Nephron Function and Physiology Quiz

Kidney Diseases & Kidney Function Tests

• The kidneys are a pair of organs located in the posterior part of the abdomen, on both sides of the vertebral column.
• The outer part of the kidney is the cortex, containing glomeruli and renal tubules.
• The inner part is the medulla, containing tubules and collecting ducts.
• The renal pelvis diminishes and merges into the ureter.
• Each ureter descends in the abdomen and joins the bladder.
• The bladder is the storage organ for urine, which is voided to the urethra and then to the exterior.
• Each kidney is composed of approximately 1 million nephrons, the functional units.
• The nephron begins with the glomerulus, a tuft of capillaries.
• The glomerulus is surrounded by Bowman's capsule.
• The proximal convoluted tubule runs through the cortex, enters the medulla, forming the descending limb of the loop of Henle, and then the ascending limb of the loop of Henle.
• The thick section of the ascending limb of the loop of Henle returns to the cortex and forms the distal convoluted tubule.
• The distal convoluted tubule merges with the collecting duct.
• The collecting duct descends through the cortex and medulla, increasing in size as it passes down the medulla.
• At the end, the collecting duct drains to the pelvis.

Renal Physiology

• The kidney is the chief regulator of all body fluid and electrolytes.
• It has six main functions: urine formation, regulation of fluid and electrolyte balance, regulation of acid-base balance, excretion of end products of protein metabolism, hormonal function, and protein conservation.

Urine Formation

• The removal of waste products is accomplished with the formation of urine.
• Urine formation covers three major processes: filtration, reabsorption, and secretion.

Glomerular Filtration

• 1000-1200 mL of blood passes through the kidneys per minute.
• The glomerulus has a semipermeable membrane that allows free passage of water and electrolytes but is relatively impermeable to large molecules.
• In filtration, fluid transfers from the capillary lumen to Bowman's space (urinary space).
• The hydrostatic pressure in glomerular capillaries is approximately 3 times greater than in other capillaries.
• This pressure pushes fluid to the semipermeable membrane, resulting in filtration.
• The rate of this process is 130 mL/minute and is called Glomerular Filtration Rate (GFR).
• GFR is a critical parameter in assessing renal glomerular function.

Proximal Tubule

• Approximately 80% of water and salts are reabsorbed from the glomerular filtrate in the proximal tubule.
• All filtered glucose and most filtered amino acids are reabsorbed in this part of the tubule.
• Low molecular weight proteins, urea, uric acid, bicarbonate, phosphate, chloride, potassium, magnesium, and calcium are reabsorbed to varying degrees.

Loop of Henle

• The loop of Henle is a long U-shaped portion of the tubule within each nephron.
• It segments are the thick descending limb, a thin descending limb, and a thick ascending limb.
• Its main functions are to reabsorb water and sodium chloride from the filtrate.
• This conserves water for the body and produces highly concentrated urine.

Distal Convulated Tubule

• A small amount of filtered sodium, chloride, and water is reabsorbed in the distal convoluted tubule.
• The distal tubule responds to antidiuretic hormone (ADH). In the presence of ADH, its water permeability is high.
• Potassium can be reabsorbed or secreted in this segment.
• Aldosterone stimulates sodium reabsorption and potassium secretion in the distal tubule.
• The secretion of hydrogen, ammonia, ammonium ions, and uric acid; while the reabsorption of bicarbonate occurs.
• This segment has low permeability to urea.

Collecting Duct

• ADH controls the water permeability of the collecting duct throughout its length.
• In the presence of ADH, the hypotonic tubular fluid entering the duct loses water.
• Aldosterone stimulates sodium reabsorption, and chloride reabsorption follows sodium reabsorption.

Water

• Water is the most abundant component of the body; 60% of the body.
• Kidneys regulate body water to maintain serum osmolality.
• Osmolality is the measurement of the number of moles of particles per kilogram of water. A normal range is typically 275 to 295 mOsm/kg.
• Serum osmolality remains constant daily (though daily water and salt intake can vary).
• Kidneys can form more concentrated/diluted urine.
• If water needs to be conserved, kidneys form concentrated urine (about 1200 mOsm/kg).
• If excess water is present, diluting mechanisms can reduce osmolality (to as low as 50 mOsm/kg).

