The Urinary System

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Human Anatomy
Chapter 26
Long Answer
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Describe the structure and function of the filtration slit and membrane.

Blood is filtered through filtration slits in the glomerular capillaries. The slits consist of
three layers. The fenestrations in the endothelium of the capillaries create large pores through
which the blood filtrate passes. A basement membrane superficial to the endothelium provides a
second layer of the barrier. It prevents most large proteins from passing. Finally, the pedicles of
the podocytes form the third layer of the membrane. These also prevent proteins from passing
through the slits.


Describe how the structure of the proximal convoluted tubule walls reflects its function in
filtrate processing.

The walls of the proximal tubule are simple cuboidal epithelium. The luminal surface
of the cells is covered by microvilli. These structures increase the surface area of the proximal
tubule thereby greatly enhancing the resorption of water, electrolytes, and other solutes from the


Compare and contrast the construction of the bladder and ureter walls.

The walls of both the bladder and ureter consist of three layers: a mucosa of
transitional epithelium, a muscularis layer, and an external adventitia. The muscularis of the
ureter consists of two layers of smooth muscle, circular and inner longitudinal, for most of its
length, then adds a third external longitudinal layer at its distal end. The bladder has all three
layers of smooth muscle. The adventitia of the bladder is replaced with parietal peritoneum on its
superior surface.


Explain the significance of the intimate anatomical relationship between the nephrons and the
peritubular capillaries and vasa recta.

The vast majority, 99%, of the filtrate that is removed from the blood must be resorbed
from the nephrons. If this fluid were not resorbed, the body would quickly dehydrate. All ions,
solutes, and water that are resorbed from the nephron pass into the surrounding peritubular
capillaries and return to the cortical radiate vein. Sodium and water resorbed from the nephron
loop are picked up by the vasa recta and returned to the cortical radiate vein.


Describe the structure, function, and control of the internal and external urethral sphincters in

The internal urethral sphincter is composed of smooth muscle and is under involuntary
control. In contrast, the external urethral sphincter is skeletal muscle that can be voluntarily
controlled. The internal sphincter prevents leakage of urine between voiding, and the external
sphincter prevents urination until consciously desired.