Balance WK1 LG1 Introduction to the Kidney
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94 Terms
What is the main job of the kidneys?
Answer: Maintain homeostasis by filtering blood, keeping what the body needs, and removing what it does not need.
Extra Information:
The kidneys regulate water, electrolytes, osmolarity, acid-base balance, and blood pressure.
They also excrete metabolic wastes and foreign chemicals.
Think: kidneys keep the body's internal environment balanced.
What are important functions of the kidneys besides removing waste?
Answer: Erythrocyte production, vitamin D synthesis, gluconeogenesis, and hormone metabolism.
Extra Information:
The kidney does much more than make urine.
Its overall goal is homeostasis.
Why can the kidney be thought of as a recycling plant rather than a sewer?
Answer: It filters substances first, then reabsorbs what the body needs and excretes what should be removed.
Extra Information:
Filtration = blood → tubule.
Reabsorption = tubule → blood.
Secretion = blood → tubule.
Excretion = leaves the body in urine.
What is the functional unit of the kidney?
Answer: The nephron.
Extra Information:
Each kidney contains approximately 1.2 million nephrons.
A nephron is essentially a hollow tube made of a single epithelial cell layer.
What are the major parts of the nephron in order?
Answer: Renal corpuscle → proximal tubule → loop of Henle → distal tubule → collecting duct system.
Extra Information:
Know this order well enough to draw it.
What is the main function of the renal corpuscle?
Answer: Filter blood to create the initial filtrate.
Extra Information:
The renal corpuscle contains the glomerulus and Bowman's capsule.
Blood enters through the afferent arteriole and leaves through the efferent arteriole.
What is the main job of the proximal tubule?
Answer: Bulk reabsorption.
Extra Information:
It reabsorbs most filtered sodium, water, and bicarbonate.
It reabsorbs nearly all glucose and amino acids under normal conditions.
Think: proximal tubule = take back MOST of the good stuff.
Why does the proximal tubule look very metabolically active?
Answer: It has many mitochondria and a brush border because it performs a large amount of reabsorption.
Extra Information:
The brush border increases surface area.
Mitochondria provide energy for transport.
What is the main job of the thin descending limb of the loop of Henle?
Answer: Reabsorb water.
Extra Information:
It is highly permeable to water.
As water leaves, the tubular fluid becomes more concentrated.
Think: descending = water leaves.
What happens to tubular fluid as it travels down the thin descending limb?
Answer: It becomes more concentrated.
Extra Information:
The tubule travels into the hyperosmotic medulla.
Water leaves the tubule.
What is the main job of the thick ascending limb of the loop of Henle?
Answer: Reabsorb sodium, potassium, and chloride without reabsorbing water.
Extra Information:
It uses the NKCC2 transporter.
NKCC2 moves Na+, K+, and 2 Cl−.
The thick ascending limb is relatively impermeable to water.
Why does tubular fluid become more dilute in the thick ascending limb?
Answer: Salt is reabsorbed while water stays in the tubule.
Extra Information:
Think: ascending limb = SALT leaves, water stays.
What transporter is important in the thick ascending limb?
Answer: NKCC2.
Extra Information:
NKCC2 = sodium-potassium-2 chloride cotransporter.
It reabsorbs Na+, K+, and Cl−.
What is the overall purpose of the loop of Henle?
Answer: Establish the medullary osmotic gradient needed to concentrate urine.
Extra Information:
The descending and ascending limbs have different water and solute permeabilities.
Together they help create the concentration gradient in the medulla.
What is the main job of the distal convoluted tubule?
Answer: Continue sodium chloride reabsorption.
Extra Information:
Compared with the proximal tubule, this is a more selective part of nephron handling.
What is the main job of the collecting duct?
Answer: Make the final regulated adjustments to sodium, potassium, water, and acid-base balance.
Extra Information:
Think: collecting duct = final decision maker.
This is where the kidney fine-tunes the final urine.
Which parts of the nephron perform bulk reabsorption versus highly regulated fine-tuning?
Answer: The proximal tubule performs major bulk reabsorption, while the distal nephron and collecting duct perform more selective regulation.
Extra Information:
This helps explain why different nephron segments have different functions.
What are the two major types of nephrons?
Answer: Cortical nephrons and juxtamedullary nephrons.
