Unit One: Introduction to Physiology: The Cell and General Physiology

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

Tubular Reabsorption is Quantitatively Large andHighly Selective

Amount FilteredAmount ReabsorbedAmount Excreted% Filtered ReabsorbedGlucose g/day1801800100Bicarbonate mEq/day432043182>99.9Sodium mEq/day255602541015099.4Chloride mEq/day194401926018099.1Potassium mEq/day7566649287.8Urea g/day46.823.423.450Creatinine g/day1.801.80Table 27.1 Filtration, Excretion, and Reabsorption Rates of Different Substances by the KidneyTubular Reabsorption

Fig. 27.1Tubular Reabsorption (cont.) Active Transport

Solutes transported through epithelial cells orbetween 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 transportTubular Reabsorption (cont.)Secondary active secretion into the tubules (i.e.counter-transport)

Pinocytosis-active reabsorption of proteins

Transport maximum-limit to the rate at whicha 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.SubstanceTransport MaximumGlucose375 mg/minPhosphate0.10 mM/minSulfate.06 mM/minAmino Acids1.5 mM/minUrate15 mg/minLactate75 mg/minPlasma protein30 mg/minTransport Maximums for Substances Actively Reabsorbed by the TubulesSubstanceTransport MaximumCreatinine16 mg/minPara-aminohippuric acid80 mg/minTransport Maximums for Substances Actively SecretedSubstances that are actively transported but do notexhibit a transport maximum (i.e. Na in the proximaltubule

h. Passive reabsorption coupled to sodium reabsorption

Fig. 27.5 Mechanisms by which water, chloride, and urea reabsorption are coupled with sodium reabsorptionReabsorption and Secretion Along Different Parts of the Nephron Proximal Tubular Reabsorption - normally about 65% offiltered load of Na and water and slightly lower percentof chloride is reabsorbed before the loop of Henle

Have a high capacity for both active and passivereabsorption

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. 27.7Reabsorption and Secretion (cont.) Secretion of Organic Acids and Bases

End products of metabolism

Harmful drugs and toxins

PAH (para-aminohippuric acid) clearance: used toestimate renal plasma flowReabsorption and Secretion (cont.) Solute and Water Transport in the Loop of Henle

Fig. 27.8Reabsorption 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 complexprovides feedback control for GFR

Fig. 27.11Reabsorption and Secretion (cont.) Late Distal Tubule and Cortical Collecting Tubule

Principal cells reabsorb sodium and secrete potassium

Fig. 27.12Reabsorption and Secretion (cont.) Late Distal Tubule and Cortical Collecting Tubule (cont.)

Intercalated cells secrete hydrogen and reabsorbbicarbonate and potassium ions

c.Permeability is controlled by concentrations of ADH

Reabsorption and Secretion (cont.) Medullary Collecting Duct - absorb less than 10% of thefiltered water and sodium; final site for processingurine

Permeability controlled by ADH

Permeable to urea

Can secrete H+ against a concentration gradient (helpsregulate acid-base balance

Fig. 27.13Summary of Concentrations of Different Solutesin the Different Tubular Segments

Fig. 27.14Regulation of Tubular Reabsorption

Glomerulotubular Balance- ability of the tubules toincrease reabsorption rate in response to increasedtubular load

Peritubular Capillary and Renal Interstitial FluidPhysical Forces - hydrostatic and colloid osmotic forces govern the rate of reabsorption acrossthe peritubular capillariesRegulation of Tubular Reabsorption (cont.)

Fig. 27.15Regulation (cont.) Regulation of Peritubular Capillary Physical Forces

Peritubular capillary hydrostatic pressure is influencedby arterial pressure and resistance of the afferent andefferent arterioles

Increases in these pressures tend to raise peritubularhydrostatic pressure and decrease reabsorption rate

Increases in the resistance of the arterioles reduces thehydrostatic pressure and increases the reabsorption rateRegulation (cont.) Regulation of Peritubular Capillary Physical Forces

Raising the colloid osmotic pressure increases peritubularcapillary reabsorption

Colloid osmotic pressure is determined by

Systemic plasma colloid osmotic pressure

Filtration fractionRegulation (cont.) Renal Interstitial Hydrostatic and Colloid Osmotic Pressures

Fig. 27.16

Regulation (cont.) Hormonal Control of Tubular Reabsorption

Aldosterone increases Na reabsorption and stimulatesK secretion

Site of action is on the principal cells of the corticalcollecting tubule

Most important stimuli for aldosterone are increasedK 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 inthe proximal tubules, the loops of Henle, the distaltubules, and the collecting tubules

Fig. 27.17 Direct effects of angiotensin II to increase proximal tubular sodium reabsorptionRegulation (cont.) ADH Increases Water Reabsorption

Fig. 27.18Regulation (cont.) ANP Decreases Na and Water Reabsorption

PTH Increases Ca Reabsorption

Sympathetic Nervous System Activation IncreasesNa 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 PAGSubstanceClearance Rate (ml/min)Glucose0Sodium0.9Chloride1.3Potassium12.0Phosphate25.0Inulin125.0Creatinine140.0

Approximate Clearance Rates for Some of the Substances Normally Handled by the Kidneys