fluid and electrolyte balance

Net secretion

clearanceX > GFR

Net reabsorbed

clearanceX < GFR

1o regulated by renal-related hormones:

Aldosterone (ANG II), ADH, ANP

MEMORIZE FLOWCHART

Aldosterone

Diffuses across membrane because of cholesterol backbone and increases/modulates activity of sodium and potassium channels and atpase pump

More sodium absorbed

More potassium secreted

Binds to intracellular receptor

Membrane Recycling

1. Vasopressin binds to membrane receptor.

2. Receptor activates CAMP second messenger system.

3. Cell inserts AQP2 water pores into apical membrane. AQP2 increase or decrease depending on how much ADH there is

4. Water is absorbed by osmosis into the blood.

Drinking large amount of water

Vol inc

Osm dec

Ingestion of isotonic saline

Vol inc

Osm no change

Ingestion of hypertonic saline

Vol inc

Osm inc

Replacement of sweat loss with plain water

Vol no change

Osm dec

Normal volume and osmolarity

No changes to vol and Osm

Eating salt without drinking water

Vol no change

Osm inc

Incomplete compensation for dehydration

Vol and Osm dec

Hemorrhage

Vol Dec

Osm no change

Dehydration (e.g., sweat loss or diarrhea)

Vol Dec

Osm inc

osmoregulation

Body fluid osmolarity is maintained at a value of about 290mOsm/L (for simplicity, 300mOsm/L)

It occurs in late distal tubule and collecting duct.

Water reabsorption is responsible for maintaining constant body fluid osmolarity.

Normal urine

50 to 1200 mOsM

isosomatic urine

urine osmolarity =blood osmolarity

Hyperosmotic urine

urine osmolarity > blood osmolarity

Hypoosmotic urine

urine osmolarity < blood osmolarity

ADH is main determinant to osmolarity of urine

MEMORIZE

loop of Henle

 Functions: Site of production of dilute urine, Create/maintain osmotic gradient in medulla

 Mechanisms: Countercurrent multiplication (loop of Henle), Countercurrent exchange (vasa recta)

Countercurrent: flow is parallel, opposite directions

establishes the medullary concentration gradient

Countercurrent multiplication (loop of henle)

active process that establishes the medullary osmotic gradient.

Countercurrent exchange (vasa recta)

passive process that helps maintain the gradient. The passive properties of the vasa recta are the same as for other capillaries: They are freely permeable to small solutes and water. Blood flow through the vasa recta is slow, and solutes and water can move in and out, allowing for efficient countercurrent exchange.

Countercurrent heat exchanger

allows warm blood entering the limb to transfer heat directly to blood flowing back into the body.

Proximal tubule

Isoosmotic fluid

300 mOsm

Loop of henle

Hyperosmotic fluid

Higher osmolarity

Distal tubule

Hyposomotic fluid

~100mOsm

Loop of henle and vasa recta