Review of Normal Renal Physiology Syed I. A. Zaidi, Ph.D. Assistant Professor Department of Physiology & Biophysics Phone (202) 806-9867 sizaidi@howard.edu

Review of Normal Renal Physiology Anatomy of the excretory system How the kidney is organized How the nephron works to filter blood, recycle, secrete, and excrete How filtration is regulated Urination reflex

Kidney Functions: Overview Homeostatic regulation: ECF volume, osmolarity, ion & pH balance Excretion: Metabolic wastes & foreign molecules Regulating hormones & enzymes Blood pressure regulation Gluconeogenisis Erythropoises (erythropoitin) Vitamin D activation (Calcitriol ---- reabsorption of Ca++)

Regulation of body fluid osmolarity and volmue Regulation of electrolyte balance Regulation of acid-base balance Excretion of metabolic products Excretion of foreign substances (pesticides, chemicals etc.) Gluconeogenisis Blood pressure regulation Erythropoises (erythropoitin) Vitamin D activation (Calcitriol reabsorption of Ca++) -kidney can only conduct fluid it cannot restore lost volume. If volume drops too low the GFR will stop -kidney is a retroperitoneal organ, it is behind the peritoneal membrane.

Cortical Juxtaglomerular

Figure 19-9: The juxtaglomerular apparatus Regulation of GFR Figure 19-9: The juxtaglomerular apparatus

Blood flow in the kidney -above 100mmhg is the starting blood flow, at the end it is only 8mmhg. The distribution of the blood flow is divided into two beds, one being high pressure the other being low pressure -there is a shunt in which about 1% of blood will go (vasa recta shunt), this will go around the henle loop. There is minimal pressure in this area. The vasa recta are responsible for the countercurrent mechanism. -you need a slow moving fluid and low pressure so that ion transport can occur Vesa recta (1%)

Blood flow in kidneys and other organs Approx. blood flow (mg/min/g of tissue) A-V O2 difference (ml/L) Kidney 4.00 12-15 (depends on reabsorption of Na+ ) Heart 0.80 96 Brain 0.50 48 Skeletal muscle (rest) 0.05 - Skeletal muscle (max. exercise) 1.00 Filterablility of plasma constituents vs. water -Kidney receives most of the blood but it is only for filtration purposes. -Consumption of O2 in the kidney depends on reabsorption of sodium. Maximum oxygen is being used to provide ATP for the Na/K+ pump. -The filtration ratio shows that there is no restriction if the ratio is one, and if it is smaller this, indicates higher resistance -In the Bowman’s capsule, negatively charged glycoproteins and a small pore size regulate materials that pass through. The normal pore size is about 42A. When the membrane loose its charge you will see a lot of albumin and other proteins in the urine. This is common in certain disease states Constituent Mol. Wt. Filteration ratio Urea 60 1.00 Glucose 180 Inulin 5,500 Myoglobin 17,000 0.75 Hemoglobin 64,000 0.03 Serum albumin 69,000 0.01

Nephron Functions: Filtration, secretion and excretion There are four processes that occur in the nephron -excretion- about 1% of the ultra filtrate is excreted -secretion- means from the peritublar capillaries and vasa recta some products are secreted back into the distal tubule. -most reabsorption takes place in the proximal tubule (water and sodium). Some drugs can pass through the capillary system to be secreted back into the proximal tubule. -the remaining water and sodium reabsorption will take place in the loop of henle

Filtration pressure in the renal corpuscle Pressure at the afferent end is 55mmhg, inside it is only 10mmhg. There is a negative pressure colloid that will absorb water back, EXAM SLIDE

GFR  125 ml/min (180L/day) (about 1% is excreted) 80% of the blood goes back, only 20% of the volume is filtered. Of this 20%, only 19% will be reabsorbed. -total volume that is filtered is only about 180L/day, and 1% of this will excreted.

Regulation of GFR Autoregulation Myogenic stretch Tubuloglomerular feedback Macula densa J G cells ANS-Sympathetic Arteriole vasoconstriction Hormones/paracrines Angiotensin II Prostaglandins Catecholamine Serotonin Vasopressin Extrinsic factors influencing renal flow Stress (exercise, cold, heat) Vasomotor activity Pyrogens General anesthesia Auto regulation is controlled by two mechanisms -Myogenic stretch- a local mechanism -Tubuloglomerular feedback- macula densa cells and JG cells play a critical role

