Ch 26-29 Urine Formation & Electrolyte Regulation Flashcards
What percentage of blood flow do the kidneys require?
About 22%
With which region of the kidney tubules is the macula densa associated?
The thick ascending loop of henle (at the end)
What percentage of nephrons are cortical vs juxtamedullary?
20-30% are juxtamedullary
What are the vasa recta?
They are specialized peritubular capillaries in the outer medulla that aid in developing concentrated urine
What is the name of the urinary bladder’s smooth muscle?
detrusor muscle
Describe the pathway of the sympathetic innervation, (hypogastric nerve) of the bladder from SC segment to neuromuscular junction
From L2, to sympathetic chain ganglion, to the body of the bladder, mostly blood vessels. Some sensory innervation to detect pain and fullness
Which is the principle nerve supply of the bladder? What it its pathway?
The pelvic nerves from the sacral plexus at S2 and S3 then split into the sensory and motor nerve fibers. The Sensory fibers detect stretch and synapse on the body, neck, and external sphincter. The motor PSNS fibers terminate at the bladder wall to ganglion cells.
What is the route and function of the pudendal nerve to the bladder?
Innervates skeletal motor fibers, traveling from the pudendal nerve to the external sphincter for voluntary control
What is the ureterorenal reflex?
When a ureter is blocked, the smooth muscle reflexively constricts powerfully, causing pain. The pain impulses cause a sympathetic reflex that causes renal arteriolar constriction to reduce urine production
How is urine propelled through the collecting ducts to the ureter?
It enters the calyces and stretches them, initiating pacemaker activity, which causes further contractions until the urine enters the bladder
In a stepwise description, how does the micturition reflex occur?
- Urine fills bladder and initiates a stretch reflex via sensory stretch receptors in the posterior urethra
- Stretch receptor signals conduct through the pelvic nerves to the sacral SC
- Signals travel back through PSNS fibers via the same nerves.
- If the bladder keeps filling, these reflexes become more frequent and cause greater contractions of the detrusor muscle
- The reflex is self-regenerative until the bladder reaches a strong degree of contraction, until it fatigues and bladder relaxes
- Or if it becomes powerful enough, it causes a reflex traveling through the pudendal nerves to the external sphincter to inhibit it. Only if the brain overrides this will urine NOT exit.
Which brain centers can inhibit or facilitate micturition reflex?
- Pons of the brain stem ( facilitative and inhibitory centers)
- Inhibitory centers in the cerebral cortex
Which nerves are disrupted in overflow incompetence?
Sensory fibers. Then only a few drops are urinated at a time
How does uninhibited neurogenic bladder abnormality develop?
Partial damage in the spina cord or brian stem interrupting inhibitory signals, so facilitative impulses pass continuously down the cord. This keeps the sacral centers so excitable that even small amounts of urine elicit uncontrollable micturition
simply, how do you calculate urinary excretion rate?
Filtration rate - reabsorption rate + Secretion rate
Which factors determine how easily a molecule is filtered at the glomerulus?
Charge (negative charge is more difficult to filter) and size (molecular weight)
How do you calculate GFR?
GFR= Kf x (Pc - Ps- PiG + PiB). Parentheses are equal to net filtration pressure
Which forces comprise net filtration pressure?
glomerular hydrostatic pressure, bowman’s capsule hydrostatic pressure, colloid osmotic pressure of the glomerular capillary plasma proteins, and colloid osmotic pressure of proteins in the bowman’s capsule.
Which forces of filtration oppose vs promote filtration?
opposes: Bowman’s capsule hydrostatic pressure; glomerular capillary colloid osmotic pressure
promote: glomerular hydrostatic pressure. Bowman’s colloid osmotic is basically = 0
How do you estimate Kf?
Kf = GFR/ Net filtration pressure
How does increasing the filtration fraction affect plasma protein levels and glomerular colloid osmotic pressure?
It concentrates the plasma proteins and raises the pressure
What three forces determine glomerular hydrostatic pressure?
- arterial pressure (incr = incr prsr)
- afferent arteriolar resistance (constriction reduces ghr and gfr, dilation increases it)
- efferent arteriolar resistance (3fold constriction reduces gfr, slight constriction increases ghr and gfr)
In order of decreasing resistance, how does vascular resistance change in kidney circulation?
1) renal artery 2) afferent arteriole 3) glomerular capillaries 4) efferent arterioles 5) peritubular capillaries 6) interlobar/intrarenal veins 7) renal vein
What portion of renal blood flow is dedicated to the cortex vs medulla?
cortex gets almost 100%, and 1-2% total blood flow goes to the medulla
Which blood vessels are preferentially constricted by Angiotensin II?
Efferent arterioles are highly sensitive to it. So increased angiotensin II raises glomerular hydrostatic pressure and reduces renal blood flow
When an animal is volume depleted, angiotensin II helps prevent GFR decreases by constricting the efferent arterioles, and the decreased blood flow through peritubular capillaries facilitates increased sodium and water reabsorption to increase blood volume
What is the function of endothelial-derived nitric oxide?
