Difficulty: Advanced | Prerequisites: Parts 1 and 2 of these notes (kidney anatomy, nephron structure, filtration, reabsorption, secretion).
You now know how the nephron filters blood and reabsorbs useful substances. This final section explains how the kidney decides whether to produce concentrated or dilute urine, and how hormones orchestrate the process. It also covers the countercurrent mechanisms that maintain the medullary concentration gradient, the hormonal regulation of urine volume (ADH, aldosterone, ANH), and the anatomy and reflexes involved in getting urine out of the body. This is where physiology meets clinical medicine: drugs that treat hypertension (ACE inhibitors), heart failure (diuretics), and diabetes insipidus all target the mechanisms discussed here.
The kidneys can produce urine ranging from 65 to 1200 mOsm/kg, depending on body needs. Countercurrent mechanisms in the loop of Henle and vasa recta build and maintain a concentration gradient in the medulla. ADH controls how much water the collecting duct reabsorbs, while aldosterone regulates sodium reabsorption and the renin-angiotensin system links kidney function to blood pressure. Urine exits via the ureters, bladder, and urethra, with the micturition reflex controlling when you go.
Countercurrent mechanism
A system in which fluid in adjacent structures flows in opposite directions, allowing exchange of materials between them. In the kidney, it maintains the medullary concentration gradient.
Countercurrent multiplier
The mechanism operating in the nephron loop (loop of Henle) that builds the high solute concentration in the medullary interstitial fluid. It depends on active ion transport in the thick ascending limb and the impermeability of the ascending limb to water.
Countercurrent exchanger
The mechanism operating in the vasa recta that maintains the medullary concentration gradient. Blood flows in opposite directions through the descending and ascending vasa recta, exchanging water and solutes with the interstitial fluid without washing away the gradient.
Medullary concentration gradient
The progressive increase in solute concentration of the interstitial fluid from the cortex (about 300 mOsm/kg) to the tip of the medullary pyramids (about 1200 mOsm/kg). This gradient is essential for concentrating urine.
Urea cycling
The recycling of urea between the collecting ducts and the loop of Henle that contributes to the high osmolality in the medulla. Urea diffuses out of the collecting ducts into the interstitial fluid, enters the descending limb, and cycles back through. The ascending limb and distal tubule are impermeable to urea, so it accumulates in the medullary interstitial fluid.
Obligatory reabsorption
The constant, non-regulated reabsorption that occurs in the PCT and descending limb of the loop of Henle. It accounts for roughly 80% of filtrate volume and does not change with body conditions.
Facultative reabsorption
The variable, hormonally regulated reabsorption that occurs in the DCT and collecting ducts. This is how the body fine-tunes urine concentration and volume.
Antidiuretic hormone (ADH) / vasopressin
A hormone produced by hypothalamic neurons and stored in the posterior pituitary gland. It increases water reabsorption in the DCT and collecting ducts by promoting the insertion of aquaporin-2 water channels into the apical membrane. Think of ADH as the hormone that says "keep the water."
Aquaporins
Water channel proteins embedded in cell membranes. ADH causes aquaporin-2-containing vesicles to fuse with the apical membrane of DCT and collecting duct cells, making those cells permeable to water. Aquaporin-3 and aquaporin-4 are present in the basal membrane and allow water to exit the cell into the interstitial fluid.
Osmoreceptors
Specialised neurons in the hypothalamus that detect increases in the osmolality of interstitial fluid and trigger the release of ADH.
Baroreceptors
Pressure-sensing receptors in the atria of the heart and certain blood vessels that stimulate ADH release when blood pressure drops.
Diabetes insipidus
A condition caused by insufficient ADH secretion (or lack of kidney response to ADH), resulting in the production of large volumes of very dilute urine.
Renin-angiotensin-aldosterone system (RAAS)
A hormonal cascade initiated by the release of renin from juxtaglomerular cells when blood pressure drops. Renin converts angiotensinogen (from the liver) to angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme (ACE) in the lungs. Angiotensin II raises blood pressure directly (vasoconstriction) and indirectly (stimulating aldosterone, ADH, and thirst).
Renin
An enzyme secreted by juxtaglomerular cells in response to low blood pressure or low Na+ concentration in the filtrate (detected by the macula densa). It is the first step in the RAAS.
