CH 25 - Urinary System
What is the Colloid osmotic pressure in capillaries (OPgc)?
"Pull" of proteins in blood
What are the hormones that regulate DCT and collecting duct?
1. **ADH** = promotes reabsorption of H2O 2. **Aldosterone** = promotes reabsorption of Na+ (therefore H2O) 3. **Atrial natriuretic peptide (ANP)** = inhibits reabsorption of Na+ (opp. of aldosterone) 4. **PTH** = promotes reabsorption of Ca2+
What chemicals enhance urinary output?
1. ADH inhibitors (alcohol) 2. Na+ reabsorption inhibitors (coffee, drugs) 3. Loop diuretics = inhibit medullary gradient formation 4. Osmotic diuretics = no substances reabsorbed lots of water stays in urine
Why is the BP in glomerulus high?
1. Afferent arterioles larger in diameter than efferent arterioles 2. Arterioles are high-resistance vessels
What does the ascending limb of the nephron loop do? What segments does this include?
H2O cannot leave; Solutes can (selective) 1. Thin segment = passive Na+ movement 2. Thick segment = active Na+-K+-2Cl-symporter and Na+-H+ antiporter; some passes paracellular route
Where does water enter the ascending vasa recta from?
H2O entering ascending vasa recta from descending vasa recta or reabsorbed from nephron loop and collecting duct
What does the descending limb of the nephron loop do?
H2O leaves; solutes cannot - Filtrate osmolarity increases
What are the major excretory organs?
Kidneys
What is the vasa recta?
Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons Arise from efferent arterioles serving juxtamedulllary nephrons *instead of peritubular capillaries*
What are Pertibular Capillaries?
Low-pressure, porous capillaries adapted for absorption Cling to adjacent renal tubules in cortex
What is the Renin-Angiotensin-Aldosterone Mechanism?
Main mechanism for increasing BP
What is the goal of the intrinsic controls in GFR?
Maintain GFR in kidney
What is the goal of the extrinsic controls in GFR?
Maintain systemic blood pressure ( increased GFR = Inc urine output = dec. BP)
What do the extraglomerular mesential cells do?
May pass signals between macula densa and granular cells
What is the most abundant cation in filtrate?
Na+
What are the granular cells (JG cells)?
- Secretory granules contain enzyme renin - Mechanoreceptors; sense BP in afferent arteriole
What are juxtamedullary nephrons?
-Long nephron loops *deeply invade medulla* -*Ascending limbs have thick and thin segments* -Important in production of *concentrated urine*
what are results when there is a homeostatic imbalance in the GFR?
1. Chronic renal disease (GFR < 60) 2. Renal failure (GFR < 15) - Causes uremia syndrome - Treated with hemodialysis 3. Renal calculi (stones) - Crystallized Ca+, Mg, or uric acid salts - Treatment shock wave lithoripsy
What are the classes of nephrons?
1. Cortical nephron 2. Juxtamedullary nephron
What are the three pathways to renin released by granular cells?
1. Direct stimulation of granular cells by SNS 2. Stimulation by activated macula densa cells when filtrate NaCl concentration low 3. Reduced stretch of granular cells
What are the two parts of the nephron loop?
1. Distal descending limb 2. Thick ascending limb
What are the 3 layers of the filtration membrane?
1. Fenestrated Endothelium 2. Basesment Membrane 3. Foot Processes of Podocytes with filtration slits; slit diaphragms repel macromolecules
What are the 3 processes in urine formation and adjustment of blood composition?
1. Glomerular filtration 2. Tubular reabsorption 3. Tubular secretion
What are the two parts of the Renal Corpuscle?
1. Glomerulus = Highly porous and allows filtrate formation 2. Bowman's Capsule = Cup-shaped, hollow structure surrounding glomerulus
What are the cells that compose the juxtaglomerular complex?
1. Granular cells 2. Extraglomerular mesangial cells 3. Macula densa cells
What regulates the Glomerular Filtration?
1. Intrinsic Controls = act locally within kidney to maintain GFR 2. Extrinsic Controls = Nervous and endocrine mechanisms that maintain BP
What are the two types of Renal autoregulation (intrinsic control)?
1. Myogenic Mechanism 2. Tubuloglomerular Feedback Mechanism
What is the Glomerular Filtration Rate (GFR) directly porpotional to?
1. NFP = primary pressure is hydrostatic pressure in glomerulus 2. Total SA available for filtration = glomerular mesangial cells control by contracting 3. Filtration membrane permeability = much more permeable than other capillaries
What are the two cell types of the collecting ducts?
