Lecture 20: Renal 3 — Reabsorption, Secretion & Excretion -

What is glomerular filtration?

Movement of fluid from blood in the glomerulus into the glomerular capsule.

What is tubular reabsorption?

Movement of useful substances from kidney tubules back into the blood.

What is tubular secretion?

Movement of selected substances from blood into kidney tubules.

What is excretion?

Removal of substances from the body in urine.

How much fluid is filtered by the kidneys per day?

About 180 liters.

About how much urine is produced per day according to the overview slide?

About 1.5 liters.

Where does filtrate enter after glomerular filtration?

The proximal convoluted tubule (PCT).

What blood vessels receive substances reabsorbed from renal tubules?

Peritubular capillaries.

Why are peritubular capillaries located close to kidney tubules?

To allow substances to move between the tubules and blood.

What does a net reabsorption pressure of −20 mmHg favor?

Movement of fluid into peritubular capillaries (reabsorption).

What does HP mean in the renal pressure equation?

Hydrostatic pressure.

What does OP mean in the renal pressure equation?

Oncotic pressure.

Why is tubular reabsorption essential?

It returns useful water and solutes to the blood instead of losing them in urine.

Where does most tubular reabsorption occur?

The proximal convoluted tubule (PCT).

Approximately what percentage of filtrate is reabsorbed in the PCT?

About 65–67%.

What percentage of filtered glucose is normally reabsorbed?

About 100%.

What percentage of filtered amino acids is normally reabsorbed?

About 100%.

Approximately what percentage of filtered water is reabsorbed in the PCT?

About 65%.

Is reabsorption of every ion fixed at the same percentage?

No. Ion reabsorption varies depending on the ion and the body's needs.

What epithelial cells line the PCT?

Simple cuboidal epithelial cells with microvilli.

Why do PCT cells have microvilli?

To increase surface area for reabsorption.

Does tubular reabsorption use only passive transport?

No. It uses both passive and active transport; much is active.

What can cross the thin descending limb of the loop of Henle easily?

Water.

Is the thin descending limb of the loop of Henle permeable to NaCl?

No. The slide describes it as impermeable to ions such as Na+ and Cl−.

What is the thick ascending limb of the loop of Henle permeable to?

Ions, including Na+ and Cl−.

Is the thick ascending limb permeable to water?

No. It is impermeable to water.

What does DCT stand for?

Distal convoluted tubule.

What does CD stand for in these slides?

Collecting duct.

What does ADH stand for?

Antidiuretic hormone, also called vasopressin.

What does ANP stand for?

Atrial natriuretic peptide.

Which later nephron regions have hormone-regulated reabsorption?

The distal convoluted tubule and collecting duct.

What does the Na+/K+ ATPase pump use to transport ions?

ATP (primary active transport).

What is primary active transport?

Movement of substances using energy directly from ATP.

What is secondary active transport?

Transport powered by an ion gradient created by another active transporter.

What does SGLT transport in the PCT?

Sodium and glucose together.

What is the role of the Na+/K+ ATPase in PCT reabsorption?

It maintains the sodium gradient that helps drive transport of other substances.

What does 'water follows solutes' mean?

Water moves by osmosis toward regions with more dissolved particles.

How can increased sodium reabsorption affect blood volume?

Water follows sodium, so blood volume can increase.

Which ions are especially important for regulation of extracellular fluid osmolarity?

Na+, K+, and Cl−.

Which two substances are important for renal pH regulation?

H+ and HCO3− (bicarbonate).

What does GFR stand for?

Glomerular filtration rate.

What happens to GFR initially when arterial pressure falls?

GFR tends to decrease.

What structure senses NaCl delivery in tubuloglomerular feedback?

The macula densa of the juxtaglomerular apparatus (JGA).

What happens to NaCl delivery to the macula densa when pressure and tubular flow decrease?

NaCl delivery decreases.

How does afferent arteriole dilation affect glomerular hydrostatic pressure?

It increases glomerular hydrostatic pressure.

How does increasing glomerular hydrostatic pressure affect GFR?

It tends to increase GFR.

What hormone/enzyme is released by juxtaglomerular cells during low-pressure RAAS activation?

Renin.

