urinalysis and body fluids

©2008 F. A. Davis
CHAPTER 2 • Renal Function 15
Afferent
arteriole
Distal tubule
Macula
densa
Efferent
arteriole
Juxtaglomerular
cells
Glomerulus
Figure 2–4 Close contact of the distal tubule with the afferent
arteriole, macula densa, and the juxtaglomerular cells
within the juxtaglomerular apparatus.
trate enters the proximal convoluted tubule, the nephrons,
through cellular transport mechanisms, begin reabsorbing
these essential substances and water (Table 2–2).
Reabsorption Mechanisms
The cellular mechanisms involved in tubular reabsorption
are termed active and passive transport. For active transport
to occur, the substance to be reabsorbed must combine
with a carrier protein contained in the membranes of the renal
tubular cells. The electrochemical energy created by this
interaction transfers the substance across the cell membranes
and back into the bloodstream. Active transport is
responsible for the reabsorption of glucose, amino acids,
and salts in the proximal convoluted tubule, chloride in the
ascending loop of Henle, and sodium in the distal convoluted
tubule.
Passive transport is the movement of molecules across a
membrane as a result of differences in their concentration or
electrical potential on opposite sides of the membrane. These
Low blood pressure
Low plasma sodium
Renin secretion
Angiotensinogen
Angiotensin I
Angiotensin II
Angiotensin
converting enzymes
Vasoconstriction
Proximal convoluted tubule
Sodium reabsorption
Aldosterone
ADH
Figure 2–5 Algorithm of the reninangiotensin-aldosterone
system.
Distal convoluted tubule
Sodium reabsorption
Collecting duct
Water reabsorption