1. Principal cells = maintain H2O and Na+ balance 2. Intercalated cells = acid-base regulation
Define the 3 parts of the Renal Tubule
1. Proximal Tubule (closest to renal corpuscle) 2. Nephron loop (loop of Henle) 3. Distal Tubule (furthest from renal corpuscle)
What are the functions of the kidneys?
1. Regulate total H2O volume & total solute concentration in H2O 2. Regulating ECF ion concentrations 3. Ensuring long-term acid-base balance 4. Removal of metabolic wastes, toxins, drugs 5. Endocrine Functions: - Renin - Erythropoetin 6. Activation of vitamin D 7. Gluconeogenesis during prolonged fasting
What are the two main parts of the nephrons? What are they responsible for
1. Renal Corpuscle = collection of filtrate 2. Renal Tubule = Secretion and reabsorption role; carries filtrate to collecting ducts
What are the two types of extrinsic controls?
1. Sympathetic Nervous System 2. Renin-Angiotensin-Aldosterone Mechanism
How is Aldosterone responsible in the reabsorptive capabilities of renal tubules and collecting ducts?
1. Targets collecting ducts and distal DCT 2. Promotes synthesis of luminal Na+/K+ channels and basolateral Na+-K+ ATPases 3. Functions - Increase BP - Secrete K
What are the two routes in tubular reabsorption?
1. Transcellular route 2. Paracellular route
What composes the nitrogenous wastes?
1. Urea (largest solute) 2. Uric aid 3. Creatinine
What is the normal Glomerular Filtration Rate (GFR)?
120-125 ml/min
What are cortical nephrons?
85% of nephrons; almost entirely in cortex
What is urine?
<1% of total filtrate Contains metabolic wastes and unneeded substances
What is the paracellular route?
Between tubule cells - limited by tight junctions, but leaky in proximal nephron
How is PTH responsible in the reabsorptive capabilities of renal tubules and collecting ducts?
Acts on DCT to increase Ca2+ reabsorption
What is filtrate?
Blood plasma minus proteins
What is the transcellular route?
Apical membrane of tubule cells --> cystol of tubule cells --> basolateral membranes of tubule cells --> endothelium of peritubular caps
What happens in intrinsic controls if the MAP range of 80-180 mm Hg is not achieved?
Autoregulation ceases
How is ADH responsible in the reabsorptive capabilities of renal tubules and collecting ducts?
Causes principal cells of collecting ducts to insert aquaporins = promote H2O reabsorption
What happens if the solute concentration in filtrate increases?
Concentration gradients for solutes increase
What are the organic nutrients reabsorbed by secondary active transport?
Cotransported with Na+ - Glucose - AA - ions - Vitamins
What is the difference between countercurrent multiplier and countercurrent exchange?
Countercurrent Multiplier = interaction of filtrate flow in ascending/descending limbs (of juxtamedullary nephrons) Countercurrent Exchange = Blood flow in ascending/descending limbs of vasa recta
What is the vasa recta an example of? What is it?
Countercurrent exchanger 1. Preserve medullary gradient - NaCl is not rapidly removed from interstitial space
Where is regulated reabsorption and secretion occuring?
DCT and collecting ducts
What is the Tubuloglomerular Feeback Mechanism?
Directed by macula densa cells Respond to filtrate NaCL concentration If GFR inc. = filtrate flow rate inc. = dec. reabsorption time = high filtrate NaCL levels = constriction of afferent arteriole = dec. NFP and GFR = more time for NaCl reabsorption
What makes the nephron capillary beds different?
Fed and drained by arteriole - Afferent arteriole --> glomerulus --> efferent arteriole
Since fat-soluble substances, ions, and urea are harder to excrete, what happens to them?
Follow water into peritubular capillaries down concentration gradients
What is the function of the vasa recta?
Formation of concentrated urine
What is the function and characteristic of the Distal convoluted tubule (DCT)?
Function more in secretion than reabsorption Confined to cortex
What are the functions and characteristics of the Proximated convoluted tubule (PCT)?
Functions in reabsorption and secretion Confined to cortex
What is the Hydrostatic pressure in glomerular capillaries
Glomerular BP - Chief force pushing water, solutes out of blood
In the kidney, how does it affect the blood?
It cleanses blood and adjusts its composition
What happens if C = 125 ml/min?
No net reabsorption/secretion
What occurs in the sympathetic nervous system if extracellular fluid volume is extremely low?
Norepinephrine & epinephrine releasesd = - Systemic vasoconstriction = inc. BP - Constriction of afferent arterioles = dec. GFR = Inc. blood volume and BP
What is the purpose of tubular secretion?