Extra Information:
They differ mainly in glomerular location, loop length, blood supply, and role in urine concentration.
Where are cortical nephron glomeruli located?
Answer: In the outer cortex.
Extra Information:
Cortical nephrons make up about 70–80% of nephrons.
Where are juxtamedullary nephron glomeruli located?
Answer: Deep in the cortex near the corticomedullary junction.
Extra Information:
Juxtamedullary nephrons make up about 20–30% of nephrons.
Which nephron has a short loop of Henle?
Answer: Cortical nephron.
Extra Information:
Its loop penetrates only a short distance into the medulla.
Which nephron has a long loop of Henle?
Answer: Juxtamedullary nephron.
Extra Information:
Its loop extends deep into the medulla.
Some reach near the renal papillae.
Which nephron type is best at concentrating urine?
Answer: Juxtamedullary nephrons.
Extra Information:
Their long loops extend deep into the medulla.
This helps establish the medullary osmotic gradient needed for water conservation.
What is the main role of cortical nephrons?
Answer: Bulk filtration and solute reabsorption.
Extra Information:
Think: cortical = everyday workhorses.
What is the main role of juxtamedullary nephrons?
Answer: Concentrating urine and conserving water.
Extra Information:
Think: juxtamedullary = concentration specialists.
What capillary network mainly surrounds cortical nephrons?
Answer: Peritubular capillaries.
Extra Information:
The efferent arteriole forms a dense network around the cortical tubules.
What specialized vessels are associated with juxtamedullary nephrons?
Answer: Vasa recta.
Extra Information:
The vasa recta run alongside the long loops of Henle.
They are important for maintaining the medullary osmotic gradient.
Where is essentially all filtered glucose normally reabsorbed?
Answer: Proximal tubule.
Extra Information:
Within the physiological range, essentially 100% of filtered glucose is reabsorbed.
Therefore, little or no glucose normally appears in urine.
What transporter reabsorbs most filtered glucose in the proximal tubule?
Answer: SGLT2.
Extra Information:
SGLT2 = sodium-glucose cotransporter-2.
It uses the sodium gradient to help move glucose from the tubular lumen into the cell.
What normally happens to filtered glucose?
Answer: It is reabsorbed back into the bloodstream in the proximal tubule.
Extra Information:
This is why normal urine contains little or no glucose.
Why can glucose begin appearing in the urine when blood glucose becomes very high?
Answer: The glucose transporters become saturated and cannot reabsorb all of the filtered glucose.
Extra Information:
The filtered load can exceed the tubule's reabsorptive capacity.
The extra glucose stays in the tubular fluid and is excreted.
What are two major ways glucosuria can occur according to the slides?
Answer: The filtered glucose load exceeds reabsorptive capacity, or SGLT2 is inhibited.
Extra Information:
Diabetes can increase the filtered glucose load.
SGLT2 inhibitor medications intentionally decrease glucose reabsorption.
How does empagliflozin cause glucosuria?
Answer: It inhibits SGLT2 and decreases glucose reabsorption in the proximal tubule.
Extra Information:
More glucose remains in the tubular fluid and is excreted in urine.
Why can someone taking empagliflozin have glucose in the urine even if blood glucose is not extremely high?
Answer: The medication directly decreases renal glucose reabsorption.
Extra Information:
The lecture case had a blood glucose of 170 mg/dL but positive urine glucose.
The medication changes the kidney's handling of glucose.
Why can SGLT2 inhibitors increase the risk of genital fungal infections?
Answer: They increase glucose in the urine.
Extra Information:
The lecture case connects empagliflozin-associated glucosuria with vulvovaginal candidiasis.
What is filtration?
Answer: Movement of substances from the blood into the nephron at the glomerulus.
Extra Information:
This is the first step in renal handling.
What is reabsorption?
Answer: Movement of a substance from the tubular fluid back into the blood.
Extra Information:
Think: REabsorb = REturn to blood.
What is secretion?
Answer: Movement of a substance from the blood into the renal tubule.
Extra Information:
Secretion adds substances to tubular fluid after filtration.
What is excretion?
Answer: The amount of a substance that ultimately leaves the body in the urine.
Extra Information:
Excretion reflects the combined effects of filtration, reabsorption, and secretion.
What is renal clearance in simple terms?
