Introduction to Nephrology

[Kidney functions] What is the kidney’s overall job?

Keep the internal environment stable by adjusting what leaves in urine and what stays in the blood.

[Kidney functions] What are the five major kidney functions in this lecture?

Urine formation; fluid and electrolyte balance; acid-base balance; long-term blood pressure regulation; erythropoietin release for red blood cell production.

[Kidney functions] How do kidneys help regulate blood pH?

They secrete hydrogen ions (H+) and conserve or make bicarbonate (HCO3−), which helps remove acid.

[Kidney functions] What happens when the blood becomes too acidic?

Kidneys increase H+ secretion and conserve or generate more bicarbonate, helping bring pH back toward normal.

[Kidney functions] What is erythropoietin (EPO)?

A kidney-released hormone that tells bone marrow to make more red blood cells when kidney oxygen availability remains low.

[Kidney functions] How do kidneys regulate blood pressure over the long term?

By controlling sodium and water excretion, which changes blood volume, and by releasing renin to activate RAAS.

[Body fluids] About what fraction of adult body weight is water?

Roughly 50–60% on average; it varies with age, sex, and body fat.

[Body fluids] How is total body water divided?

About two-thirds is inside cells (intracellular fluid, ICF), and one-third is outside cells (extracellular fluid, ECF).

[Body fluids] What are the main extracellular fluid compartments?

Interstitial fluid around cells and plasma within blood vessels.

[Body fluids] What does ECF stand for?

Extracellular fluid: fluid outside cells, including plasma and interstitial fluid.

[Body fluids] What does osmolality mean?

The concentration of dissolved particles per kilogram of water; higher osmolality means more concentrated fluid.

[Body fluids] What must happen to water intake and output over time?

They must approximately match for stable body water; kidneys adjust urine output to help achieve this.

[Body fluids] What are common routes of daily water loss besides urine?

Skin evaporation, breathing, sweat, and feces.

[Body fluids] How does prolonged heavy exercise change water loss?

Sweat and respiratory water loss can rise greatly, so drinking and kidney water conservation become more important.

[Body fluids] What is an insensible water loss?

Water lost through skin evaporation or breathing that is not easily noticed or measured.

[Anatomy and blood flow] What is a nephron?

The kidney’s functional unit that filters blood and modifies the filtrate to make urine.

[Anatomy and blood flow] What is the path of urine through the urinary tract?

Kidneys → ureters → bladder → urethra.

[Anatomy and blood flow] What is the path of fluid through the nephron?

Bowman’s capsule → proximal tubule → descending limb → ascending limb → distal tubule → collecting duct.

[Anatomy and blood flow] What is a glomerulus?

A tuft of tiny capillaries where plasma water and small solutes are filtered into Bowman’s capsule.

[Anatomy and blood flow] What is Bowman’s capsule?

The cup-like structure surrounding the glomerulus that receives the initial filtrate.

[Anatomy and blood flow] What is the cortex versus medulla?

The cortex is the outer kidney region; the medulla is the inner region that becomes progressively more concentrated toward the papilla.

[Anatomy and blood flow] What is a juxtamedullary nephron?

A nephron with a long loop of Henle extending deep into the medulla; especially important for concentrated urine.

[Anatomy and blood flow] What are peritubular capillaries?

Small vessels around renal tubules that receive reabsorbed material and supply substances for secretion.

[Anatomy and blood flow] What are the vasa recta?

Long, hairpin-shaped vessels next to deep loops of Henle that preserve the medullary osmotic gradient.

[Anatomy and blood flow] Approximately what share of resting cardiac output goes to the kidneys?

About 20–25%, corresponding to roughly 1,200 mL/min of renal blood flow in the lecture.

[Anatomy and blood flow] What is the difference between renal blood flow and GFR?

Renal blood flow is blood delivered to the kidneys; GFR is the volume of plasma filtrate produced by glomeruli per minute.

[Filtration and urine formation] What are the three processes that create urine?

Glomerular filtration, tubular reabsorption, and tubular secretion.

[Filtration and urine formation] What is filtration?

Movement of water and small solutes from glomerular blood into Bowman’s capsule.

[Filtration and urine formation] What is reabsorption?

Movement from tubular fluid back into the blood; it keeps water and useful substances.

[Filtration and urine formation] What is secretion?

Movement from peritubular blood or tubular cells into the tubule to add substances to urine.

[Filtration and urine formation] What is excretion?

Material that actually leaves the body in urine.

[Filtration and urine formation] What is the urine excretion equation?

Excretion = filtration − reabsorption + secretion.

[Filtration and urine formation] Does glucose normally enter the filtrate?

Yes. It is filtered, then normally almost completely reabsorbed in the proximal tubule.

[Filtration and urine formation] What normally cannot pass the filtration barrier?

Blood cells and most large plasma proteins.

