The Genitourinary (Urogenital) System Concept Help (page 2)

By — McGraw-Hill Professional
Updated on Aug 30, 2011

The Urinary Pathway

Figure 20.2 shows the rest of the urinary pathway, lying beyond the kidney. The right and left ureters both dump urine into the urocyst ( YUR -oh- sist ) or urinary bladder (cyst). The urocyst (urinary bladder) is a hollow, muscular-walled pouch that temporarily stores the urine before it is excreted.

Urine and (Gulp!) Sex in Animals The Genitourinary (Urogenital) System Concept The Urinary Pathway

Fig. 20.2 The urinary pathway.

The urocyst empties into the urethra (you- REETH -rah), the tube that helps a person literally “make water” (urethr ) – that is, urinate ( YUR -ih- nayt ). Surrounding the upper neck of the urethra is the urinary sphincter. Much like the external anal sphincter in the digestive pathway (Chapter 19), the urinary sphincter is a ring of voluntary striated muscle. This means, of course, that the contraction and relaxation of this sphincter is under our voluntary control. Thus, after we have been adequately “potty-trained” during early childhood, we can voluntarily relax the urinary sphincter whenever the place and time are right for urination!

Finally, urine exits out of the body through the urinary orifice ( OR -ih- fis ), a tiny, “mouth” (or) -like opening.

We have, from time to time, been using some very basic descriptive equations in this book. Let us do so, again. We can state the urinary excretion equation, for example:

E = FR + S

(This equation is put into visual form within Figure 20.3 below.)

In this equation, E stands for “excretion,” F for amount filtered, R for the amount of tubular reabsorption, and S for tubular secretion. The urinary filtrate ( FIL -trayt), or filtration product, comes from the pushing force of the blood pressure against the walls of the renal capillaries in the glomeruli (glah- MEHR -you- lie ). This quantity of filtrate is huge, averaging about 180 liters of fluid per day, in an average adult! [ Study suggestion: Assume that an adult has a blood volume of about 6 liters. Then, on average, how many times is this person’s entire blood volume filtered out of his glomeruli, each day?]

While it may seem wasteful, the huge volume of urinary filtrate ( F ) acts as the starting point for the urine. Because there is so much of this filtrate, the body can adjust many factors to influence how much urine is actually excreted, under particular current conditions.

After urinary filtration, one of the chief processes is tubular reabsorption ( R ). Reabsorption is the movement of material out of the filtrate, across the walls of the urinary tubules, and back into the bloodstream. Consider, for example, the tubular reabsorption of glucose. Under normal conditions, almost 100% of the glucose that is filtered into the urinary tubules is eventually reabsorbed back into the bloodstream. As a result, the urine excreted from the body is nearly free of glucose. Several hormones also control the amount of sodium (Na + ions) and water (H 2 O) molecules that are reabsorbed back into the bloodstream. Because the amount of sodium (salt) and water reabsorbed can vary greatly, the kidneys play a critical role in regulating the salt–water balance of the human bloodstream.

  • nder typical conditions, about 99%, or 179 liters, of the urinary filtrate (mostly water) is reabsorbed. [ Study suggestion: If a person becomes extremely dehydrated, as after excessive sweating, then what do you predict will happen to the amount of H 2 O reabsorbed? Will the percent (%) reabsorbed increase above typical conditions, or decrease below it? Why?]

Another process, tubular secretion ( S ), involves the active (ATP-requiring) addition of small quantities of particular chemicals from the bloodstream, into the urinary tubules. Molecules of penicillin (pen-ih- SILL -in) and many other antibiotics ( an -tih-buy- AH -ticks), for instance, are just too large to be filtered across the walls of the glomeruli. Hence, the epithelial cells lining the blood vessels actively pump the penicillin into the urinary tubules. Therefore, penicillin is excreted ( E ) out of the body, via the urine. Although only a few milliliters (ml) of fluid are generally secreted each day, they still have an important influence.

Urine and (Gulp!) Sex in Animals The Process of (“Making Water”) or Excreting Urine

Fig. 20.3 Processes involved in excretion of urine.

Summarizing our previous urinary excretion equation and plugging in some numbers, we get:

Urine and (Gulp!) Sex in Animals The Process of (“Making Water”) or Excreting Urine

Practice problems for these concepts can be found at:  Urine and Sex in Animals Test

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