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Chapter 26: Urine Formation By the Kidneys. I. Glomerular Filtration, Renal Blood Flow and Their Control Guyton and Hall, Textbook of Medical Physiology, 12 th edition

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Multiple Functions of the Kidney Excretion of Metabolic Waste Products, Foreign Chemicals, Drugs, and Hormone Metabolites a.Eliminate waste products of metabolism that are no longer needed ( i.e. urea, creatinine, uric acid, hb breakdown products) b.Also rapidly eliminate most toxins, pesticides, drugs, food additives

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Functions (cont.) Regulation of Water and Electrolyte Balances a.For homeostatsis, excretion of electrolytes must match intake b. Governed mostly by eating habits

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Functions (cont.) Regulation of Arterial Pressure a. By excreting variable amounts of sodium and water b.Short-term-by secreting hormones and vasoactive factors (i.e. renin) Glucose Synthesis a.Gluconeogenesis-synthesize glucose from amino acids and other precursors during prolonged fasting.

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Functions (cont.) Regulation of Acid-Base Balance a.By excreting acids and by regulating the body fluid buffer stores b.Kidney is the only means of eliminating sulfuric and phosphoric acids (generated by the metabolism of proteins

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Functions (cont.) Regulation of Erythrocyte Production a.Secretes erythropoietin which stimulates the production of red blood cells Regulation of 1,25-Dihydroxyvitamin D 3 Production a.Produces the active form (calcitriol) b. Calcitriol is necessary for the normal calcium deposition in bone and calcium reabsorption by the gastrointestinal tract

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Physiologic Anatomy of the Kidneys General Organization of the Kidneys and Urinary Tract Fig. 26.2 General Organization of the Kidneys and Urinary System

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Physiologic Anatomy of the Kidneys Renal Blood Supply Fig. 26.3 Renal Blood Supply

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Physiologic Anatomy of the Kidneys The Nephron-functional unit of the kidney a.Each kidney contains 800,000-1,000,000 b.Decrease with age or injury (cannot be replaced) c.Contains the glomerulus (Bowmans capsule) d.Proximal and distal convoluted tubules e.Loop of Henle f.Macula densa g.Collecting ducts

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Physiologic Anatomy of the Kidneys Figure 26.4 Basic tubular segments of the nephron

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Physiologic Anatomy of the Kidneys Figure 26.5 Cortical and juxtaglomerular nephrons Regional Differences in Nephron Structure: Cortical and Juxtaglomerular Nephrons

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Physiologic Anatomy of Urinary Bladder Micturition-process by which the urinary bladder empties when it becomes filled a.The bladder fills progressively until the tension within the walls rises above a threshold level b.The micturition reflex empties the bladder or stimulates a conscious desire to urinate c.It is an autonomic reflex that can be altered by centers in the cerebral cortex

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Physiologic Anatomy of Urinary Bladder Bladder Anatomy-smooth muscle chamber composed of two main parts: a.Body-where the urine collects b.Neck-funnel shaped extension which connects with the urethra The smooth muscle of the bladder is the detrusor muscle and its contraction is a major step in emptying the bladder.

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Fig. 26.6 Anatomy of the urinary bladder in males and females

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Innervation of the Bladder-pelvic nerves through the saccral plexus Fig. 26.7 Innervation of the urinary bladder

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Transport of Urine No significant changes in urine composition from the renal calyces to the ureters to the bladder Peristaltic contractions in the smooth muscle of the ureter are enhanced by parasympathetic stimulation and inhibited by sympathetic stimulation

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Micturition Reflex Fig. 26.8 Normalcystometrogram showing acute pressure waves caused by micturition reflexes

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Micturition Reflex As the bladder fills, micturition contractions occur Initiated by stretch receptors in the bladder When partially filled, the contractions relax spontaneously The reflex is self-regenerative

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Micturition Reflex Reflex is a Single Complete Cycle a.Progressive and rapid increase of pressure b.A period of sustained pressure c.Return of the pressure to the basal tone

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Urine Formation Fig. 26.9 Basic kidney processes that determine the composition of urine

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Urine Formation Sum of three renal processes

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Fig. 26.10 A.The substance is freely filtered but not reabsorbed B.The substance is freely filtered but part is reabsorbed C.The substance is freely filtered but not excreted because all is reabsorbed. D.The substance is freely filtered but is not reabsorbed but secreted from the peritubular capillaries into the renal tubules

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Glomerular Filtration Composition of the Filtrate a.Protein free and cell free b.Salts and organic molecules are similar to those in plasma c.Does not include calcium and fatty acids that are bound to proteins

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Glomerular Filtration (cont.) GFR (Glomerular Filtration Rate) a.Determined by (1) the balance of hydrostatic and colloid osmotic forces acting on the capillary membrane, and (2) the product of the permeability and filtering surface area of the capillaries b.