[Filtration and urine formation] What normally passes the filtration barrier?

Water and small dissolved substances such as electrolytes, glucose, and wastes.

[Filtration and urine formation] What is GFR?

Glomerular filtration rate: the volume of filtrate made by all glomeruli each minute.

[Filtration and urine formation] What is the approximate normal GFR used in this lecture?

About 125 mL/min, or about 180 L/day.

[Filtration and urine formation] How does 125 mL/min become about 180 L/day?

125 × 1,440 minutes/day = 180,000 mL/day = 180 L/day.

[Filtration and urine formation] How much urine is typically excreted each day in the lecture?

Roughly 1–2 L/day, because almost all filtered fluid is reabsorbed.

[Filtration and urine formation] What is the main force pushing fluid out of glomerular capillaries?

Glomerular capillary hydrostatic pressure.

[Filtration and urine formation] What forces oppose glomerular filtration?

Plasma protein oncotic pressure pulling water into capillaries and Bowman’s space hydrostatic pressure pushing back.

[Filtration and urine formation] What is filtrate like when it first enters the nephron?

Its small-solute concentration is approximately isotonic with plasma, around 290–300 mOsm/kg, but it has very little protein.

[Filtration and urine formation] Name substances that may be secreted into the tubule.

H+, K+, ammonium/ammonia, certain drugs, creatinine, and organic waste products.

[Tubular transport] What does the proximal convoluted tubule (PCT) do?

It performs bulk reabsorption of filtered water and sodium and reabsorbs almost all glucose and amino acids under normal conditions.

[Tubular transport] About what fraction of filtered sodium and water is reabsorbed in the PCT?

Approximately 65% is the lecture’s key summary value; some older slide diagrams use rougher 75–80% estimates.

[Tubular transport] What happens to filtered glucose and amino acids in a healthy person?

They are normally almost completely reabsorbed in the PCT.

[Tubular transport] What powers much tubular reabsorption?

The basolateral Na+/K+ ATPase uses ATP to pump sodium out of tubule cells, creating a sodium gradient for transport.

[Tubular transport] What is active transport?

Movement against a gradient that requires energy directly or indirectly.

[Tubular transport] What is passive transport?

Movement down an electrochemical or osmotic gradient without direct ATP use by that step.

[Tubular transport] Why does water often follow sodium?

Sodium reabsorption creates an osmotic pull, causing water to move toward the more concentrated side where the segment is water-permeable.

[Tubular transport] Is every nephron segment equally water-permeable?

No. The descending limb is water-permeable, the thick ascending limb is essentially water-impermeable, and collecting duct permeability depends strongly on ADH.

[Tubular transport] Where does aldosterone act in the nephron?

Mainly the late distal tubule and cortical collecting duct, increasing sodium reabsorption and potassium secretion.

[Tubular transport] Where does ADH act in the nephron?

Mainly the collecting duct, increasing its water permeability; it also contributes to water reabsorption in the late distal nephron.

[Loop and countercurrent system] What happens in the descending limb?

Water can leave into the concentrated medulla, so the tubular fluid becomes more concentrated.

[Loop and countercurrent system] What happens in the thick ascending limb?

Na+, K+, and Cl− are reabsorbed through NKCC2, but water cannot follow, so tubular fluid becomes more dilute.

[Loop and countercurrent system] What is NKCC2?

The Na+-K+-2Cl− cotransporter in the thick ascending limb.

[Loop and countercurrent system] Why is the thick ascending limb called a diluting segment?

It removes salt without removing water.

[Loop and countercurrent system] What is the corticomedullary osmotic gradient?

The kidney tissue becomes progressively more concentrated from the cortex toward the inner medulla.

[Loop and countercurrent system] What is countercurrent multiplication?

Repeated salt removal in the ascending limb and water movement in the descending limb amplify small local differences into a large medullary gradient.

[Loop and countercurrent system] Why is it called countercurrent?

Fluid travels in opposite directions in the two adjacent limbs of the loop.

[Loop and countercurrent system] What does the loop of Henle do for the gradient?

Creates and amplifies it.

[Loop and countercurrent system] What does the vasa recta do for the gradient?

Preserves it by countercurrent exchange and limiting washout while carrying away reabsorbed water.

[Loop and countercurrent system] What does the collecting duct do with the gradient?

Uses it to reabsorb water when ADH makes the duct water-permeable.

[Loop and countercurrent system] How does urea help concentrate urine?

Urea recycling adds solute to the inner medulla and supports the high osmotic gradient.

[Loop and countercurrent system] What happens to the collecting duct without ADH?

It remains relatively impermeable to water, so more dilute urine is excreted.

[Loop and countercurrent system] What happens to the collecting duct with ADH?

Aquaporin-2 channels appear on the luminal membrane, letting water leave toward the hyperosmotic medulla and concentrating urine.

