Key Features of the Urinary System

Parts of the System: Paired Kidneys, paired Ureters, Urinary Bladder, Urethra
Major Tissue Type: Epithelium
Characteristic Cell Types: Simple squamous, cuboidal and columnar epithelia with and without a brush border
Functional Unit: Nephron

The retroperitoneal kidneys are familiar to most students of Anatomy. You will appreciate the position of these organs better in lab when you have to manually strip the parietal peritoneum from the posterior abdominal wall to view the kidneys.

The kidneys are an excretory organ, which means that they form a body waste we call urine. The kidneys accomplish their goal by filtration and reabsorption. Substances in the blood move into the lumen of the nephron. Just like the rain falls on both good and evil, so substances the body will keep and those the body will discard are filtered. Substances that should be retained by the body must move back to the blood through a process called reabsorption.

These events occur in the functional unit of the kidney, the nephron. The first challenge for students is to memorize the parts of the nephron. A flow diagram helps with this process.

The loop of Henle extends into the renal medulla in juxtamedullarynephrons. Cortical nephrons have very short descending and ascending limbs of the loop of Henle. For Physiology, thin and thick segments rather than ascending and descending limbs are identified in juxtamedullary nephrons. Notice that the descending limb is thin; whereas the ascending limb is both thin and thick in the drawing below.

The parts of the nephron are purple or orange. Bowman's capsule and the proximal convoluted tubule are purple. The descending limb, loop of Henle, ascending limb, distal convoluted tubule, collecting tubule and collecting duct are orange. Each part of the nephron contains a lumen bordered by epithelial cells.

Now study the circulatory pattern associated with the nephron. Oxygenated blood is brought to the kidneys by the renal arteries; and deoxygenated blood is returned to the heart by the renal veins. However, the distribution of branches of the renal arteries relate to the special function of the kidneys, that is, the filtration of blood for the elimination of waste products. Note that the flowchart below shows two capillary beds, instead of one capillary network--the usual pattern in the body. The afferent (going toward) and efferent (going away from) arterioles are named for their relationship to the first capillary network, the glomerulus. Filtration occurs when substances in blood move from the glomerulus into Bowman's capsule. Since the glomerulus and Bowman's capsule function as a unit, the term applied to this unit is renal corpuscle.

Finally, put the two together. In other words, decide which part of the vascular tree is immediately adjacent to a part of the nephron. Pay attention to the short, straight vessels in the peritubular capillary network called the vasa rectae. These channels are physiologically important! Have you ever eaten kidney? They explain why the renal medulla is salty.

Your textbook describes a very important, very specialized structure at the point where the convolutions of the nephronare entwined by the afferent arteriole. This structure is called the juxtaglomerular apparatus or JGA. The JGA consists of:

1. Specialized, epithelial cells in the wall of the nephron called the macula densa.
2. Specialized, smooth muscle cells in the wall of the afferent arteriole called juxtaglomerular cells.

Which part of the nephron contains the macula densa? Current editions of most textbooks call this section the "ascending limb of the loop of Henle"; it's definitely the end of the ascending limb! Some textbooks still refer to this area as the "beginning" of the distal convoluted tubule. I am continuing my research on this topic.

The next part of the system--the ureters--are also retroperitoneal. The ureters begin as the enlarged renal pelvis. The renal pelvis sits in a space called the renal sinus. The ureters begin at the hilus of the kidneys and end at the urinary bladder.

The urethra carries urine to the outside. Notice how short the urethra is in females compared to males.

It's so important to teach your daughters to "wipe from front to back" when cleaning themselves after a bowel movement. This cleanliness procedure helps women avoid painful, urinary tract infections.

In fact, in males, the urethra is long enough to be divided into three parts:

1. The prostatic urethra leaves the urinary bladder and passes through the substance of the prostate gland.
2. The membranous urethra passes through the floor of the pelvis known as the urogenital diaphragm.
3. The cavernous urethra is the penile portion aka spongy urethra.