• Typical plasma glucose concentration is between 70-100 milligrams/deciliters.
• Glucose is completely reabsorbed from the proximal tubule via secondary active transport mechanisms as long as plasma glucose concentrations do not exceed this concentration.

REABSORPTION

• Sodium-potassium ATPase extrudes sodium from the cell in exchange for potassium.
  – This exchange creates the electrochemical gradient that drives the SGLT co-transportation of sodium and glucose into the cell.
• As sodium moves down its concentration gradient, glucose moves against its concentration gradient.
• From here, GLUT transporters facilitate glucose diffusion out of the cell to return it to the blood (again, there are multiple types of GLUT transporters, but here we’ll simply generalize).
  – Within the healthy physiological glucose range, these transporters can completely reabsorb glucose from the proximal tubule; however, as we’ll see, at higher plasma concentrations, the transporters are overwhelmed, glucose is incompletely reabsorbed, and thus is excreted in the urine.

GLUCOSE TITRATION CURVE

Explains relationships between glucose plasma concentration, reabsorption, and excretion.

• Filtered load of glucose increases in proportion to plasma glucose levels; this makes sense, because we know that glucose is freely filtered within the renal corpuscle. (A)
• Glucose reabsorption follows filtration until plasma glucose concentration reaches approximately 200 milligrams per deciliter. (B)
  – At that point, the glucose reabsorption curve begins to bend because glucose transporters are approaching saturation.
• At plasma glucose concentrations above 350 milligrams per deciliter, glucose reabsorption plateaus as the transporters reach full saturation. (D)
• Glucose excretion remains near zero until glucose plasma concentration rises above 200 milligrams per deciliter (C)
  – Once this threshold is surpassed, glucose begins to appear in the urine (aka, glucosuria).
• Then, when plasma glucose concentration rises above of 350-400 milligrams per deciliter, excretion rises in parallel with filtered load. (E)

Summary of some key points in the glucose titration curve.

• Filtered load increases in proportion to plasma glucose concentrations (even when outside of the typical physiological range).
• Reabsorption matches filtered load when plasma glucose concentrations remain below 200 milligrams per deciliter.
• Above this threshold, glucose begins to appear in the urine (aka, glucosuria).
• Once plasma glucose concentration rises above 350 milligrams per deciliter, all glucose transporters are saturated.
• Transport maximum (Tm) is reached, and glucose reabsorption plateaus.
• Once transport maximum is reached, excretion rate rises linearly with filtered load.
• Because of a phenomenon called splay, threshold occurs before Tm because of variation in the transport maximum of individual nephrons due to differences in transport number and types.

Clinical causes of glucosuria.

• Glucosuria occurs when the filtered load of glucose exceeds the resorptive capabilities, and glucose is excreted in the urine.
• Diabetes mellitus, the body’s inability to make and/or use insulin results in excessive plasma glucose concentrations, which increases its filtered load.
• Some women experience pregnancy-related glucosuria when increased GFR increases filtered load. This is often benign, and is not synonymous with gestational diabetes.

• Key function of the kidneys is to ensure sodium balance:
  Sodium intake = sodium excretion.

Sodium Reabsorption by Segment:

• 67% of filtered load is reabsorbed in proximal tubule.
• 25% in thick ascending limb
• 5% in the early distal tubule
• 3% in the late distal tubule and collecting duct.
• Less than 1% of the filtered load is excreted in the urine.

Load-dependent Sodium Reabsorption

• In the distal segments, ensures that reabsorption rate remains relatively constant despite changes in filtered load.

Thick ascending limb:

• Sodium reabsorption is linked to potassium and chloride reabsorption.
• Sodium-potassium ATPase on basolateral membrane pumps sodium out of cell, potassium into it.
• Drives cotransport of sodium, potassium, and chloride into the cell.
  In presence of ADH, activity of cotransporter is increased;
  In presence of Loop diuretics, chloride-binding site is blocked, cotransport ceases, and sodium is not reabsorbed.
• Chloride and potassium diffuse out of the cell through the basolateral membrane;
  some potassium “leaks” back into the lumen.
• Thick ascending limb is impermeable to water.

Early distal tubule:

• Sodium and chloride reabsorption are linked.
• Sodium-potassium ATPase creates electrochemical gradient that drives cotransport of sodium and chloride from lumen into cell.
  – Thiazide diuretics block chloride binding site on contransporter, so sodium is not reabsorbed.
• Chloride exits via simple diffusion.
• Early distal tubule is impermeable to water.