Occurs in PCT (Blood to Tubules) 1. Eliminates undesirable substances 2. Rids body of excess K+ 3. Controls blood pH by altering amounts of H+/HCO3- in urine
What is a countercurrent mechanism?
Occurs when fluid flows in opposite directions in two adjacent segments of the same tube
In the juxtaglomerular complex, what is the macula densa?
One of three cell populations - Tall, closely packed cells of ascending limb *Chemoreceptors;* sense NaCL content of filtrate - *can sense filtration flow*
What results from the movement of Na+ and other solutes?
Osmotic gradient for H2O created
What pressures affect filtration?
Outward = Hydrostatic pressure in glomerular capillaries 2. Inward = - Hydrostatic pressure in capsular space (HPcs) - Colloid osmotic pressure in capillaries (OPgc)
What is the difference in urine when a person is overhydrated? Dehydrated?
Overhydrated = large volume of dilute urine - ADH production down - Aldosterone present? = more ions removed Dehydrated = small volume of concentrated urine - ADH released
Where is most reabsorption and secretion occuring?
PCT
What is the site for the most reabsorption?
PCT - All nutrients - 65% H2O
Tubular secretion occurs in
PCT with selected substances moving from peritubular capillaries into filtrate
What is the filtration membrane?
Porous membrane between the blood and the capsular space - No cells pass normally - H2O, solutes smaller than plasma proteins pass
What is the Hydrostatic pressure in capsular space (HPcs)?
Pressure of filtrate in capsule
How does Na+ transport across the basolateral membrane?
Primary active transport out of tubule cell by Na+K+ ATPase pump --> peritubular capillaries
What is Glomerular filtration?
Produces cell- and protein-free filtrate Passive process Hydrostatic pressure = forces fluids and solutes through filtration membrane NO reabsorption
If Na+ is reabsorbed through primary active transports what does this result in?
Provides energy and means for reabsorbing other substances = creates electrochemical gradient
What is the function of the collecting ducts?
Receive filtrate from many nephrons
How is ANP responsible in the reabsorptive capabilities of renal tubules and collecting ducts?
Reduces blood Na+ = decreased blood volume and BP
What is omsolality? How is it regulated?
Reflects ability to cause osmosis - Kidneys maintain osmolality of plasma by regulating urine concentration & volume - Kidneys regulate with countercurrent mechanism
Why is the juxtaglomerular complex important?
Regulates rate of filtrate formation and BP
What are the plasma proteins?
Remain in blood and maintain colloid osmotic pressure = prevents loss of all water to capsular space
What is the granular-appearing superficial region of the kidney?
Renal Cortex
What is the part of the urinary system that is composed of cone-shaped medullary (renal) pyramids? What are they separated by?
Renal Medulla - Pyramids are separated by renal columns = inward extensions of cortical tissue
What is the funnel-shaped tube continuous with ureter?
Renal pelvis
How does Na+ transport across the apical membrane?
Secondary active transport or faciliated diffusion mechanisms
What is Tubular secretion?
Selectively moves substances from blood to filtrate in renal tubules and collecting ducts
What is Tubular reabsorption?
Selectively returns 99% of substances from filtrate to blood in renal tubules and collecting ducts
What occurs in the Myogenic mechanism?
Smooth muscles contract when stretched - Inc. BP = muscle stretch = constriction of afferent arterioles = restricts blood flow into glomerulus - Dec. BP = dilation of afferent arterioles
What happens if C = 0?
Substance completely reabsorbed/not filtered
What happens if C < 125 ml/min?
Substance reabsorbed
What happens if C > 125 ml/min?
Substance secreted
What is the Net filtration pressure (NFP)? What is the Net outward force?
Sum of forces Net outward force of 10 mm Hg
What is the juxtaglomerular complex?
There's one per nephron Involves modified portions of *distal portion of ascending limb of nephron loop*
What happens to macromolecules in the filtration membrane?
They get stuck; recall normal cells cannot pass Only H2O, glucose, AA, nitrogenous wastes
What is the renal clearance of insulin?
This standard used renal clearance = GFR = 125 ml/min
What is tubular reabsorption?
Tubules to blood. Most of tubular contents reabsorbed to blood - All organic nutrients reabsorbed - H2O and ion reabsorption Includes active and passive tubular reapsorption
What transports urine from kidneys to urinary bladder?
Ureters
What transports urine out of body?
Urethra
What is the temporary storage reservoir for urine?
Urinary bladder
What is renal clearance? What is it an indicator of?
Volume of plasma kidneys clear of particular substance in given time - Used to determine GFR
What are aquaporins?
Water-filled pores = always present in PCT --> obligatory water reabsorption = inserted in collecting ducts IF ADH present = faculatative H2O reabsorption
