anatomy and basics of urine formation

functions of the kidneys

• regulation of water, inorganic ion, and acid-base balance

• removal of metabolic waste products from the blood and their excretion in the urine

• removal of foreign chemicals from the blood and their excretion in the urine

• production of hormones and enzymes

hormones and enzymes produced in the kidneys

• erythropoietin

• renin

• 1,25-dihydroxyvitamin D

erythropoietin

hormone that controls erythrocyte production

renin

enzyme that controls the formation of angiotensin and influences blood pressure and sodium balance

1,25-dihydroxyvitamin D

active vitamin that influences calcium balance

gross renal anatomy

• kidneys are paired organs

• ~150 grams each

• each about the size of a fist

• located behind the peritoneum on either side of the vertebral column against the posterior abdominal wall

major structures of the kidney

• renal cortex

• renal medulla

• renal pelvis

blood flow within the kidneys (incoming)

1. renal artery

2. interlobar artery

3. arcuate artery (at border between cortex and medulla)

4. interlobular artery

5. afferent arteriole (supplies blood to nephron)

nephron

• functional unit of the kidney

• ~1 million per kidney

• consists of renal corpuscle and tubule

renal corpuscle

• glomerulus (glomerular capillaries)

• Bowman’s capsule

• Bowman’s space

renal tubule

1. proximal tubule

   1. proximal convoluted tubule

   2. proximal straight tubule

2. loop of Henle

   1. descending thin limb

   2. ascending thin limb

   3. ascending thick limb (contains macula densa at end)

3. distal convoluted tubule

4. collecting duct system

   1. cortical collecting duct

   2. medullary collecting duct

Bowman’s capsule

• parietal layer

• Bowman’s space

• visceral layer (podocytes)

  • attached to glomerulus

juxtaglomerular apparatus

• juxtaglomerular cells

• macula densa

glomerular capillary wall

• podocyte (visceral layer of Bowman’s capsule)

  • cell processes → interdigitate

  • cell body

• endothelial layer

  • fenestrae

filtration slits

spaces between interdigitated processes of podocytes, allowing the passage of filtrate

glomerulus

• entangled capillary loops surrounded by Bowman’s capsule

• wall consists of endothelial cells, glomerular basement membrane, visceral epithelial cells (podocytes)

• filters blood to make urine

peritubular capillaries

supply cells of the tubules with blood

processes of urine formation

• glomerular filtration

• tubular secretion

• tubular reabsorption

glomerular filtration

• filtration of plasma from the glomerular capillaries into Bowman’s space

• filtrate is cell-free and, except for proteins, contains all the substances of plasma in virtually the same concentrations

tubular secretion / reabsorption

• composition of glomerular filtrate is altered by movements of substances as it passes through the tubules

• reabsorption: tubules → peritubular capillaries

• secretion: peritubular capillaries → tubules

elimination of a substance

amount excreted = amount filtered + amount secreted - amount reabsorbed

renal handling of para-amino-hippurate (PAH)

• some filtered at glomerulus

• completely secreted

• entirely excreted

renal handling of sodium or water

• some filtered at glomerulus

• some reabsorbed

• mostly maintained, some excreted

renal handling of glucose

• some filtered at glomerulus

• completely reabsorbed

• all maintained (none excreted)

control of urine formation

• rate of filtration, reabsorption, or secretion is subject to physiological control (particularly reabsorption and secretion)

• when body content of a substance goes above or below normal, homeostatic mechanisms can regulate the substance’s bodily balance by changing these rates

  • too much → secrete more / reabsorb less

  • not enough → secrete less / reabsorb more