L3 Renal Physiology 2

Overview

This lecture covers the processes the nephron uses to turn plasma into urine and where each occurs: filtration at the glomerulus, bulk reabsorption in the proximal tubule, medullary gradient generation in the loop of Henle, fine-tuning of electrolytes in the distal tubule, and fine-tuning of water reabsorption in the collecting duct. It works through the glomerular filtration barrier and the Starling forces that drive filtration, the quantitative language of renal function (GFR, clearance, filtration fraction, filtered load), the renal blood supply and its intrinsic (autoregulatory) and extrinsic control, and then the transport routes and segment-by-segment handling of solutes and water.

Nephron processes: the map

Different processes occur in different parts of the nephron:

• Glomerulus - filtration.
• Proximal tubule - bulk reabsorption.
• Loop of Henle - puts salt into the medulla to provide the gradient for water movement in the collecting duct.
• Distal tubule - fine-tuning of electrolytes.
• Collecting duct - fine-tuning of water reabsorption.

Filtration at the glomerulus is not particularly selective. About 180 litres per day are filtered across the glomeruli, but only about 1.5 litres per day of urine are produced, so reabsorption (with or without secretion) is essential. It occurs in the tubules and may be active or passive. All tubule segments reabsorb to varying degrees, and some segments also secrete.

Glomerular filtration

Glomerular filtration is a specialised form of ultrafiltration, with roughly 100 times the permeability of normal capillaries. It creates a plasma-like filtrate of the blood, with a molecular weight cut-off of 100,000, or 70,000 for negatively charged molecules. The glomerulus filters about 180 L/day, or 125 ml/minute.

What moves where:

• Filtered: plasma water and small solutes.
• Retained: blood cells and proteins.
• From: the lumen of the glomerular capillary.
• Via: the filtration barrier.
• To: Bowman’s capsule.

The filtration barrier

Three layers in series, each with a distinct selectivity mechanism:

1. Fenestrations of the endothelial cells of the glomerular capillary. A size barrier, 70 daltons.
2. Basement membrane, common to capillary and podocytes. Negatively charged, so it excludes proteins.
3. Filtration slits of the podocytes, between the foot processes. Size selective, via the slit diaphragm (nephrins).

What determines filtration

• The forces, that is the pressure gradient between the glomerular capillary and Bowman’s space.
• Permeability of the glomerular capillary.
• Surface area of the glomerular capillary.
• These last two are combined in $K_f$, the ultrafiltration coefficient, which is about 100 times greater than in systemic capillaries.

Starling forces at the glomerulus

$\text{Net filtration pressure}=\text{glomerular hydrostatic pressure}-\text{Bowman’s capsule pressure}-\text{glomerular oncotic pressure}$

With the lecture’s values:

• Glomerular hydrostatic pressure 60 mmHg (favours filtration).
• Glomerular colloid osmotic (oncotic) pressure 32 mmHg (opposes filtration).
• Bowman’s capsule pressure 18 mmHg (opposes filtration).
• Net filtration pressure = 60 − 18 − 32 = 10 mmHg.

Quantifying renal function

• GFR (glomerular filtration rate): the amount of fluid filtered across the glomerulus per unit time. Typically 180 L/day or 125 ml/min. Tightly regulated, both intrinsically and extrinsically, with some person-to-person variation. Normalised, 70 ml/min/m² body surface area (1.73 m²). Declines slowly from age 30.
• SNGFR (single-nephron GFR): 60 nl/min, or 90 µL/day.
• Renal plasma flow (RPF) = RBF × (1 − Hct). Estimated with PAH.
• Filtration fraction = GFR/RPF, about 20%.
• Filtered load = amount of a solute filtered per minute = GFR × plasma concentration (P).
• Clearance = UV/P, where P = plasma concentration, U = urine concentration, and V = urine production rate in ml/min. Clearance indicates how a substance is handled by the kidney and can be calculated for any substance.