Answer: The volume of plasma completely cleared of a specific substance per unit time.
Extra Information:
Clearance tells you how the kidney handles a specific substance.
It is substance-specific.
What is the renal clearance equation?
Answer: Cx = (Ux × V) / Px.
Extra Information:
Cx = clearance of substance x.
Ux = urine concentration.
V = urine flow rate.
Px = plasma concentration.
What is glomerular filtration rate in simple terms?
Answer: The rate at which plasma is filtered through the glomeruli.
Extra Information:
GFR is a global measure of kidney filtration.
A normal value presented in the slides is approximately 120–125 mL/min.
What is the main difference between GFR and clearance?
Answer: GFR measures overall filtration, while clearance tells how the kidney handles a specific substance.
Extra Information:
GFR concerns the kidney's overall filtration rate.
Clearance can reflect filtration plus reabsorption and/or secretion depending on the substance.
If a freely filtered substance has a clearance LESS than GFR, what is happening?
Answer: Net reabsorption.
Extra Information:
Some of the filtered substance is returned to the blood.
Memory trick: clearance LOW → kidney kept some.
If a freely filtered substance has a clearance EQUAL to GFR, what is happening?
Answer: There is no net tubular reabsorption or secretion.
Extra Information:
The amount excreted equals the amount filtered.
Inulin behaves this way.
If a freely filtered substance has a clearance GREATER than GFR, what is happening?
Answer: Net secretion.
Extra Information:
The tubules added more of the substance to the urine after filtration.
Memory trick: clearance HIGH → tubule added more.
What does a very low sodium clearance tell you?
Answer: Most filtered sodium is being reabsorbed.
Extra Information:
More than 99% of filtered Na+ is normally reabsorbed.
The lecture example calculates sodium clearance as only 0.07 mL/min.
What is the normal clearance of glucose under physiological conditions?
Answer: Approximately zero.
Extra Information:
Filtered glucose is normally completely reabsorbed.
Therefore almost none is excreted.
What properties would make a substance an ideal marker for measuring GFR?
Answer: It is freely filtered and neither reabsorbed nor secreted.
Extra Information:
Then the amount filtered equals the amount excreted.
Therefore, its clearance equals GFR.
Why does inulin clearance equal GFR?
Answer: Inulin is freely filtered but is neither reabsorbed nor secreted.
Extra Information:
Filtered load = excretion rate.
Therefore: inulin clearance = GFR.
Why is inulin considered an excellent GFR marker?
Answer: Its renal handling closely matches the requirements for an ideal filtration marker.
Extra Information:
It is freely filtered.
It is not reabsorbed.
It is not secreted.
Why is inulin not routinely used clinically to measure GFR?
Answer: It must be infused and requires timed blood and urine measurements.
Extra Information:
Inulin is not naturally present in the body.
Testing is time-consuming, expensive, and inconvenient.
Think: accurate but impractical.
What is used clinically instead of inulin to estimate GFR?
Answer: Serum creatinine and estimated GFR.
Extra Information:
Creatinine is endogenous, inexpensive, and readily available.
It is sufficiently accurate for most clinical decisions.
Where does creatinine come from?
Answer: Breakdown of creatine phosphate in skeletal muscle.
Extra Information:
It is produced at a relatively constant rate in many people.
How does the kidney handle creatinine?
Answer: Creatinine is freely filtered, not reabsorbed, and slightly secreted.
Extra Information:
That slight secretion is an important difference from inulin.
Why does creatinine clearance approximate GFR?
Answer: Creatinine is freely filtered and not reabsorbed, with only slight secretion.
Extra Information:
Because of the small amount of secretion, creatinine clearance is not as perfect a GFR marker as inulin.
What is a major advantage of creatinine over inulin?
Answer: Creatinine is naturally produced by the body.
Extra Information:
It does not require an intravenous infusion.
This makes it much more practical clinically.
What factors can affect serum creatinine besides kidney function?
Answer: Muscle mass, diet, age, sex, and hydration.
Extra Information:
This is why the same creatinine value can mean different things in different patients.
Why can the same serum creatinine mean different kidney function in different people?
Answer: Creatinine production depends partly on characteristics such as muscle mass, age, and sex.
Extra Information:
The slides give the example that 1.5 mg/dL may have different significance in a muscular man versus a woman.
