Chapter 26

 

I.                    INTRODUCTION

A.    The urinary system consists of two kidneys, two ureters, one urinary bladder, and one urethra (Figure 26.1).

B.    Urine is excreted from each kidney through its ureter and is stored in the urinary bladder until it is expelled from the body through the urethra.

C.    The specialized branch of medicine that deals with structure, function, and diseases of the male and female urinary systems and the male reproductive system is known as nephrology. The branch of surgery related to male and female urinary systems and the male reproductive system is called urology.

II.   OVERVIEW OF KIDNEY FUNCTIONS

A.    The major work of the urinary system is done by the kidneys.

B.    Kidneys contribute to homeostasis of body fluids by regulation of blood ionic composition, regulation of pH, regulation of blood volume, regulation of blood pressure, maintenance of blood osmolarity, producing hormones, regulating blood glucose level, and excreting wastes and foreign substances.  Please refer to lecture notes for further detail.

III. ANATOMY AND HISTOLOGY OF THE KIDNEYS

A.    The paired kidneys are retroperitoneal organs (Figure 26.2).

B.    External Anatomy of the Kidney

1.     Near the center of the concave medial border of the kidney is a vertical fissure called the hilus, through which the ureter leaves and blood vessels, lymphatic vessels, and nerves enter and exit (Figure 26.3).

2.     Three layers of tissue surround each kidney: the innermost renal capsule, the adipose capsule, and the outer renal fascia.

 

C.    Internal Anatomy of the Kidney

1.     Internally, the kidneys consist of cortex, medulla, pyramids, papillae, columns, calyces, and pelves (Figure 26.3).

2.     The renal cortex and renal pyramids constitute the functional portion or parenchyma of the kidney.

3.     The nephron is the functional unit of the kidney. 

D.    Blood and Nerve Supply of the Kidneys

1.     Blood enters the kidney through the renal artery and exits via the renal vein. Figure 26.4 shows the branching pattern of renal blood vessels and the path of blood flow through the kidneys. You are responsible for knowing the blood vessels and path of blood flow through the kidney.

2.     The nerve supply to the kidney is derived from the renal plexus (sympathetic division of ANS).

E.     Nephrons

1.         A nephron consists of a renal corpuscle where fluid is filtered, and a renal tubule into which the filtered fluid passes (Figure 26.5).

a.      Nephrons perform three basic functions: glomerular filtration, tubular reabsorption, and tubular secretion.

b.     A renal tubule consists of a proximal convoluted tubule (PCT), loop of Henle (nephron loop), and distal convoluted tubule (DCT). Distal convoluted tubules of several nephrons drain into to a single collecting duct and many collecting ducts drain into a small number of papillary ducts.

c.      The loop of Henle consists of a descending limb, a thin ascending limb, and a thick ascending limb (Figure 26.5).

d.     There are two types of nephrons that have differing structure and function.

1)     A cortical nephron usually has its glomerulus in the outer portion of the cortex and a short loop of Henle that penetrates only into the outer region of the medulla (Figure 26.5a).

2)     A juxtamedullary nephron usually has its glomerulus deep in the cortex close to the medulla; its long loop of Henle stretches through the medulla and almost reaches the renal papilla (Figure 26.5b).

2.     Histology of the Nephron and Collecting Duct

a.      Glomerular Capsule

1)     The glomerular capsule consists of visceral and parietal layers (Figure 26.6).

2)     The visceral layer consists of modified simple squamous epithelial cells called podocytes.

3)     The parietal layer consists of simple squamous epithelium and forms the outer wall of the capsule.

4)     Fluid filtered from the glomerular capillaries enters the capsular space, the space between the two layers of the glomerular capsule.

b.     Renal Tubule and Collecting Duct

1)     Table 26.1 illustrates the histology of the cells that form the renal tubule and collecting duct.

a)     The juxtaglomerular apparatus (JGA) consists of the juxtaglomerular cells of an afferent arteriole and the macula densa. The JGA helps regulate blood pressure and the rate of blood filtration by the kidneys (Figure 26.6).

 

 

 

IV. OVERVIEW OF RENAL PHYSIOLOGY

A.    Nephrons and collecting ducts perform three basic processes while producing urine: glomerular filtration, tubular secretion, and tubular reabsorption (Figure 26.7).

B.    By filtering, reabsorbing, and secreting, nephrons maintain homeostasis of blood.

V.    GLOMERULAR FILTRATION

A.    Introduction

1.     The fluid that enters the capsular space is termed glomerular filtrate.

2.     The fraction of plasma in the afferent arterioles of the kidneys that becomes filtrate is termed the filtration fraction.