Using clearance to estimate GFR

If a substance is filtered but neither reabsorbed nor secreted, then the amount filtered equals the amount appearing in urine:

1. GFR × plasma concentration = urine concentration × urine rate.
2. GFR × P = U × V.
3. GFR = U × V / P = clearance.

Creatinine is such a substance, so creatinine clearance is used to estimate GFR. Inulin is better, but it has to be injected.

Renal blood supply

• Receives 20-25% of cardiac output (brain 13%, heart 4%).
• 1-1.2 L/min, about 400 mL/100 g/min (brain about 50 ml/100 g/min, heart about 80 ml/100 g/min).
• 90% goes to the cortex, 1% to the inner medulla.
• The high flow is for filtration rather than for metabolism.

Route: renal arteries -> afferent arteriole -> glomerular capillaries -> efferent arteriole -> peritubular capillaries -> renal vein. The key structural point is that the glomerular capillaries sit between two arterioles, which gives high pressure for filtration and easy control of blood flow.

Pressure profile along the renal vessels: arteries about 93 mmHg, afferent arterioles about 90, glomerular capillaries about 60, efferent arterioles about 60 with a sharp drop after them, peritubular capillaries about 10, veins about 3. The steep drop across the efferent arteriole is what preserves high glomerular capillary pressure while leaving the peritubular capillaries at low pressure.

Arteriolar control of GFR

Four combinations, each acting through glomerular capillary pressure ($P_{GC}$):

• Constrict afferent arteriole -> decreased $P_{GC}$ -> decreased GFR.
• Constrict efferent arteriole -> increased $P_{GC}$ -> increased GFR.
• Dilate efferent arteriole -> decreased $P_{GC}$ -> decreased GFR.
• Dilate afferent arteriole -> increased $P_{GC}$ -> increased GFR.

Autoregulation (intrinsic control)

Renal blood flow is regulated so that filtration stays relatively constant despite variations in blood pressure. Plotted against mean arterial pressure, both RBF and GFR rise steeply from zero, then plateau over roughly 80-200 mmHg, and rise again above about 220 mmHg.

Mechanisms:

• Afferent and efferent arteriolar resistance is the effector.
• Myogenic: stretch of vascular smooth muscle causes it to contract.
• Tubulo-glomerular feedback, sensed by the macula densa:
  • Less NaCl in the distal tubule -> less NaCl enters macula densa cells -> afferent vasodilation -> increases GFR.
  • Angiotensin II enhances this, acting on the efferent arteriole (the most sensitive) to cause vasoconstriction -> increases GFR.
  • More NaCl in the distal tubule -> more NaCl enters macula densa cells -> afferent vasoconstriction -> decreases GFR (with or without less angiotensin II -> less efferent vasoconstriction -> decreases GFR).

Warning

The slide itself marks the mediator of the low-NaCl afferent response as uncertain, writing “??adenosine/renin”. The transcript carries this as a flag rather than resolving it.

The juxtaglomerular apparatus

The anatomical basis of tubulo-glomerular feedback. The afferent arteriole carries juxtaglomerular cells and an internal elastic lamina; the efferent arteriole carries smooth muscle fibres; the macula densa sits in the wall of the distal tubule where it contacts the glomerular vascular pole, adjacent to the glomerular epithelium of the capillary tuft.

The feedback loop in sequence

1. Decreased arterial pressure.
2. -> decreased glomerular hydrostatic pressure.
3. -> decreased GFR.
4. -> decreased macula densa NaCl (increased proximal NaCl reabsorption also feeds into this step).
5. Two branches follow: increased renin -> increased angiotensin II -> increased efferent arteriolar resistance; and decreased afferent arteriolar resistance.
6. Both branches feed back negatively to raise glomerular hydrostatic pressure.

Extrinsic control of renal blood flow

Operates outside the range or capacity of autoregulation, for example with systemic vascular alterations such as cold or bleeding. Mediators listed: sympathetic innervation, angiotensin II, prostaglandins, NO, endothelin, bradykinins, natriuretic peptides, glucocorticoids, adenosine, ATP, and arteriolar endothelial cells.