Late distal tubule and collecting duct:

• Principal cells link sodium reabsorption to potassium secretion.
• Sodium-potassium ATPase creates electrochemical gradient that drives sodium diffusion via epithelial sodium channels.
• Potassium is secreted into lumen.
• Aldosterone increases sodium reabsorption and potassium secretion.
• Late distal tubule and collecting duct are only permeable to water in presence of ADH, which increases aquaporin-2 water channels.
• Water exits cell via aquaporin 3 and 4 water channels.
  – K-sparing diuretics act on late distal tubule and collecting ducts to reduce sodium reabsorption and potassium secretion.

• Sodium reabsorption in the proximal nephron tubule is coupledwith reabsorption of other key solutes and water, and with secretion of hydrogen.
• Sodium reabsorption maintains sodium balance, so that sodium intake equals sodium excretion.
  – This is one of the most important functions of the kidney because:
  As the major cation of the extracellular fluid, the amount of sodium determines extracellular fluid volume (because water follows osmotic gradients), and extracellular fluid volume determines plasma volume, blood volume, and blood pressure, which are critical physiological determinants.
• Two-thirds of sodium reabsorption occurs within the proximal tubule, which comprises both early and late segments.

Early proximal tubule:

• Sodium reabsorption is linked to reabsorption of nutrients, inorganic and organic acids, and the secretion of hydrogen.
• Transport occurs via two pathways:
  – The transcellular pathway, which transports substances through tubule cells;
  – The paracellular pathway, which moves substances through “leaky” tight junctions between tubule cells.
• Sodium-potassium ATPase (aka, pump) actively pumps sodium out of the tubule cell, and brings potassium into it.
  – This exchange creates the electrochemical gradient that drives secondary transport of sodium and other solutes across the luminal membrane and into the tubule cell.
• Luminal membrane cotransporters couple movement of sodium into the cell with movement of other solutes, including:
  – Glucose
  – Amino acids
  – Inorganic and organic acids.
• These solutes move out of the cell via facilitated diffusion.
• As a general rule, water follows sodium:
  – Transcellularly, it passes through aquaporin 1 channelslocated on the luminal and basolateral membranes;
  – Paracellularly, it passes between the tight junctions of tubule cells.
• Countertransporter exchanges sodium for hydrogen, which it secretes into the tubular lumen.
• Reactions within the cell produce bicarbonate, which is reabsorbed via facilitated diffusion.

Late Proximal Tubule:

• Sodium and chloride reabsorption are linked in the late proximal tubule.
  – To understand why this is so, consider that most other solutes were reabsorbed within the early proximal tubule; chloride is not, and, therefore, remains in the tubule fluid as it enters the late proximal tubule.
• Sodium-potassium pump creates gradient that drives luminal membrane counter transporters:
  – One moves sodium into the cell and hydrogen into the lumen.
  – The other moves chloride into the cell and formate into the lumen (formate ions are metabolic byproducts).
• Chloride then passes through the basolateral membrane down its concentration gradient via simple diffusion.
• Both sodium and chloride pass through “leaky” tight junctions between the tubule cells; this is another example of the paracellular pathway.

Glomerulotubular balance

• Maintains a nearly constant rate of sodium reabsorption, despite changes in GFR.
• In response to changes in GFR, the proximal tubule alters the total amount of sodium it reabsorbs so that the rate of sodium reabsorption is held at ~67%.
• However, glomerulotubular balance changes in response to changes in extracellular fluid volume:
  – When ECF volume contracts, total sodium and water reabsorption is increased; this reflects the body’s attempt to increase ECF volume.
  – When ECF volume expands, total sodium and water reabsorption is decreased; excess sodium and water are excreted in the urine in attempt to decrease ECF volume.

Reabsorption

• Removes solutes and water from the tubular fluid and returns them to the blood;
• It reclaims much of the water, ions, and nearly all of the nutrients that are filtered.

Secretion

• Moves solutes from the blood and nephron tubule cells into the tubular fluid;
• Secretion is important for removal of substances that aren’t filtered (such as drugs and metabolites) and for fine-tuning the final urine composition.

Key principles of transport:

• Vasculature:
  – Efferent arteriole leaves glomerulus, gives rise to peritubular capillaries.
  – Peritubular capillaries give rise to vasa recta of juxtamedullary nephrons.
  – Vasa recta drains deoxygenated blood into the interlobular vien.

