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21: The Urinary System

  • Page ID
    12541
  • Gross Anatomy of Urine Transport

    • Identify the ureters, urinary bladder, and urethra, as well as their location, structure, and histology

    • Compare and contrast male and female urethras

    Urethra

    The urethra transports urine from the bladder to the outside of the body for disposal. The urethra is the only urologic organ that shows any significant anatomic difference between males and females; all other urine transport structures are identical (Figure 21.1).

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    The urethra in both males and females begins inferior and central to the two ureteral openings forming the three points of a triangular-shaped area at the base of the bladder called the trigone (Greek tri- = “triangle” and the root of the word “trigonometry”). The urethra tracks posterior and inferior to the pubic symphysis (see Figure 21.1). In both males and females, the proximal urethra is lined by transitional epithelium, whereas the terminal portion is a nonkeratinized, stratified squamous epithelium. In the male, pseudostratified columnar epithelium lines the urethra between these two cell types. Voiding is regulated by an involuntary autonomic nervous system-controlled internal urinary sphincter, consisting of smooth muscle and voluntary skeletal muscle that forms the external urinary sphincter below it.

    Figure 21.1 Female and Male Urethras The urethra transports urine from the bladder to the outside of the body. This image shows (a) a female urethra and (b) a male urethra.

    Bladder

    The urinary bladder collects urine from both ureters (Figure 21.2). The bladder lies anterior to the uterus in females, posterior to the pubic bone and anterior to the rectum. During late pregnancy, its capacity is reduced due to compression by the enlarging uterus, resulting in increased frequency of urination. In males, the anatomy is similar, minus the uterus, and with the addition of the prostate inferior to the bladder. The bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered “dome” projecting into the abdomen when the bladder is distended with urine.

    The bladder is a highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle. The interior surface is made of transitional cellular epithelium that is structurally suited for the large volume fluctuations of the bladder. When empty, it resembles columnar epithelia, but when stretched, it “transitions” (hence the name) to a squamous appearance (see Figure 21.2). Volumes in adults can range from nearly zero to 500–600 mL.

    The detrusor muscle contracts with significant force in the young. The bladder’s strength diminishes with age, but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying. Such voluntary contraction is also used in forceful defecation and childbirth.

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    Figure 21.2 Bladder (a) Anterior cross section of the bladder. (b) The detrusor muscle of the bladder (source: monkey tissue) LM × 448. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)

    Ureters

    The kidneys and ureters are completely retroperitoneal, and the bladder has a peritoneal covering only over the dome. As urine is formed, it drains into the calyces of the kidney, which merge to form the funnel-shaped renal pelvis in the hilum of each kidney. The renal pelvis narrows to become the ureter of each kidney. As urine passes through the ureter, it does not passively drain into the bladder but rather is propelled by waves of peristalsis. As the ureters enter the pelvis, they sweep laterally, hugging the pelvic walls. As they approach the bladder, they turn medially and pierce the bladder wall obliquely. This is important because it creates an one-way valve (a physiological sphincter rather than an anatomical sphincter) that allows urine into the bladder but prevents reflux of urine from the bladder back into the ureter. Children born lacking this oblique course of the ureter through the bladder wall are susceptible to “vesicoureteral reflux,” which dramatically increases their risk of serious UTI. Pregnancy also increases the likelihood of reflux and UTI.

    The ureters are approximately 30 cm long. The inner mucosa is lined with transitional epithelium (Figure 21.3) and scattered goblet cells that secrete protective mucus. The muscular layer of the ureter consists of longitudinal and circular smooth muscles that create the peristaltic contractions to move the urine into the bladder without the aid of gravity. Finally, a loose adventitial layer composed of collagen and fat anchors the ureters between the parietal peritoneum and the posterior abdominal wall.

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    Figure 21.3 Ureter Peristaltic contractions help to move urine through the lumen with contributions from fluid pressure and gravity. LM × 128. (Micrograph provided by the Regents of the University of Michigan Medical School © 2012)

    Gross Anatomy of the Kidney

    External Anatomy

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    The left kidney is located at about the T12 to L3 vertebrae, whereas the right is lower due to slight displacement by the liver. Upper portions of the kidneys are somewhat protected by the eleventh and twelfth ribs (Figure 21.4). Each kidney weighs about 125–175 g in males and 115–155 g in females. They are about 11–14 cm in length, 6 cm wide, and 4 cm thick, and are directly covered by a fibrous capsule composed of dense, irregular connective tissue that helps to hold their shape and protect them. This capsule is covered by a shock-absorbing layer of adipose tissue called the renal fat pad, which in turn is encompassed by a tough renal fascia. The fascia and, to a lesser extent, the overlying peritoneum serve to firmly anchor the kidneys to the posterior abdominal wall in a retroperitoneal position.