• Sympathetic innervation: acts on afferent and efferent arterioles, but is minimal when ECF volume is normal. α1-mediated, mainly afferent. Reduces RBF and GFR.
• Angiotensin II: acts systemically and locally. Constricts both afferent and efferent arterioles, with the efferent more sensitive, giving a “low dose” efferent-predominant effect.
• Prostaglandins: limited role in the healthy person at rest. With volume depletion, stress, sympathetic activity or angiotensin II, prostaglandins are produced locally, modulate the sympathetic and angiotensin II inputs by attenuating vasoconstriction, and so prevent renal ischaemia. Important in maintaining RBF with age.

Important

NSAIDs inhibit prostaglandins and therefore cause renal ischaemia, particularly where prostaglandin-mediated protection is being relied upon.

How solutes are handled: reabsorption and secretion

• Reabsorbed only (not secreted): glucose, water, Na⁺, Cl⁻, PO₄³⁻, Ca²⁺. Illustrated with glucose: 98% reabsorbed in the proximal tubule, 2% in the remaining segments, and 0% of the filtered load excreted in urine.
• Secreted only (not reabsorbed): organic cations, including monoamines such as dopamine and histamine and drugs such as morphine; and organic anions, including endogenous compounds such as bile salts and drugs such as penicillin. A variable percentage is secreted, mainly in the proximal tubule, so more than 100% of the filtered load can remain by the inner medullary collecting duct.
• Both secreted and reabsorbed: K⁺, NH₃, H⁺, HCO₃⁻ (secretion/absorption regulated according to homeostatic requirements) and urea (secretion/absorption not regulated).

Nephron structure and tubular epithelia

Two nephron types drain into a shared collecting duct: the juxtamedullary nephron, with a long loop of Henle reaching the inner medulla (glomerulus, proximal tubule, thin descending limb, thick ascending limb, distal tubule), and the cortical nephron, with a short loop of Henle.

Reabsorption and secretion occur across the tubular epithelium. There is a variety of epithelial types; cells are held together by tight junctions, and microvilli increase surface area. The epithelium is polarised: the apical side faces the tubule lumen (the “urine side”) and carries the brush border of microvilli; the basolateral side faces the interstitium (the “blood side”), with lateral interstitial spaces (LIS) between adjacent cells. Tight junctions sit between cells near the apical side.

Two routes of transport

• Paracellular (between cells): a single barrier, connecting tubule lumen to the LIS. No requirement for transport proteins, so limited selectivity. Permeability depends on the “tightness” of the tight junctions. “Leaky” epithelium gives bulk reabsorption, as in the proximal tubule.
• Transcellular (through cells): two barriers, the apical and basolateral membranes, connecting tubule lumen to the LIS or peritubular space. Usually involves membrane transport proteins, so highly selective, requiring channels and energy, and open to hormonal control.

Proximal tubule

The site where most reabsorption occurs, that is bulk reabsorption:

• 65% of sodium, water and chloride.
• All of the filtered glucose.
• Almost all of the filtered amino acids.
• Most of the K⁺ (90%), PO₄ and Ca.
• Half of the urea.

It also secretes organic acids, drugs and H⁺.

Mechanism

Paracellular movement is isosmotic. Transcellular reabsorption is driven by Na⁺ reabsorption:

1. Na⁺ moves into the cell down its concentration gradient.
2. The Na⁺-K⁺-ATPase pump keeps intracellular Na⁺ concentration low so that Na⁺ can keep moving into the cell, which is where the energy cost sits.
3. Many solutes are coupled to Na⁺ absorption, for example glucose and amino acids, that is secondary active transport.

Transporters shown for the proximal tubule cell:

• Apical (lumen-facing): Na⁺/glucose cotransport, Na⁺/amino acid cotransport, Na⁺/H⁺ exchange (with formic acid/formate cycling), Cl⁻ channel.
• Basolateral (interstitium-facing): facilitated transporters for glucose and amino acids, Na⁺/3HCO₃⁻ cotransport, a Cl⁻ channel (marked uncertain on the figure), a K⁺ channel, and the Na⁺-K⁺-ATPase pump moving 3 Na⁺ out and K⁺ in.
• Paracellular: Na⁺, K⁺, Cl⁻ and H₂O also move between cells.

