01: Body Fluid Compartments

Roles of the kidney

• Filtration and osmolality

• Blood pressure and volume

• Acid/Base balance

• Gluconeogenesis

Hormones that act on the kidneys to control blood pressure

• Vasopressin/ADH

• Aldosterone

• Renin

• ANP

How does the kidney modulate acid/base balance

• Produces and secretes HCO₃^-

• Secretes H⁺

When do the kidneys release EPO

Low O₂/RBCs

How do the kidneys modulate Ca⁺⁺

Vit D activation

What is the goal when kidneys modulate fluid volume

It is trying to match intake to output

Blood flow through the nephrons

Renal artery → afferent arteriole → glomerular capillary → efferent arteriole → renal vein

What blood components are filtered out the glomerulus

Plasma

What blood components are usually not filtered out the glomerulus

Cells and proteins

Features of the glomerulus that keep proteins in the blood

• Small holes/cellular junctions

• Negative charge

Parts of the uriniferous tubule

• Proximal convoluted tubule

• Thin descending loop of Henle

• Thick ascending loop of Henle

• Distal convoluted tubule

Where does most of the filtrate reabsorption happen

Ascending loop of Henle (double check)

How is most of the body water excreted

Urine

Factors that influence water loss

• Ambient conditions

• Amount of particles

• Activity

• Intake

• Physiological and anatomical adaptations

Term for water that is always lost during homeostatic mechanisms

Insensible water loss

Where is most of the body water found

ICF

Fluid compartments in the body

• ECF: interstitial fluid, plasma

• ICF

• Transcellular: CSF, joint fluid, other special compartments

Forces that move water between body compartments

Hydrostatic pressure and osmotic pressure

Forces that move water in/out of cells

Osmotic forces

What does water in the plasma have to cross to get to the interstitium

Capillary membrane

How does plasma prevent loss of lots of water as it travels through capillary beds

Osmotic pressure retains water in the blood/plasma

Which ions are most abundant in the ECF

NaCl

Which ions are most abundant in the ICF

K

How do osmotically active ions impact water distribution

Water will move to an area of high ionic concentration

If the blood is anisotonic, how is it sensed and fixed by th body

Pituitary neurons sense cell swelling/shrinking, and will modulate ADH release accordingly

What causes cells to shrink

Hypertonic ECF

What causes cells to swell

Hypotonic ECF

What determines osmotic pressure

Number of solutes (NOT the size of solutes!)

Definition of osmotic pressure

The force of a concentration gradient exerted on water

Physiological unit used to measure osmolarity in the body

mOsm

Pressure exerted by 1 mOsm

19.3 mmHg

When assessing the osmotic pressure provided by a solution, what are the factors you use to calculate the osmotic pressure

• Molecular weight

• Concentration

• Dissociation

• Osmotic coefficient (how much of the compound actually dissociates in solution)

What type of solution would you give to a patient with intracellular edema

Hypertonic solution → into ECF → pulls water from ICF

What type of solution would you give to a dehydrated patient

Hypotonic solution → pushes water into ICF

Do water or ions move faster

Water

Effect on body compartments if you give isotonic fluid

It just increased the ECF volume, does nothing to the ICF

Causes of hyponatremia

• Excessive Na loss

• Excessive water intake

Causes of hypernatremia

• Dehydration

• Overconsumption or retention of Na

Affect of hyponatremia on the brain

Swelling as water moves from ECF → ICF

Causes of extracellular/pitting edema

• Increased hydrostatic pressure

• Decreased oncotic pressure

• Poor lymphatic drainage

Factors that increase capillary filtration

• Increased capillary hydrostatic pressure

• Decreased tissue hydrostatic pressure

• Decreased capillary osmotic pressure

• Increased tissue osmotic pressure

How does heart failure cause extracellular edema

• Back up of the venous system

• Decreased GFR → increased blood volume

• Increased RAAS → salt and water retention

How does kidney failure cause extracellular edema

Failure to filter adequate amounts of fluid and solutes

How does decreased plasma proteins cause extracellular edema

Decreased oncotic pressure

Cases of intracellular edema

• Na⁺ pump dysfunction

• Inflammation

Physiological mechanisms that prevent edema

• Tissues have low compliance

• Lymphatics have a huge reserve drainage capacity

• Proteins are routinely washed out of the interstitium to decrease osmotic pressure and prevent the build up of fluid

Effusion v edema

Edema is fluid in tissues. Effusion is accumulation of fluid in potential spaces