Angiotensinogen
A plasma protein produced by the liver that is the precursor to angiotensin I.
Angiotensin I
An inactive peptide formed when renin cleaves angiotensinogen.
Angiotensin-converting enzyme (ACE)
A proteolytic enzyme produced by the capillary endothelium of the lungs. It converts angiotensin I to the active angiotensin II.
Angiotensin II
A potent vasoconstrictor that also stimulates aldosterone secretion from the adrenal cortex, ADH release from the posterior pituitary, and the sensation of thirst and salt appetite. It is rapidly broken down, so its effects are short-lived.
Aldosterone
A steroid hormone secreted by the cortex of the adrenal glands. It acts on the DCT and collecting ducts to increase synthesis of Na+-K+ pumps and Na+ transport proteins, which increases sodium (and therefore water) reabsorption while increasing K+ secretion.
Atrial natriuretic hormone (ANH) / atrial natriuretic peptide (ANP)
A hormone produced by cells in the right atrium when the atrial wall is stretched by high blood volume. ANH lowers blood volume by inhibiting Na+ reabsorption, inhibiting ADH production, and increasing urine output.
Autoregulation of GFR
Intrinsic mechanisms that keep GFR relatively constant despite changes in systemic blood pressure. Includes the myogenic mechanism (afferent arteriole constricts in response to rising BP) and tubuloglomerular feedback (macula densa signals the afferent arteriole to constrict when filtrate flow increases).
Myogenic mechanism
A component of autoregulation in which the smooth muscle of the afferent arteriole constricts in response to increased stretch (from rising systemic blood pressure), preventing excessive increases in renal blood flow and GFR.
Tubuloglomerular feedback
A component of autoregulation in which an increased rate of filtrate flow past the macula densa triggers a signal to the juxtaglomerular cells, causing the afferent arteriole to constrict and reduce GFR back toward normal.
Detrusor muscle
The smooth muscle in the wall of the urinary bladder. Its contraction during the micturition reflex expels urine.
Trigone
A triangular area on the interior of the urinary bladder between the openings of the two ureters and the urethra. It expands less than the rest of the bladder during filling.
Internal urinary sphincter
In males, a ring of elastic connective tissue and smooth muscle at the neck of the bladder that prevents semen from entering the bladder during ejaculation. (Functionally less distinct in females.)
External urinary sphincter
A ring of skeletal muscle surrounding the urethra as it passes through the pelvic floor. It is under voluntary control and acts as a valve to prevent urination until consciously relaxed.
Micturition reflex
The reflex that controls urination. When the bladder wall is stretched, stretch receptors trigger parasympathetic signals that contract the detrusor muscle and decrease somatic motor signals to the external urethral sphincter, allowing it to relax.
The kidney's ability to produce urine that ranges from very dilute (65 mOsm/kg) to very concentrated (1200 mOsm/kg) depends on three things:
Countercurrent mechanisms (multiplier and exchanger)
The medullary concentration gradient
Hormonal regulation (primarily ADH and aldosterone)
The countercurrent multiplier operates in the nephron loop and is responsible for building the high solute concentration in the medullary interstitial fluid. It works because:
The thick ascending limb actively pumps Na+, K+, and Cl- out of the filtrate into the interstitial fluid.
Both the thin and thick portions of the ascending limb are impermeable to water, so water cannot follow the ions.
The descending limb is permeable to water, so water is drawn out by osmosis into the increasingly concentrated medullary interstitial fluid.
Ions are also actively transported out of the collecting ducts into the medullary interstitial fluid.
The net result: the interstitial fluid concentration increases progressively from about 300 mOsm/kg in the cortex to about 1200 mOsm/kg at the tips of the medullary pyramids.
The vasa recta supply blood to the medulla without destroying the concentration gradient. They achieve this because their walls are permeable to both water and solutes, and blood flows in opposite directions through the descending and ascending limbs:
As blood descends into the medulla, water moves out and solutes move in (blood becomes more concentrated).
As blood ascends back toward the cortex, water moves in and solutes move out (blood becomes more dilute again).
Slightly more water and solute are carried away from the medulla than are delivered to it, which removes the excess reabsorbed from the nephron without collapsing the gradient.