B.    The Filtration Membrane

1.     The filtering unit of a nephron is the endothelial-capsular membrane. It consists of the glomerular endothelium, glomerular basement membrane, and slit membranes between pedicels of podocytes.

2.     Filtered substances move from the blood stream through three barriers: a glomerular endothelial cell, the basal lamina, and a filtration slit formed by a podocyte (Figure 26.8). Please refer to lecture notes for further detail.

3.     The principle of filtration - to force fluids and solutes through a membrane by pressure - is the same in glomerular capillaries as in capillaries elsewhere in the body.

4.     The three features of the renal corpuscle that enhance its filtering capacity include the large surface area across which filtration can occur, the thin and porus nature of the filtration membrane, and the high level of glomerular capillary blood pressure.

C.    Net Filtration Pressure

1.     Glomerular filtration depends on three main pressures, one that promotes and two that oppose filtration (Figure 26.9).

2.     Filtration of blood is promoted by glomerular blood hydrostatic pressure (BGHP) and opposed by capsular hydrostatic pressure (CHP) and blood colloid osmotic pressure (BCOP).  The net filtration pressure (NFP) is about 10 mm Hg.

            NFP = BGHP – (CHP + BCOP)

3.     In some kidney diseases, damaged glomerular capillaries become so permeable that plasma proteins enter the filtrate, causing an increase in NFP and GFR and a decrease in BCOP.

D.    Glomerular Filtration Rate

1.     Glomerular filtration rate (GFR) is the amount of filtrate formed by both kidneys per minute; in a normal adult, it is about 125 ml/minute. This amounts to 180 liters per day.

a.      GFR is directly related to the pressures that determine net filtration pressure.

b.     Surprisingly, when system blood pressure rises above the normal resting level, net filtration pressure and GFR increase very little.

2.     Regulation of GFR

a.      The mechanisms that regulate GFR adjust blood flow into and out of the glomerulus and alter the glomerular capillary surface area available for filtration.

b.     The three principal mechanisms that control GFR are renal autoregulation, neural regulation, and hormonal regulation.

1.)    Renal Autoregulation of GFR

a.)    This is an intrinsic mechanism with the kidneys consisting of the myogenic mechanism and tubuloglomerular feedback.

b.)    The myogenic mechanism occurs because stretching causes contraction of smooth muscle cells in the wall of the afferent arteriole.

c.)    Tubuloglomerular feedback occurs as the macula densa provides feedback to the glomerulus (Figure 26.10). Please refer to lecture notes for more detail.

2.)    Neural regulation of GFR is through the ANS.

3.)    Hormonal regulation of GFR is through the action of angiotensin II

c.      Table 26.2 summarizes the way in which GFR is regulated.

VI. TUBULAR REABSORPTION AND TUBULAR SECRETION

A.    Principles of Renal Transport

1.     Introduction

a.      The normal rate of glomerular filtration is so high that the volume of fluid entering the proximal convoluted tubule in half an hour is greater than the total plasma volume.

b.     Reabsorption returns most of the filtered water and many of the filtered solutes to the bloodstream using both active and passive transport processes.

c.      Table 26.3 compares the amounts of substances that are filtered, reabsorbed, and excreted in urine with the amounts present in blood plasma.

d.     Tubular secretion, the transfer of materials from the blood and tubule cells into tubular fluid, helps control blood pH and helps eliminate other substances from the body.

2.  Water Reabsorption

Solute reabsorption drives water reabsorption. The mechanisms that accomplish Na+ reabsorption in each portion of the renal tubule and collecting duct recover not only filtered Na+ but also other electrolytes, nutrients, and water.

1)     The mechanism for water reabsorption by the renal tubule and collecting duct is osmosis.

a)     About 90% of the filtered water reabsorbed by the kidneys occurs together with the reabsorption of solutes such as Na+, Cl-, and glucose.

b)     Water reabsorption together with solutes in tubular fluid is called obligatory water reabsorption.

c)     Reabsorption of the final water, facultative reabsorption, is based on need and occurs in the collecting ducts and is regulated by ADH.

B.    Hormonal Regulation of Tubular Reabsorption and Secretion

1.     Four hormones affect the extent of Na+, Cl-, and H2O reabsorption and K+ secretion by the renal tubules.

2.     In the renin-angiotensin-aldosterone system, angiotensin II increases blood volume and blood pressure and is a major regulator of electrolyte reabsorption and secretion along with aldosterone which also increases reabsorption of water in the collecting duct. Please refer to p. 610 – 611 and lecture notes for explanation.

3.     Antidiuretic hormone (ADH) regulates facultative water reabsorption by increasing the water permeability of the collecting duct (Figure 26.17).