    Figure 21.4 Kidneys The kidneys are slightly protected by the ribs and are surrounded by fat for protection (not shown).

    Internal Anatomy

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    A frontal section through the kidney reveals an outer region called the renal cortex and an inner region called the medulla (Figure 21.5). The renal columns are connective tissue extensions that radiate downward from the cortex through the medulla to separate the most characteristic features of the medulla, the renal pyramids and renal papillae. The papillae are bundles of collecting ducts that transport urine made by nephrons to the calyces of the kidney for excretion. The renal columns also serve to divide the kidney into 6–8 lobes and provide a supportive framework for vessels that enter and exit the cortex. The pyramids and renal columns taken together constitute the kidney lobes.

    Figure 21.5 Left Kidney

    Nephrons and Vessels

    The renal artery first divides into segmental arteries, followed by further branching to form interlobar arteries that pass through the renal columns to reach the cortex (Figure 21.6). The interlobar arteries, in turn, branch into arcuate arteries, cortical radiate arteries, and then into afferent arterioles. The afferent arterioles service about 1.3 million nephrons in each kidney.

    Nephrons are the “functional units” of the kidney; they cleanse the blood and balance the constituents of the circulation. The afferent arterioles form a tuft of high-pressure capillaries about 200 μm in diameter, the glomerulus. The rest of the nephron consists of a continuous sophisticated tubule whose proximal end surrounds the glomerulus in an intimate embrace—this is Bowman’s capsule. The glomerulus and Bowman’s capsule together form the renal corpuscle. As mentioned earlier, these glomerular capillaries filter the blood based on particle size. After passing through the renal corpuscle, the capillaries form a second arteriole, the efferent arteriole (Figure 21.7). These will next form a capillary network around the more distal portions of the nephron tubule, the peritubular capillaries and vasa recta, before returning to the venous system. As the glomerular filtrate progresses through the nephron, these capillary networks recover most of the solutes and water, and return them to the circulation. Since a capillary bed (the glomerulus) drains into a vessel that in turn forms a second capillary bed, the definition of a portal system is met. This is the only portal system in which an arteriole is found between the first and second capillary beds. (Portal systems also link the hypothalamus to the anterior pituitary, and the blood vessels of the digestive viscera to the liver.)

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    Figure 21.6 Blood Flow in the Kidney

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    Figure 21.7 Blood Flow in the Nephron The two capillary beds are clearly shown in this figure. The efferent arteriole is the connecting vessel between the glomerulus and the peritubular capillaries and vasa recta.

    Microscopic Anatomy of the Kidney

    Nephrons: The Functional Unit

    Nephrons take a simple filtrate of the blood and modify it into urine. Many changes take place in the different parts of the nephron before urine is created for disposal. The term forming urine will be used hereafter to describe the filtrate as it is modified into true urine. The principle task of the nephron population is to balance the plasma to homeostatic set points and excrete potential toxins in the urine. They do this by accomplishing three principle functions—filtration, reabsorption, and secretion. They also have additional secondary functions that exert control in three areas: blood pressure (via production of renin), red blood cell production (via the hormone EPO), and calcium absorption (via conversion of calcidiol into calcitriol, the active form of vitamin D).

    Renal Corpuscle

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    As discussed earlier, the renal corpuscle consists of a tuft of capillaries called the glomerulus that is largely surrounded by Bowman’s (glomerular) capsule. The glomerulus is a high-pressure capillary bed between afferent and efferent arterioles. Bowman’s capsule surrounds the glomerulus to form a lumen, and captures and directs this filtrate to the PCT. The outermost part of Bowman’s capsule, the parietal layer, is a simple squamous epithelium. It transitions onto the glomerular capillaries in an intimate embrace to form the visceral layer of the capsule. Here, the cells are not squamous, but uniquely shaped cells (podocytes) extending finger-like arms (pedicels) to cover the glomerular capillaries (Figure 21.8). These projections interdigitate to form filtration slits, leaving small gaps between the digits to form a sieve. As blood passes through the glomerulus, 10 to 20 percent of the plasma filters between these sieve-like fingers to be captured by Bowman’s capsule and funneled to the PCT. Where the fenestrae (windows) in the glomerular capillaries match the spaces between the podocyte “fingers,” the only thing separating the capillary lumen and the lumen of Bowman’s capsule is their shared basement membrane (Figure 21.9). These three features comprise what is known as the filtration membrane. This membrane permits very rapid movement of filtrate from capillary to capsule though pores that are only 70 nm in diameter.

