Kidneys Salt and Water Balance

Kidney Structure and Function

Three fundamental mechanisms characterize kidney function: (1) large quantities of water and solutes are filtered from the blood. (2) This primary urine enters the tubule, where most of it is reabsorbed, i.e., it exits the tubule and passes back into the blood. (3) Certain substances (e.g., toxins) are not only not reabsorbed but actively secreted into the tubule lumen. The non-reabsorbed residual filtrate is excreted together with the secreted substances in the final urine.

Functions: The kidneys (1) adjust salt and water excretion to maintain a constant extracellular fluid volume and osmolality; (2) they help to maintain acid-base homeostasis; (3) they eliminate end-products of metabolism and foreign substances while (4) preserving useful compounds (e.g., glucose) by reabsorption; (5) the produce hormones (e.g., erythropoietin) and hormone activators (renin), and (6) have metabolic functions (protein and peptide cata-bolism, gluconeogenesis, etc.).

Nephron Structure

Each kidney contains about 106 nephrons, each consisting of the malpighian body and the tubule. The malpighian body is located in the renal cortex (^ A) and consists of a tuft of capillaries (glomerulus) surrounded by a double-walled capsule (Bowman's capsule). The primary urine accumulates in the capsular space between its two layers (^ B). Blood enters the glomerulus by an afferent arteriole (vas afferens) and exits via an efferent arteriole (vas efferens) from which the peritubular capillary network arises (^ p. 150). The glomerular filter (^ B) separates the blood side from the Bowman's capsular space.

The glomerular filter comprises the fenestrated endothelium of the glomerular capillaries (50-100 nm pore size) followed by the basal membrane as the second layer and the visceral membrane of Bowman's capsule on the urine side. The latter is formed by podocytes with numerous interdigitating footlike processes (pedicels). The slit-like spaces between them are covered by the slit membrane, the pores of 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.

♦ The proximal tubule (^ A, darkgreen) is the longest part of a nephron (ca. 10 mm). Its twisted initial segment (proximal convoluted tubule, PCT; ^ A3) merges into a straight part, PST (pars recta; ^ A4).

♦ The loop of Henle consists of a thick descending limb that extends into the renal medulla (^ A4 = PST), a thin descending limb (^ A5), a thin ascending limb (only in jux-tamedullary nephrons which have long loops), and a thick ascending limb, TAL (^ A6). It contains the macula densa (^ p. 184), a group of specialized cells that closely communicate with the glomerulus of the respective ne-phron. Only about 20% of all Henle's loops (those of the deep juxtamedullary nephrons) are long enough to penetrate into the inner medulla. Cortical nephrons have shorter loops (^ A and p. 150).

♦ The distal tubule (^ A, grayish green) has an initially straight part (=TAL of Henle's loop; ^ A6) that merges with a convoluted part (distal convoluted tubule, DCT; ^ A7).

The DCT merges with a connecting tubule (^ A8). Many of them lead into a collecting duct, CD (^ A9) which extends through the renal cortex (cortical CD) and medulla (medullary CD). At the renal papilla the collecting ducts opens in the renal pelvis. From there, the urine (propelled by peristaltic contractions) passes via the ureter into the urinary bladder and, finally, into the urethra, through which the urine exits the body.

Micturition. Voiding of the bladder is controlled by reflexes. Filling of the bladder activates the smooth detrusor muscle of the bladder wall via stretch sensors and parasympathetic neurons (S2-S4, ^ p. 78ff.). At low filling volumes, the wall relaxes via sympathetic neurons (L1-L2) controlled by supraspinal centers (pons). At higher filling volumes (> 0.3 L), the threshold pressure (about 1 kPa) that triggers the micturition reflex via a positive feedback loop is reached: The detrusor muscle contracts ^ pressure f^contraction ft and so on until the internal (smooth m.) and external sphincter (striated m.) open so the urine can exit the body.

|— A. Anatomy of the kidney (schematic diagram)

Afferent arteriole

Peritubular capillary network

Cortex

Interlobular artery Glomerulus

Outer stripe Outer medulla —

Inner stripe Arcuate artery

Cortical nephron

Juxtamedullary nephron

Inner medulla

Afferent arteriole

Peritubular capillary network

Interlobular artery Glomerulus

Inner medulla

Vasa recta

Outer Stripe Medulla

Loop of Henle

Collecting duct

Ureter

Papilla

Vasa recta

Loop of Henle

Collecting duct

Ureter

Papilla l~ B. Glomerulus and Bowman's capsule

Afferent arteriole Efferent arteriole

Afferent arteriole Efferent arteriole

Afferent Arterios

Capsular space

Glomerular capillaries

Bowman's capsule Glomerulus

Podocyte

Origin of Pedicel proximal tubule

Vas Afferens Renin Smooth

Endothelium Basement membrane

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