Glucose handling

At normal filtered loads all glucose is reabsorbed and none appears in urine. At high plasma glucose, for example in diabetes mellitus, the filtered load exceeds the cell’s reabsorptive capacity and glucose appears in urine. On the plot of renal glucose handling against plasma glucose: the filtered line rises linearly throughout; the reabsorbed line rises linearly then plateaus at the transport maximum ($T_m$, about 1.9 mmol/min) at a plasma glucose of about 20 mmol/L; the excreted line stays at zero until plasma glucose exceeds the threshold of about 10 mmol/L, then rises.

Bicarbonate handling

The filtered HCO₃⁻ ion is not actually reabsorbed. Instead a new HCO₃⁻ is synthesised in the tubule cell to replace the filtered ion, with H⁺ secreted at the same time. The steps:

1. In the lumen, filtered HCO₃⁻ combines with secreted H⁺ and, via carbonic anhydrase, forms CO₂ + H₂O.
2. CO₂ and H₂O diffuse into the cell.
3. Inside the cell, carbonic anhydrase reforms H⁺ + HCO₃⁻.
4. The new HCO₃⁻ exits across the basolateral membrane on the 3HCO₃⁻/Na⁺ cotransporter into the interstitial fluid, alongside the Na⁺-K⁺-ATPase pump.

Loop of Henle

• The thick ascending limb is water impermeable.
• It performs energy-dependent removal of NaCl (via the NaK2Cl channel) and urea from the filtrate.
• This makes the medullary interstitium hyperosmotic, generating the hyperosmotic medullary gradient (HOMG).
• It leaves the filtrate very dilute.
• Water is then reabsorbed in the collecting duct as required.

Transport pathways in the thick ascending limb cell:

• Apical: Na⁺/K⁺/2Cl⁻ cotransporter bringing Na⁺, K⁺ and 2Cl⁻ into the cell, plus a Na⁺ channel/leak.
• Basolateral: Cl⁻ channel, K⁺ channel, and the Na⁺-K⁺-ATPase pump moving Na⁺ out and K⁺ in.

Distal tubule and collecting duct

These segments fine-tune electrolytes, pH and water:

• Reabsorb most of the remaining NaCl (10%) and water (15%).
• Secrete K⁺ and H⁺.
• Under hormonal control: sodium reabsorption by aldosterone, water reabsorption by ADH.

Daily handling of Na⁺ and water

Following the numbers along the nephron, with medullary osmolarity rising from 285 to 1400 mosmol/L:

1. Filtered: 180 L H₂O and 27,000 mmol Na⁺.
2. Proximal tubule (cortex, osmolarity 285) reabsorbs 120 L and 18,000 mmol Na⁺.
3. Loop of Henle reabsorbs NaCl and urea into the medulla, leaving 30 L of water.
4. Distal tubule and collecting duct handle 1200 mmol/15 L in the cortex and 600 mmol in the medulla, both segments influenced by ADH and aldosterone, with up to 14.5 L reabsorbed.
5. Excreted: 0.5-30 L H₂O and 200 mmol Na⁺.

