamin D.

The kidney converts:

25-hydroxyvitamin D

into:

1,25-dihydroxyvitamin D

through the enzyme:

1α-hydroxylase.

Main Effects of Calcitriol

The most important action of calcitriol is actually outside the kidney:

It increases intestinal calcium and phosphate absorption.

It also participates in calcium and phosphate homeostasis and has renal effects on mineral handling.

Therefore, the original table’s statement that calcitriol simply “increases tubular calcium reabsorption” is incomplete.

The major high-yield concept is:

KIDNEY ACTIVATES VITAMIN D → CALCITRIOL → ↑ INTESTINAL Ca²⁺ AND PHOSPHATE ABSORPTION.

5. Kidney Disease and Calcitriol

In advanced CKD, functioning renal mass and renal 1α-hydroxylase activity decline.

Therefore:

↓ Calcitriol production

↓

↓ Intestinal calcium absorption

↓

Tendency toward hypocalcaemic stimulation

↓

↑ PTH

↓

Secondary hyperparathyroidism.

This is an important mechanism in:

CKD-mineral and bone disorder.

6. Erythropoietin

Erythropoietin – EPO is produced predominantly by specialised:

Renal interstitial cells

in response to reduced tissue oxygen availability.

Effect of Erythropoietin

EPO travels to the:

Bone marrow

where it stimulates:

Erythropoiesis.

This increases production of:

Red blood cells.

Therefore:

RENAL HYPOXIA → ↑ EPO → BONE MARROW → ↑ RBC PRODUCTION.

7. Erythropoietin and CKD

In chronic kidney disease, the kidneys lose their ability to produce an appropriate amount of EPO.

This contributes to:

Anaemia of CKD.

The typical anaemia is:

Normocytic and normochromic.

Therefore:

CKD → RELATIVE EPO DEFICIENCY → ANAEMIA.

8. Prostaglandins

The kidneys produce prostaglandins, particularly:

PGE₂

and

PGI₂ – prostacyclin.

These have important local effects on:

Renal vascular tone.

Renal Effects of Prostaglandins

Renal prostaglandins promote:

Vasodilation, particularly helping preserve afferent arteriolar blood flow under physiological stress.

They therefore help maintain:

Renal blood flow

and

GFR

when vasoconstrictor systems are activated.

They can also facilitate:

Renin release.

9. Prostaglandins and NSAIDs

This explains an important clinical effect of:

NSAIDs.

NSAIDs inhibit:

Cyclo-oxygenase – COX

↓

reduce:

Prostaglandin synthesis

↓

reduce protective:

Afferent arteriolar vasodilation

↓

may decrease:

Renal blood flow and GFR.

Clinical Consequence

This is particularly dangerous in patients whose renal perfusion is already compromised, such as those with:

Dehydration.

Heart failure.

Advanced CKD.

Cirrhosis.

Therefore:

NSAID → ↓ PROSTAGLANDINS → AFFERENT CONSTRICTION → ↓ GFR → AKI RISK.

10. Parathyroid Hormone

Parathyroid hormone – PTH is produced by the:

Parathyroid glands

and has several important renal effects.

Its overall role is to increase:

Serum calcium

while reducing:

Serum phosphate.

11. PTH and Calcium

PTH increases renal:

Calcium reabsorption, particularly in the distal nephron.

Therefore:

Less calcium is lost in urine.

This contributes to the rise in:

Serum calcium.

12. PTH and Phosphate

PTH decreases proximal tubular:

Phosphate reabsorption.

Therefore more phosphate is excreted in urine.

This is called:

Phosphaturia.

Therefore:

PTH → ↑ PHOSPHATE EXCRETION.

13. PTH and Bicarbonate

PTH also decreases proximal tubular bicarbonate reabsorption to some extent.

Therefore it can increase:

Bicarbonate excretion.

This effect is less clinically emphasized than its actions on calcium and phosphate.

14. PTH and Vitamin D

PTH stimulates renal:

1α-hydroxylase.

This increases conversion of:

25-hydroxyvitamin D

to

1,25-dihydroxyvitamin D – calcitriol.

Therefore:

PTH → ↑ CALCITRIOL SYNTHESIS.

15. Overall Renal Effects of PTH

The easiest pattern to remember is:

PTH SAVES CALCIUM.

PTH WASTES PHOSPHATE.

PTH ACTIVATES VITAMIN D.

Therefore:

PTH → ↑ Ca²⁺ reabsorption + ↑ phosphate excretion + ↑ calcitriol synthesis.

16. Vasopressin

Vasopressin, also called:

Antidiuretic hormone – ADH,

is synthesised in the:

Hypothalamus

and released from the:

Posterior pituitary.

Its major renal function is regulation of:

Water balance.

17. Renal Effects of ADH

ADH binds to:

V₂ receptors

on principal cells in the collecting ducts.

This activates intracellular signalling that inserts:

Aquaporin-2 water channels

into the apical membrane.

Result

More water is reabsorbed from the collecting duct.

Therefore:

Urine volume decreases

and

Urine becomes more concentrated.

This is:

Antidiuresis.

Therefore:

ADH → V₂ → AQUAPORIN-2 → ↑ WATER REABSORPTION → ↓ URINE VOLUME.

18. ADH Deficiency

If ADH is deficient, as in:

Central diabetes insipidus,

the collecting ducts cannot appropriately concentrate urine.

This produces:

Large volumes of dilute urine.

Polyuria.

Polydipsia.

19. Excess ADH

Excessive ADH activity occurs in:

SIADH.

This causes excessive water retention and produces:

Dilutional hyponatraemia.

Therefore:

EXCESS ADH → WATER RETENTION → HYPONATRAEMIA.

20. Renin

Renin is an enzyme produced by:

Juxtaglomerular cells of the kidney.