Reabsorption and Secretion by Segment:

• Proximal tubule:
  Reabsorbs:
  Water
  Sodium
  Chloride
  Potassium
  Calcium
  Phosphate
  Urea
  Bicarbonate
  Glucose, amino acids, and other nutrients.
  Secretes:
  Hydrogen
  PAH (para-aminohippurate)
  Ammonium ions
  Certain drugs
  Organic acids and bases (such as creatinine)

• Nephron loop:
  Concentrates or dilutes urine.
  Thin limb:
  Water is reabsorbed.
  Thick ascending limb:
  Sodium
  Potassium
  Chloride
  Calcium
  Bicarbonate
  Magnesium
  (no water reabsorption)

• Distal tubule
  Reabsorption and secretion are hormonally regulated to fine-tune tubular fluid, to maintain ECF volume and osmolarity homeostasis.
  Early distal tubule, aka, diluting segment:
  Sodium
  Chloride
  Potassium
  Calcium
  (no water reabsorption).
  Late distal tubule:
  Reabsorbs:
  Water
  Sodium
  Chloride
  Bicarbonate
  Urea.
  Secretes:
  Potassium.

• Collecting duct:
  Secretes OR Reabsorbs
  Potassium
  Hydrogen
  Bicarbonate
  Ammonium ions

Filtration

• The first step in urine formation is filtration, in which water and solutes, including ions and nutrients, are filtered from the blood to produce ultrafiltrate.
• Filtration is a passive process, and relies on pressures within the renal blood vessels and nephron.
• Ultrafiltrate passes through the nephron tubule as tubular fluid.

Reabsorption

• Removes solutes and water from the tubular fluid and returns them to the blood;
• It reclaims much much of the water, ions, and nearly all of the nutrients that are filtered are reclaimed via reabsorption.

Secretion

• Moves solutes from the blood and nephron tubule cells into the tubular fluid;
• Secretion is important for removal of substances that aren’t filtered (such as drugs and metabolites) and for fine-tuning the final urine composition.

Nephron segments

• Renal corpuscle is where the blood is filtered and tubular fluid is formed. .
• Proximal tubule is the primary site of reabsorption of water, ions, and nutrients.
  Specifically: sodium, potassium, water, and nutrients are reabsorbed from the proximal tubule; hydrogen, which plays a major role in acid/base balance, is secreted into the proximal tubule.
• The nephron loops is the U-shaped segment:
  It comprises descending and ascending limbs.
  Water is reabsorbed from the descending limb; solutes are reabsorbed from the ascending limb. .
  This is where urine is either diluted or concentrated, depending on the body’s needs.
• Distal tubule is the distal portion of the nephron.
• The collecting duct delivers urine to the renal pelvis.
  In these last two segments, reabsorption and secretion are hormonally regulated to maintain water and ion homeostasis.

Specifically:

• Sodium and water are reabsorbed from the distal tubule.
• Hydrogen and potassium are secreted into it.
• Sodium and water are reabsorbed from the collecting duct;
• Potassium and hydrogen are either reabsorbed or secreted, depending on the body’s needs. It is also a primary site of secretion of substances that weren’t filtered but need to be excreted in the urine.
• The nephron loop either dilutes or concentrates the tubular fluid, depending on the body’s needs.
• Activity of the the distal tubule and collecting duct is hormonally regulated to maintain ECF volume and osmolarity homeostasis.

Key ways that reabsorption and secretion are regulated:

• First, it’s important to recognize that, in general, water and other solutes follow sodium: when sodium is reabsorbed, they usually are, too (the exception is the ascending limb of the nephron loop).
• Therefore, altering sodium reabsorption is an effective way to alter the reabsorption of other solutes and water.
• This may be necessary, for example, in the case of high blood pressure:
• To help reduce blood volume, diuretics reduce sodium reabsorption to facilitate increased excretion of solutes and water.
• On the other hand, in cases of low blood pressure, the body needs to conserve solutes and water.
• Two key hormones that facilitate this are:
• Aldosterone increases reabsorption of sodium within the distal nephron; since water follows sodium, it also increases blood volume.
• Anti-diuretic hormone directly increases the rate of water absorption in the distal nephron, and, therefore, blood volume.

Renin-angiotensin system

(aka, renin-angiotensin-aldosterone system)

• Extrinsic mechanism of GFR regulation that is activated when mean arterial pressure drops below 80 mmHg; recall that too-low blood pressure can cause tissue necrosis.
• To raise blood volume and pressure, the renin-angiotensin system produces hormones that act on multiple organs, including the renal arterioles.
• Because it requires the production, secretion, and transport of hormones throughout the blood circulation, it is a relatively slow mechanism of GFR regulation.
• One of these hormones, angiotensin II, has dose-dependent effects on GFR:
  – At low levels, angiotensin II reduces renal blood flow but increases GFR.
  – At higher levels, it reduces both renal blood flow and GFR.