Tubuloglomeruar feedback Autoregulation of GFR Urine (6 ml/min) Myogenic mechanism eg. Local infusion of cyanide Tubuloglomeruar feedback e.g. response to increased arterial pressure Plateau in the graph means that in a wide range of BP the kidney will try to regulate the GFR (from 80mmhg to about 180mmhg). When you infuse cyanide locally the Myogenic control is lost, there is no Autoregulation and you won’t see the plateau. This is proof of this mechanism. -When you decrease the GFR and RBF by constriction of the afferent end, there will be less supply to the bowman's capsule. -You can increase the GFR by constricting the efferent end. -by dilating the efferent end you will decrease the GFR. This will reduce the pressure in the bowman capsule and because of the low pressure there is decreased GFR -by dilating afferent there will be increase in pressure, more blood will come in to the renal system, this will cause an increase in GFR. -you can increase renal blood flow by dilation, or decrease the renal blood flow by constriction. -remember that GRF is pressure and flow dependent -THE MYOGENIC MECHANISM WILL BE AN EXAM QUESTION Tubuloglomerular when the BP is increased, there will be a response, so this means that there is a feed back mechanism going on. BP increase in detected by the macula densa, and vasoactive substance will be secreted to increase afferent arteriole vasoconstriction.

Endothilin and ATII cause vasoconstriction Vasoconstrictors Sympathetic nerves Angiotensin II Endothelin Vasodilators Prostaglandings (PG1, PG2) Nitric oxide Bradykinin There are different types of hormones that can vasodilate or vasoconstrict. Endothilin and ATII cause vasoconstriction Ach, ATP, Histamine, Bardykinin, shear stress all contribute to N.O. being given off and this will lead to vasodilatation.

Qualities of agents to measure GFR Inulin: (Polysaccharide from Dahalia plant) Freely filterable at glomerulus Does not bind to plasma proteins Biologically inert Non-toxic, neither synthesized nor metabolized in kidney Neither absorbed nor secreted Does not alter renal function Can be accurately quantified Low concentrations are enough (10-20 mg/100 ml plasma) Creatinine: End product of muscle creatine metabolism Used in clinical setting to measure GFR but less accurate than inulin method Small amount secrete from the tubule High readings because of non-specific chromagens in plasma Para-aminohippurate (PAH): An organic anion not present in body Freely filtered, secreted but not reabsorbed by nephron Low concentrations are enough (10 mg/100 ml plasma) GFR is a direct correlation to kidney function. If kidneys are jacked up the GFR will decrease. If there is toxicity from any source this will also effect the GFR. When new drugs are being made, measurement of clearance rate and absorption can be measured by these agents. INULIUN and INULIN LIKE DRUGS -if there is a charge in a drug (like dextran which is positively charge) this will effect the extent of its reabsorption. If they are negatively charged they will be filtered less, and if they are positively charged they will be filtered more. -If drug binds to plasma protein this will reduce secretion -drug should be inert and should not be of a hormonal nature -if they are made or metabolized in the kidney this will change the quantity of the drug in the kidney -drug should not be a vasoconstrictor, or vasodilator -inulin is not made in our bodies, but it is infused. Low concentrations are sufficient enough to measure and quantify GFR. In clinical setting most of the time creatinine is used. -inulin will give the total GFR value CREATININE -90% of creatinine will be filtered into the kidney and excreted, the remaining 10% will go into the peritubular capillary system and will later be secreted into the collecting ducts. -there are non specific chromagens in the plasma that can give off readings similar to that of creatinine (As far as color change when you are looking at optic properties) PAH- will give the total plasma value -there is a limitation on the kidney. IF there is an overload on the system it cannot filter all of the substance. CONCENRATIONS WILL BE ON EXAM

Solute Clearance: Rate of removal from the Blood Under 10 mg condition. When 10mg/ml of inulin is passed thru the bowmans capsule, it will be filtered. What ever amount is filtered none will be reabsorbed, and none will be secreted. You can measure the GFR with this because whatever is filtered in will be excreted out. Figure 19-16: Inulin clearance

THERE WILL BE A QUESTION ON THIS SLIDE AS WELL /PAH THERE WILL BE A QUESTION ON THIS SLIDE AS WELL Glucose most of the substance that is secreted will be reabsorbed, none will be excreted. PAH, or Penicillin a certain amount goes in the capsule and is filtered, the amount left over will go into the capillaries. It is not desirable to be in the body so the body will secrete it so that eventually all of it will be excreted out into the urine -when injection is given of penicillin, all of the penicillin will be secreted. It is not given IV on a constant basis, you only get one injection or one tablet each day. Once the blood goes into the kidney it will all be secreted. -penicillin is designed so that it will be bound to proteins in the blood, to be able to have a bigger effect. Urea 4 molecules are secreted and 2 are reabsorbed. Urea is a toxic substance to the body. Urea is used in the system to maintain osmolarity. The osmolarity is higher as you go deeper into the kidney. Urea will help to maintain this concentration gradient.