Its secretion increases when the kidney detects reduced effective circulating volume or reduced renal perfusion.

21. Stimuli for Renin Release

Important stimuli include:

Reduced renal perfusion pressure.

Reduced NaCl delivery to the macula densa.

β₁ sympathetic stimulation.

Therefore:

LOW PERFUSION + LOW NaCl DELIVERY + β₁ STIMULATION → ↑ RENIN.

22. Renin–Angiotensin–Aldosterone System

Renin converts:

Angiotensinogen

to

Angiotensin I.

ACE then converts:

Angiotensin I

to

Angiotensin II.

Angiotensin II

Angiotensin II causes:

Vasoconstriction.

It preferentially constricts the:

Efferent arteriole at physiologically relevant levels.

It also stimulates:

Aldosterone release.

ADH release.

Thirst.

Proximal sodium reabsorption.

23. Overall RAAS Effect

The overall purpose of RAAS is to defend:

Blood pressure

and

Effective circulating volume.

The sequence is:

↓ Renal perfusion

↓

↑ Renin

↓

↑ Angiotensin II

↓

Vasoconstriction + ↑ aldosterone

↓

↑ Na⁺ and water retention

↓

↑ Blood pressure and circulating volume.

24. Important Correction – Renin

The original table states:

“Renin – autoregulation of renal blood flow.”

This is an oversimplification.

Renin is primarily the initiating enzyme of the:

Renin–angiotensin–aldosterone system.

It participates in the response to reduced renal perfusion, but classic renal autoregulation itself depends importantly on:

Myogenic mechanisms

and

Tubuloglomerular feedback.

Therefore the better high-yield statement is:

RENIN → ACTIVATES RAAS → REGULATES BLOOD PRESSURE, SODIUM BALANCE AND EFFECTIVE CIRCULATING VOLUME.

25. Aldosterone – Note Form

Source:

Adrenal cortex, zona glomerulosa.

Kidney effect:

↑ Na⁺ reabsorption.

↑ K⁺ secretion.

↑ H⁺ secretion.

Memory point:

ALDOSTERONE SAVES Na⁺, LOSES K⁺ AND H⁺.

26. ANP – Note Form

Source:

Atrial myocardium.

Stimulus:

Atrial stretch/volume expansion.

Kidney effect:

↑ Na⁺ excretion.

↑ Water excretion.

Suppresses renin/aldosterone.

Memory point:

ANP GETS RID OF SALT AND WATER.

27. Catecholamines – Note Form

Main renal effect:

β₁ sympathetic stimulation of juxtaglomerular cells.

↓

↑ Renin secretion.

Strong sympathetic activity also:

↓ Renal blood flow through vasoconstriction.

28. Calcitriol – Note Form

1,25-dihydroxyvitamin D = calcitriol.

Kidney activates vitamin D through:

1α-hydroxylase.

Major effect:

↑ Intestinal calcium and phosphate absorption.

CKD:

↓ Calcitriol → contributes to secondary hyperparathyroidism.

29. Erythropoietin – Note Form

Produced mainly by:

Kidney.

Stimulus:

Tissue hypoxia.

Effect:

↑ Bone marrow erythropoiesis.

CKD:

↓ Appropriate EPO production → anaemia.

30. Prostaglandins – Note Form

Effect:

Maintain renal perfusion through vasodilator effects, particularly at the afferent arteriole.

Can facilitate:

Renin release.

NSAIDs:

↓ Prostaglandins → ↓ afferent vasodilation → ↓ GFR → AKI risk.

31. PTH – Note Form

Renal effects:

↑ Calcium reabsorption.

↑ Phosphate excretion.

↑ Bicarbonate excretion to some degree.

↑ 1α-hydroxylase.

↑ Calcitriol synthesis.

Memory point:

PTH SAVES Ca²⁺, WASTES PHOSPHATE AND ACTIVATES VITAMIN D.

32. ADH – Note Form

ADH = vasopressin.

Acts on:

V₂ receptors in collecting ducts.

Causes:

Aquaporin-2 insertion.

Therefore:

↑ Water reabsorption.

↓ Urine volume.

↑ Urine concentration.

33. Renin – Note Form

Produced by:

Juxtaglomerular cells.

Stimulated by:

↓ Renal perfusion.

↓ NaCl delivery to macula densa.

β₁ sympathetic activity.

Effect:

Activates RAAS.

↓

↑ Angiotensin II.

↓

↑ Aldosterone.

↓

↑ Sodium/water retention and blood pressure.

Key Clinical Pattern

For rapid recall:

ALDOSTERONE → ↑ Na⁺ REABSORPTION + ↑ K⁺/H⁺ SECRETION.

ANP → ↑ Na⁺ + WATER EXCRETION.

CATECHOLAMINES → β₁ → ↑ RENIN.

CALCITRIOL → ↑ INTESTINAL Ca²⁺ + PHOSPHATE ABSORPTION.

ERYTHROPOIETIN → ↑ RBC PRODUCTION.

PROSTAGLANDINS → HELP MAINTAIN RENAL PERFUSION.

PTH → ↑ Ca²⁺ REABSORPTION + ↑ PHOSPHATE EXCRETION + ↑ CALCITRIOL.

ADH → ↑ WATER REABSORPTION VIA AQUAPORIN-2.

RENIN → ACTIVATES RAAS → ↑ BLOOD PRESSURE + Na⁺/WATER RETENTION.

A useful final memory sequence is:

ADH SAVES WATER.

ALDOSTERONE SAVES SODIUM.

PTH SAVES CALCIUM BUT WASTES PHOSPHATE.

ANP WASTES SODIUM AND WATER.

EPO MAKES RED CELLS.

RENIN ACTIVATES RAAS.