mod 4

What does ECF include?

lasma, interstitial fluid, lymph, transcellular fluid

What separates plasma and interstitial fluid?

capillary walls

What can cross between them?

everything except proteins

What separates ICF and ECF?

cell (plasma) membrane

Ion distribution (VERY TESTED):

K⁺ high in ICF
→ Na⁺ high in ECF

Why regulate ECF volume?

maintains blood pressure

Why regulate ECF osmolarity?

prevents cell swelling or shrinking

ECF volume ↑ → what happens?

plasma volume ↑ → BP ↑

What detects BP changes?

baroreceptors (carotid + aortic arch)

xvIf BP ↓ → what happens?

CO ↑ + TPR ↑ → BP ↑

If BP ↑ → what happens?

CO ↓ + TPR ↓

If plasma volume ↓ → what happens?

fluid moves from interstitial → plasma

If plasma volume ↑ → what happens?

fluid moves to interstitial

Who controls long-term ECF volume?

kidneys + thirst

why is Na important in regulating ECF volume

Key concept:
→ salt movement → water follows (osmosis)

BIG IDEA:
→ controlling salt = controlling ECF volume

Salt output pathways:

feces, sweat, kidneys

what is hypertonicity

water enters the cell and decreasesi n the ecf

cells shrink

what is hypotonic

water leaves the cell and increases in the ecf

cells swell

Why ADH causes hypotonicity?

ncreases water retention

in the isotonic solution where does fluid go first

plasma (ECF) and increases volume

isotonic effect on osomalarity and cells

Effect on osmolarity?
→ no change

Effect on cells?
→ no swelling or shrinking

Where are osmoreceptors?

hypothalamus

what happens when osmolarity increases

ADH ↑
→ thirst ↑

what happens if osomolarity decrease

ADH ↓
→ thirst ↓
→ ↑ water loss

wjat is the short term regulation of fluid balance

• baroreceptor reflex

• fluid shifts

• CO + TPR changes

longterm regulation fo fluid balance

• kidneys (urine output)

• thirst

What are the basic structural regions of the kidney?

Cortex (outer)
→ Medulla (inner)
→ Renal pelvis (center, drains urine)

What is the role of the renal pelvis?

Collects urine and funnels it into the ureter

What are the 2 major components of the nephron?

• Vascular component → handles blood

• Tubular component → handles filtrate (urine formation)

What is the glomerulus?

A ball of capillaries where filtration occurs

What is the role of the efferent arteriole?

Carries blood AWAY from glomerulus
→ then forms peritubular capillaries

What do peritubular capillaries do?

→ Deliver O₂ to kidney tissue
→ Receive reabsorbed substances

Where does filtration first enter the tubule?

Bowman’s capsule

Where does the collecting duct lead?

Renal pelvis → urine exits kidney

What is the juxtaglomerular apparatus (JGA)?

→ Region where distal tubule contacts afferent/efferent arterioles
→ Important for regulation (GFR + BP)

what are characteristics of cortical nephrons

• outer cortex, loop of henle barely dip into the medulla

what are characteristics of the juxtamedullary nephrons

inner cortex, maintains urine concentration and dilution

long loop of henle

What are the 3 basic renal processes? (KNOW THIS PERFECTLY)

• Glomerular filtration (GF)

• Tubular reabsorption (TR)

• Tubular secretion (TS)

what gets filtered

Water + small solutes
→ NO proteins

what gets reabsorbed

glucose, water, Na⁺

what is the glomerular capilalry wall of the glomerulus

• has large pores

• very permeable (100x normal capillaries)

what is the basement membrane of the glomerulus

• made of collagen + glycoproteins

• negatively charged → repels proteins

what is the inner layer of the bowman’s capsule

• form filtration slits

Why don’t proteins get filtered easily?

size + negative charge barrier

what is the glomerular capillary blood pressure

→ pushes fluid OUT of capillaries
→ promotes filtration

what is the Plasma-colloid oncotic pressure

due toplasma proteins in blood
→ pulls water INTO capillaries
→ opposes filtration

what ist he bowman’s capsule hydrostatic pressure

→ pressure from fluid already in capsule
→ pushes back on incoming fluid
→ opposes filtration

how do u calculate gfr

GFR=Kf​×NFP

What is Kf (filtration coefficient)?

• surface area

• permeability of membrane

Which pressures usually stay constant?

oncotic pressure + Bowman’s pressure

What is autoregulation?

kidney maintains stable GFR despite BP changes

by changing afferent arteriole diameter

how does afferent arterioel diameter change gfr

• Afferent constriction → ↓ GFR

• Afferent dilation → ↑ GFR

What triggers the myogenic response?

Stretch of the afferent arteriole

what does myogenic response do for bp changes

If BP increases → what happens?
→ arteriole constricts → prevents excessive filtration
If BP decreases → what happens?
→ arteriole dilates → maintains filtration

What structure detects changes in filtrate in the tubuloglomerular feedback?

Macula densa → NaCl concentration in tubule

what happens if BP is too high in the TGF

• more filtrate and Na+

• ATP secreted into adenosine

→ afferent constricts
→ ↓ GFR

When is sympathetic control activated?

When is sympathetic control activated?

constricts arteriole

why is cardiac output so high for kidneys

for filtration, not oxygen delivery

How does tubular reabsorption differ from glomerular filtration?

filtration is mostly non selective, while reabsorption is highly selective

What are the TWO main steps of tubular reabsorption?

Step 1: Substances move from the tubular lumen into the interstitial fluid (via active or passive transport).
Step 2: Substances then move from the interstitial fluid into the peritubular capillaries (back into the blood).

