Physiology 441 The Renal System, Chp. 14 Text: Human Physiology (Sherwood), 6 th Ed. Julie Balch Samora, MPA, MPH , Room 3145

Renal Processes 1

Tubular Reabsorption All plasma constituents (except proteins) are filtered through the glomerular capillaries, including needed nutrients and electrolytes Tubular reabsorption is the discrete transfer of substances from the tubular lumen into the peritubular capillaries (to be returned to the body) VERY SELECTIVE PROCESS

Tubular Reabsorption Tubules have a high reabsorptive capacity for substances needed by the body Only excess amounts of essential materials (eg elctrolytes) end up being excreted in the urine Out of the 125 ml/min filtered, ~124 ml/min are reabsorbed! (178.5/180 L filtered/day are reabsorbed) 99% of filtered water, 100% filtered sugar, and 99.5% of filtered salt gets reabsorbed

Steps of Transepithelial Transport A Substance must: Cross luminal membrane of tubular cell Pass through cytosol from one side of tubular cell to the other Cross the basolateral membrane of the tubular cell to inter the IF Diffuse through the intersitial fluid Penetrate the capillary wall to enter the blood plasma

Tubular lumen Tubular epithelial cell Interstitial fluid Peritubular capillary Plasma Lateral space Capillary wall Basolateral membrane Luminal membrane Tight junction Filtrate Fig , p. 515

Active Reabsorption Major substances are reabsorbed via active transport. These are substances that are needed by the body (e.g. Na +, glucose, aa’s, other elctrolytes) Sodium reabsorption- 99.5% of filtered sodium is absorbed –Proximal tubules (67%) –Loop of Henle (25%) –Distal/Collecting tubules (8%)

Sodium Reabsorption Na + -K + ATPase located at the basolateral membrane of tubular cells. Pumps Sodium out of cell into lateral space. Keeps tubular cell ICF [Na + ] low, creating concentration gradient for Na + to diffuse in from lumen. Keeps lateral space [Na + ] high creating concentration gradient for Na + to diffuse into blood (against gradient)

LumenTubular cellInterstitial fluid Peritubular capillary Diffusion Na + channel Active transport Basolateral Na + – K + ATPase carrier Lateral spaceDiffusion Fig , p. 516 Mechanism of Na + reabsorption

Control of Na + Absorption Renin-Ang II-Aldo – goal to keep Na + If ↓ NaCl, ECF volume, arterial blood pressure, the JGA secretes renin into blood Ang I  Ang II in lung Ang II stimulates release of Aldo from Adrenal Cortex Aldo stimulates distal/collecting tubules to reabsorb sodium, thereby conserving it

Aldosterone Only the sodium reabsorbed in the distal and collecting tubules is subject to hormonal control The amount of this sodium depends on amt of Aldo secreted Without Aldo, can excrete lots of NaCl daily However, maximum Aldo secretion leads to complete reabsorption of sodium

RAAS

Diuretics Drugs/drinks that increase urinary output Promote loss of fluid from body Function by inhibiting tubular reabsorption of sodium Examples include thiazide, loop and potassium-sparing diuretics (medicinal- used for ↑ BP, CHF, Edema, Diabetes insipidus), coffee, sodas, and alcohol

Atrial Natriuretic Peptide (ANP) Goal to rid body of Sodium ANP released from cardiac atria when senses ↑ECF Decreases sodium reabsorption and increases sodium loss in urine Result- decrease sodium load and ECF volume (not nearly as powerful as renin-Ang-Aldo system!)