    Figure 21.8 Podocytes Podocytes interdigitate with structures called pedicels and filter substances in a way similar to fenestrations. In (a), the large cell body can be seen at the top right corner, with branches extending from the cell body. The smallest finger-like extensions are the pedicels. Pedicels on one podocyte always interdigitate with the pedicels of another podocyte. (b) This capillary has three podocytes wrapped around it.

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    Figure 21.9 Fenestrated Capillary Fenestrations allow many substances to diffuse from the blood based primarily on size.

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    Figure 21.10 Juxtaglomerular Apparatus and Glomerulus (a) The JGA allows specialized cells to monitor the composition of the fluid in the DCT and adjust the glomerular filtration rate. (b) This micrograph shows the glomerulus and surrounding structures. LM × 1540. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    LAB 21 EXERCISES 21-1

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    B

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    Identify the following: L. and R. Kidneys *

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    Adrenal glands * Ureters * Bladder.

    Identify all of the blood vessels shown.

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    LAB 21 EXERCISES 21-2

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    GROSS ANATOMY JUXTAGLOMELLULAR

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    APPARATUS

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    Identify the following: Cortex * Medulla * Capsule * Renal Pyramid * Renal Column * Renal sinus * Minor Calyx * Major Calyx * Renal Pelvis * Ureter * Papilla * Hilus.

    Label the following: Juxtaglomerular apparatus * JG cells * Macula densa * Distal convoluted tubule * Proximal convoluted tubule

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    LAB 21 EXERCISES 21-3

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    GLOMERULUSSES

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    Label the following: Afferent arteriole *

    Efferent arteriole * Bowman’s capsule *

    Podocyte * Fenestrated endothelium

    Label the following: Filtration membrane *

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    Fenestrated endothelia * Podocyte foot process

    * Filtration slit * Basement membrane * Plasma * Filtrate.

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    LAB 21 EXERCISES 21-4

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    NEPHRON & BLADDER

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    Label the parts of a nephron: Bowman’s capsule * Proximal convoluted tubule * Distal convoluted tubule * Loop of Henle * Collecting duct * Afferent arteriole * Efferent arteriole * Peritubular capillaries * Glomerulus.

    Label the following: Detrusor muscle * Ureters * Ureteral slits * Urethra * Internal urethral sphincter * External urethral sphincter * Trigone * Rugae.

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    LAB 21 EXERCISES 21-5

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    HISTOLOGY

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    1. Name this organ

    2. Identify 3 tissue types

    1. Name this organ

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    2. Identify 3 tissue types

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    Organ: Organ:

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    LAB 21 EXERCISES 21-6

    HISTOLOGY

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    Label the following: Afferent arteriole *

    Glomerulus * Peritubular capillaries

    1. Name this organ

    2. Label the following: Glomerulus *

      Bowman’s capsule * Convoluted tubule.

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      Organ:

      MODELS: Kidney, Nephron, Torso and Hemi-Pelvis

      Blood Supply:

      • Internal & external sphincter

         Renal artery  segmental  interlobar  arcuate  cortical  Kidney anatomy (model)

        radiate  afferent arteriole glomerular capillaries efferent arteriole peritubular capillaries cortical radiate venules (interlobular)  arcuate veins  interlobar veins  (segmental veins) renal vein

        Nephron: (model)

        • Renal corpuscle

        • Glomerulus

        • Afferent + efferent arteriole

        • Peritubular capillaries

        • Vasa recta

        • Glomerular (Bowman’s) capsule

        • Podocytes

        • Renal tubules:

        • Proximal Convoluted Tubule (PCT)

        • Loop of Henle

          • Descending & ascending limbs

          • Thick & thin segments

        • Distal Convoluted Tubule (DCT)

        • Collecting Duct

        • Papillary duct

        • Juxtaglomerular Apparatus (JGA)

        • JG cells

        • Macula Densa

          Ureters (torso)

        • Urinary Bladder

        • Trigone (2 ureters * 1 urethra)

        • Rugae

        • Detrusor muscle

        • Urethra

      • Renal capsule

      • Hilum

      • Nephrons

        • Juxtamedullary nephron

        • Cortical nephron

      • Renal cortex

      • Renal medulla:

        • Columns

        • Renal sinus

        • Renal Pyramids

        • Papilla

      • Minor Calyx (calyces)

      • Major Calyx (calyces)

      • Pelvis

        Histology:

         Kidney:

      • Bowman’s capsule

      • Glomerulus

      • Afferent & efferent arterioles

      • simple cuboidal epithelium

      • convoluted tubules

      • Adipose capsule

         Bladder/Ureters

      • Mucosa:

        • Transitional epithelium

      • Submucosa

        • Areolar CT (& other CT)

      • Muscularis externa (detrusor muscle)

        • Smooth muscle