Ch. 17 - Urinary Physiology

What are the 3 main functions of the kidneys?

1. Filtrate blood and remove wastes

2. Produce urine to eliminate waste

3. Maintain homeostasis of body fluids

How do the kidneys regulate the extracellular environment/fluid to maintain homeostasis?

1. Maintain volume of blood plasma affecting BP

2. Remove wastes

3. Regulate pH

4. Regulate electrolytes/ions

5. Secrete erytrhoprotein

Where is urine made?

• In kidney nephrons

What are the main regions of the kidneys?

1. Renal cortex

2. Renal medulla

nephron loops through both

What is the renal medulla made up of?

• Renal pyramids (triangles)

From the nephron in the renal medulla, what pathway does urine follow?

1. Renal pyramid

2. Minor Calyx

3. Major Calyx

4. Renal pelvis

5. Ureter

6. Urinary bladder

7. Urethra

8. Exits body

How is pushed out or transported?

• Peristalsis; circular and longitudinal contractions

What is a nephron? How many nephrons per kidney?

• Filtering unit of the kidney

Per kidney; over a million nephrons

What is a nephron densely innervated with?

• Blood vessels; peritubular capillaries

What segments is a nephron made up of?

1. Glomerulus capsule + glomerulus = renal corpuscle

2. Proximal Convoluted Tubule (PCT)

3. Descending Loop (loop of henle)

4. Ascending Loop (loop of henle)

5. Distal Convoluted Tubule (DCT)

6. Collecting duct 

After collecting duct where does urine go?

• Minor calyx and so on and so forth

What is filtrate?

• Filtered blood containing water, glucose, AA, salts, HCO3-, and urea

When does blood become filtrate in the kidney structure?

• Blood becomes filtrate; once it enters glomerulus capsule

What are the 3 main processes of urine production? Where do they each occur?

1. Filtration; glomerulus

2. Reabsorption; PCT (mostly) and loop of henle

3. Secretion; DCT

What is reabsorption versus secretion?

• Reabsorption; lumen of tubules to peritubular capillaries

• Secretion; peritubular capilarries to lumen of tubules

What is a renal corpuscle made up of?

• Glomerulus

• Bowman’s/Glomerular capsule

What is glomerulus?

• High pressure fenestrated capillaries

connects to afferent/efferent arterioles

What are fenestrated capillaries?

• Capillaries with large pores; allowing solutes to leave but keeping plasma proteins and blood cels

How is blood moved or pushed through the small and narrow glomerulus capillaries?

• Wide to narrow diameter of BV; builds pressure pushing blood filtration 

What are podocytes? Where are they located?

• Unique cells located on visceral layer of glomerlus capsule 

What structure do podocytes have? What is it called?

• Extending finger like arms/projections; pedicels

What do the pedicels of the podocytes interlock to form?

• Interlock to form filtration slits

What are the pores in the filtration slits called?

• Slit diaphragm pores

What is the function of the slit diaphragm pores?

• Major barrier for keeping of plasma proteins

What can a defect at the slit diaphragm pores cause?

• Can cause proteinuria: proteins in urine

Why is it called reabsorption in the kidney tubules?

• Have filtered out water and nutrients from blood; most will return back to the blood

inefficient

What useful substances will the blood reabsorb from the kidney tubules?

• Water, glucose, amino acids, and ions (filtrate)

How are substances transported from the kidney tubules to the blood/peritubular capillaries?

• Diffusion, passive, and active transport

How much water and ions are filtered out from blood? How much is actually excreted as urine?

• 180 L of water filtered; 1-2 L excreted

When does these 1-2 L change based on hydration levels?

• Inc; overhydration

• Dec; dehydration

Where does 85% of reabsorption occur in the kidney tubules?

Reabsorption; back into blood

• Proximal convoluted tubule

What is osmolality?

• Concentration of dissolved particles in blood

In the glomerular capsule, what is the osmolality of the filtrate versus the blood plasma in peritubular capillaries?

• Osmolality equal; isosmotic

Once filtrate enters PCT, what is actively transported? Where?

• Na+ actively transported OUT of filtrate into interstitial space to peritubular capillaries

sets up concentration gradient

What is the concentration gradient of active transport of Na+?

• Low (filtrate) to high concentration in interstitial space

As Na+ moves out of filtrate to insterstial space, what mainly follows?

• Water passively follows Na+; to maintain isosmotic fluid

What is obligatory water absorption?

• Water obliged to follow Na+ as it’s actively transported to interstitial fluid

How is Cl- also passively transporting to the interstitial space?

• Na+ actively transported into interstitial space; attract Cl-

Where does water follow Na+ and Cl- ions to?

• Follows to interstitial space. to tubular cells (peritubular capillaries)

What does the symport mechanism involve?

• Cotransport: 2 or more substances being transported at the same time

What are some ions that are cotransported with Na+?

• Cl-, Ca2+, AA, and glucose

using the same pump

In summary, what the mechanisms for reabsorption?

• Na+ actively transported (main); driving osmosis of H2O, passive transport, and symport (cotransport)

What is the descending loop of henle made up of?

• Simple squamous epithelial cells

Can water move freely in the descending loop? How?

• Yes; contain aquaproin channel proteins

allow unrestricted movement of water from descending loop to interstitial fluid

What substance is mostly entering in the descending loop?

• Urea

Where does the ascending loop start? Is it permeable to water, why?