Self-test

1. State which nephron segment performs each of filtration, bulk reabsorption, generation of the medullary gradient, electrolyte fine-tuning and water fine-tuning.
2. List the three layers of the glomerular filtration barrier and give the selectivity mechanism of each.
3. Calculate the net filtration pressure from a glomerular hydrostatic pressure of 60 mmHg, a Bowman’s capsule pressure of 18 mmHg and a glomerular oncotic pressure of 32 mmHg, writing out the equation.
4. Explain what $K_f$ represents and how it compares with systemic capillaries.
5. Define GFR and give its typical value in both L/day and ml/min.
6. Derive GFR = U × V / P from first principles, stating the assumption the substance must satisfy.
7. Explain why creatinine is used clinically to estimate GFR rather than inulin.
8. Define filtration fraction and give its approximate value, and state how renal plasma flow is calculated from renal blood flow.
9. State the proportion of cardiac output the kidney receives and explain why renal blood flow is so high relative to organ mass.
10. Predict the effect on glomerular capillary pressure and GFR of constricting the efferent arteriole, and of constricting the afferent arteriole.
11. Describe the steps of tubulo-glomerular feedback when NaCl delivery to the distal tubule falls.
12. Describe, in order, the feedback sequence triggered by a fall in arterial pressure, from the pressure drop through to the correction of glomerular hydrostatic pressure.
13. Distinguish the actions of sympathetic innervation and angiotensin II on the renal arterioles.
14. Explain why NSAIDs can precipitate renal ischaemia, and name two situations in which the risk is greatest.
15. Distinguish the paracellular from the transcellular route of epithelial transport on barriers, selectivity and control.
16. List the solutes that are only reabsorbed, only secreted, and both secreted and reabsorbed, giving an example of a drug handled by secretion.
17. Describe how sodium reabsorption in the proximal tubule drives the reabsorption of glucose and amino acids, and say where the energy is spent.
18. Explain why glucose appears in the urine in diabetes mellitus, referring to the transport maximum and the threshold value.
19. Describe the steps by which the proximal tubule handles filtered bicarbonate, and explain why it is not strictly reabsorption.
20. Explain how the properties of the thick ascending limb generate the hyperosmotic medullary gradient and why this matters for the collecting duct.
21. Name the hormone controlling sodium reabsorption and the hormone controlling water reabsorption in the distal nephron, and state the fraction of filtered NaCl and water handled there.
22. Integrative: trace 180 L of filtrate through the nephron to a final urine output of about 1.5 L, naming at each segment the volume handled and the mechanism responsible.