4.     Table 26.4 summarizes the hormonal regulation of tubular reabsorption and tubular secretion.

VII. PRODUCTION OF DILUTE AND CONCENTRATED URINE

A.    The rate at which water is lost from the body depends mainly on ADH, which controls water permeability of principal cells in the collecting duct (and in the last portion of the distal convoluted tubule).

B.    When ADH level is very low, the kidneys produce dilute urine and excrete excess water; in other words, renal tubules absorb more solutes than water (Figure 26.18). Please refer to lecture notes for further detail.

 

 

C.    Formation of Concentrated Urine

1.     When ADH level is high, the kidneys secrete concentrated urine and conserve water; a large volume of water is reabsorbed from the tubular fluid into interstitial fluid, and the solute concentration of urine is high.

2.     Production of concentrated urine involves ascending limb cells of the loop of Henle establishing the osmotic gradient in the renal medulla, collecting ducts reabsorbing more water and urea, and urea recycling causing a build up of urea in the renal medulla (Figure 26.19).

3.     The descending limb of Henle is permeable to water, but not solutes so the filtrate becomes more concentrated as it rounds the loop of Henle.

4.     The thick ascending limb of Henle and first part of the distal convoluted tubule is impermeable to water, but actively transports ions out. The filtrate becomes increasingly more dilute as it goes up the ascending limb of Henle. 

5.     If ADH is present, the collecting duct becomes more permeable to water and water leaves the filtrate because of the hypertonic interstitium in the medulla of the kidney (which was created by the above tubule permeabilities as well as the countercurrent mechanism and maintained by the vasa recta). Urea recycling also contributes to the hypertonic interstitium.  Please refer to lecture notes for more detail on the formation of dilute and concentrated urine.

D.    Figure 26.20 summarizes the processes of filtration, reabsorption, and secretion in each segment of the nephron and collecting ducts. Hormonal effects are also noted.

VIII. EVALUATION OF KIDNEY FUNCTION

A.    An analysis of the volume and physical, chemical, and microscopic properties of urine, called urinalysis, reveals much about the state of the body.

1.     Table 26.5 summarizes the principal physical characteristics of urine.

2.     Table 26.6 lists several abnormal constituents of urine that may be detected as part of a urinalysis.  You should know the normal and abnormal constituents of urine.

IX. URINE STORAGE, TRANSPORTATION, AND ELIMINATION

A.    Urine drains through papillary ducts into minor calyces, which joint to become major calyces that unite to form the renal pelvis (Figure 26.3).  From the renal pelvis, urine drains into the ureters and then into the urinary bladder, and finally, out of the body by way of the urethra (Figure 26.1).

B.    Ureters

1.     Each of the two ureters connects the renal pelvis of one kidney to the urinary bladder (Figure 26.21).

2.     The ureters transport urine from the renal pelvis to the urinary bladder, primarily by peristalsis, but hydrostatic pressure and gravity also contribute.

3.     The ureters are retroperitoneal and consist of a mucosa, muscularis, and fibrous coat.

C.    Urinary Bladder

1.     The urinary bladder is a hollow muscular organ situated in the pelvic cavity posterior to the pubic symphysis.

2.     Anatomy and Histology of the Urinary Bladder

a.      In the floor of the urinary bladder is a small, smooth triangular area, the trigone. The ureters enter the urinary bladder near two posterior points in the triangle; the urethra drains the urinary bladder from the anterior point of the triangle (Figure 26.21).

3.     Micturition Reflex

a.      Urine is expelled from the urinary bladder by an act called micturition, commonly known as urination or voiding.

b.     When the volume of urine in the bladder reaches a certain amount (usually 200-400 ml), stretch receptors in the urinary bladder wall transmit impulses that initiate a spinal micturition reflex. Older children and adults may also initiate or inhibit micturition voluntarily.

D.    Urethra

1.      The urethra is a tube leading from the floor of the urinary bladder to the exterior (Figure 26.1).

2.      The function of the urethra is to discharge urine from the body. The male urethra also serves as the duct for ejaculation of semen (reproductive fluid).

3.      A lack of voluntary control over micturition is referred to as incontinence; failure to void urine completely or normally is termed retention.

X.    WASTE MANAGEMENT IN OTHER BODY SYSTEMS

A.    One of the many functions of the urinary system is to rid the body of waste materials.

B.    Other organs, tissues and processes contribute to waste management.

1.     Buffers prevent an increase in the acidity of body fluids.

2.     The blood transports wastes.

3.     The liver is the primary site for metabolic recycling.

4.     The lungs excrete CO2. H2O, and heat.

5.     Sweat glands eliminate excess heat, water, and CO2, plus small quantities of salts and urea.

6.     The GI tract eliminates solid, undigested foods, waste, some CO2, H2O, salts and heat.