• Thick portion

• Impermeable to water; lack of aquaporins 

What is actively transported out of the ascending loop, what ions follow?

• Na+ actively transported; Cl-, Ca2+, glucose follow 

What is hypoosmotic versus hyperosmotic?

• Hypoosmotic; lower solute concentration 

• Hyperosmotic; higher solute concentration

What does the removal of Na+ and Cl- into the interstitial space do to the osmolality of the filtrate?

• Produces hypoosmotic filtrate

removes solutes

What does the pumping of Na+ and Cl- into the interstitial space do the osmolality of the interstitial space of the kidneys?

• Contributes to hyperosmotic interstitial space/environment of kidneys

Where does the countercurrent multiplier system occur?

• Loop of henle

What does the countercurrent part of the name describe in the loop of henle?

• Descending and ascending loops are next to each other; but fluid flows in opposite directions

What does the descending loop mainly control? The ascending loop?

• Descending; control of water

• Ascending; control of ions/solutes

What does the multiplier name of the system describe in the kidneys?

• Action of solute pumps that increase Na+ and urea 

How is positive feedback occurring as Na+ is pumped out of the ascending loop?

• The more Na+ being pumped out; the more water that follows

As filtrate moves through the descending and ascending loop, what is occurring to its solute concentration?

• Water drawn out of filtrate; more solute concentrated

perfect for salt transport out of filtrate in ascending loop 

What contributes to hypertonic environment/interstitial space of kidneys?

• Na+ actively pumped out; Cl- following from ascending limb

What occurs to water in the descending limb as Na+ is actively transported out?

• Water from descending limb follows Na+; producing hypertonic filtrate

As NaCl is continuously pumped out of ascending limb, what occurs to water in descending limb?

• Water continuously drawn out of descending limb; due to greater concentration of ions in interstitial space

Out of what kidney structure, does urea diffuse out of?

• Diffuses out of collecting duct

What is the function of urea?

• Removal of salts and nitrogen in urine

What is the function of hormones in the kidneys?

• Stimulate or inhibit blood flow in kidneys

How do paracrine hormones function? Endocrine?

• Paracrine; function locally

• Endocrine; function distantly 

What is the aldosterone known as? What is its function?

• Salt retaining hormone; promotes Na+ reabsorption (reabsorbs water) 

What integrating center commands the release of aldosterone? In response to what?

• Released from adrenal cortex; in response to angiotensin II or inc plasma K+

when BP low

What is the function of the antidiuretic hormone (ADH)?

• Promotes recovery of water, dec. urine volume, and maintains blood pressure

What is the negative feedback from ADH with lower water intake, dehydration?

• Inc. ADH in blood; increasing water reabsorption

less water in urine; more water in blood

What is the negative feedback from ADH with more water intake, overhydration?

• Dec. ADH in blood; de. water reabsorption

more water in urine; less water in blood

What is renal clearance?

• Kidneys removing excess ions and wastes from blood

What initiates the process of renal clearance?

• Filtration; from glomerulus to glomerulus capsule

What occurs to renal clearance in reabsorption?

• Reabsorption; in PCT returning water and ions back into blood DEC. RENAL CLEARANCE 

What occurs to renal clearance in secretion?

• Secretion; substances from peritubular capillaries moving into tubules INC. RENAL CLEARANCE 

What does excretion rate measure?

• Measures glomerular filtration rate, indicator of renal health 

What two types of membrane carriers specific to foreign substances secrete drugs into kidney tubules?

• Organic anion transporters

• Inorganic anion transporters

What do kidneys match electrolyte/ion secretion to?

• Match electrolyte secretion; to ingestion

What does the control of Na+ affect?

• Affect BP and BV

What does the control of K+ affect?

• Affect skeletal /cardiac muscle 

What does electrolytes does aldosterone balance?

• Balances Na+ and K+

When BP is high, what do the kidneys do?

• Kidneys reduce reabsorption of water; reducing blood volume, thus BP

reduce reabsorption; inc. water in urine

When BP is low, what do the kidneys do?

• Kidneys produce renin

What is the function of renin? What hormone does it use?

• Triggers/stimulates vasoconstriction of BV, inc. BP

uses angiotensin II to constrict

How do kidneys maintain blood pH? What pH does this make urnie?

• Reabsorb bicarbonate (HCO3-) into blood; secrete H+ ions in kidneys

urine acidic

What pump does the proximal convoluted tubule use making urine acidic? What is this known as?

• Na+/H+ pump; Na+ OUT of kidneys, H+ IN

Antiport secondary active transport

Can HCO3- (bicarbonate) cross the kidney membrane? What must it be converted to?

• Cannot cross; must combine with H+ —> H2CO3 —> CO2 and H2O 

CO2 can cross 

What reaction does carbonic anhydrase specifically catalyze?

• H2CO3 —> H2O and CO2

Once CO2 enters interstitial space, what does it convert back into?

• Picks up H2O; H2CO3 —> HCO3- and H+

What pH of urine can nephrons not produce?

• Cannot product urine with ph BELOW 4.5

Since nephrons cannot produce really acidic urine, what must be done to it? Using what?

• Must be buffered; use phosphates and ammonia

How does phosphate and ammonia buffer, or make urine less acidic?

• Combine with H+ ions 

How do phosphates enter urine?

• Via filtration

How does ammonia enter urine?

• From deamination of amino acids