Chapter 19b The Kidneys

Principles governing the tubular reabsorption of solutes and water Filtrate is similar to interstitial fluid. 1 1 Na+ Na+ is reabsorbed by active transport. 2 Anions 2 Electrochemical gradient drives anion reabsorption. 3 3 H2O Water moves by osmosis, following solute reabsorption. 4 K+, Ca2+, urea 4 Concentrations of other solutes increase as fluid volume in lumen decreases. Permeable solutes are reabsorbed by diffusion. Tubular epithelium Tubule lumen Extracellular fluid Figure 19-11

Transepithelial transport Reabsorption Transepithelial transport Substances cross both apical (lumen side) and basolateral membrane Paracellular pathway Substances pass through the junction between two adjacent cells

Sodium reabsorption in the proximal tubule Filtrate is similar to interstitial fluid. 1 Na+ reabsorbed Na+ enters cell through membrane proteins, moving down its electrochemical gradient. 2 Na+ is pumped out the basolateral side of cell by the Na+-K+-ATPase. [Na+] high [Na+] low [Na+] high 2 1 Na+ Na+ ATP K+ Tubule lumen Interstitial fluid KEY Proximal tubule cell = Membrane protein ATP = Active transporter Figure 19-12

Glucose and Na+ reabsorbed Reabsorption Sodium-linked glucose reabsorption in the proximal tubule Filtrate is similar to interstitial fluid. 1 Na+ moving down its electrochemical gradient using the SGLT protein pulls glucose into the cell against its concentration gradient. Glucose and Na+ reabsorbed + [Na+] high [glu] low [Na+] low [glu] high 2 [glu] low 2 Glucose diffuses out the basolateral side of the cell using the GLUT protein. glu glu 1 Na+ Na+ 3 3 [Na+] high Na+ is pumped out by Na+-K+-ATPase. ATP K+ KEY ATP = Active transporter = SGLT secondary active transporter Tubule lumen Proximal tubule cell Interstitial fluid = GLUT facilitated diffusion carrier Figure 19-13

Reabsorption Urea Passive reabsorption Plasma proteins Transcytosis

Saturation of mediated transport Reabsorption Saturation of mediated transport Transport maximum (Tm) is transport rate at saturation. Saturation occurs. Transport rate of substrate (mg/min) Renal threshold is plasma concentration at which saturation occurs. Plasma [substrate] (mg/mL) Figure 19-14

Glucose handling by the nephron Reabsorption Glucose handling by the nephron Figure 19-15a

Reabsorption Figure 19-15b

Reabsorption Figure 19-15c

Reabsorption Figure 19-15d

Important in homeostatic regulation Secretion Transfer of molecules from extracellular fluid into lumen of the nephron Active process Important in homeostatic regulation K+ and H+ Increasing secretion enhances nephron excretion A competitive process Penicillin and probenecid

Excretion = filtration – reabsorption + secretion Clearance Rate at which a solute disappears from the body by excretion or by metabolism Non-invasive way to measure GFR Inulin and creatinine used to measure GFR

Inulin concentration is 4/100 mL. Inulin Clearance Inulin clearance is equal to GFR Efferent arteriole Filtration (100 mL/min) Peritubular capillaries Glomerulus 2 Afferent arteriole 1 Inulin molecules Nephron KEY = 100 mL of plasma or filtrate 1 Inulin concentration is 4/100 mL. 3 100 mL, 0% inulin reabsorbed 2 GFR = 100 mL /min 3 100 mL plasma is reabsorbed. No inulin is reabsorbed. 4 Inulin clearance = 100 mL/min 4 100% of inulin is excreted so inulin clearance = 100 mL/min. 100% inulin excreted Figure 19-16

Inulin concentration is 4/100 mL. Inulin Clearance Efferent arteriole Filtration (100 mL/min) Peritubular capillaries Glomerulus 2 Afferent arteriole 1 Inulin molecules Nephron KEY = 100 mL of plasma or filtrate 1 Inulin concentration is 4/100 mL. 3 100 mL, 0% inulin reabsorbed 2 GFR = 100 mL /min 3 100 mL plasma is reabsorbed. No inulin is reabsorbed. 4 Inulin clearance = 100 mL/min 4 100% of inulin is excreted so inulin clearance = 100 mL/min. 100% inulin excreted Figure 19-16, steps 1–4