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Glomerular Capillary Membrane Consists of Three Major Layers a.Endothelium of the capillary b.Basement membrane c.Layer of epithelial cells (podocytes)

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Fig. 26.11 A: Basic ultrastructure of the glomerular capillaries; B: Cross-section of the capillary membrane and its major components

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Membrane (cont.) Filterability of Solutes is Inversely Related to Their Size SubstanceMolecular WeightFilterability Water181.0 Sodium231.0 Glucose1801.0 Inulin5,5001.0 Myoglobin17,0000.75 Albumin69,0000.005 Table 26.1 Filterability of substances by glomerular capillaries based on molecular weight

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Membrane (cont.) Negatively Charged Large Molecules Are Filtered Less Easily Than Positively Charged Molecules of Equal Molecular Size Fig. 26.12 Effect of molecular radius and electrical charge of dextran on its filterability by glomerular capillaries

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Determinants of the GFR GFR is determined by (1) sum of the hydrostatic and colloid forces across the glomerular membrane (net filtration pressure), and (2) the glomerular capillary filtration coefficient. Fig. 26.13

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Determinants of the GFR Increased glomerular capillary filtration coefficient increases GFR Increased Bowmans capsule hydrostatic pressure decreases GFR Increased glomerular capillary colloid osmotic pressure decreases GFR

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Fig. 26.14 Increase in colloid osmotic pressure in plasma flowing through the glomerular capillary.

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Determinants of the GFR (cont.) Increased glomerular capillary hydrostatic pressure increases GFR Fig. 26.15 Effect of change in afferent or efferent arteriole resistance on GFR and renal flow.

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Renal Blood Flow Renal Blood Flow and Oxygen Consumption Fig. 26.16 Relationship between sodium reabsorption and oxygen consumption

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Renal Blood Flow (cont.) Determinants of Renal Blood Flow VesselBeginningEnd Percentage of Total Renal Vascular Resistance Renal Artery 100 0 Interlobar, arcuate, and interlobular arteries Approx. 1008516 Afferent arteriole 856026 Glomerular capill. 60591 Efferent arteriole 591843 Peritubular capill. 18810 Interlobar, arcuate, and interlobular veins 844 Renal vein 4Approx. 40 Table 26.3 Approximate pressures and vascular resistances in the circulation of a normal kidney

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Physiologic Control of GFR and Renal Blood Flow Sympathetic Nervous System Activation Decreases GFR Hormonal and Autacoid Control of Renal Circulation Hormone or AutacoidEffect on GFR NorepinephrineDecreases EpinephrineDecreases EndothelinDecreases Angiotensin IIConstricts efferent arterioles Endothelian NOIncreases ProstaglandinsIncreases

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Autoregulation of GFR and Renal Blood Flow Fig. 26.17 Autoregulation of renal blood flow and GFR but lack of autoregulation of urine flow during changes in renal arterial pressure

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Tubuloglomerular Feedback and Autoregulation of GFR Links changes in NaCl concentration at the macula densa with the control of renal arteriolar resistance Two components a.An afferent arteriolar feedback mechanism b.An efferent arteriolar feedback mechanism

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Fig. 26.18 Structure of the juxtaglomerular apparatus, demonstrating its possible feedback role in the control of nephron function.

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Decreased macula densa NaCl causes dilation of afferent arterioles and increased renin release Fig. 26.19

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Myogenic Autoregulation of Renal Blood Flow and GFR a.Ability of blood vessels to resist stretching during increased arterial pressure b.Stretch allows the release of calcium from ECF into the cells causing them to contract High protein intake and increased blood glucose also increase renal blood flow