Page 161 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

7 Kidneys, Salt, and Water Balance

! The proximal tubule (! A, dark green) is the
Kidney Structure and Function
longest part of a nephron (ca. 10 mm). Its
Three fundamental mechanisms characterize twisted initial segment (proximal convoluted
kidney function: (1) large quantities of water tubule, PCT; ! A3) merges into a straight part,
and solutes are filtered from the blood. (2) This PST (pars recta; ! A4).
primary urine enters the tubule, where most of ! The loop of Henle consists of a thick de-
it is reabsorbed, i.e., it exits the tubule and scending limb that extends into the renal
passes back into the blood. (3) Certain sub- medulla (! A4 = PST), a thin descending limb
stances (e.g., toxins) are not only not reab- (! A5), a thin ascending limb (only in jux-
sorbed but actively secreted into the tubule tamedullary nephrons which have long loops),
lumen. The non-reabsorbed residual filtrate is and a thick ascending limb, TAL (! A6). It con-
excreted together with the secreted sub- tains the macula densa (! p. 184), a group of
stances in the final urine. specialized cells that closely communicate
Functions: The kidneys (1) adjust salt and with the glomerulus of the respective ne-
water excretion to maintain a constant extra- phron. Only about 20% of all Henle’s loops
cellular fluid volume and osmolality; (2) they (those of the deep juxtamedullary nephrons)
help to maintain acid-base homeostasis; (3) are long enough to penetrate into the inner
they eliminate end-products of metabolism and medulla. Cortical nephrons have shorter loops
foreign substances while (4) preserving useful (! A and p. 150).
compounds (e.g., glucose) by reabsorption; (5) ! The distal tubule (! A, grayish green) has an
the produce hormones (e.g., erythropoietin) initially straight part (= TAL of Henle’s loop;
and hormone activators (renin), and (6) have ! A6) that merges with a convoluted part (dis-
metabolic functions (protein and peptide cata- tal convoluted tubule, DCT; ! A7).
bolism, gluconeogenesis, etc.). The DCT merges with a connecting tubule
(! A8). Many of them lead into a collecting
Nephron Structure duct, CD (! A9) which extends through the
6
Each kidney contains about 10 nephrons, each renal cortex (cortical CD) and medulla (medul-
consisting of the malpighian body and the lary CD). At the renal papilla the collecting
tubule. The malpighian body is located in the ducts opens in the renal pelvis. From there, the
renal cortex (! A) and consists of a tuft of urine (propelled by peristaltic contractions)
capillaries (glomerulus) surrounded by a passes via the ureter into the urinary bladder
double-walled capsule (Bowman’s capsule). and, finally, into the urethra, through which
The primary urine accumulates in the capsular the urine exits the body.
space between its two layers (! B). Blood en- Micturition. Voiding of the bladder is con-
ters the glomerulus by an afferent arteriole trolled by reflexes. Filling of the bladder acti-
(vas afferens) and exits via an efferent arteriole vates the smooth detrusor muscle of the blad-
(vas efferens) from which the peritubular capil- der wall via stretch sensors and parasympa-
lary network arises (! p. 150). The glomerular thetic neurons (S 2–S 4, ! p. 78ff.). At low filling
filter (! B) separates the blood side from the volumes, the wall relaxes via sympathetic neu-
Bowman’s capsular space. rons (L 1–L 2) controlled by supraspinal centers
(pons). At higher filling volumes (! 0.3 L), the
The glomerular filter comprises the fenestrated en-
dothelium of the glomerular capillaries (50–100 nm threshold pressure (about 1 kPa) that triggers
pore size) followed by the basal membrane as the the micturition reflex via a positive feedback
second layer and the visceral membrane of Bow- loop is reached: The detrusor muscle contracts
man’s capsule on the urine side. The latter is formed ! pressure"!contraction""and so on until
by podocytes with numerous interdigitating footlike the internal (smooth m.) and external sphincter
processes (pedicels). The slit-like spaces between (striated m.) open so the urine can exit the
them are covered by the slit membrane, the pores of body.
which are about 5 nm in diameter. They are shaped
148 by the protein nephrine, which is anchored to the cy-
toskeleton of the podocytes.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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