182 ❯ STEP 4. Review the Knowledge You Need to Score Highbody. It receives some help in the handling of the fat from a product made in the liver called
bile.Bile contains bile salts, phospholipids, cholesterol, and bile pigments such as bilirubin.
The bile is stored in the gallbladderand is dumped into the small intestine upon the arrival
of food. The bile salts help digest the fat by emulsifyingit into small droplets contained in
water. (Emulsification is a physical change—bile does not contain any enzymes.) Amylase
continues the breakdown of carbohydrates into simpler sugars. Maltase, lactase,andsucrase
break maltose, lactose, and sucrose, respectively, into monosaccharides. Trypsin and chy-
motrypsin work together to handle the digestion of the peptides in our diet. Trypsin cuts
peptide bonds next to arginine and lysine; chymotrypsin cuts bonds by phenyalanine, tryp-
tophan, and tyrosine. Like pepsin, these two proteolytic enzymes are secreted as inactive
forms: trypsinogen and chymotrypsinogen. Trypsinogen is activated first to become trypsin,
which, in turn, activates chymotrypsin. Some of you might ask “If the proteolytic enzymes
only cut at certain sites, how do we finish digesting the proteins?” Trypsin and chymotrypsin
are examples of enteropeptidases.It is the exopeptidasesthat complete the digestion of pro-
teins by hydrolyzing all the amino acids of the remaining fragments.
After the small intestine comes the large intestine (which includes the cecum, colon,
and rectum). The two meet up in the lower right corner of the abdomen. The colon has
three main parts: the ascending, transverse,anddescending colon.There are two major func-
tions for this part of the system—the primary function is to reabsorb water and electrolytes.
A second function is to serve as a passageway for the waste material as it moves toward the
rectum. The food enters the large intestine, travels up the ascending colon, across the trans-
verse colon, down the descending colon into the rectum, where it is stored until it gets
eliminated... but we don’t need to go there. We’ve seen enough for now.Control of the Internal Environment
The next stop on our tour is the kidney (see Figure 15.3 for an overview of the human
excretory system). The kidneys lie on the posterior wall of the abdomen. The renal artery
and vein bring blood to and from the kidney, respectively. Kidneys are divided into two
major regions: an outer region called the cortex,and an inner region called the medulla.
These two regions are full of nephrons,the functional units of the kidney. The medulla is
divided into structures called renal pyramids,which dump urine into the major and minor
calyces.From here, the urine is sent toward the bladdervia the ureter.When contracted to
urinate, the bladder sends the urine through the urethrato the outside world.
We’ve pulled the bus right up to one of over a million nephrons in each kidney. The
nephron is composed of a renal corpuscle, proximal convoluted tubule, loop of Henle, distal
convoluted tubule,andcollecting duct system.If you look closely, you will see that the renal
corpuscle is made up of glomerular capillariessurrounded by Bowman’s capsule.Osmoregulation and Excretion
The blood that enters via the renal artery is sent to the various nephrons by the branching
of the renal artery into smaller and smaller vessels that culminate in the capillaries of the
glomerulus. The blood pressureis the force that leads to the movement of solutes such as
water, urea, and salts into the lumen of Bowman’s capsule from the glomerular capillaries.
From here, the fluids pass down the proximal tubule, through the loop of Henle, and into
the distal tubule, which dumps into the collecting duct. The various collecting ducts of the
kidney collectively merge into the renal pelvis, which leads via the ureter to the bladder.
As we mentioned moments ago, fluid moves from the capillaries into the lumen of the
nephron as a result of the force of blood pressure. During this process of filtration,the
capillaries are able to let small particles through the pores of their endothelial linings, butBIG IDEA 2.D.2
Homeostatic mech-
anisms reflect
common ancestry.