Unit Five: The Body Fluids and Kidneys Chapter 27: Urine Formation By the Kidneys. II. Tubular Reabsorption and Secretion Guyton and Hall, Textbook of Medical Physiology, 12th edition

Renal Tubular Reabsorption and Secretion Tubular Reabsorption is Quantitatively Large and Highly Selective

Table 27.1 Filtration, Excretion, and Reabsorption Rates of Different Substances by the Kidney Amount Filtered Amount Reabsorbed Amount Excreted % Filtered Reabsorbed Glucose g/day 180 100 Bicarbonate mEq/day 4320 4318 2 >99.9 Sodium 25560 25410 150 99.4 Chloride 19440 19260 99.1 Potassium 756 664 92 87.8 Urea g/day 46.8 23.4 50 Creatinine g/day 1.8

Tubular Reabsorption Fig. 27.1

Tubular Reabsorption (cont.) Active Transport Solutes transported through epithelial cells or between cells Primary active transport (i.e. Na+ and K+) Secondary active transport

Fig. 27.2 Basic mechanism for transport of sodium through the tubular epithelial cell

Fig. 27.3 Mechanisms of secondary active transport

Tubular Reabsorption (cont.) Secondary active secretion into the tubules (i.e. counter-transport) Pinocytosis-active reabsorption of proteins Transport maximum-limit to the rate at which a solute can be transported in secretion or reabsorption

Fig. 27.4 Relations among the filtered load of glucose, the rate of glucose reabsorption by the renal tubules, and the rate of glucose excretion in the urine.

Substance Transport Maximum Transport Maximums for Substances Actively Reabsorbed by the Tubules Substance Transport Maximum Glucose 375 mg/min Phosphate 0.10 mM/min Sulfate .06 mM/min Amino Acids 1.5 mM/min Urate 15 mg/min Lactate 75 mg/min Plasma protein 30 mg/min

Substances that are actively transported but do not Transport Maximums for Substances Actively Secreted Substance Transport Maximum Creatinine 16 mg/min Para-aminohippuric acid 80 mg/min Substances that are actively transported but do not exhibit a transport maximum (i.e. Na in the proximal tubule h. Passive reabsorption coupled to sodium reabsorption

Fig. 27.5 Mechanisms by which water, chloride, and urea reabsorption are coupled with sodium reabsorption

Reabsorption and Secretion Along Different Parts of the Nephron Proximal Tubular Reabsorption - normally about 65% of filtered load of Na and water and slightly lower percent of chloride is reabsorbed before the loop of Henle Have a high capacity for both active and passive reabsorption b. Co-transport and counter-transport occurs

Fig. 27.6 Cellular ultrastructure and primary transport characteristics of the proximal tubule.

Reabsorption and Secretion (cont.) Concentrations of Solutes Along the Proximal Tubule Fig. 27.7

Reabsorption and Secretion (cont.) Secretion of Organic Acids and Bases End products of metabolism Harmful drugs and toxins PAH (para-aminohippuric acid) clearance: used to estimate renal plasma flow

Reabsorption and Secretion (cont.) Solute and Water Transport in the Loop of Henle Fig. 27.8

Reabsorption and Secretion (cont.) Solute and Water Transport in the Loop of Henle Fig. 27.9

Reabsorption and Secretion (cont.) Distal Tubule - Macula densa of the juxtaglomerular complex provides feedback control for GFR Fig. 27.11

Reabsorption and Secretion (cont.) Late Distal Tubule and Cortical Collecting Tubule Principal cells reabsorb sodium and secrete potassium Fig. 27.12

Reabsorption and Secretion (cont.) Late Distal Tubule and Cortical Collecting Tubule (cont.) Intercalated cells secrete hydrogen and reabsorb bicarbonate and potassium ions c. Permeability is controlled by concentrations of ADH

Reabsorption and Secretion (cont.) Medullary Collecting Duct - absorb less than 10% of the filtered water and sodium; final site for processing urine Permeability controlled by ADH Permeable to urea Can secrete H+ against a concentration gradient (helps regulate acid-base balance

Fig. 27.13

Summary of Concentrations of Different Solutes in the Different Tubular Segments Fig. 27.14

Regulation of Tubular Reabsorption Glomerulotubular Balance- ability of the tubules to increase reabsorption rate in response to increased tubular load Peritubular Capillary and Renal Interstitial Fluid Physical Forces - hydrostatic and colloid osmotic forces govern the rate of reabsorption across the peritubular capillaries

Regulation of Tubular Reabsorption (cont.) Fig. 27.15

Regulation (cont.) Regulation of Peritubular Capillary Physical Forces Peritubular capillary hydrostatic pressure is influenced by arterial pressure and resistance of the afferent and efferent arterioles Increases in these pressures tend to raise peritubular hydrostatic pressure and decrease reabsorption rate Increases in the resistance of the arterioles reduces the hydrostatic pressure and increases the reabsorption rate

Regulation (cont.) Regulation of Peritubular Capillary Physical Forces Raising the colloid osmotic pressure increases peritubular capillary reabsorption Colloid osmotic pressure is determined by Systemic plasma colloid osmotic pressure Filtration fraction

Renal Interstitial Hydrostatic and Colloid Osmotic Pressures Regulation (cont.) Renal Interstitial Hydrostatic and Colloid Osmotic Pressures Fig. 27.16

Regulation (cont.) Hormonal Control of Tubular Reabsorption Aldosterone increases Na reabsorption and stimulates K secretion Site of action is on the principal cells of the cortical collecting tubule Most important stimuli for aldosterone are increased K and angiotensin II levels

Regulation (cont.) Angiotensin II Increases Na and Water Reabsorption Angiotensin II stimulates aldosterone secretion Angiotensin II constricts the efferent arterioles Angiotensin II directly stimulates Na reabsorption in the proximal tubules, the loops of Henle, the distal tubules, and the collecting tubules

Fig. 27.17 Direct effects of angiotensin II to increase proximal tubular sodium reabsorption

ADH Increases Water Reabsorption Regulation (cont.) ADH Increases Water Reabsorption Fig. 27.18

Regulation (cont.) ANP Decreases Na and Water Reabsorption PTH Increases Ca Reabsorption Sympathetic Nervous System Activation Increases Na Reabsorption

Quantifying Kidney Function Inulin Clearance Creatinine Clearance PAH (para-aminohippuric acid) Clearance

Fig. 27.19 Measurement of the GFR from the renal clearance of inulin

Fig. 27.20 Effect of reducing GFR by 50% on serum creatinine concentration and on creatinine excretion rate when the production rate of creatinine remains constant

Fig. 27.21 Approximate relationship between GFR and plasma creatinine concentration under steady-state conditions

Fig. 27.22 Measurement of renal plasma flow from the renal clearance of PAG

Clearance Rate (ml/min) Approximate Clearance Rates for Some of the Substances Normally Handled by the Kidneys Substance Clearance Rate (ml/min) Glucose Sodium 0.9 Chloride 1.3 Potassium 12.0 Phosphate 25.0 Inulin 125.0 Creatinine 140.0