5 Steps to a 5 AP Biology, 2014-2015 Edition

184 ❯ STEP 4. Review the Knowledge You Need to Score High

large molecules such as proteins, platelets, and blood cells tend to remain in the vessel. As

the filtrate progresses along the tubule, plasma solutes such as urea are added by the process

ofsecretion,a selective process that helps to create a solute gradient. It is important to real-

ize that much of what is dumped into the tubule originally is reabsorbed—nearly all the

sugars, water, and organic nutrients. The combination of reabsorption and secretion help

the nephron to control what gets released in the urine. The following chart outlines in

detail what happens in the various parts of the nephron:

Proximal tubule Reabsorbs 75 percent of NaCl and water of filtrate.

Nutrients such as glucose and amino acids are reabsorbed

unless their concentration is higher than the absorptive

capacity. Glucose in urine is an indicator of diabetes, for

this reason.

Descending loop of Henle Freely permeable to H 2 O but not NaCl. Assists in control

of water and salt concentrations.

Ascending loop of Henle Freely permeable to NaCl but not water. Assists in control

of salt concentration.

Distal tubule Regulates concentration of K+and NaCl. Helps control pH

by reabsorbing HCO 3 −and secreting H+.

Collecting duct Determines how much water is actually lost in urine. The

osmotic gradient created in the earlier regions of the nephron

allows the kidney control in the final concentration of the

urine.

The body controls the concentration of the urine according to the needs of the system.

When dehydrated, the body can excrete a small volume of hypertonic concentrated urine

(little water in the urine; it is dark yellow). But in times of excessive fluid, the body will

excrete a large volume of hypotonic dilute urine to conserve the necessary salts (lots of water

in the urine; it is clear). This is controlled by hormones and is discussed in more detail in

a later section, but briefly: ADH(antidiuretic hormone) is released by the pituitary gland;

it increases permeability of the collecting duct to water, leading to more concentrated urine.

Aldosterone,released from the adrenal gland, acts on the distal tubules to cause the reab-

sorption of more Na+and water to increase blood volume and pressure.

Figure 15.3 The human excretory system on four size scales. (a) The kidneys produce urine and

regulate the composition of the blood. Urine is conveyed to the urinary bladder via the ureter and to

the outside via the urethra. Branches of the aorta, the renal arteries convey blood to the kidneys; renal

veins drain blood from the kidneys into the posterior vena cava. (b) Urine is formed in two distinct

regions of the kidney: the outer renal cortex and inner renal medulla. It then drains into a central

chamber, the renal pelvis, and into the ureters. (c) Excretory tubules (nephrons and collecting ducts)

and associated blood vessels pack the cortex and medulla. The human kidney has about a million

nephrons, representing about 80 km of tubules. Cortical nephrons are restricted mainly to the renal

cortex. Juxtamedullary nephrons have a long, hairpinlike portion that extends into the renal medulla.

Several nephrons empty into each collecting duct, which drains into the renal pelvis.

(Adapted from Biology,8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the

McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies.)

KEY IDEA

BIG IDEA 2.C.1
Animals use feed-
back mechanisms
(e.g., ADH) to
maintain internal
conditions.