Page 1031 - Fundamentals of anatomy physiology
P. 1031
1018 Unit 5 Environmental Exchange
over the course of a long event. Proteinuria (protein in the urine) 2. Active Reabsorption of Ions. The PCT actively transports sev-
commonly occurs after such events because the glomerular cells eral ions, including sodium, potassium, and bicarbon-
have been injured by prolonged hypoxia (low oxygen levels). ate ions, plus magnesium, phosphate, and sulfate ions
If the damage is substantial, hematuria (blood in the urine) (Figure 26–12). Although the ion pumps involved are in-
occurs. Hematuria develops in about 18 percent of marathon dividually regulated, they may be influenced by circulat-
runners. The cause is trauma to the bladder epithelium from ing ion or hormone levels. For example, angiotensin II
the shocks of running. Proteinuria and hematuria generally dis- stimulates Na+ reabsorption along the PCT. By absorb-
appear within 48 hours as the glomerular tissues are repaired. ing carbon dioxide, the PCT indirectly recaptures about
However, a small number of marathon and ultramarathon run- 90 percent of the bicarbonate ions from tubular fluid.
ners experience acute renal failure, with permanent impairment Bicarbonate is important in stabilizing blood pH. We
of kidney function. examine this process further in Chapter 27.
Checkpoint 3. Reabsorption of Water. The reabsorptive processes have a di-
rect effect on the solute concentrations inside and outside
9. What nephron structures are involved in filtration? the tubules. The filtrate entering the PCT has the same os-
10. List the factors that influence net filtration pressure. motic concentration as that of the surrounding peritubular
11. List the factors that influence the rate of filtrate fluid. As reabsorption proceeds, the solute concentration
of tubular fluid decreases, and that of peritubular fluid and
formation. adjacent capillaries increases. Osmosis then pulls water out
1 2. How would a decrease in blood pressure affect the of the tubular fluid and into the peritubular fluid. Along
the PCT, this mechanism results in the reabsorption of
GFR? about 108 liters of water each day.
See the blue Answers tab at the back of the book. 4. Passive Reabsorption of Ions. As active reabsorption of ions
takes place and water leaves the tubular fluid by osmosis,
26-5 Countercurrent multiplication, the concentration of other solutes in the tubular fluid in-
creases above that in the peritubular fluid. If the tubular
antidiuretic hormone, and aldosterone cells are permeable to them, those solutes move across
affect reabsorption and secretion the tubular cells and into the peritubular fluid by passive
diffusion. Urea, chloride ions, and lipid-soluble materi-
Learning Outcome Identify the types and functions of transport als may diffuse out of the PCT in this way. Such diffusion
mechanisms found along each segment of the nephron, explain the role of further decreases the solute concentration of the tubu-
countercurrent multiplication, describe hormonal influence on the volume lar fluid and promotes additional water reabsorption by
and concentration of urine, and describe the characteristics of a normal osmosis.
urine sample.
5. Secretion. Active secretion of hydrogen ions also takes place
26 Reabsorption recovers useful materials that have entered the along the PCT (Table 26–4). Because the PCT and DCT
filtrate. Secretion ejects wastes, toxins, or other undesirable secrete similar substances, and the DCT carries out com-
solutes that did not leave the bloodstream at the glomerulus. paratively little reabsorption, we will consider secretory
Both processes take place in every segment of the nephron mechanisms when we discuss the DCT.
except the renal corpuscle. Their relative importance changes
from segment to segment. Sodium ion reabsorption plays an important role in all
of these processes. Sodium ions may enter tubular cells by
Reabsorption and Secretion at the PCT diffusion through Na+ leak channels; by the sodium-linked
cotransport of glucose, amino acids, or other organic solutes;
The cells of the proximal convoluted tubule normally reabsorb or by countertransport for hydrogen ions (Figure 26–12). Once
60–70 percent of the volume of the filtrate produced in the inside the tubular cells, sodium ions diffuse toward the base-
renal corpuscle. The reabsorbed materials enter the peritubular ment membrane. The plasma membrane in this area contains
fluid, diffuse into peritubular capillaries, and are quickly re- sodium–potassium exchange pumps that eject sodium ions in
turned to the circulation. exchange for extracellular potassium ions. Reabsorbed sodium
ions then diffuse through the peritubular fluid and into adja-
The PCT has five major functions: cent peritubular capillaries.
1. Reabsorption of Organic Nutrients. Under normal circum- The reabsorption of ions and compounds along the PCT
stances, before the tubular fluid enters the nephron loop, involves many different carrier proteins. Some people have an
the PCT reabsorbs more than 99 percent of the glucose, inherited inability to manufacture one or more of these carrier
amino acids, and other organic nutrients in the fluid. This
reabsorption involves a combination of facilitated trans-
port and cotransport.