Urea contributes significantly to the high osmolality of the medullary interstitial fluid:
Collecting ducts are permeable to urea; some urea diffuses out into the medullary interstitial fluid.
The descending limbs of the loops of Henle are also permeable to urea, so urea re-enters the filtrate.
The ascending limbs and distal tubules are impermeable to urea, trapping it in the medullary interstitial fluid.
This continuous cycling maintains a high urea concentration in the medulla.
Autoregulation keeps GFR relatively stable across a range of systemic blood pressures:
Myogenic mechanism: when systemic BP rises, the afferent arteriole smooth muscle is stretched, causing it to constrict reflexively. This prevents excessive blood flow into the glomerulus.
Tubuloglomerular feedback: when filtrate flow rate increases (because GFR is too high), the macula densa cells detect the change and send a signal to the juxtaglomerular cells, causing the afferent arteriole to constrict.
In severe conditions such as haemorrhage or dehydration:
The sympathetic nervous system constricts small arteries and afferent arterioles, reducing renal blood flow and filtrate formation. This diverts blood to more immediately vital organs.
Renin is secreted from juxtaglomerular cells, leading to angiotensin II production, which stimulates vasoconstriction and helps maintain GFR through efferent arteriole constriction.
This cascade is triggered by low blood pressure:
Blood pressure drops. Juxtaglomerular cells detect reduced stretch of the afferent arteriole. The macula densa also detects low Na+ concentration in the filtrate.
Juxtaglomerular cells secrete renin into the blood.
Renin converts angiotensinogen (a plasma protein from the liver) to angiotensin I.
ACE in the lungs converts angiotensin I to angiotensin II.
Angiotensin II acts in several ways: it is a potent vasoconstrictor (raises BP directly), it stimulates aldosterone secretion from the adrenal cortex, it triggers the sensation of thirst and salt appetite, and it stimulates ADH release.
Aldosterone binds to nuclear receptors in cells of the DCT and collecting ducts, increasing synthesis of Na+-K+ pumps and Na+ transport proteins.
The Na+-K+ pump increases Na+ reabsorption (and therefore water reabsorption by osmosis) while simultaneously increasing K+ secretion.
Renin secretion decreases when blood pressure in the afferent arteriole normalises or when the Na+ concentration of the filtrate rises past the macula densa.
A large drop in blood Na+ can also directly stimulate aldosterone secretion from the adrenal cortex, though angiotensin II is the more important regulator.
ADH is produced by hypothalamic neurons and released from the posterior pituitary:
Osmoreceptors in the hypothalamus detect increased osmolality of interstitial fluid and trigger ADH release.
Baroreceptors in the atria and blood vessels also trigger ADH release when blood pressure drops.
ADH binds to receptors on the basal membrane of DCT and collecting duct cells.
This activates a G protein mechanism, which activates adenylate cyclase, increasing cAMP synthesis.
cAMP promotes the insertion of aquaporin-2 vesicles into the apical membrane, making the cells permeable to water.
Water moves by osmosis from the filtrate into the cells through aquaporin-2, then exits into the interstitial fluid through aquaporin-3 and aquaporin-4 in the basal membrane.
Result: small volume of concentrated urine.
When ADH is absent, the collecting duct is impermeable to water. Water stays in the tubule and a large volume of dilute urine is produced.
Insufficient ADH secretion causes diabetes insipidus, characterised by excessive dilute urine output and intense thirst.
ANH acts as a counterbalance to the RAAS when blood volume is too high:
Released by cells in the right atrium when the atrial wall is stretched beyond normal (indicating high blood volume).
ANH decreases blood volume by: inhibiting Na+ reabsorption (so less water follows), inhibiting ADH production, and increasing the volume of urine produced.
The resulting drop in blood volume reduces venous return, lowering the stretch on the right atrium and completing the negative feedback loop.
When blood volume increases above normal:
Control centres respond by decreasing ADH and aldosterone secretion, reducing water reabsorption.
Renal arteries dilate, increasing urine production.
The heart secretes ANH, further increasing urine output.
Blood volume falls, and homeostasis is restored.
The reverse occurs when blood volume drops below normal: ADH and aldosterone secretion increase, renal blood flow decreases, and the body conserves water.
Bring urine from the renal pelvis to the urinary bladder.
Lined with transitional epithelium.
Peristalsis moves urine along, occurring every few seconds to every 2 to 3 minutes.