Concept of clearance Qx extracted = Qx excreted Px . Cx = Ux . V Where, Cx = Clearance of substance X (mg/min) Ux = Urine concentration of X (mg/ml) Px = Plasma concentration of X (mg/ml) V = Urine flow rate of X (ml/min) GFR = Cx = Px Ux . V Qx extracted = Qx excreted Px . Cx = Ux . V Effective renal plasma flow =GFR ERBF = Cx = 1 - Hct ERPF Renal blood flow = RBF = Extraction ratio ERBF Effective renal blood flow = Extraction ratio (0.9) = APAH APAH - VPAH Hct=hematocrit VPAH = vein plasma PAH EXAM SLIDE -GFR can be calculated using inulin or creatinine. -ERPF can be calculated using PAH -Penicillin, urea, and Glucose cannot give a good measurement of GFR APAH = arterial plasma PAH

Net reabsorption rate: Clearance Ratio of X = Cin Cx Cx < 1 means reabsorption Cx > 1 means secretion Net reabsorption rate: Tx = (GFR . Px) – (Ux . V) Filtration - Excretion GFR = Cx = Clearance of inuline (mg/min) Ux = Urine concentration of X (mg/ml) Px = Plasma concentration of X (mg/ml) V = Urine flow rate of X (ml/min) EXAM SLIDE Net secretion rate rate: Tx = (Ux . V) – (GFR . Px) Excretion - Filtration

Reabsorption: Passive Transport & Trancytosis -this will be covered in later lectures. -once plasma is filtered most of the NaCl will be reabsorbed (Some in the soluble form and some in the ionic form) -Na in the interstitium space will increase the osmolarity -water will follow the flow of the ions. -these three molecules are passively reabsorbed (urea, water, and sodium) Passive reabsorption of urea in the proximal tubule

Reabsorption: 10 Active Transport (Passive diffusion in) Active Transport Na+ to ECF K+ into cell ATP-ase Uses energy Na+ ECF peritubular capillaries Reabsorption  blood

Reabsorption: Secondary Active Transport Na+ linked 20 transport Symport Glucose Ions Amino acids Proximal tubule, key site Figure 19-12: Sodium-linked glucose reabsorption in the proximal tubule

Most of the NaCl is reabsorbed in the proximal tubule EXAM QUESTION. In the early distal tubule almost water is reabsorbed, in the late distal tubule and CT there is 8-17% reabsorption. If there is any disturbance in the hormones, urine will get concentrated or too diluted this will lead crystal formation (if the concentration is outside of this 8-17% regulation range). -below pH 6.8 death will occur. With kidney failure there will be a change in pH, the kidney is an important regulator of pH -In acidosis- hyperventilation will occur to adjust pH, and the kidney will excrete more acidic urine.

Reabsorption: Passive Transport & Trancytosis Passive Transport of urea Na+ pumped out H2O follows Passive  [urea] [urea] higher than ECF passive diffusion to ECF Trancytosis of proteins

Reabsorption: Receptors can Limit Transport maximum Saturation (# of receptors) Competition Specificity Renal Threshold Example: glucosuria

Reabsorption: Receptors can Limit Renal threshold of the plasma- past this point the kidney cannot reabsorb any longer and substance will be secreted (ie: too much glucose). Figure 19-15: Glucose handling by the nephron

Secretion: From Peritubular Blood vessels & ECF Active Transport into nephron tubules Example: K+ & H+ (more on this next unit)

Secretion: From Peritubular Blood vessels & ECF Figure 19-2 : Filtration, reabsorption, secretion, and excretion

Excretion: All Filtration Products that are not reabsorbed Figure 19-5: The filtration fraction

Urination: Micturation reflex Rugae folds Detrusor a-Adrenergic receptors Hypogastic nerves (L1, L2, L3) Sympathetic Pelvic nerve Visceral afferent pathway There is a higher CNS input to excrete urine. Urine is secreted and held in the bladder. From the lumbar region the hypo gastric sympathetic nerves will supply the motor neurons. Normally in a filling phase the ruggae folds will be relaxed, and the sphincters are closed due to firing of the neurons to contract the internal sphincter. This is a passive processes -when urine is collected, it will stretch the stretch receptors, sensory neurons will bring the message to the CNS, the sacral parasympathetic fibers send the message to contract the bladder. The sacral pudential fibers will cause the external sphincter to relax and allow for urine to come out. Tonic discharge will be inhibited by the sensory neurons -the body is naturally designed for urine to be collected and excreted out without any control. Kids are trained to use their higher CNS input to control urine excretion. -Bed wetting at night is related to this pathway. It is related to sphincter control. There is medicine that can be spread at night in the nose. Vasopressin is in this combination and it will eliminate excess secretion to keep them from urinating in their sleep. Fundus Sacral Parasympathetic (S1, S2, S3) Sacral Pudential nerves Skeletal muscle Figure 19-18: The micturition reflex

Formation of Water Pores: Mechanism of Vasopressin Action

Kidney Hydrogen Ion Balancing: Proximal Tubule