Lesson 1, Topic 1
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Anatomy of the Urinary System

April 11, 2024

Learning Objective: Examine the anatomy of the urinary system.
      The urinary system is composed of two kidneys, two ureters, a urinary bladder, and a urethra (Figures 42.1 and 42.2). The kidneys filter the blood and eliminate waste through the passage of urine. The ureters move urine from the kidneys to the bladder. The urinary bladder stores the urine until it is excreted. The urethra is the tube that conducts the urine out of the bladder.

Learning Objective: Describe the location, structure, and function of the kidneys.
      The kidneys are bean-shaped organs that are the size of a fist. The hilum (an indentation) is the location on the kidney where the ureter and renal vein leave the kidney and the renal artery enters. The kidneys are located posterior to the peritoneum, the muscles of the back, and between the T12 and L3 vertebrae. The left kidney is situated about 1 inch (2 cm) higher than the right because of the location of the liver.
      The kidneys remove unwanted substances from the blood and form urine for excretion. Blood is delivered to the two kidneys by the renal arteries, which branch off the abdominal aorta (FIGURE 27.3). Blood flows from the renal arteries into afferent arterioles (smaller arteries) and through a network of capillaries called the glomeruli. Filtration of the blood occurs in the glomerulus. Blood from the glomerulus moves into the efferent arterioles and into the peritubular capillaries. As the blood leaves the kidneys, it moves from the peritubular capillaries to venules and then empties into the renal veins. The renal veins take the blood out of the kidneys. Blood from the renal veins flows into the inferior vena cava as it heads to the heart to become oxygenated again (FIGURE 27.4).

FIGURE 27.1  Male urinary system. From Shiland B: Medical terminology & anatomy for ICD-10 coding, ed 2, St. Louis, 2016, Mosby.

FIGURE 27.2  Female urinary system. From Shiland B: Medical terminology & anatomy for ICD-10 coding, ed 2, St. Louis, 2016, Mosby.

FIGURE 27.3  Cross section of a kidney.

The Kidney Structures
If a kidney were sliced open, you would see the following:
                • Capsule: The fibrous outer covering of the kidney
                • Cortex: The outer portion of the kidney
                • Renal column: An extension of the cortex that dips down between the medullary pyramids
                • Medulla: The inner portion that extends from the end of the cortex to the calyces
                • Medullary pyramid: A cone-shaped structure located in the medulla and containing straight tubular structures and blood vessels
                • Minor and major calyces and the renal pelvis: Extensions of the ureter inside the kidney

Each kidney contains tissue with millions of microscopic units called nephrons. Nephrons are the functional units of the kidneys. They are located in the cortex and extend into the medulla. Each neph ron is a very long tube or tubule. One end of the nephron is the renal corpuscle, and the remaining section of the nephron is the renal tubule. The renal corpuscle consists of the following:

FIGURE 27.4  Blood flow to and from the kidney.

                • Bowman capsule: A cup-shaped structure of the nephron that surrounds the glomerulus
                • Glomerulus: A cluster of capillaries inside the Bowman capsule
      The afferent arteriole brings blood into the glomerulus, and the efferent arteriole takes blood away from the glomerulus. The diameter of the afferent arteriole is much larger than the diameter of the efferent arteriole. The importance of the narrowed diameter will be discussed in the Physiology of the Urinary System section of this chapter.
      The renal tubule is a long, thin, twisted tube that brings the filtrate (which will become urine) from the Bowman capsule to the renal pelvis. During the passage of filtrate from the Bowman capsule to the renal pelvis, water and electrolytes move between the urine and the blood to maintain homeostasis. The formation of urine will be discussed later in the chapter. The renal tubule is made up of the following:
                • Proximal (convoluted) tubule (PCT): The first section of the renal tubule that is attached to the Bowman capsule.
                • Henle loop: Follows the PCT and includes a straight descending section, a loop, and then a straight ascending section.
                • Distal (convoluted) tubule (DCT): Follows the ascending section of the Henle loop.
                • Collecting duct (CD): Multiple distal tubules join and form this straight collecting duct, which empties into the calyx.
      To summarize, the nephron begins on one end with the Bowman capsule, which is filled with capillaries. The proximal tubule is the next section of the nephron. It is followed by the Henle loop, the distal tubule, and finally, the collecting duct. Many collecting ducts empty into one calyx. Many calyces merge into the renal pelvis, which is an extension of the ureter inside the kidney. The Bowman capsule, proximal tubules, and distal tubes are in the cortex. The Henle loop and the collecting ducts are in the medulla of the kidney.

27.1 Critical Thinking Application
Hannah is working with Mrs. Green, a urology patient. Mrs. Green has kidney disease and asks Hannah what the kidneys do. How should Hannah explain the role of the kidneys to Mrs. Green?

Learning Objective: Describe the location, structure, and function of the ureters.
      The bilateral ureters are thin, muscular tubes approximately 10 inches long. The tube’s muscular layer creates peristaltic waves that help move the urine to the bladder. The ureters’ walls are made of transitional epithelium, which allows the ureters to stretch to accommodate urine flow. Many nerve endings are also in the walls. A stone in the ureter can cause severe pain.
      The point where the ureter enters the bladder is called the ureterovesical junction. The ureters enter the bladder at an angle. This creates a flap over the end of the ureter, which serves as a valve. Urine can empty into the bladder, but it cannot back up from the bladder and reenter the ureter. This mechanism also prevents bacteria from moving from the bladder up to the kidneys.

Learning Objective: Describe the location, structure, and function of the bladder.
The urinary bladder is a hollow organ in the pelvic cavity. It serves as a holding tank for urine. Urine enters the bladder through the ureters and leaves the bladder through the urethra. The triangular area in the bladder between the ureters’ entrance and the urethral outlet is called the trigone.
      Transitional epithelium creates the inner lining of the bladder. When the bladder is empty, it flattens, and the walls overlap in folds, also known as rugae. This creates a wrinkled appearance. As the bladder starts to fill with urine, the rugae and transitional epithelium allow for greater bladder volume. Think of a balloon that was blown up and then emptied of air. The balloon is smaller and wrinkly in appearance. When inflated, the wrinkles are gone, and it can accommodate a large volume of air. The same thing happens with the bladder and urine.
       The bladder is also composed of smooth muscle fibers that make up the detrusor muscle. The detrusor muscle relaxes when the bladder fills. It contracts to push urine out of the bladder and into the urethra. Normally, the bladder holds about 1.5 to 2 cups (360 to 480 mL) of urine during the day and doubles that amount at night. As the bladder fills to about 150 mL, stretch receptors in the detrusor send a message to the central nervous system. As more urine enters the bladder, the urge to urinate increases. After urination or micturition, residual urine remains (usually less than 50 mL) in the bladder.

Learning Objective: Describe the location, structure, and function of the urethra.
The urethra is a mucous membrane–lined tube that drains urine out of the bladder. In males, the internal urethral sphincter is located at the bladder end (or proximal end) of the urethra. The sphincter is made of smooth involuntary muscles. The external sphincter is located in different places in males and females (TABLE 27.1). The external sphincter is made of skeletal muscles and is a voluntary muscle. A voluntary muscle provides you with the ability to control when you urinate. The distal end of the urethra is called the urinary meatus. The urinary meatus is the final structure before the urine leaves the body.

27.2 Critical Thinking Application
Hannah is working with Katrina, the urology medical assistant. Hannah asks Katrina what the major differences are between the male and female urinary systems. How would Katrina respond to Hannah’s question?