An autocoid, decreased renal vascular resistance, from the vascular endothelium. Basal tone of this allows the kidneys to excrete normal amounts of sodium and water
What is meant by Autoregulation of GFR and renal blood flow?
Blood flow autoregulation in the kidneys maintains a constant GFR despite changes in arterial pressure.
Why do changes in arterial pressure not exert much effect on urine production?
- Renal autoregulation prevents large changes in GFR
- Glomerulotubular balance causes the renal tubules to increase their reabsorption rate when GFR rises
What effect(s) does reduced NaCl sensed by the macula densa have?
- Decreased blood flow resistance in the afferent arterioles to raise GFR back to normal
- Increases renin release from juxtaglomerular cells of afferent and efferent arterioles
How does a high protein diet increase renal blood flow and GFR?
-Amino acids from the high protein meal are reabsorbed at the proximal tubule. Amino acids are reabsorbed with sodium, so this stimulates Na+ reabsorption in the proximal tubules.
-Na+ delivery to the macula densa decreases
-Afferent arteriolar resistance drops in response to the reduced Na+ at the MD
- Renal blood flow and GFR raise
-Therefore Na+ excretion stays stable while increasing excretion of urea from the increased protein metabolism
Briefly, how do you calculate Urinary excretion?
Excretion = Glomerular filtration - tubular reabsorption + tubular secretion
Glom filtration = Glomerular filtration RATE x plasma concentration
What are the primary active transporters in the kidney?
- Na/K ATPase
- H+ ATPase
- H/K ATPase
- Ca ATPase
What are the three major steps required for Na+ net reabsorption from the tubular lumen back to the blood?
- Diffusion across the luminal/apical membrane down an electrochemical gradient established by the Na/K ATPase
- Na+ is transported across the basolateral membrane against an EC gradient by the Na/K ATPase
- Na, water, and other substances are reabsorbed from the interstitial fluid into the peritubular capillaries by ultrafiltration, a passive process driven by the hydrostatic and colloid osmotic pressure gradients
Where and how is most of the glucose that enters the kidney tubules reabsorbed INTO THE CELL?
Where: 90% of filtered glucose is reabsorbed by SGLT2 in the early (S1) segment of the proximal tubule (other 10% is via SGLT 1 in the later segment of the PT).
How: Secondary active transport.
The Sodium glucose co-transporters on the brush border of the prox tubular cells carries glucose against the conc gradient, powered by the energy liberated by sodium moving down its own EC gradient
How is glucose reabsorbed from the tubular cells back into the bloodstream?
Passive facilitated diffusion!
Glucose diffuses out of the cells via the glucose transporters.:
In the S1 segment: GLUT2
In the latter part/S3: GLUT1
How is hydrogen transported from the tubular cell into the tubular lumen?
Via secondary active transport! The sodium-hydrogen exchanger in the luminal membrane carries Na into the cell and forces out H+. Then a Na/K transporter in the basolateral membrane forces out Na and brings K+ intracellularly
How are proteins reabsorbed from the tubular lumen?
By active transport: Pinocytosis!
Protein attaches to the brush border and the membrane invaginates and is digested intracellularly, then its constituents are reabsorbed through the BL membrane
Which substances are actively reabsorbed and have a transport maximum?
Glucose, phosphate, sulfate, amino acids, urate, lactate, and plasma proteins
What is a transport maximum? What is its relevance?
It’s a limit to the rate at which a solute can be transported, created by a limitation in the number of transporters and enzymes available.
Ex// glucose reaches a transport maximum when plasma glucose exceeds the kidney’s ability to actively transport glucose into cells. Not all nephrons have the same transport maximum, so glucose appears in urine before the transport maximum is reached
What other factors, besides transport max, can determine rate of transport?
- electrochem gradient
- membrane permeability
- time that solute-containing fluid spends in the tubule (gradient time transport, determined by flow rate)
How is sodium reabsorption in the proximal tubule an example of gradient time transport?
*Remember a lot of Na transported out of the cell leaks back into the tubular lumen through epithelial tight junctions, and this rate depends on…
-Tight junction permeability
-Interstitial physical forces
SO the greater the conc of Na in the proximal tubules, and the slower the flow rate of tubular fluid, the greater its reabosorption rate
Which hormone can change the transport maximum of sodium in the more distal nephron?
Aldosterone
How does water movement across the tubular epithelium change throughout the nephron?
- Proximal tubule: high permeability, reabsorbed as quickly as the solutes
- Ascending LoH: almost no water is reabsorbed, poor permeability
- Distal tubules, collecting tubules, collecting ducts: high or low, depending on ADH influence
By which types of transport is chloride reabsorbed?
- Passively via paracellular pathway, following EC gradient of Na+
- Secondary active transport: co-transport with Na+
By which types of transport is urea reabsorbed from the tubules?
- Passive reabsorption:
as water is reabsorbed, urea luminal conc increases. Sometimes using passive transporters. Only 1/2 of filtered urea is reabsorbed though
Which characteristics of proximal tubule epithelial cells enable its high reabsorptive capacity?
- Lots of mitochondria to power active transporters
- Extensive apical brush border
- Extensive intercellular and basal channel labrynths
- Lots of protein carrier molecules
