Na and Water Balance

-quickly reclaims most of tubular contents and returns them to blood

Tubular reabsorption

Where does most tubular reabsorption occur?

In the proximal convoluted tubule (PCT)

What type of process is tubular reabsorption?

A selective transepithelial process

What substances are almost entirely reabsorbed during tubular reabsorption?

Almost all organic nutrients

Water and ion reabsorption is ________ regulated and adjusted.

hormonally

-Site of most reabsorption

-All nutrients, such as glucose and amino acids, are reabsorbed

Proximal convoluted tubule

______% of Na+ and water are reabsorbed in the PCT.

65%

About _____% of urea is reabsorbed and later secreted back into ______.

50%; filtrate

T/F: Almost all uric acid is reabsorbed in the PCT.

true

What can/can't leave the descending limb of the nephron loop?

Water (H₂O) can leave, but solutes cannot

What can/can't leave the ascending limb of the nephron loop?

Solutes can leave, but water (H₂O) cannot

How many segments does the ascending limb of the nephron loop have? What are they called?

2; thick segment and thin segment

Thin segment is passive to ______ movement.

Na+

What transporters (symporters and antiporters) are found in the thick segment of the ascending limb?

Symporters: Na⁺−K⁺−2Cl⁻

Antiporters: Na⁺−H⁺

What is the function of the transporters in the thick segment?

They transport Na⁺ (sodium) into the cell

Besides transporters, how else can Na⁺ enter the cell in the ascending limb?

By the paracellular route (between cells)

How is reabsorption regulated in the distal convoluted tubule and collecting duct?

Hormonally

What hormone causes principal cells of collecting ducts to insert aquaporins in apical membranes? What effect does this have?

Antidiuretic hormone (ADH); increases water reabsorption

Targets the collecting ducts (principle cells) and the distal convoluted tubule

Aldosterone

What does aldosterone promote the synthesis of?

- Apical Na⁺ and K⁺ channels and basolateral Na⁺−K⁺ ATPases for Na+ reabsorption

What is the effect of aldosterone on sodium reabsorption?

It increases Na⁺ reabsorption, and water follows, reducing Na⁺ loss in urine

Without aldosterone, daily loss of filtered Na+ would be _____%, which is incompatible with life.

2%

What are the functions of aldosterone?

Increase blood pressure and decrease K⁺ levels

-Reduces blood Na+, resulting in decreased blood volume and blood pressure

-Released by cardiac atrial cells if blood volume or pressure elevated

Atrial natriuretic peptide

Acts on DCT to increase Ca2+reabsorption

Parathyroid hormone

The wall of the afferent arteriole of the Juxtaglomerular Apparatus contains secretory cells known as what?

juxtaglomerular (JG)/granular cells.

Juxtaglomerular (granular) cells secrete this enzyme when stretch decreases...

Renin

What is the function of renin?

It influences sodium balance and blood pressure

This structure is in the wall of the ascending loop of Henle as it becomes the DCT

macula densa

What does the macula densa sense?

Changes in NaCl content of the filtrate

What is the function of the macula densa?

Helps regulate sodium balance and blood pressure.

What structures make up the juxtaglomerular apparatus (JGA)?

The macula densa and juxtaglomerular (JG) cells

This is known as reabsorption in reverse.

tubular secretion

Where are selected substances secreted from during tubular secretion?

From peritubular capillaries through tubule cells into the filtrate

Where does most tubular secretion occur?

Mostly in the distal convoluted tubule (DCT)

Tubular secretion is important for...

- Disposing drugs or metabolites, that are bound to plasma proteins

- Eliminating undesirable substances that were passively reabsorbed (e.g., urea and uric acid)

- Ridding body of excess K+ (aldosterone effect)

- Controlling blood pH by altering amounts of H+ or HCO3- in urine

What undesirable substances does tubular secretion help eliminate?

Substances like urea and uric acid that were passively reabsorbed

How does tubular secretion help regulate blood pH?

By altering the amounts of H⁺ or HCO₃⁻ (bicarbonate) in the urine

What is one of the main functions of the kidneys?

To adjust and maintain body fluid osmotic concentration around 300 mOsm

The number of solute particles in 1 kg of H₂O

Osmolality

How much is 1 osmol?

1 mole of solute particles per kg of water

How is osmolality usually expressed in the body and why?

In milliosmols (mOsm), or 0.001 osmol because body fluids have much smaller amounts

Kidneys produce _________ of urine if the body is dehydrated, and _______ urine if overhydrated.

small amounts; dilute

Fluid flows in opposite directions in two adjacent segments of same tube with hairpin turn

countercurrent mechanism

What mechanism do the kidneys use to regulate urine concentration?

The countercurrent mechanism

What happens to urine volume when the body is overhydrated?

A large volume of dilute urine is produced

What happens to ADH levels during overhydration?

ADH production decreases

What happens if aldosterone is present during overhydration?

Additional ions are removed, allowing urine to dilute further to about 50 mOsm.

What is the typical osmolality of urine when ADH is low?

Around 100 mOsm

What happens to urine volume during dehydration?

A small volume of concentrated urine is produced

What hormone is released at maximal levels during dehydration?

Antidiuretic hormone (ADH)

What is the typical osmolality of urine when ADH is at its maximum?

Around 1200 mOsm

How much water can be reabsorbed during severe dehydration?

Up to 99% of filtered water

Where does active Na⁺ reabsorption occur?

In all tubular segments except the descending limb of the loop of Henle

How does water reabsorption occur and what is it dependent on?

By osmosis (a passive process) and is dependent on Na⁺ reabsorption

What structures allow water to move through the tubules?

Aquaporin channels

Where are aquaporin channels highly expressed?

In the proximal tubules

When are aquaporins present in the collecting ducts?

Only if antidiuretic hormone (ADH) is present