What is estimated glomerular filtration rate, or eGFR?
Answer: An estimate of GFR calculated from serum creatinine and patient-specific factors.
Extra Information:
The lecture uses the 2021 CKD-EPI creatinine equation.
It is easier to obtain than measured creatinine clearance.
Why is eGFR generally more useful than looking at serum creatinine alone?
Answer: It puts the creatinine value into the context of patient characteristics.
Extra Information:
The slides emphasize age and sex.
Raw creatinine alone can be misleading.
Is race included in the current 2021 CKD-EPI creatinine equation presented in the slides?
Answer: No.
Extra Information:
The lecture states that race was historically considered but is no longer used in this equation.
Why is creatinine an imperfect marker of early kidney function loss?
Answer: GFR can decrease substantially before serum creatinine rises dramatically.
Extra Information:
The relationship between GFR and plasma creatinine is inverse and nonlinear.
How are GFR and plasma creatinine related?
Answer: As GFR decreases, plasma creatinine increases.
Extra Information:
The relationship is inverse, not linear.
When GFR is already low, a small further decrease can produce a large rise in creatinine.
What does an approximate doubling of plasma creatinine suggest about GFR?
Answer: About a 50% decrease in GFR.
Extra Information:
This is a clinical interpretation tip given in the slides.
Why can serum creatinine lag behind an acute kidney injury?
Answer: Its blood level does not rise immediately when GFR suddenly decreases.
Extra Information:
Therefore creatinine is not a real-time marker of acute changes.
What is BUN?
Answer: Blood urea nitrogen.
Extra Information:
Urea is a nitrogen-containing waste product.
Its kidney handling differs from creatinine because urea is reabsorbed.
How is urea handled at the glomerulus?
Answer: It is freely filtered.
Extra Information:
After filtration, part of the urea is reabsorbed.
How much urea is reabsorbed in the proximal tubule according to the slides?
Answer: About 50%.
Extra Information:
The reabsorption is passive.
Additional urea can be reabsorbed in the inner medullary collecting duct under ADH regulation.
Why is BUN more affected by hydration status than creatinine?
Answer: Urea is reabsorbed, so changes in tubular water handling can change how much urea returns to the blood.
Extra Information:
Creatinine is not reabsorbed.
Therefore BUN reflects filtration plus reabsorption and other nonrenal factors.
What is the normal BUN-to-creatinine ratio in the lecture?
Answer: About 10:1 to 20:1.
Extra Information:
The ratio is a physiologic clue rather than a standalone diagnosis.
Why can the BUN-to-creatinine ratio increase in volume depletion or pre-renal acute kidney injury?
Answer: More urea is reabsorbed.
Extra Information:
BUN rises disproportionately compared with creatinine.
The take-home slide associates a ratio above about 20:1 with volume depletion or pre-renal AKI.
What can cause a high BUN besides reduced kidney filtration?
Answer: Volume depletion, dehydration, high protein breakdown, gastrointestinal bleeding, or reduced renal clearance.
Extra Information:
This is why BUN is influenced by more than just GFR.
What can cause a low BUN according to the lecture?
Answer: Liver disease or overhydration.
Extra Information:
Liver disease can decrease urea production.
Overhydration can lower BUN.
Why should the BUN-to-creatinine ratio not be used by itself to make a diagnosis?
Answer: Many kidney and non-kidney factors can change BUN and creatinine.
Extra Information:
The lecture emphasizes using the ratio as a physiologic clue within the broader clinical picture.
What substance is used to estimate effective renal plasma flow?
Answer: Para-aminohippurate, or PAH.
Extra Information:
PAH is a synthetic substance that is infused during testing.
How is PAH handled by the kidney?
Answer: It is freely filtered, not reabsorbed, and strongly secreted.
Extra Information:
PAH entering the kidney is therefore largely removed from the plasma into the urine.
Why does PAH clearance estimate effective renal plasma flow?
Answer: Because PAH is filtered and secreted, so most PAH delivered to the kidney is removed from the plasma in one pass.
Extra Information:
Its clearance therefore approximates the amount of plasma flowing through functioning renal tissue.
What is the equation used to estimate renal plasma flow with PAH?
Answer: RPF = (UPAH × V) / PPAH.