Stimuli for Renin Release:

• Mechanoreceptors within the juxtaglomerular cells respond to reduced stretch of the afferent arteriole.
• The macula densa of the distal tubule detects decreased salt concentration of the filtrate, and sends signals to the juxtaglomerular cells.
• Sympathetic stimulation, which is activated by the baroreceptor reflex, activates beta 1 receptors of the juxtaglomerular cells. Notice that this pathway involves both the nervous and endocrine systems.

Steps of RAS:

• First, reduced renal blood flow induces the juxtaglomerular cells to secrete renin, which is an enzyme.
• Within the blood stream, renin catalyzes the conversion of angiotensinogen to angiotensin I, which is a hormone with only mild vasoconstrictor effects.
• However, as it travels through the blood, *angiotensin-converting enzyme8 converts angiotensin I to angiotensin II.
• Angiotensin II has widespread effects throughout the body that raise blood volume and pressure

GFR regulation:

• Angiotensin II induces arteriole vasoconstriction, especially of the efferent arteriole, which has more angiotensin-sensitive receptors than the afferent arteriole does.
• Reduced blood flow through the efferent arteriole raises hydrostatic capillary pressure within the glomerulus, which helps to maintain GFR within the homeostatic range.
• However, if efferent arteriole blood flow is reduced too much, the oncotic capillary pressures overwhelm the hydrostatic pressures, and GFR is reduced (filtration pressures are discussed in detail, elsewhere).

Clinical Correlations:

• RAS can be life-saving in the case of systemic hypotension, but consider the clinical consequences of renal artery stenosis, which reduces blood flow through the renal artery.
  – In response, the renin-angiotensin system activates to elevate blood volume and pressure throughout the body, which can cause renovascular hypertension and kidney damage.
• A pharmacologic correlate, antihypertensive medications were developed to regulate the renin-angiotensin-aldosterone system, which reduce vasoconstriction and lower blood pressure.
  – Angiotensin converting enzyme inhibitors (ACE inhibitors); inhibit ACE.
  Unfortunately, ACE inhibitors can cause an irritating dry cough,
  So angiotensin 2 receptor inhibitors were also developed to block the angiotensin 2, type 1 receptors.

• The sympathetic division of the nervous system is an extrinsic mechanism of GFR regulation.
• It is activated when mean arterial blood pressure drops below 80 mmHg.
• Sympathetic activation rapidly induces vasoconstriction of the arterioles to the glomerulus to reduce GFR and to divert blood away from the kidneys to support other body tissues: the kidneys typically receive 20-25 % of total cardiac output, when this extrinsic regulator is activated, the percentage is much less.
• This is particularly important in response to hemorrhage, in which loss of blood volume lowers blood pressure, so blood is shunted away from the kidneys to avoid body tissue necrosis.

Sympathetic Regulation Steps:

• Low blood pressure decreases carotid sinus activity.
• Cardiovascular centers of medulla in brainstem respond by releasing norepinephrine via sympathetic nerves.
• Norepinephrine activates alpha 1 receptors on arterioles, initiates vasoconstriction.
  – Because afferent arteriole has more alpha 1 receptors, it constricts to a greater degree.
  – This reduces renal blood flow, hydrostatic capillary pressure, and GFR.
• Norepinephrine also activates beta 1 receptors of juxtaglomerular cells of afferent arteriole, which induces reninrelease.
• In the bloodstream, renin catalyzes hormonal conversion that eventually leads to production of angiotensin II.
• Angiotensin II induces vasoconstriction, especially of the efferent arteriole.
• Ultimately, both RBF and GFR are reduced in response to sympathetic stimulation.

• Glomerular filtration rate (GFR) is the total volume of ultrafiltrate formed by the collective kidney nephrons per minute;
• GFR is closely regulated to balance potentially opposing requirements:
  – Excess solutes and water needs to be removed from the blood.
  – The body tissues need nearly constant blood volume and pressure.

