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Renal Physiology II. Urination Tubular Transport Countercurrent. Getting Urine from the kidney to the outside. (Urination or micturition) Processed tubular fluid is dumped by the collecting system into the renal pelvis where it enters the ureters.
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Renal Physiology II • Urination • Tubular Transport • Countercurrent
Getting Urine from the kidney to the outside. (Urination or micturition) Processed tubular fluid is dumped by the collecting system into the renal pelvis where it enters the ureters. Ureters: conduits that propel urine by peristaltic contractions toward the bladder. Bladder: a muscular “bag” that holds urine and forces it by contration. Urethra: the conduit for urine from the baldder to the outside
Pelvis: Collects urine from collecting ducts. In the pelvis there are “electrical pacemaker” cells that initiate peristaltic waves in the (2) smooth muscle sheaths of the ureteral wall. (The pelvis to ureter is a functional syncitium, not unlike the muscular wall of the heart). The frequency of the waves is 2-6/min. The pacemaker cells seem to be stimulated by the stretch of urine filling the pelvis. The movement of the peristaltic wave is about 2-6 cm/sec., traveling from its origin at the pelvis down to the bladder. Urethra
Anatomy of the bladder and ureter. On the right is the electrical profile of a peristaltic wave passing down the muscular wall of the ureter. Page 752
The peristaltic waves propel the urine along the ureter, generating a pressure head of which changes from a baseline of 2-5 cm H2O up to 20-80 cm H2O. While peristalysis is independent of nerve input, the action of symapthetic nerves innervating the ureter may modify the rate or force of peristalsis. Interruption of the flow of urine by an obstruction (such as a kidney stone) stops flow, increases pressure which can back up through the ureter into the pelvis, and increase the nephron and subcapsular hydrostatic pressure. This may result in the condition hydronephrosis in which the medulla is damaged and may even be sloughed off, leaving a hollow kidney. Obviously this condition impares the concentrating ability fo the kidney. There are autonomic pain fibers in the ureter which account for the acute pain when a kidney stone is formed.
The bladder and its sphincters is also innervated with sympathetic, parasympathetic and somatic (voluntary) nerves. The wall of the bladder is composed of three muscular layers, called the “detrusor muscle.” A triangular membrane called the “trigone” acts as a valve system along with the internal sphinctors of the muscular wall to prevent urine reflux into the ureters.
Sympathetic nerves originate from the neurons ot the intermediallateral cell column from T-10 to L-2. they innervate the body of the bladder and the trigone. Parasympathetic nerves originate from S2-S4 of the sacral spinal cord. They innervate the body and neck of the bladder. Somatic innervation (voluntary or pudendal) originates from the motor neurons arising from S-2 to S-4. They innervate and control the voluntary muscles of the external sphinctor.
Bladder tone is derived from the volume and pressure exerted on the inside of the bladder (intervesical pressure). Increasing bladder volume by 50 ml increases pressure. As volume inceases further, the intervesical pressure increases, but not much until you get above 300 ml. then the pressure rises steeply with additional volume. (see next slide-blue line). This increase in volume and pressure increases bladder “tone” triggering the mictiurition reflex (open the flood-gates!) Efferent impulses from the brain supress the reflex (a learned reflex) until a decision is made to relax the external sphinctor using voluntary nerves. Voiding begins with relaxation of the external sphinctor, then the internal sphinctor.
Next, the detrussor muscle of the bladder wall contracts in waves (see red lines in previous figure) to expell the urine. Voluntary contraction of the abdonimal muscles further contracts the bladder, increasing the voiding. Once the bladder is empty, we are back down to the “no tone” phase (in the lower left corner of figure 32-14) and the sphinctors can close again. The process is sterile until it leaves the body. However, because of all the organic and waste material, once out is it a good culture media.