Sodium

• Sodium is the main cation of extracellular fluid.
• Sodium is freely filtered by the glomerulus and actively reabsorbed by tubules.
• When sodium ions are actively reabsorbed by proximal tubule cells, this reabsorption causes passive reabsorption of bicarbonate and chloride (as counter ions), and water.
• In normal persons, sodium content is constant.
• In a normal person, the kidneys reabsorb more than 99% of the filtered sodium.
• Sodium reabsorption is controlled by the renin-angiotensin-aldosterone system.

Chloride

• The concentration of chloride in extracellular fluids is similar to sodium concentration and is influenced by similar factors.

Potassium

• Potassium is the main cation of intracellular fluid.
• Maintaining potassium concentration is essential for cells.
• The ingested daily potassium amount is equal to the potassium excreted by kidneys (a small amount of potassium is eliminated by feces and sweat).
• The kidney is the major regulator of potassium in the body.
• Potassium is filtered freely at the glomerulus, and reabsorption occurs throughout the tubules (except for the descending loop of Henle).
• The distal tubule and collecting ducts have the ability to reabsorb and secrete potassium.
• Aldosterone enhances potassium secretion in the distal tubule.

Acid-Base Balance

• The body produces acidic ions/compounds.
• In a person with a normal diet, about 50-100 mmol of hydrogen ions are generated each day.
• These must be effectively disposed of from the body; otherwise, they cause cellular damage.
• The body's acid-base (pH) balance is regulated by three systems: buffer systems (acid-base buffers), the lung, and the kidneys.
• The kidney has four mechanisms to control acid-base balance: excretion of hydrogen ions, reabsorption of bicarbonate, secretion of ammonia, and secretion of hydrogen ion.

Urea

• Ammonia is removed from amino acids by deamination reactions.
• The toxic levels of ammonia are prevented by urea production in the liver.
• Urea is generally reported as blood urea nitrogen (BUN) in lab results.
• BUN measures the amount of urea nitrogen in a patient's blood.
• High BUN levels may indicate protein-rich diets, tissue breakdown, and decreased protein synthesis.
• Low BUN levels may indicate protein-poor diets or severe liver disease.
• Urea is readily filtered. Approximately 40-50% of filtered urea is normally reabsorbed by the proximal tubule.
• BUN levels are variable due to several factors.
• BUN is not a precise indicator of renal disease.

Creatinine

• Serum creatinine levels and urinary creatinine excretion depend on muscle mass in normal persons.
• Dietary changes have little effect on serum levels.
• Creatinine is filtered freely by the glomerulus and is not absorbed by tubules.
• A small amount is secreted into urine via tubular secretion.
• Creatinine clearance can be used to estimate GFR.
• A 24-hour urine sample is used for measurement.
• Creatinine clearance is calculated using the formula: Creatinine clearance = UxV/P, where U is urinary creatinine, V is the volume of 24-hour urine, and P is plasma creatinine.
• Creatinine clearance generally parallels GFR, but it overestimates because of tubular secretion of creatinine.
• eGFR (estimated GFR) is often used instead of creatinine clearance.

Uric Acid

• Uric acid is derived from the oxidation of purine bases.
• Plasma uric acid levels are variable.
• It is completely filtered, and both proximal tubule reabsorption and distal tubular secretion may occur.
• Advanced chronic renal failure is associated with a progressive increase in uric acid levels.

Hormonal Functions of Kidneys

• Kidney is the production site of active vitamin D (1,25-dihydroxycholecalciferol).
• The enzyme responsible for vitamin D production is exclusively present in the renal cortex.
• Patients with chronic kidney disease have an exceptionally high rate of severe vitamin D deficiency.
• Kidney releases renin in response to decreased afferent arteriolar pressure or increased sympathetic nervous system activity.
• Renin is a member of the renin-angiotensin-aldosterone hormonal axis. Stimulation of this axis results in sodium conservation.
• This hormone stimulates red blood cell production. The kidney's role in EPO production explains anemia associated with chronic renal disease.