Parasympathetic stimulation increases peristaltic frequency; sympathetic stimulation decreases it.
Ureters enter the bladder at an oblique angle through the trigone. Pressure in the filling bladder compresses the ureter opening, preventing backflow.
A hollow muscular container in the pelvic cavity, posterior to the symphysis pubis.
Lined with transitional epithelium; the muscular layer is the detrusor muscle.
The trigone is a triangular area between the two ureteral openings and the urethral opening. It expands less than the rest of the bladder during filling.
The bladder can stretch to hold about 1 litre, thanks to the folds in its wall, the elastic transitional epithelium, and the stretch capacity of the smooth muscle.
Internal urinary sphincter (males): elastic connective tissue and smooth muscle at the bladder neck; prevents semen entering the bladder during ejaculation.
External urinary sphincter: skeletal muscle surrounding the urethra as it passes through the pelvic floor. Under voluntary control.
Male urethra: extends from the bladder through the prostate (prostatic urethra), through the pelvic floor (membranous urethra), and through the penis (spongy urethra).
Female urethra: shorter, opening into the vestibule anterior to the vaginal opening. The shorter length makes females more prone to urinary tract infections (UTIs).
Stretch receptors in the bladder wall detect filling.
Parasympathetic signals cause the detrusor muscle to contract.
Somatic motor signals to the external urethral sphincter decrease, allowing it to relax.
The result is urination. Higher brain centres can override the reflex to delay micturition when socially appropriate.
Glomerulonephritis: inflammation of the filtration membrane; increases permeability so proteins and blood cells enter the filtrate, raising osmotic pressure and increasing urine volume. Acute form often follows bacterial infection (e.g. strep throat). Chronic form leads to progressive loss of kidney function.
Pyelonephritis: bacterial infection (often E. coli) of the renal pelvis that can spread and destroy nephrons, reducing the kidney's concentrating ability.
Acute renal failure: rapid, extensive kidney damage leading to accumulation of wastes. Can be fatal within 1 to 2 weeks if complete.
Chronic renal failure: permanent loss of so many nephrons that the remaining ones cannot maintain normal function. Causes include chronic glomerulonephritis, trauma, tumours, and kidney stones.
Medullary concentration gradient (approximate values):
Cortex: 300 mOsm/kg, increasing progressively through the medulla to 1200 mOsm/kg at the papillary tips.
RAAS cascade:
Low BP detected by JG cells / low Na+ detected by macula densa, then renin released, then angiotensinogen converted to angiotensin I, then ACE converts angiotensin I to angiotensin II, then angiotensin II causes vasoconstriction + aldosterone + ADH + thirst.
ADH signalling in tubule cells:
ADH binds receptor on basal membrane, then G protein activated, then adenylate cyclase activated, then cAMP increases, then aquaporin-2 vesicles inserted into apical membrane, then water crosses through aquaporin-2 (apical) and exits through aquaporin-3/4 (basal).
ACE inhibitors (e.g. lisinopril, enalapril) are among the most widely prescribed drugs for hypertension. They work by blocking the conversion of angiotensin I to angiotensin II, reducing vasoconstriction and aldosterone secretion. Understanding the RAAS explains exactly why these drugs lower blood pressure, and also why they can cause elevated potassium (because less aldosterone means less K+ secretion). Desmopressin, a synthetic analogue of ADH, is used to treat diabetes insipidus by restoring the water reabsorption that the patient's own ADH system cannot provide.
Students often think ADH causes the kidneys to "make less urine." More precisely, ADH makes the collecting duct permeable to water so that water is reabsorbed from the filtrate back into the blood. The filtrate volume was already established by filtration; ADH determines how much of the remaining water is reclaimed.
Students sometimes confuse the countercurrent multiplier (loop of Henle, builds the gradient) with the countercurrent exchanger (vasa recta, maintains the gradient). They are complementary but distinct.
Aldosterone is sometimes thought to directly reabsorb water. It does not. Aldosterone increases Na+ reabsorption, and water follows the sodium passively by osmosis.
The micturition reflex is sometimes assumed to be purely involuntary. While the reflex itself is autonomic (parasympathetic), the external urethral sphincter is skeletal muscle under voluntary control, which is why adults can delay urination.
