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THE URINARY SYSTEM

INTRODUCTION

1. The integument, respiratory system, and digestive system assist the urinary system in excreting wastes and regulating the water and electrolyte composition of body fluids. The urinary system plays the dominant role through its influence on the composition of the circulating blood.

2. The urinary system, which includes the kidneys, the ureters, the urinary bladder, and the urethra, preserves homeostasis by adjusting the composition of the circulating blood by conserving water, excreting metabolic wastes, reabsorbing useful compounds, and regulating the pH and electrolyte profile.

GROSS ANATOMY

The Kidneys

1. Important anatomical landmarks on each kidney include the hilus, renal sinus, cortex, medulla, renal lobes, papillae, and renal columns.The kidney is anchored by the renal fascia and surrounded by adipose tissue.

2. The vasculature of the kidneys includes the renal, interlobar, arcuate, and interlobular arteries and corresponding veins.

3. The ureter communicates with the renal pelvis. This chamber branches into two major calyces, each connected to 4-5 minor calyces that enclose the renal papillae.

The Ureters and Bladder

1. The ureters extend from the renal pelvis to the urinary bladder.

2. The bladder is stabilized by the urachus and the lateral umbilical ligaments.

3. The mucosal lining of the empty bladder contains prominent rugae.Other internal features and structures include the trigone and the internal and external sphincters.

HISTOLOGICAL ORGANIZATION

The Kidney

The Nephron:

1. The nephron represents the basic functional unit of the kidney. The nephron includes the glomerulus and renal tubule that empty into a collecting tubule, a small tributary of a collecting duct.

2. Cortical nephrons are found within the cortex; juxtamedullary nephrons are situated close to the cortical-medullary border and portions of the tubule extend into the medulla.

3. The capsular epithelium lines the outer wall of the renal corpuscle. Podocytes cover the lamina densa of the capillaries that project into the capsular space. The pedicels of the podocytes are separated by narrow slit pores.

4. The proximal convoluted tubule draining the capsular space extends to the loop of Henle. The PCT actively reabsorbs nutrients, plasma proteins, and electrolytes from the filtrate.

5. The loop of Henle includes descending and ascending limbs. The ascending limb delivers the urine to the distal convoluted tubule.

6. The DCT opens into a collecting tubule that joins its neighbors to form a collecting duct. The collecting ducts descend through the medulla and empty into the minor calyx at the base of the renal lobe.

The Blood Supply to the Nephron:

1. Blood arrives at the glomerulus via the afferent arteriole and leaves in the efferent arteriole headed toward the peritubular capillaries and the vasa recta.

The Renal Pelvis, Ureters, and Urinary Bladder

1. The renal pelvis, ureter, and bladder are lined by a transitional epithelium. Several layers of smooth muscle surround this epithelium and their peristaltic contractions move urine along the urinary tract.

2. The outer smooth muscle layers of the bladder form the detrusor muscle whose contractions compress the contents and expel the urine.

The Urethra

1. In the female the urethra conducts urine but has no role in reproduction. The longer male urethra carries urine and reproductive products.

2. The urethral lining consists of a stratified epithelium that varies from transitional near the bladder to stratified squamous near the urethral opening.

3. As the urethra passes through the urogenital diaphragm a circular band of skeletal muscle forms the external sphincter.

RENAL PHYSIOLOGY

1. Urine formation involves filtration, active transport (both reabsorption and secretion), and osmosis. NOTE: This differs from the more traditional breakdown into filtration, reabsorption, and secretion; this arrangement permits a more logical flow of information.

Filtration

1. The difference between the glomerular and capsular pressures determines the net filtration pressure forcing materials into the filtrate.

2. The filtration pressure determines the glomerular filtration rate. The normal GFR averages around 125 ml/min.

Active Transport

Tubular Reabsorption and the Proximal Convoluted Tubule

1. The PCT normally manages to reabsorb 80 percent of the filtrate generated by the glomerulus, including all of the nutrients and plasma proteins up to the transport maximum. It also absorbs electrolytes, and performs secretory functions comparable to those of the distal convoluted tubule.