What does RAAS stand for?

Renin–angiotensin–aldosterone system.

What major vasoconstrictor is produced through RAAS?

Angiotensin II.

What does angiotensin II do to systemic blood vessels?

It causes vasoconstriction, increasing blood pressure.

Which hormone increases sodium reabsorption in the DCT and collecting duct?

Aldosterone.

How does aldosterone increase blood volume?

It increases sodium reabsorption; water follows sodium into the body.

What happens to potassium excretion when aldosterone acts?

Potassium excretion increases.

What are the two intrinsic mechanisms of GFR autoregulation?

Myogenic regulation and tubuloglomerular feedback.

What is the purpose of intrinsic GFR autoregulation?

To help maintain GFR despite changes in blood pressure.

What mean arterial pressure range is given for intrinsic autoregulation?

Approximately 80–180 mmHg.

Which systems provide extrinsic regulation of GFR and blood pressure?

The sympathetic nervous system and hormones such as angiotensin II, aldosterone, and ADH.

During extreme blood-pressure changes, which regulation dominates?

Extrinsic regulation.

What reaction links carbon dioxide to bicarbonate and hydrogen ions?

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3−.

What enzyme speeds the carbon dioxide–bicarbonate reaction?

Carbonic anhydrase.

What is the purpose of bicarbonate reabsorption?

To help buffer acids and regulate blood pH.

In which nephron regions can hydrogen ions be secreted?

PCT, DCT, and collecting duct.

What do type A (alpha) intercalated cells do during acidosis?

Excrete H+ and reabsorb HCO3− and K+ (as described in the slides).

What do type B (beta) intercalated cells do during alkalosis?

Reabsorb H+ and secrete HCO3− and K+ (as described in the slides).

How do type A and type B intercalated cells differ?

They move H+ and HCO3− in opposite directions.

Name one substance that can be secreted into kidney tubules.

Potassium, hydrogen ions, phosphate, creatinine, choline, or penicillin.

Why is potassium secretion important?

It helps regulate membrane excitability.

Why is hydrogen ion secretion important?

It helps regulate blood pH.

How do you calculate the amount of a substance excreted?

Excreted = filtered − reabsorbed + secreted.

What is obligatory urine?

The minimum urine volume needed to eliminate wastes, even during dehydration.

About how much obligatory urine must be produced per day?

Approximately 500 mL.

Approximately what fraction of filtrate volume is reabsorbed overall?

About 99%.

What does ADH do in the collecting duct?

Increases water reabsorption by increasing aquaporins in principal cells.

What stimulates ADH release according to the concept review?

High body-fluid osmolarity.

What triggers ANP release?

Stretching of the atria due to high blood volume or pressure.

How does ANP affect sodium and water excretion?

It promotes sodium excretion, and water follows, increasing urine output.

What happens to urine output when ADH increases?

Urine output generally decreases because more water is reabsorbed.

What happens to urine output when ANP increases?

Urine output generally increases because more sodium and water are excreted.

What is hydrostatic pressure?

The pressure exerted by fluid against a vessel wall.

What does hydrostatic pressure do in peritubular capillaries?

It pushes fluid out of the capillaries and opposes reabsorption.

What is oncotic pressure?

The osmotic pulling force created mainly by proteins in the blood.

What does oncotic pressure do in peritubular capillaries?

It pulls water into the capillaries and favors reabsorption.

What is the difference between hydrostatic and oncotic pressure?

Hydrostatic pressure pushes fluid out of capillaries; oncotic pressure pulls fluid into capillaries.

What does NRP stand for?

Net reabsorption pressure.

How do you calculate net pressure using peritubular capillary hydrostatic and oncotic pressures?

Using the simplified model: hydrostatic pressure minus oncotic pressure, with outward movement defined as positive.

If hydrostatic pressure is 15 mmHg and oncotic pressure is 35 mmHg, what is the net pressure?

15 - 35 = -20 mmHg, which favors reabsorption into the capillaries.

What does a negative net pressure mean in the peritubular capillaries?

Fluid moves into the capillaries; reabsorption is favored.

What happens to peritubular capillary reabsorption if oncotic pressure increases?

Reabsorption increases because the inward pulling force becomes stronger.