SECTION II Transport along the nephron:
Transport of Sodium (Na+) and Chloride (Cl-) Chapter 34 The filtered load of Na+ is the product of the glomerular filtration rate (GFR, 180 liters/day) and the plasma Na+ concentration (142 mM), or approximately 25,500 mM/day (equivalent to the Na+ in approximately 1.5 kg of table salt, more than nine times the total quantity of Na+ present in the body fluids. With a typical Western diet consuming approximately 120 mM of Na+ per day, the kidneys reabsorb approximately 99.6% of the filtered Na+ by the time the tubule fluid reaches the renal pelvis. Therefore, even minute variations in the fractional reabsorptive rate could lead to changes in total-body Na+ that markedly alter ECF volume, sodium balance, blood pressure, body weight and many other
The filtered load of Na is 25,500 mM/day, but the intake is only 120 mM/day and the output is 100 mM/day excreted plus about 20 mM in the feces and sweat. Thus, the intake equals output, so the body is in sodium balance.
Reabsorption of filtered Na load along the each nephron segment. Yellow boxes are the amount of filtered which is reabsorbed. Green boxes represent the amount of filtered which remains in each portion of the nephron. Pg 776
Transport of ions, and particularly of sodium from the lumen to the blood across the tubular wall is through two pathways; transcellular and paracellular. • Transport is driven by two general mechanisms; • active transport in an energy (ATP) utilizing fashion where ions are pumped against their electrochemical gradient (“uphill”), and • passively down their electrochemical gradient (“downhill”) along the gradients created by the active transport.
Tubular epithelial wall Capillary wall Start here with Na delivery End here with sodium reabsorbed and recovered Tubular lumen Interstitial space Capillary Lumen Na Apical Membrane Basal-lateral membrane Na Na Na Na Net pathway for sodium (Na) reabsorption from tubular lumen to capillary
Para-cellular movement incorporates The transepithelial electrochemical Na gradient drives passive Na reabsorption in the proximal and thick ascending limb of the nephron. Not so for later nephron segments where the net (passive) force favors movement into the lumen. Na can also move passively in the proximal tubule (without active transport) via “solvent drag” where the movement of water (driven by active Na transport) sweeps additional Na and Cl along with it (a sort of mass-action) out of the lumen into the lateral intracellular space. The leakiness of the nephron (facillitating passive reabsorption) is greatest in the proximal, and decreases along the nephron to the papillary collecting ducts.
Trans-cellular movement incorporates 1) passive entry from the lumen via the “apical” membrane into the cell down an electro-chemical gradient. The proximal, TAL and DCT use various co-transporters and exchangers, while in the collecting ducts Na enters via Na channels. 2) Active extrusion of Na out the basal-lateral membrane via a Na-K+ pump which maintains intracellular Na low and K high. This exchange keeps the voltage at 70 mV (cell interior negative vs interstitium, or lumen) depending on pump activity and the voltage gradient it creates.
There is a net driving force due to the active pump forcing Na into the interstitium, but a net negative change favoring the lumen to draw Na back via extracellular junctions. “Downhill” refers to a passive flow along an electrochemical gradient not requiring active transport.
Proximal: the Na-K pump on the apical (interstitial side) membrane is the driving force for the electrochemical gradient which drives passive transport into the cell and keeps intracellular Na low, pumping against the gradient into the basal-lateral space. Passive entry into the cell is by diffusion, facillitated diffusion through a transporter or co-transporter, and by electroneutral “exchange” with hydrogen ions (H+). Na+ Na+ Interstitial space Tubular lumen pump Na+
The sodium movement across the thin limbs (decending and ascending limbs) of the loop of Henle are virtually entirely passive down its electrochemical gradient and paracellular. Keep this in mind when we return to the countercurrent system!.
Thick Ascending Limb (TAL) of the loop of Henle. • Transcellular Na reabsorption includes • the Na/K/2Cl co-transporter (NKCC2) which couples inward movement of these three ions in an electroneutral (2+ &2-) process driven by the downhill gradient of Na and Cl into the cell. (Note that this pump is the target of loop diuretics). Much of the K+ entering the cell is extruded via K channels down its own gradient. • The Na+-H+ exchanger exchanging sodium for hydrogen in an electroneutral process.
