RENAL SYSTEM

URINARY SYSTEM

• Two kidneys on the posterior wall of abdomen

• Two ureters (25cm), muscular, move urine by peristalsis

• Bladder, stores urine up to 700-800mL

• Urethra, emptying bladder through voluntary contraction (learned)

The kidneys form a filtrate of the blood that is modified by reabsorption and secretion. Urine destined for excretion moves along ureters to bladder.

KIDNEY FUNCTION

• regulation of water and electrolyte volume and osmolarity

• regulation of acid-base balance (pH)

• excretion of metabolic waste products

• removal of foreign chemical from blood

• secretion of hormones

  • renin

  • erythropoietin

  • vitamin D3

KIDNEY STRUCTURE

• Renal cortex

• Renal medulla

• Renal pelvis

• Nephrons

RENAL CORTEX

• outer layer

• site of glomerular filtration and the convoluted tubules

RENAL MEDULLA

• inner, pyramid shaped

• location of long loops of Henle

• Collecting ducts drain here into the pelvis

RENAL PELVIS

• innermost, broadest top of ureter

• kidney/tubules drain into it

• collects urine and funnels into ureter/bladder

KIDNEY ARTERIAL PATHWAY

Blood into the kidneys

Aorta → renal artery → segmental artery → interlobar artery → arcuate artery → cortical radiate artery → afferent arteriole → glomerulus

KIDNEY VENOUS PATHWAY

Blood out of the kidney

Glomerulus → efferent arteriole → peritubular capillaries → cortical radiate vein → arcuate vein → interlobar vein → renal vein → inferior vena cava

NEPHRONS

• filtration of nephrons occurs in renal cortex

• 800,000 to 1.5 million per kidney

• Filtration component = glomerulus + bowman’s capsule

• Renal tubules - divided into structural and functional regions

NEPHRON BLOOD VESSELS

Afferent arterioles

• supply blood to glomerulus

Glomerulus

• ball of capillaries where filtration takes place

Efferent arterioles

• carry blood away from glomerulus

Peritubular capillaries

• exchange with tubular fluid in lumen

• runs close to tubule, enabling ‘cross talk’

NEPHRON TUBULAR SYSTEM

Proximal tubules

• convoluted and straight

• drains Bowman’s capsule

Loops of Henle

• thin ascending and descending limbs

• thick ascending limb

• produces urine of varying concentrations

Distal tubule

• contains macula densa

Collecting duct

• cortical and medullary portions

NEPHRON TUBULAR SEGMENTS

Bowman’s capsule

• surrounds glomerulus

• collects filtrate

Proximal tubule

• where majority of unregulated reabsorption takes place

Loop of Henle

• produces urine of varying concentrations

Distal tubule/collecting duct

• controlled reabsorption and secretion

Renal pelvis

• collect urine and funnel to ureter/bladder

JUXTAGLOMERULAR APPARATUS

• formed by distal convoluted tubules and glomerular afferent arterioles meeting

• regulates blood pressure and filtration rate of glomerulus

FILTRATION, REABSORPTION, SECRETION

Excretion = filtration - (reabsorption + secretion)

GLOMERULAR FILTRATION

• movement of fluid/solutes from glomerular capillaries to Bowman’s space

• Blood → tubule

TUBULAR REABSORPTION

• movement of materials in filtrate in tubules to peritubular capillaries

• Tubules → blood

TUBULAR SECRETION

• movement of solutes from peritubular capillaries into tubules

• Blood → tubules

REABSORPTION %

20% - plasma volume entering afferent arteriole is filtered to Bowman’s capsule

80% - plasma volume exits efferent arteriole (not filtered)

>99% - filtered volume reabsorbed into peritubular capillaries

<1% - filtered volume is excreted

SODIUM REABSORPTION

64% - proximal tubule

25% - loop of Henle (ascending limb)

7% - distal tubule and collecting duct

1% - excreted

WATER REABSORPTION

67% - proximal tubule

25% - loop of Henle (descending limb)

9% - distal tubule and collecting duct

<1% - excreted

VASOPRESSIN (ADH)

Increases tubular permeability to water in the collecting duct - influencing urine concentration and osmolarity.

Hydrated = less ADH needed = less water reabsorption = more water excreted = restore homeostasis.

Dehydrated = more ADH need = more water reabsorbed in collecting duct + distal tubule = fluid loss minimised.

VASOPRESSIN STEPS

1. ADH synthesised in hypothalamus

2. Travels down posterior pituitary and stored until released

3. Released into circulation in response to dehydration

4. Delivered via blood and stimulates V2 receptors on membrane of collecting duct

5. Activates adenylyl cyclase, triggering cascade that inserts pre-formed aquaporins into membrane of collecting duct

6. Increased water absorption

ALDOSTERONE

• selected in response to increase plasma K+ and angiotensin II

• acts on renal distal tubule to increase Na+ reabsorption

• Aldersterone increases sodium reabsorption (water follows sodium osmotically) therefore it also increases plasma volume.

TRANSPORT MAXIMUM

For most substances actively reabsorbed/secreted, there is a maximum rate the solute can be transported at.

• due to saturation of available carrier proteins

• any excess above transport maximum is excreted

WHY TUBULAR SECRETION MATTERS

• disposes substances bound to plasma proteins and too large to be filtered at glomerulus

• disposes substances that were passively reabsorbed

• disposes excess K+

• controls blood pH

VARIATIONS OF FILTRATION, REABSORPTION AND SECRETION