Anatomy of the Urinary System

The goal this week is to learn the gross anatomy of the kidney and urinary tract.  In lecture, we will look at the histology of the nephron, which is the functional unit of the kidney.

Recommended Reading

Read section 19.1 and 19.2 pp. 588-592 in Silverthorn (function of the kidneys and anatomy of the urinary system).

Gross Anatomy of the Kidney

The figure at the right depicts the structures that are visible when you section a kidney along its long axis. There are two basic regions in the kidney, an outer cortex, and an inner medulla.  As you will see, certain parts of the nephrons are located in the cortex, and certain parts are located in the medulla.

The medulla contains dark structures known as pyramids. Each pyramid has a tip known as the papilla. Urine forms in the nephrons and drains from collecting ducts in the papilla. The papilla fits into a cup-like structure called a calyx.  The calyces form short branches that attach to a wide space called the renal pelvis.  The renal pelvis then narrows to form the ureter, a tube that conveys urine to the bladder where it is stored.  The calyces, renal pelvis, ureters, and bladder are all lined with a specialized type of epithelium called uroepithelium (or transitional epithelium).  The uroepithelium is a stratified epithelium with tight junctions between adjacent cells so that it acts as an important permeability barrier.  Urine can be made more concentrated than the extracellular fluid, so it is important that the structures that conduct and store urine don't allow water to be drawn into the urine.  Another unique feature about the uroepithelium is that the epithelium can stretch when the bladder is full and its walls are distended.  In class we will look at the uroepithelium when we review kidney histology.

The key function of the kidneys is to regulate the composition and volume of the extracellular fluid. Critical to this function is a high rate of blood flow. The renal blood flow is typically about a fifth of the total cardiac output. The renal artery carries blood from the aorta to the kidney, and the renal vein returns blood from the kidney to the inferior vena cava and heart.  The renal artery, renal vein, and renal pelvis are all located in a region of the kidney called the hilum.

Model of the Kidney

Be able to identify the following structures in the model of the kidney:

Model of the Urinary Tract

The second model shows the the major blood vessels, and the urinary tract. The ureters connect the kidneys to the bladder, which stores urine.  The urethra is the tube that carries urine from the bladder to the outside of the body.  In males, the urethra passes through the prostate gland and penis, and also conveys semen.  What is the gender of the model?

The bladder is sectioned to show you the structures in its interior. The wall of the bladder consists of the detrusor muscle, which is smooth muscle, with an overlying mucosal layer that has uroepithelium on its surface. The trigone is a thickened region of bladder wall on the posterior side near the base of the bladder. The openings of each ureter are in the lateral portion of the trigone.  The ureters penetrate the wall of the bladder in an oblique orientation, and their openings are narrow and slit-like.  This arrangement ensures that the openings of the ureters will squeeze shut when the detrusor muscle contracts during urination.  This prevents reflux of urine back towards the kidneys.

Urinary Anatomy: Clinical Example

The first segment of the urethra in males passes through the prostate gland and is called the prostatic urethra (refer to figure 26.7b on p. 812 of the textbook).  As men age, many will experience non-cancerous growth of the prostate gland called benign prostatic hyperplasia.  Benign prostatic hyperplasia can cause difficulty in urination because the enlarged prostate presses on the urethra.  Common symptoms are increased frequency of urination, difficulty in starting urination or fully emptying the bladder, and a weak flow of urine. 

The prostate gland is in a difficult place to access surgically, so benign prostatic hyperplasia is usually treated with drugs.

• The initial approach to treatment is to promote relaxation of smooth muscle surrounding the bladder outlet and urethra. Smooth muscle in the urethra and prostate gland is innervated by sympathetic neurons, which excite smooth muscle contraction. Thus, alpha-adrenergic antagonists that block this excitation are used to relax smooth muscle and promote urine flow.
  
• In some cases, the prostate gland volume is much increased, so treatment that limits further prostate growth is desirable. The prostate gland grows in response to a type of androgen hormone known as dihydrotestosterone (DHT). 5-alpha-reductase inhibitors are enzyme inhibitors that reduce the formation of DHT in order to limit prostate growth.