[Loop and countercurrent system] What is the key loop-vasa-duct memory line?

Loop makes the gradient; vasa recta maintains it; ADH and collecting duct use it.

[ADH and water balance] What is ADH also called?

Antidiuretic hormone, or vasopressin.

[ADH and water balance] Where is ADH released from?

The posterior pituitary, after signals from hypothalamic neurons and osmoreceptors.

[ADH and water balance] What do hypothalamic osmoreceptors detect?

Changes in plasma osmolality.

[ADH and water balance] What happens to osmoreceptors when plasma osmolality rises?

They shrink and stimulate ADH release.

[ADH and water balance] What is the ADH pathway in dehydration?

Water loss → higher plasma osmolality → more ADH → more collecting duct water reabsorption → lower volume, concentrated urine.

[ADH and water balance] What is the ADH pathway after drinking excess water?

More body water → lower plasma osmolality → less ADH → less collecting duct water reabsorption → larger volume, dilute urine.

[ADH and water balance] What receptor does ADH use to raise collecting duct water permeability?

V2 receptors, which lead to aquaporin-2 insertion.

[ADH and water balance] Does ADH create the medullary gradient?

No. The loop helps create it; ADH makes the collecting duct able to use it.

[ADH and water balance] What does antidiuresis mean?

Reduced urine output as water is conserved.

[ADH and water balance] Can urine be more concentrated or more dilute than plasma?

Yes. ADH and the medullary gradient allow concentrated urine; low ADH permits dilute urine.

[RAAS] What does RAAS stand for?

Renin–angiotensin–aldosterone system.

[RAAS] What problem is RAAS trying to solve?

Low effective circulating volume or blood pressure; it helps preserve sodium, water, and arterial pressure.

[RAAS] What commonly triggers renin release?

Lower pressure in the afferent arteriole, less NaCl delivered to the macula densa, or increased renal sympathetic beta-1 stimulation.

[RAAS] Where is renin released?

From juxtaglomerular (granular) cells near the afferent arteriole of the kidney.

[RAAS] What is renin?

An enzyme released by the kidney; it starts the RAAS cascade by cutting angiotensinogen.

[RAAS] Where does angiotensinogen come from?

The liver makes angiotensinogen, a circulating precursor protein.

[RAAS] What does renin do to angiotensinogen?

Converts it to angiotensin I.

[RAAS] What does ACE do?

Angiotensin-converting enzyme converts angiotensin I into angiotensin II, especially in vascular endothelium including the lungs.

[RAAS] Which angiotensin form has the major immediate effects?

Angiotensin II, the active form that strongly constricts blood vessels and promotes volume retention.

[RAAS] How does angiotensin II affect blood vessels?

It causes vasoconstriction, raising vascular resistance and helping support blood pressure.

[RAAS] How does angiotensin II affect aldosterone?

It stimulates the adrenal cortex to release aldosterone.

[RAAS] Where does aldosterone come from?

The adrenal cortex, specifically the zona glomerulosa.

[RAAS] What does aldosterone do in simple terms?

Tells the late distal nephron to keep more sodium and excrete more potassium; water tends to follow retained sodium when available.

[RAAS] What happens to potassium when aldosterone rises?

Potassium secretion and urinary potassium loss generally increase.

[RAAS] How does aldosterone affect blood volume?

Retained sodium promotes water retention, expanding extracellular fluid and blood volume over time.

[RAAS] How is aldosterone different from ADH?

Aldosterone chiefly increases sodium reabsorption (and potassium secretion); ADH chiefly increases water permeability and water reabsorption.

[RAAS] How does angiotensin II also support water conservation?

It can stimulate thirst and ADH release and also increases renal sodium reabsorption.

[RAAS] What is the full RAAS sequence?

Low renal perfusion or low distal NaCl → renin → angiotensinogen becomes angiotensin I → ACE makes angiotensin II → vasoconstriction + aldosterone (and thirst/ADH) → sodium and water retention → blood pressure support.

[RAAS] Why does renin fall after volume and pressure recover?

The original low-perfusion signal weakens and feedback from angiotensin II reduces further renin release.

[RAAS] What happens to RAAS after significant blood loss?

Renin rises, angiotensin II rises, aldosterone rises, and the kidneys conserve sodium and water.

[RAAS] Is renin a hormone that directly reabsorbs sodium?

No. Renin is an enzyme that initiates the cascade; angiotensin II and aldosterone produce the major downstream effects.

[Urine characteristics and synthesis] What are common normal urine solutes?

Urea, creatinine, uric acid, and varying amounts of sodium, potassium, ammonium, bicarbonate, and other ions.

[Urine characteristics and synthesis] What is the typical urine pH mentioned in the lecture?

Often near pH 6, but it can vary with diet and acid-base status.