Know the difference between the countercurrent multiplier (nephron loop, builds gradient) and countercurrent exchanger (vasa recta, maintains gradient).
Be able to trace the full RAAS pathway from low blood pressure through to aldosterone's effects on the DCT.
Understand the ADH mechanism at the cellular level: receptor, G protein, adenylate cyclase, cAMP, aquaporin-2 insertion.
Know what happens to urine volume and concentration in the presence vs. absence of ADH.
Be able to explain how ANH opposes the RAAS.
Know the two components of GFR autoregulation (myogenic mechanism and tubuloglomerular feedback).
Be able to describe the micturition reflex and identify which parts are voluntary vs. involuntary.
1. True or false: The countercurrent multiplier is located in the vasa recta. A: False. The countercurrent multiplier operates in the nephron loop (loop of Henle). The vasa recta house the countercurrent exchanger.
2. Fill in the blank: ADH increases water reabsorption by inserting ______ water channels into the apical membrane of DCT and collecting duct cells. A: Aquaporin-2.
3. True or false: Aldosterone acts on the DCT and collecting duct to increase potassium reabsorption. A: False. Aldosterone increases Na+ reabsorption and K+ secretion.
4. Fill in the blank: The enzyme that converts angiotensin I to angiotensin II is ______, located in the ______. A: Angiotensin-converting enzyme (ACE), located in the lungs.
5. True or false: The external urethral sphincter is smooth muscle under involuntary control. A: False. It is skeletal muscle under voluntary control.
Q: Explain how the countercurrent multiplier in the loop of Henle creates the medullary concentration gradient.
A: The thick ascending limb actively transports Na+, K+, and Cl- from the filtrate into the medullary interstitial fluid. Because the ascending limb is impermeable to water, water cannot follow these ions, so the interstitial fluid becomes increasingly concentrated. The descending limb, which is permeable to water, loses water by osmosis into this concentrated interstitial fluid. The opposing flow directions in the descending and ascending limbs amplify the concentration difference, resulting in a gradient from about 300 mOsm/kg at the cortex to 1200 mOsm/kg at the papillary tips.
Q: A patient has very low ADH levels. Describe the expected effect on urine volume and concentration.
A: Without ADH, the collecting ducts remain impermeable to water. Water cannot be reabsorbed from the filtrate, so the patient produces a large volume of very dilute urine. This condition is called diabetes insipidus.
Q: Trace the RAAS pathway from stimulus to effect on the nephron.
A: Low blood pressure (detected by reduced stretch in the afferent arteriole) or low Na+ in the filtrate (detected by the macula densa) triggers juxtaglomerular cells to release renin. Renin converts angiotensinogen to angiotensin I. ACE in the lungs converts angiotensin I to angiotensin II. Angiotensin II causes vasoconstriction (raising BP), stimulates aldosterone release from the adrenal cortex, and promotes ADH secretion and thirst. Aldosterone acts on the DCT and collecting ducts to increase Na+ reabsorption (and K+ secretion) by upregulating Na+-K+ pumps and Na+ transport proteins. Water follows the reabsorbed sodium by osmosis, increasing blood volume and pressure.
Q: How does ANH counteract the effects of high blood volume?
A: ANH is released from the right atrium when blood volume stretches the atrial wall. It inhibits Na+ reabsorption in the nephron (so water stays in the filtrate), inhibits ADH production (so the collecting ducts remain less permeable to water), and increases urine output. The net result is reduced blood volume.
Q: What is the difference between the internal and external urinary sphincters?
A: The internal urinary sphincter (prominent in males) is made of smooth muscle and elastic connective tissue at the bladder neck; it prevents semen from entering the bladder. The external urinary sphincter is skeletal muscle surrounding the urethra at the pelvic floor; it is under voluntary control and acts as a valve to delay urination.
The RAAS and ADH mechanisms connect directly to the endocrine system (hypothalamus, posterior pituitary, adrenal cortex) and cardiovascular regulation of blood pressure. The role of ANH ties into cardiac physiology and the body's response to volume overload, which becomes clinically important in heart failure. The acid-base regulation touched on here (H+ secretion) will be explored further in fluid and electrolyte balance (Chapter 27). The micturition reflex ties into the autonomic nervous system, with parasympathetic control of the detrusor and somatic control of the external sphincter.
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