Extra Information:
UPAH = urine PAH concentration.
V = urine flow rate.
PPAH = plasma PAH concentration.
What is the difference between renal plasma flow and renal blood flow?
Answer: Renal plasma flow measures only plasma, while renal blood flow includes plasma plus blood cells.
Extra Information:
Red blood cells make up an important cellular portion of blood.
What does hematocrit represent?
Answer: The fraction of blood volume occupied by red blood cells.
Extra Information:
Therefore, 1 − hematocrit represents the plasma fraction of blood.
How can renal blood flow be calculated from renal plasma flow?
Answer: RBF = RPF / (1 − hematocrit).
Extra Information:
RPF only measures plasma.
Dividing by the plasma fraction converts plasma flow to total blood flow.
If a substance has a clearance of 300 mL/min and GFR is 120 mL/min, what does that tell you?
Answer: The substance undergoes net tubular secretion.
Extra Information:
Clearance is greater than GFR.
The tubules must be adding substance to the filtrate.
A compound has Ux = 150 mg/dL, V = 2 mL/min, and Px = 1 mg/dL. What is its clearance?
Answer: 300 mL/min.
Extra Information:
Cx = (Ux × V) / Px.
Cx = (150 × 2) / 1 = 300 mL/min.
If GFR is 120 mL/min, the compound must undergo net secretion.
What is filtered load?
Answer: The amount of a specific substance filtered into the nephron per unit time.
Extra Information:
Filtered load = plasma concentration × GFR.
It is substance-specific.
What is filtration fraction?
Answer: The fraction of renal plasma flow that gets filtered through the glomeruli.
Extra Information:
Filtration fraction = GFR / RPF.
The slides give approximately 20%.
What is the difference between filtered load and filtration fraction?
Answer: Filtered load asks how much of a specific substance is filtered, while filtration fraction asks what fraction of renal plasma is filtered.
Extra Information:
Filtered load = Px × GFR.
Filtration fraction = GFR / RPF.
What is the easiest clearance rule to memorize?
Answer:
Clearance below GFR = reabsorption
clearance equal to GFR = filtration only
clearance above GFR = secretion.
Extra Information:
LOW = kidney took some BACK.
EQUAL = nothing added or removed by tubules.
HIGH = kidney ADDED more to urine.
How do glucose, inulin, creatinine, and PAH compare by clearance?
Answer:
Glucose clearance is about 0
inulin clearance equals GFR
creatinine clearance approximates GFR
PAH clearance is greater and estimates effective RPF.
Extra Information:
Glucose → reabsorbed.
Inulin → filtered only.
Creatinine → filtered + slightly secreted.
PAH → filtered + strongly secreted.
What are the highest-yield nephron segment functions to remember?
Answer:
Proximal tubule = bulk reabsorption
descending limb = water
thick ascending limb = Na+/K+/Cl− without water
distal tubule = more NaCl reabsorption
collecting duct = final regulation.
Extra Information:
Proximal = MOST.
Descending = WATER OUT.
Thick ascending = SALT OUT, NO WATER.
Collecting duct = FINE-TUNE.
What are the major take-home points for glucose handling by the kidney?
Answer: Glucose is filtered and normally reabsorbed in the proximal tubule by SGLT2, but glucosuria occurs when reabsorptive capacity is exceeded or SGLT2 is inhibited.
Extra Information:
This connects nephron anatomy with filtration, reabsorption, and clinical pharmacology.
What are the major take-home points for measuring kidney filtration?
Answer: Inulin accurately measures GFR, creatinine is a practical GFR proxy, and eGFR estimates kidney function using serum creatinine plus patient factors.
Extra Information:
Inulin = accurate but inconvenient.
Creatinine = practical but imperfect.
eGFR = clinically useful estimate.
What are the major take-home points for BUN and creatinine?
Answer: Creatinine mainly reflects filtration, while BUN reflects filtration plus urea reabsorption and other physiologic factors.
Extra Information:
Volume depletion can increase urea reabsorption and raise the BUN/creatinine ratio.
Creatinine can lag behind acute decreases in GFR.
What is the major take-home point for PAH?
Answer: PAH clearance estimates effective renal plasma flow.
Extra Information:
PAH is filtered and strongly secreted.
This allows the kidney to remove most PAH from the plasma passing through it.