Tubular Secretion and the Distal Convoluted Tubule

1. Roughly 80 percent of the water and 65 percent of the solutes are absorbed by the PCT, and another 25 percent of the dissolved materials are pumped into the medulla along the loop of Henle.The DCT performs the final adjustments by actively secreting or absorbing materials.

2. Sodium ions are actively absorbed, in exchange for potassium or hydrogen ions discharged into the filtrate.Aldosterone secretion increases the rate of sodium resorption.

3. The production of ammonia by cells of the DCT accelerates when the pH of the blood declines. Ammonium ions then appear in the blood, and bicarbonate ions enter the circulation.

Osmosis

1. The concentration of dissolved materials determines the osmolarity of the solution, expressed in osmoles or milliosmoles per liter.

2. The reabsorption and retention of water represents a primary function of the kidney. An osmotic gradient in the medulla encourages the osmotic flow of water out of the filtrate.

3. The osmolarity of the interstitial fluid of the medulla changes gradually from about 300 mOsm near the cortex to 1200 mOsm near the renal pelvis.

The Formation and Maintenance of the Osmotic Gradient

1. The thick ascending limb is impermeable to water and electrolytes, but active transport mechanisms pump sodium ions out of the filtrate; these anions are followed passively by chloride ions.

2. The descending limb is permeable to water but impermeable to electrolytes and urea.As the filtrate moves along the descending limb, water moves into the interstitial fluid of the medulla.

3. The filtrate arriving at the bend of the loop therefore has a higher osmolarity, and the chloride and sodium ions will subsequently be pumped into the interstitial fluid surrounding the ascending limb.

4. This countercurrent multiplication between the ascending and descending limbs of the loop of Henle helps to create the osmotic gradient in the medulla.

5. Urea diffusing out of the terminal segments of the collecting duct makes a significant contribution to the osmolarity of the deeper portions of the medulla.

6. The vasa recta assists in the maintenance of the osmotic gradient by opposing the diffusion of solutes out of the medulla and by removing water and solutes retrieved from the filtrate.

The Production of Hypertonic Urine

1. Although the total volume declines, the filtrate remains isotonic with respect to the surrounding interstitial fluid as it passes along the PCT and the descending limb of the loop of Henle.

2. The ascending limb of the loop of Henle is impermeable to water, and due to the reabsorption of ions in the thick segment the DCT receives a small amount of hypotonic filtrate.

3. The permeabilities of the terminal portions of the DCT and the collecting tubules and ducts are regulated by ADH. Without ADH, the tubules remain impermeable and the individual excretes relatively large volumes of hypotonic urine.

4. Antidiuretic hormone increases the water permeabilities of these regions.Under ADH stimulation, water retention increases until the filtrate concentration reaches 1,200 mOsm; the urine produced is then isotonic with the interstitial fluids deep in the medulla.

The Regulation of Kidney Function

1. More than 99 percent of the filtrate produced each day gets reabsorbed. The composition of the excreted urine varies depending upon the metabolic and hormonal events underway at the time.

2. Urinalysis provides considerable information concerning an individual's metabolic state. The composition reflects the filtration, absorption, and secretion activities at the nephrons. The concentration depends upon the osmotic movement of water across the walls of the DCT and collecting tubules and ducts.

The Control of Glomerular Filtration:

1. The glomerulus show autoregulation through alterations in the diameters of the afferent and efferent arterioles.

2. Dropping filtration pressures stimulate the juxtaglomerular apparatus to release renin and erythropoietin.

3. ADH production and release follows stimulation of hypothalamic osmoreceptors.

4. The macula densa also responds to changes in the osmotic concentration of the filtrate.The osmotic concentration declines when glomerular filtration rates are low, and the macula densa responds by releasing renin.

5. Sympathetic stimulation causes vasoconstriction of the afferent arterioles, temporarily decreasing the glomerular filtration rate.

The Control of Tubular Transport and Osmosis:

1. Reabsorption and secretion along the PCT occurs in response to the presence of specific substances in the urine or in the surrounding interstitial fluids. The rates of secretion for calcium, hydrogen, sodium and potassium vary in the presence of specific hormones.

Urine Storage and Release

1. The micturition reflex is initiated by stretch receptors in the bladder wall. Voluntary urination involves coupling this reflex with the voluntary relaxation of the external sphincter.

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