Paracellular sodium transport by the thick ascending limb (TAL) (also known as the “diluting” segment). Because the lumen of the TAL is positive voltage due to the high density of K+ channels in the apical membrane, unlike all other nephron segment epithelia. This lumen-positive voltage drives sodium (and other positively charged ions) out of the lumen across the tight junctions between the cells. This paracellular pathway accounts for about half of the sodium movement out of the lumen to the basalateral spaces and the interstitium. The TAL has low water permeability, so removal of ions without water following leaves the lumen dilute (hypoosmotic) and the interstitium concentrated (hyperosmotic).
Transcellular and paracellular Na reabsorption in different nephron segments
Distal Convoluted Tubule (DCT) Sodium reabsorption in the distal tubule is almost entriely due to transcellular transport. Electroneutral passive apical Na entry is due to an Na/Cl cotransporter (NCC). Unlike the NKCC2, this is independent of K+ (this pump is the target of the “thiazide” diuretics, which tqarget sodium without wasting potassium) The net movement of transcellular sodium in the DCT is driven by an ATP-utilizing basal-lateral Na+-K+ pump
Transcellular and paracellular Na reabsorption in different nephron segments
Sodium transport in the Collecting Tubules: The relatively modest Sodium reabsorption in the collecting tubules is entirely transcellular via the “principal cells.” Na enters the apical membrane via a “voltage-gated sodium channel” or “ENaC.” The basolateral movement of sodium out of the cell is driven by an energy requiring Na-K pump which establishes the gradient driving apical sodium entry. The movement of Na+ out of the lumen into the cell makes the lumen negatively charged, and the movement of K out of the cell, primarily into the basolateral interstitium makes the cell negative, for a net transepithelial voltage of -40 mV. The hormones aldosterone and vasopressin can change this site of transport .
Transcellular and paracellular Na reabsorption in different nephron segments
Medullary Collecting Duct: The inner and outer medulalry collecting ducts reabsorb only a small amount of sodium (3% of filtered load) and this is probably via ENaC on the apical membrane and the Na-K pump driving Na movement on the basal lateral membrane.
Cl- transport and reabsorption: Most Cl follows along with Na reabsorption, but the exact nature fo the movement is somewhat different. In the Proximal tubule: Early proximal tubule Cl reabsorption is mostly paracellular via solvent drag driven by the lumen negative potential. However, in the late proximal it is reabsorbed by a predominantly by transcellular pathway, driven by apical H+ exchange and active transport with Na and a K cotransporter. The lumen becomes positive actually retarding Cl reabsorption.
Changes in proximal TF:P ratio along the length of the proximal tubule. Note that 65% of the water is lost so inulin continues to concentrate, while osmolality and [Na} are unchanged.
Thick ascending limb (TAL): Cl is primarly reabsorbed by the NaK2Cl co-transsporteracross the apical membrane, and basal lateral Cl channels along with active transport of sodium drive Cl into the interstieium. Distal Tubule: Apical Cl reabsorption occurs via a Na/Cl ccotransporter and is driven by Cl following Na active extrusion via a Na/K pump. Collecting ducts: The principal cell has an electrogenic pump that creates a -40 mV lumen negative potential that drives Cl- out of the lumen via paracellular routes. However, the other cell type (the intercallated cell) drives transcellular Cl movement powered by a basalateral H+ pump.
Water Reabsorption: In the proximal tubule, water follows sodium passibvely and isosmotically because the proximal tubule is very permeable to water. Water moves both transcellularly and paracellularly. The transcellular movement is facillitated by “aquaporin” water channels in both the apical and basalateral membranes.
Water reabsorption in the Thick Ascending Limb and Distal nephrons. All the distal nephrons, from TAL on, have a very low water permeability (in the presence of vasopressin). This low water permeability will be very important in understanding “countercurrent” concentration of the urine (coming later) The combination of Na transport with low water permeability produce a dilute tubular fluid with low Na and low osmolality. This facillitates later passive water reabsorption down a concentration gradient out of the nephron and into the capillary blood.