Filtered load of X = [X]plasma  GFR Filtered load of inulin = excretion rate of inulin GFR = excretion rate of inulin/[inulin]plasma = inulin clearance GFR = inulin clearance

Excretion Table 19-2

100 mL, 100% glucose reabsorbed Glucose clearance = 0 mL/min Excretion The relationship between clearance and excretion KEY Filtration (100 mL/min) = 100 mL of plasma or filtrate 1 Plasma concentration is 4/100 mL. 2 2 GFR = 100 mL /min 1 Glucose molecules 3 100 mL plasma is reabsorbed. 4 Clearance depends on renal handling of solute. 3 100 mL, 100% glucose reabsorbed 4 Glucose clearance = 0 mL/min No glucose excreted (a) Glucose clearance Figure 19-17a

Excretion Filtration (100 mL/min) = 100 mL of plasma or filtrate 1 KEY Filtration (100 mL/min) = 100 mL of plasma or filtrate 1 Plasma concentration is 4/100 mL. 2 2 GFR = 100 mL /min 1 3 100 mL plasma is reabsorbed. Urea molecules 4 Clearance depends on renal handling of solute. 3 100 mL, 50% of urea reabsorbed 4 Urea clearance = 50 mL/min 50% of urea excreted (b) Urea clearance Figure 19-17b

Excretion Filtration (100 mL/min) = 100 mL of plasma or filtrate 1 KEY Filtration (100 mL/min) = 100 mL of plasma or filtrate 1 Plasma concentration is 4/100 mL. 2 Some additional penicillin secreted. 2 GFR = 100 mL /min 1 Penicillin molecules 3 100 mL plasma is reabsorbed. 4 Clearance depends on renal handling of solute. 3 100 mL, 0 penicillin reabsorbed 4 Penicillin clearance = 150 mL/min More penicillin is excreted than was filtered. (c) Penicillin clearance Figure 19-17c

Gout Limit animal protein. Avoid or severely limit high-purine foods, including organ meats, such as liver, and herring, anchovies and mackerel. Red meat (beef, pork and lamb), fatty fish and seafood (tuna, shrimp, lobster and scallops) are associated with increased risk of gout. Because all animal protein contains purines, limit your intake. Eat more plant-based proteins. You can increase your protein by including more plant-based sources, such as beans and legumes. This switch will also help you cut down on saturated fats, which may indirectly contribute to obesity and gout. Limit or avoid alcohol. Alcohol interferes with the elimination of uric acid from your body. Drinking beer, in particular, has been linked to gout attacks

Relaxed (filling) state Micturition The storage of urine and the micturition reflex Higher CNS input Relaxed (filling) state Bladder (smooth muscle) Internal sphincter (smooth muscle) passively contracted Tonic discharge External sphincter (skeletal muscle) stays contracted (a) Bladder at rest Incontinence Figure 19-18a

Micturition Stretch receptors Higher CNS input may facilitate or inhibit reflex Sensory neuron 1 Parasympathetic neuron 1 Stretch receptors fire. 2 3 2 Parasympathetic neurons fire. Motor neurons stop firing. Motor neuron Tonic discharge inhibited 3 Smooth muscle contracts. Internal sphincter passively pulled open. External sphincter relaxes. Internal sphincter 2 3 External sphincter (b) Micturition Figure 19-18b

Functions of the kidneys Anatomy Summary Functions of the kidneys Anatomy Kidney, nephron, cortex, and medulla Renal blood flow and fluid flow from glomerulus to renal pelvis Overview of kidney function Filtration Podocytes, filtration slits, and mesangial cells Filtration fraction, GFR, and regulation of GFR

Reabsorption Secretion Excretion Micturition Summary How solutes are transported Transport maximum and renal threshold Secretion Excretion Clearance, inulin, and creatinine Micturition