Tubular Maximum (T M ) Represents the maximum amount of a substance that the tubular cells can actively transport within a given time period Carriers become saturated T M ONLY for active processes Sodium only actively reabsorbed substance w/o transport max

Glucose Normally no glucose should be in the urine However, DM patients often has glucose due to T M being overcome Amt of substance filtered = plasma concentration X GFR e.g. [plasma glucose] = 100mg/100ml Normal GFR = 125 ml/min Therefore Filtered Load = (100mg/100ml)x125ml/min = 125 mg/min

Glucose- 2° active transport Energy is not used directly to absorb glucose, even though it is being reabsorbed against its concentration gradient Carrier is driven by the sodium concentration gradient established by the N + -K + pump T M for glucose carriers is 375 mg glucose/ minute

Renal Processes 2 Glucose Reabsorption

Renal Threshold Plasma concentration at which a substance reaches the T M and 1 st starts appearing in the urine Renal threshold for glucose is 300 mg glucose/100 ml plasma

Phosphate Also exhibits a T M – Unlike glucose, kidneys DIRECTLY contribute to its regulation because the renal threshold for phosphate = nml plasma [phosphate] Whenever ↑ [plasma phosphate], the kidneys immediately excrete the excess as the T M is exceeded

Passive Reabsorption All passive reabsorption is ultimately linked to active sodium reabsorption. Includes Cl -, H 2 O, and urea Chloride moves via electrical gradient that was established by Na + Sodium creates an osmotic gradient for the passive reabsorption of H 2 O via osmosis

Urea Waste product At end of proximal tubule, urea is concentrated, creating a gradient favoring entrance into the blood However, tubular cell membranes not very permeable and only ~50% of the filtered urea is passively reabsorbed BUN- measured as a crude assessment of kidney function

Passive Reabsorption of Urea at the End of the Proximal Tubule

No Reabsorption of the other unwanted substances The other waste products are too large or are not lipid soluble, so even though the gradient favors their return to the blood, they do not go. Furthermore, there are no carriers to allow them passage. Therefore, they remain in the tubule and pass into the urine in a highly concentrated form (e.g. uric acid, sulfate, nitrates, phenols, etc)

Renal Processes 2 Urea Reabsorption

Tubular Secretion Additional mechanism to eliminate certain substances Secretion of H +, NH 3, K +, organic anions and cations Acid and ammonia secretion imp. in acid- base balance Ammonia is secreted during acidosis in order to buffer the secreted H +

H + and NH 3 secretion Hydrogen and ammonia secretion aid with acid-base balance H + secretion provides a highly discriminating mechanism for varying the amt of H + excreted in the urine, depending on the acidity of the body fluids NH 3 is secreted in pronounced acidosis to buffer the H + secreted into the urine

K + Secretion K + is actively reabsorbed in the proximal tubule, but is also actively secreted in the distal tubule. Allows fine degree of control over plasma K + concentration Secretion of K + is variable and subject to aldosterone control Even slight fluctuations in ECF [K + ] can alter nerve and muscle excitability

Organic anion and cation secretion The proximal tubule contains 2 different carriers for secreting organic ions These systems aid in secreting foreign organic substances The liver helps this process by converting many foreign substances into anionic metabolites

Process of Secretion Renal Processes 1

Urine excretion The final quantity of urine formed averages about 1ml/min This urine contains a high conc. of waste products & low or no conc. of substances needed by the body Minimum volume of urine to eliminate wastes is 500ml/day

Plasma Clearance The amount of plasma “cleared” of a substance Plasma clearance for a substance that is FILTERED, but not REABSORBED or SECRETED == GFR == 125ml/min EXAMPLE = INULIN

Renal Processes 2 INULIN- Filtered, NOT reabsorbed, NOT secreted

Plasma Clearance Plasma clearance for a substance that is FILTERED AND REABSORBED, but NOT SECRETED < GFR EXAMPLE = GLUCOSE, UREA Clearance rate can be anywhere from 0 up to normal clearance (125 ml/min) depending on amount reabsorbed

Plasma Clearance Plasma clearance for a substance that is FILTERED AND SECRETED > GFR EXAMPLE = PAH (Para-aminohippuric acid) Not only is the plasma that is filtered cleared of that substance, but additional amt cleared from plasma which was not filtered Plasma clearance rate for PAH=RENAL PLASMA FLOW

Renal Processes 2 Plasma clearance rate for PAH = Renal Plasma Flow