Intrinsic mechanisms

• Physiological responses that are initiated by renal structures to modify the hydrostatic capillary pressures; renal autoregulation.
• Maintain nearly constant GFR as long as mean arterial pressure is 80-180 mmHg, which allows for consistent kidney functioning despite changes in blood pressure.
• During daily activities, cardiac output, and, therefore, arterial blood pressure fluctuates—for example, during exercise it increases and during sleep it decreases.
• At high blood pressures, autoregulation protects the glomerulus from damage, and,
• At lower blood pressures, it ensures that the kidneys receive sufficient blood flow to filter wastes.
• If mean arterial pressure drops below 80 mmHg, such as during hemorrhage, extrinsic mechanisms activate (which we discuss in detail, elsewhere).

Myogenic mechanism:

• Relies on inherent properties of the arterioles, themselves.
• Arteriole walls comprise smooth muscle, made of vascular smooth muscle cells.
• When increased renal blood flow exerts increased hydrostatic capillary pressure on the walls, stretch receptors are activated and induce vasoconstriction.
• This reduces renal blood flow and, therefore, GFR.
• When renal blood flow is low, the stretch receptors are inactivated, and the arteriole dilates to increase GFR.

Tubuloglomerular feedback mechanism:

• Relies on interaction between the nephron tubule and glomerulus.
• As renal blood flow increases, so does hydrostatic capillary pressure, and, therefore, GFR increases.
• As the GFR increases, so does the concentration of salt in the ultrafiltrate, because high flow rate allows less time for tubular reabsorption.
• The macula densa of distal tuble senses the high salt concentration in the ultrafiltrate as it passes through the distal tubule;
• In response, it releases vasoconstrictor chemicals (the specifics of which are disputed).
• Consequently, the nearby afferent arteriole constricts, which, as we saw earlier:
  Reduces renal blood flow, hydrostatic capillary pressure, and GFR.
• When renal blood flow decreases, so does the sodium concentration, and eventually the macula densa stops releasing vasoconstrictors, which ultimately allows renal blood flow and GFR to again increase.

• Kidneys‘ objective is to maintain homeostatatic balance of blood volume, pressure, and ion concentrations via filtration of the blood, despite fluctuations in blood pressure throughout the day.
• GFR regulation achieves this balance through both intrinsic and extrinsic mechanisms.
• GFR is most easily regulated by adjusting the net filtration pressure, which is determined by the hydrostatic and oncotic forces at the filtration membrane.

Intrinsic vs Extrinsic Mechanisms

Intrinsic response

• Involve intra-renal mechanisms
• Structures within the kidneys initiate the intrinsic mechanisms
• Dominate as long as MAP is 80-180 mmHg
• Goal is to maintain nearly constant GFR over a wide range of mean arterial pressures
• Includes myogenic and tubuloglomerular feedback, which act primarily on afferent arteriole.

Extrinsic response

• Involve neural and hormonal mechanisms.
• Requires transport of neurotransmitters/hormones in bloodstream.
• Active when MAP is below 80 mmHg.
• Goal is to maintain blood volume and pressure; regulation of GFR is one facet of this.
• Sympathetic response acts primarily on afferent arteriole (Norepinephrine)
• Hormonal response acts primarily on efferent arteriole (Angiotensin II)

GLOMERULAR FILTRATION RATE

• The volume of ultrafiltrate formed by all of the nephrons of the kidneys per minute;
• Units = mL/min.
• Typical healthy GFR is between 110-130 mL/min; it varies based on sex, body composition, age, and other factors.

GFR is directly proportional to:

• Filtration membrane permeability
• Surface area available for filtration
• Net filtration pressure is largely influenced by hydrostatic glomerular capillary pressure (PGC), which is easily adjusted by altering blood flow through the glomerulus.
• Of the three variables that determine GFR, net filtration pressure is the easiest to manipulate.

Key Relationships:

Baseline:

• Constant supply of renal blood flows through the afferent arteriole, glomerulus, and efferent arteriole.
• Constant hydrostatic capillary pressure.
• Constant GFR.

Afferent Arteriole Constriction:

• Reduces renal blood flow
• Reduces hydrostatic capillary pressure
• Reduces GFR

Afferent Arteriole Dilation:

• Increases renal blood flow
• Increases hydrostatic capillary pressure
• Increases GFR

Efferent Arteriole Mild Constriction:

• Decreases renal blood flow
• Increases hydrostatic capillary pressure
• Increases GFR

Efferent Arteriole Extreme Constriction:

• Decreases renal blood flow
• Increases capillary oncotic forces
• Decreases GFR

Efferent Arteriole Dilation:

• Increases renal blood flow
• Decreases hydrostatic capillary pressure
• Decreases GFR

Clinical Correlations:

• GFR is clinically measured to evaluate kidney functioning.
• GFR can be altered by medications that cause vasoconstriction or vasodilation.