Answers

Reveal answers

1. Glomerulus filtration; proximal tubule bulk reabsorption; loop of Henle puts salt into the medulla to provide the gradient for water movement in the collecting duct; distal tubule fine-tuning of electrolytes; collecting duct fine-tuning of water reabsorption.
2. (i) Fenestrations of the glomerular capillary endothelial cells, a size barrier of 70 daltons; (ii) basement membrane, common to capillary and podocytes, negatively charged so excludes proteins; (iii) filtration slits between podocyte foot processes, size selective via the diaphragm (nephrins).
3. Net filtration pressure = glomerular hydrostatic pressure − Bowman’s capsule pressure − glomerular oncotic pressure = 60 − 18 − 32 = 10 mmHg.
4. $K_f$ is the ultrafiltration coefficient, combining the permeability and the surface area of the glomerular capillary. It is about 100 times greater than in systemic capillaries.
5. GFR is the amount of fluid filtered across the glomerulus per unit time, typically 180 L/day or 125 ml/min.
6. If a substance is filtered but neither reabsorbed nor secreted, the amount filtered equals the amount in the urine, so GFR × P = U × V, therefore GFR = U × V / P, which is the clearance of that substance.
7. Creatinine is filtered but not reabsorbed or secreted, so its clearance estimates GFR, and unlike inulin it does not have to be injected. Inulin is the better marker but requires injection.
8. Filtration fraction = GFR/RPF, about 20%. Renal plasma flow = RBF × (1 − haematocrit), and can be estimated with PAH.
9. 20-25% of cardiac output, 1-1.2 L/min or about 400 mL/100 g/min. The high flow serves filtration rather than metabolism.
10. Constricting the efferent arteriole increases glomerular capillary pressure and increases GFR. Constricting the afferent arteriole decreases glomerular capillary pressure and decreases GFR.
11. Less NaCl in the distal tubule means less NaCl enters the macula densa cells, which produces afferent vasodilation and increases GFR. Angiotensin II enhances the response by constricting the efferent arteriole, the more sensitive vessel, further increasing GFR. The slide marks the mediator of the afferent limb as uncertain (“??adenosine/renin”).
12. Decreased arterial pressure -> decreased glomerular hydrostatic pressure -> decreased GFR -> decreased macula densa NaCl (also fed by increased proximal NaCl reabsorption) -> either increased renin -> increased angiotensin II -> increased efferent arteriolar resistance, or decreased afferent arteriolar resistance; both branches feed back negatively to raise glomerular hydrostatic pressure.
13. Sympathetic innervation acts on both arterioles but mainly the afferent via α1 receptors, is minimal when ECF volume is normal, and reduces RBF and GFR. Angiotensin II acts systemically and locally, constricting both afferent and efferent arterioles, with the efferent more sensitive, giving a low-dose efferent-predominant effect.
14. NSAIDs inhibit prostaglandins, which normally attenuate sympathetic and angiotensin II vasoconstriction and so prevent renal ischaemia. Risk is greatest in volume depletion or stress (states with high sympathetic and angiotensin II drive), and with increasing age, where prostaglandins are important in maintaining RBF.
15. Paracellular: a single barrier between cells connecting lumen and lateral interstitial space, no transport proteins needed, limited selectivity, permeability set by the tightness of the tight junctions, gives bulk reabsorption in leaky epithelium such as the proximal tubule. Transcellular: two barriers (apical and basolateral membranes) connecting lumen to LIS or peritubular space, usually via membrane transport proteins, therefore highly selective, energy and channel dependent, and open to hormonal control.
16. Reabsorbed only: glucose, water, Na⁺, Cl⁻, PO₄³⁻, Ca²⁺. Secreted only: organic cations (monoamines such as dopamine and histamine, drugs such as morphine) and organic anions (bile salts, drugs such as penicillin). Both: K⁺, NH₃, H⁺, HCO₃⁻ (regulated according to homeostatic requirements) and urea (not regulated). Morphine or penicillin are the drug examples.
17. Na⁺ moves into the cell down its concentration gradient; the Na⁺-K⁺-ATPase on the basolateral membrane keeps intracellular Na⁺ low so the gradient persists, and that pump is where the energy is spent. Glucose and amino acids are cotransported with Na⁺ across the apical membrane, that is secondary active transport, and leave basolaterally by facilitated transporters.
18. At normal filtered loads all glucose is reabsorbed. In diabetes mellitus plasma glucose is high, so the filtered load exceeds the cell’s reabsorptive capacity. Reabsorption plateaus at the transport maximum of about 1.9 mmol/min (reached at a plasma glucose of about 20 mmol/L), and excretion begins once plasma glucose passes the threshold of about 10 mmol/L.
19. Filtered HCO₃⁻ combines with secreted H⁺ in the lumen and, via carbonic anhydrase, forms CO₂ + H₂O; these diffuse into the cell; carbonic anhydrase reforms H⁺ + HCO₃⁻ inside the cell; the new HCO₃⁻ leaves basolaterally on the 3HCO₃⁻/Na⁺ cotransporter. It is not true reabsorption because the filtered ion itself is not recovered, a newly synthesised HCO₃⁻ replaces it, with H⁺ secreted at the same time.
20. The thick ascending limb is water impermeable and removes NaCl (via the NaK2Cl cotransporter, energy dependent through the basolateral Na⁺-K⁺-ATPase) and urea from the filtrate. Salt therefore accumulates in the medullary interstitium without water following, making it hyperosmotic (the hyperosmotic medullary gradient) and leaving the filtrate very dilute. That gradient is the driving force for water reabsorption in the collecting duct as required.
21. Aldosterone controls sodium reabsorption and ADH controls water reabsorption. These segments reabsorb most of the remaining NaCl (10%) and water (15%), and secrete K⁺ and H⁺.
22. 180 L H₂O and 27,000 mmol Na⁺ are filtered at the glomerulus by ultrafiltration under a net filtration pressure of about 10 mmHg. The proximal tubule bulk reabsorbs 120 L and 18,000 mmol Na⁺ isosmotically, driven by Na⁺-coupled transcellular transport and leaky paracellular flow. The loop of Henle reabsorbs NaCl and urea into the medulla via the water-impermeable thick ascending limb, leaving about 30 L of dilute filtrate and a medullary osmolarity up to 1400 mosmol/L. The distal tubule and collecting duct fine-tune, handling 1200 mmol/15 L in the cortex and 600 mmol in the medulla under aldosterone and ADH control, reabsorbing up to 14.5 L. Final excretion is 0.5-30 L H₂O and 200 mmol Na⁺, in practice about 1.5 L/day of urine.