What is transepithelial transport?

Transepithelial transport is the movement of substances across an epithelial cell layer, specifically from the tubular lumen, through epithelial cells, into the interstitial fluid.

What are the two sides of epithelial cells in the nephron?

• Luminal membrane: Faces the tubule lumen (filtrate side)

• Basolateral membrane: Faces the interstitial fluid (blood side)

Why must substances move through epithelial cells instead of between them?

Because epithelial cells are connected by tight junctions, substances cannot easily pass between cells and must go through the cells themselves.

What are the FIVE steps of transepithelial transport?

1. Substance crosses the luminal membrane into the epithelial cell

2. Substance moves through the cytosol of the cell

3. Substance crosses the basolateral membrane

4. Substance diffuses through the interstitial fluid

5. Substance crosses the capillary wall into the blood

Where does MOST sodium reabsorption occur?

n the proximal tubule, then some in the ascending loop of henle

How is sodium transported across the basolateral membrane?

Sodium is actively transported out of the epithelial cell into the interstitial fluid using the Na⁺/K⁺ ATPase pump.

Why is the Na⁺/K⁺ ATPase pump important for reabsorption?

It keeps intracellular sodium levels low, which allows sodium to continue diffusing into the cell from the tubular lumen.

How does sodium enter epithelial cells from the tubular lumen?

Sodium enters passively down its concentration gradient through channels or transporters on the luminal membrane.

How is sodium linked to glucose and amino acid reabsorption?

Sodium moves into the cell together with glucose or amino acids via cotransporters, allowing these nutrients to be reabsorbed against their concentration gradients (secondary active transport).

How does sodium transport differ in the collecting duct?

In the collecting duct, sodium enters epithelial cells through sodium channels rather than cotransporters.

Why is sodium considered the “driver” of tubular reabsorption?

Sodium reabsorption creates osmotic and electrochemical gradients that allow water and many other solutes (like glucose and amino acids) to be reabsorbed along with it.

What are the THREE main hormones involved in regulating Na⁺ reabsorption?

• Renin (via RAAS system)

• Aldosterone

• Atrial Natriuretic Peptide (ANP)

When is renin released?

Renin is released when blood pressure, blood volume, or sodium levels are low.

What is the role of renin in sodium regulation?

enin initiates the renin-angiotensin-aldosterone system (RAAS), which ultimately increases sodium reabsorption to restore blood volume and pressure.

What is aldosterone and where is it produced?

Aldosterone is a hormone produced by the adrenal cortex that increases sodium reabsorption in the kidneys.

How does aldosterone increase Na⁺ reabsorption?

It increases the number and activity of sodium channels on the luminal membrane and Na⁺/K⁺ ATPase pumps on the basolateral membrane.

What happens to water when aldosterone increases sodium reabsorption?

Water follows sodium by osmosis, leading to increased water reabsorption and increased ECF volume.

What ions are secreted when aldosterone is active?

Potassium (K⁺) and hydrogen ions (H⁺) are secreted into the tubule.

When is ANP released?

ANP is released when blood volume is high and the atria of the heart are stretched.

What is the effect of ANP on sodium reabsorption?

ANP decreases sodium reabsorption in the kidneys, promoting sodium excretion.

What is the overall effect of ANP on the body?

It reduces blood volume and blood pressure by promoting sodium and water loss.

How is phosphate reabsorbed in the kidney?

Phosphate is reabsorbed via sodium-phosphate cotransporters in the proximal tubule.

ow is glucose normally handled by the kidneys?

Glucose is completely reabsorbed in the proximal tubule under normal conditions, so none appears in urine.

How is glucose reabsorbed?

Through sodium-glucose cotransporters (secondary active transport) on the luminal membrane.

What happens if glucose exceeds its Tm?

The transporters become saturated, and excess glucose is not reabsorbed and instead appears in the urine.

How is chloride (Cl⁻) reabsorbed?

Chloride usually follows sodium passively due to electrical and concentration gradients.

How is water reabsorbed in the nephron?

Water follows solutes (especially sodium) by osmosis, moving passively across membranes.

How is urea handled in the kidney?

Urea is partially reabsorbed, and it contributes to the osmotic gradient in the medulla.

What is tubular secretion?

Tubular secretion is the movement of substances from the peritubular capillaries into the tubule lumen. It is basically the opposite of tubular reabsorption, although it still occurs by transepithelial transport. It serves as an additional route, besides glomerular filtration, for removing substances from the body.

What types of substances are commonly secreted?

• Hydrogen ions (H⁺)

• Potassium ions (K⁺)

• Organic anions and cations (many are foreign substances)

Where does H⁺ secretion occur in the nephron?

• Proximal tubule

• Distal tubule

• Collecting tubules

What determines how much H⁺ is secreted?

The acidity of the plasma.

How is K⁺ handled in the nephron overall?

• Freely filtered at the glomerulus

• Actively reabsorbed in proximal tubule

• Then variably secreted later

Where does regulated K⁺ secretion occur?

Distal tubule + collecting duct

What hormone increases K⁺ secretion?

Aldosterone stimulated by increasing plasma K+

increases Na+ reabsorption

How does acid-base balance affect K⁺ secretion?

H⁺ and K⁺ compete at the Na⁺-K⁺-ATPase

What happens during acidosis (↑ H⁺)?

• More H⁺ handled

• Less K⁺ secreted
  → K⁺ builds up in plasma

Where are organic ion secretion systems located?

Proximal tubule

Why can’t filtration alone remove many organic ions?

• They are bound to plasma proteins

• Proteins are not filtered