Lecture 26

Proximal tubule

§ Most of the solute and water reabsorption

Proximal tubule also carries out secretion

Loop of Henle

Descending limb

§ Ascending limb

§ Distal tubule

Reabsorption completed for most solutes

Collecting duct

Drains multiple nephrons

Carries urine to renal pelvis

Describe the lumen or differences in epithelium of the proximal tubule

microvila on proximal side

Describe the lumen or differences in epithelium of the descending limb

Describe the lumen or differences in epithelium of the ascending limb

Describe the lumen or differences in epithelium of the collecting duct

One side no villa one side villa

what are the 4 processes of urine production

filtration, reabsorption, secretion, excretion

Filtration

§ Filtrate of blood formed at glomerulus

§ Reabsorption

§ Specific molecules in the filtrate

removed

Most water and salt in primary urine reabsorbed

using transport proteins and energy

Each zone of the nephron has transporters for

specific solutes

§ Secretion

§ Specific molecules added to the filtrate

§ Excretion

§ Urine is excreted from the body

What is Primary Urine

Initial filtrate filtered in Bowman’s capsule that is

isosmotic to blood

formed during reabsorption

Rate of reabsorption is limited by

number of transporters

renal Threshold is

Concentration of a specific solute that will overwhelm

reabsorptive capacity

What happens during reabsorption in proximal tubule

Many solutes reabsorbed by Na+ cotransport

Water follows by osmosis

How is glucose reabsorbed

secondary active transport

Reabsorbed molecules taken up by the blood

The secretion process involves

Molecules removed from blood and transported into the filtrate

• Molecules secreted include K+, NH4+, H+, pharmaceuticals, and water-soluble vitamins

• Requires transport proteins and energy

Ion and Water Transport in the Loop of Henle

Descending limb is permeable to water

Ascending limb is impermeable to water

Reabsorbed ions accumulate in interstitial fluid

What happens because Descending limb is permeable to water

Water is reabsorbed

§ Volume of primary urine decreases

§ Primary urine becomes more concentrated

What happens because Ascending limb is impermeable to water

Ions are reabsorbed

Primary urine becomes dilute

What happens as Reabsorbed ions accumulate in interstitial fluid

An osmotic gradient created in the medulla

The loop of henle is a….

Concurrent multiplier

Transport in the Distal Tubule

Distal tubule can reabsorb salts and water

• Distal tubule can secrete potassium

• Transport function of distal tubule affected by hormones

  • Parathormone increases Ca2+ reabsorption

  • Aldosteron increases K+ secretion

Process of Excretion

After urine is produced, it leaves kidney and enters urinary bladder via ureters

• Urine temporarily stored in bladder

Urine leaves bladder via urethra

• Sphincters of smooth muscle control flow of urine out

of bladder

Opening and closing of sphincters controlled by a

spinal cord reflex arc (micturition reflex) and can be

influenced by voluntary controls

Urine concentration is dependent on

Osmotic concentration of final urine depends on permeability (aquaporins) of the collecting duct,

Regulated by vasopressin (AVP)

Water concentration in impermeable parts

Water not reabsorbed from collecting duct

Dilute urine (formed in ascending limb) excreted

Water concentration in permeable parts

Water reabsorbed from collecting duct

Concentrated urine (formed in collecting duct) excreted

How is urinary function regulated

Hormones and Dietary factors

What hormones regulate urinary function

Steroid hormones

• For example, aldosterone

• Slow response

Peptide hormones

• For example, vasopressin

• Rapid response

What dietary factors regulate urinary function

Diuretics

• Stimulate excretion of water

Antidiurectis

• Reduce excretion of water

Regulators of GFR

Hormones

• Vasopressin (antidiuretic hormone, ADH)

• Renin-Angiotensin-Aldosterone (RAA)

pathway

What does vasopressin do

Increase cell permeability

Describe Vasopressin Process

What does aldosterone do

Stimulates Na+ Reabsorption

Process of aldosterone

Juxtaglomerular cells

secrete enzyme renin

3 ways renin is secreted

• Baroreceptors in juxtaglomerular cells release renin in response to low blood pressure

• Sympathetic neurons in cardiovascular control center of medulla oblongata trigger renin secretion in response to low BP

• Macula densa cells in distal tubule respond to decreases in flow by releasing a paracrine signal that induces juxtaglomerular cells to release renin

When is renin secreted

When blood pressure or GFR lower than normal

What does Renin do

converts angiotensinogen to angiotensin I

Angiotensin converting enzyme (ACE) on epithelia of blood vessels converts angiotensin I to angiotensin II

• Angiotensinogen an inactive protein in plasma

What does Angiotensin II do

Causes synthesis and release of aldosterone from adrenal cortex

• Steroid hormone

• Targets cells in distal tubule and collecting duct

• Stimulates Na+ and water reabsorption from urine

• Enhances K+ excretion

• Also stimulated by increases in circulating K

How does Renin-Angiotensin-Aldosterone Pathway control blood pressure

• Angiotensin II a vasoconstrictor Raises blood pressure by increasing resistance

• Aldosterone increases Na+ (and water) retention

  • Raises blood pressure by increasing blood volume

Visualize the feedback loop of AAP

What do ACE Inhibitors do

• ↓ Angiotensin II

• Vasodilation (blood vessels relax)

• ↓ Aldosterone → ↓ water retention