Page 1022 - Fundamentals of anatomy physiology
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Chapter 26 The Urinary System 1009
In sectional view, the DCT differs from the PCT in that 26-3 Different segments of the
the DCT has a smaller diameter and its epithelial cells lack
microvilli (Table 26–1). The DCT is an important site for three nephron form urine by filtration,
vital processes: (1) the active secretion of ions, acids, drugs, reabsorption, and secretion
and toxins into the tubule; (2) the selective reabsorption of
sodium ions and calcium ions from tubular fluid; and (3) the Learning Outcome Describe the basic processes that form urine.
selective reabsorption of water, which assists in concentrating
the tubular fluid. Most people don’t commonly think of it this way, but the goal
of urine production is to maintain homeostasis by regulating
The Juxtaglomerular Complex. The epithelial cells of the volume and composition of blood. This process involves
the DCT near the renal corpuscle are taller than those else- the excretion of solutes—specifically, metabolic wastes. Our
where along the DCT, and their nuclei are clustered together. bodies form three important organic wastes:
This region is called the macula densa (MAK-u. -la DEN-sa)
(Figure 26–8a). The cells of the macula densa are closely as- 1. Urea. Urea is the most abundant organic waste. You gener-
sociated with unusual smooth muscle fibers in the wall of the ate approximately 21 g of urea each day, most of it through
afferent arteriole. These fibers are known as juxtaglomerular the breakdown of amino acids.
cells. Together, the macula densa and juxtaglomerular cells
form the juxtaglomerular complex (JGC), an endocrine 2. Creatinine. Skeletal muscle tissue generates creatinine
structure that secretes the hormone erythropoietin and the through the breakdown of creatine phosphate, a high-energy
enzyme renin. p. 666 compound that plays an important role in muscle contrac-
tion. p. 342 Your body generates roughly 1.8 g of creati-
The Collecting System nine each day. Virtually all of it is excreted in urine.
The distal convoluted tubule, the last segment of the neph- 3. Uric Acid. Uric acid is a waste formed during the recycling
ron, opens into the collecting system (Figure 26–6). Individual of the nitrogenous bases from RNA molecules. You pro-
nephrons drain into a nearby collecting duct. Several collect- duce approximately 480 mg of uric acid each day.
ing ducts then converge into a larger papillary duct, which
in turn empties into a minor calyx. The epithelium lining the These wastes are dissolved in the bloodstream. They can
papillary duct is typically columnar (Table 26–1).
be eliminated only when dissolved in urine. For this reason,
Two main types of cells are found in the collecting duct:
intercalated cells and principal cells. Intercalated cells are their removal involves an unavoidable water loss. The kidneys
cuboidal cells with microvilli. Alpha-intercalated cells and
beta-intercalated cells make up the population of intercalated are usually capable of producing concentrated urine with an
cells. Alpha-intercalated cells secrete hydrogen ions and re-
absorb bicarbonate ions, while beta-intercalated cells secrete osmotic concentration of 1200–1400 mOsm/L, more than
bicarbonate ions and reabsorb hydrogen ions. Principal cells
are cuboidal cells that reabsorb water and secrete potas- four times that of plasma. (We discuss methods of reporting
sium. Together, these cells regulate the acid–base balance in
the blood. solute concentrations in a later section.) If the kidneys were
The collecting system does more than simply transport tu- unable to concentrate the filtrate produced by glomerular
bular fluid from the nephrons to the renal pelvis. It also adjusts
the fluid’s composition and determines the final osmotic con- filtration, fluid losses would lead to fatal dehydration in a
centration and volume of urine, the final product. We consider
these activities of the collecting system later in the chapter. matter of hours. The kidneys also ensure that the fluid that is
Checkpoint lost does not contain potentially useful organic substrates that
4. Which portions of a nephron are in the renal cortex? are present in blood plasma, such as sugars or amino acids.
5. Why don’t plasma proteins pass into the capsular space
These valuable materials must be reabsorbed and retained for
under normal circumstances?
6. Damage to which part of a nephron would interfere use by other tissues. 26
with the hormonal control of blood pressure? Basic Processes of Urine Formation
See the blue Answers tab at the back of the book. In forming urine, the kidneys use three distinct processes that
we have already mentioned:
1. Filtration. In filtration, blood pressure forces water and
solutes across the wall of the glomerular capillaries and
into the capsular space. Solute molecules small enough to
pass through the filtration membrane are carried by the
surrounding water molecules.
2. Reabsorption. Reabsorption is the removal of water and sol-
utes from the filtrate, and their movement across the tubu-
lar epithelium and into the peritubular fluid. Reabsorption