Protein Conservation

• Under physiological conditions, the kidney maintains blood proteins.
• 180 L of plasma (each liter containing 70 g of protein) is filtered each day by the glomerulus.
• A normal urine contains less than 200 mg of protein per day.
• Most of the plasma proteins (except those of very high molecular weight) can be found in the urine. Albumin excretion is less than 20-30 mg/day.
• Many proteins of non-serum origin are also found in urine. Uromucoid (Tamm-Horsfall protein-THP) is the predominant protein in urine. THP is a high-molecular-weight mucoprotein secreted by distal tubules and collecting ducts.

Proteinuria

• Glomerular proteinuria, large amounts of relatively high-molecular-weight proteins enter the glomerular filtrate and then appear in the urine.
• Heavy proteinuria (more than 2 g/day) results from increased glomerular permeability. A greater protein loss (more than 3.5 g/day) results from nephrotic syndrome.
• In tubular proteinuria, relatively low-molecular-weight proteins, which normally are filtered, appear in urine in large amounts due to impaired tubular reabsorption.
• Physiological increases in proteinuria can be seen after strenuous exercise and in normal pregnancy.

Microalbuminuria

• The first sign of renal glomerular disease is microalbuminuria, where only very small amounts of albumin are found in urine.
• The rate of urine albumin excretion (UAE) in microalbuminuria is 30 to 300 mg/d.
• It can't be detected with a dipstick. Screening patients with predisposition to renal disease (especially diabetes and hypertension) is recommended.

Kidney Function Tests

• Renal function tests (RFTs) can be evaluated as glomerular and tubular function tests.

Tests of Glomerular Function

• What is the optimal substance for measuring GFR?
• The optimal substance could be nonmetabolized, excreted only by the kidney, freely filtered by the glomerulus, and neither reabsorbed nor secreted by renal tubules.

Creatinine Clearance

• Creatinine is the most convenient compound to measure glomerular function.
• Creatinine clearance is the most convenient method to measure glomerular filtration rate.
• Clearance is a measure of the kidney's ability to remove a substance by excretion.

Advantage of Creatinine Clearance

• Creatinine is an endogenous substance.
• The amount of creatinine in the urine depends on renal excretion.
• Creatinine is freely filtered by the glomerulus, and not reabsorbed by tubules.
• It is produced at a relatively constant rate and directly proportional to body surface area.

Measurement of Osmolality

• Osmolality is measured using the freezing-point depression method.
• Dissolved salts increase osmolality by decreasing the freezing point of the solution compared to pure solvent.
• The freezing point temperature is inversely related to osmolality.

Urinalysis

• Urinalysis is a vital tool for assessing renal disease.
• Standard urinalysis includes appearance, color, pH, specific gravity, leukocytes, erythrocytes, protein, glucose, ketones, nitrite, bilirubin, urobilinogen, and microscopic examination of sediment.

Pathological Conditions of Kidney

• Acute glomerulonephritis (AG) is an acute inflammation of the glomeruli resulting in oligo- or hematuria, increased BUN and creatinine, decreased GFR, edema, and hypertension.
• Nephrotic syndrome is characterized by massive proteinuria (mostly albumin), hypoalbuminemia, hyperlipidemia, and lipiduria.
• Tubular disease results in defects of tubular function, either depressed secretion or reabsorption of specific biochemicals, or impairment of urine concentration and diluting mechanisms. Renal tubular acidosis (RTA)
• Acute renal failure (ARF) is characterized by abrupt impairment in renal function resulting in decreased GFR and/or urine output.
• Chronic kidney disease (CKD) results from progressive loss of renal function, including excretory, regulatory, and biosynthetic failures.

Test your knowledge on the nephron's roles in kidney function with this quiz. Questions cover concepts such as Glomerular Filtration Rate, water reabsorption, aldosterone effects, and electrolyte balance. Perfect for students studying renal physiology and anatomy.