Week 9 - Kidneys & Homeostasis & Disease

Kidney’s help regulate blood pressure

Speed of BP regulation:

Fast (seconds): Baroreceptor reflex

Intermediate (minutes): Renal/Kidney function - Hormones (Renin & Angiotensin II)

Slow (hrs to days): Long-term kidney function - Kidney’s water/salt intake

Hormones and Kidney function: Renin

3 pathways for Renin to be released.

Overall, a decrease in BP causes it.

What does Renin do?

• Converts inactive angiotensinogen from the liver into inactive angiotensin I.

ACE then converts inactive angiotensin I into active angiotensin II

Hormones and Kidney function: Angiotensin II

Overall, increases blood pressure

In diagram: Vasopressin = ADH

To act, Ang II binds to angiotensin type 1 receptors (AT1R)

Within the proximal tubule:

• Angiotensin II increases the activity of the apical Na+/H+ exchanger

• This increases Na+ uptake into blood

• Na+ and water reabsorption is coupled in proximal tubule

• Thus, water moves with Na+

Hormones and Kidney function: Vasopressin/ADH (anti-diuretic hormone)

ADH helps increase water absorption.

• Remember how ADH allows the collecting ducts to become permeable to water

  • Thus the sharp concentration gradient of dilute urine (low Osm) to concentrated blood (high Osm)

  • When permeable to water, gradient flattens as osmosis occurs collecting duct → blood vessel/vasa recta/peritubular network

This increases blood volume in the returning capillaries, which in turn increases BP.

Stimulus for ADH release: Released from hypothalamus

• Decrease in BP

• Decrease in stroke volume/blood volume

• Increased plasma osmolarity

ADH cannot cross cell membrane

Hormones and Kidney function: Aldosterone

How is it released?

Adrenal glands (sit on top of kidney) release Aldosterone. Released when high K+ plasma concentration or angiotenin II activated. However, high blood plasma osmolarity (e.g during dehydration) would inhibit aldosterone (to prevent Na+ from migrating out into the tubules)

What does Aldosterone do?

Within the distal tubule & collecting ducts, controls the NaK pump (active, uses ATP).

• reabsorption of Na+ and in turn, K+ secretion

• ENAC pumps - sodium channels on nephron side open up

• NaK pump on blood vessel side open up

Water follow Na+, so more sodium reabsorption, more water retained. Only works if tubule is permeable to water (ADH present)

Overall diagram of Angiotensin

K+ regulation in the kidney

1. K+ enters nephron in renal corpuscle (glomerulus → bowman’s capsule_

2. 90% of this k+ in the nephron is reabsorbed in the proximal tubule and (ascending) loop of Henle

3. Distal tubule is where hormones (aldosterone) play a part in controlling how much K+ is actually secreted in the urine, depending on the body’s needs at the time

K+ is TIGHTLY regulated

Natriuretic peptides - ANP & BNP

ANP & BNP - Oppose the function of the renin-angiotensin hormones

• Perform diuresis & Natriuresis - excretion of water & sodium

ANP released by atria, BNP released by ventricles.

When more blood in atria, more blood volume, more stretch, ANP released, decreases BP.

Symptoms of Kidney disease

Kidney itself has no pain receptors, only on the surface. Distension of the organ may cause abdominal/lower back pain.

Diseases mainly caught in urine. (e.g presence of blood, glucose, proteins or crystals in urine).

2 main causes of CHRONIC kidney disease: Hypertention & diabetes

• Occurs if >75% of nephrons have lost function

• Measured by observing GFR for 3 months min with <60 ml/min/1.73 m2 produced

• Only dialysis or organ transplantation for treatment