Fluids & Electrolytes (Dr. Arnolds Slides)

Define Homeostasis

Maintaining intravascular volume to facilitate tissue perfusion and oxygenation, necessary to ensure cellular functions. “Keeping the right amount of water in the right places.”

When solutes are too concentrated from loss of water what happens to the cell?

The cell undergoes crenation, losing water and resulting in shrinkage.

When theres too much dilute from increased water uptake into the cells what happens to the cell?

The cell swells and bursts (lysis) due to excess of fluid

Where does water flow?

Water flows from areas of low solute concentration to areas of high solute concentration, following the osmotic gradient, to “dilute” solutes.

Define Total Body Water (TBW).

The total amount of water in the body.

What is the percent of TBW in the body?

~ 60% in Men

~ 50% in Women

How many liters is TBW?

42 liters

How is TBW split up in the body?

Intracellular fluid (ICF) and Extracellular fluid (ECF)

What is Intracellular fluid?

The fluid contained within the cells of the body, accounting for about 2/3’s of TBW

How many liters make up intercellular fluid in TBW?

28 liters

What is Extracelluar fluid?

The fluid outside the cells, 1/3 of TBW

How is extracelluar fluid split up?

Interstitial fluid = surrounds the cells, and plasma = the liquid component of blood.

How many liters make up extracellular fluid in TBW?

14 liters

Of the 14 liters in extracellular fluid, how much is plasma?

1/4, 3 liters

Of the 14 liters in extracellular fluid, how much is interstitial fluid?

¾, 11 liters

How is water fluid lost?

Through Excretion, Injury/Trama/Disease, Insensible Fluid Loss

Define Excretion in the sense of water loss.

Urination

What are examples of injury, trama, or disease that can cause water/fluid loss from the body?

Bleeding, diarrhea, vomiting or severe burns

Define Insensible fluid loss.

Sweating and breathing, not easily measured, estimates of 40-800 mL/day

What other factors can alter water/fluid balance?

Alcohol and Drugs

What does a semi-permeable barrier do?

It does not let every substance cross equally.

Is the cell membrane a semi-permeable barrier?

Yes

What is the capillary wall?

A thin semi-permeable membrane made up of endothelial cells. It regulates exchange of fluids, solutes, and nutrients between blood and surrounding tissues.

How do substances pass through the capillary wall?

Through junctions between endothelial cells and fenestrations when present. Some also pass through by vesicular transport.

What other factors are responsible for transport across the capillary wall?

Diffusion and filtration

Define Diffusion (Passive Transport)

The movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached. Requires no energy.

Define Filtration.

Water and solutes are forced through a membrane or capillary wall due to pressure differences, usually from an area of higher hydrostatic pressure to lower.

Within blood, what is one of the large molecules/proteins that is largely responible for osmotic pressure?

Albumin

What does albumin do?

Helps hold water in the blood to maintain osmotic pressure.

Define Starling forces.

Describe how much fluids move across capillary membranes, including hydrostatic and osmotic pressures.

What are “sink conditions”?

A situation where the substance is removed from the the other side, keeping concentration low and allowed continued diffusion.

Under “sink conditions” movement is dependent on concentration, what is concentration dependent on?

A concentration gradient (the difference in concentration between the two sides of the membrane)

What are ion channels?

Gates that can open or close in response to several environmental signals (Ligand & Voltage gating, and posttranslational modification) ; help control which ions can move across the membrane and when

Define Ligand gating.

An Ion channel opens or closes in response to the binding of a specific molecule (ligand) to the channel, allowing ions to pass through the membrane.

Define Voltage gating?

An ion channel opens or closes in response to changes in the membrane potential, allowing ions to flow across the membrane.

Define Posttranslational modification.

Any chemical changes to a protein after its translation that can change whether it opens or closes the channel, such as phosphorylation

What do active transporters do?

They work against gradients to move ions or molecules across the membrane; requires energy

Define Na/K ATPase.

An active transporter that uses ATP to pump 3 Na out of the cell and 2 K into the cell.

What is the major ion found mainly outside the cell (ECF)?

Na+

What is the major ion found mainly inside the cell (ICF)?

K+

What substance binds to and inhibits the Na/K ATPase?

Ouabian

Fill in the blank. H2O is ___ actively transported across cell membranes, it flows across membranes ______ through _____ or _______ diffusion called _____?

not; passively; simple; facilitated; osmosis

What do you need to understand about how water moves?

That water flows in the direction to “dilute” solutes, so if one side of a membrane has more solute water will move toward that side to help dilute it. This is called osmosis.

Define Hydrostatic pressure.

Force excerted by fluid against a wall that results in movement of fluid between compartments.

What does Hydrostatic pressure do in summary?

Forces plasma (water) and nutrients out of the capillaries and into the surrounding tissues

What is an example of hydrostatic pressure?

Capillary hydrostatic pressure: the pumping action of the heart. It pushes blood against the walls of the blood vessels which moves the fluid out of the capillaries into surrounding tissues.

What is capillary hydrostatic pressure (CHP)?

The pressure of blood against the capillary wall that pushes water out of the capillary and into the surrounding tissue.

What is blood colloid osmotic pressure (BCOP)?

The force created mainly by plasma proteins that pulls water INTO the capillary.

Which direction does capillary hydrostatic pressure push fluid?

OUT of the capillary into interstitial fluid/tissuesand INTO the surrounding tissues.

What protein is mainly responsible for BCOP?

Albumin, because it stays mostly inside the bloodstream and attracts water.

What is the main effect of capillary hydrostatic pressure at the arterial end of a capillary?

CHP is greater than BCOP, so fluid is pushed OUT —> filtration occurs

What happens when CHP equals BCOP?

The outward push and inward pull are equal so —> no net fluid movement

What happens when CHP becomes lower than BCOP?

The inward osmotic pull becomes stronger so fluid moves back INTO the capillary —> reabsorption

What is osmotic pressure?

Pressure that pulls H2O toward the higher solute concentration; the goal is to establish equal concentration (equilibrium)

Easy distinction between hydrostatic and osmotic.

Hydrostatic = Pushes fluids

Osmotic = Pulls fluids

Define Tonicity.

How a solution affects the movement of water into or out of a cell

What determines the direction water moves during changes in tonicity?

Water moves toward the side with the higher effective solute concentration

What is an isotonic solution?

A solution with the same effective solute concentration as the cell, resulting in no net movement of water and no change in cell size.

What happens to a cell in an isotonic solution?

A solution with the same effective solute concentration as the cell, resulting in no net movement of water and no change in cell size.

What is a hypertonic solution?

A solution with a higher effective solute concentration than the cell, causing water to move out of the cell.

What happens to a cell in a hypertonic solution?

Water moves out of the cell, causing it to shrink (crenation)

What is a hyptonic solution?

A solution with a lower effective solute concentration than the cell, causing water to move into the cell.

What happens to a cell in a hypotonic solution?

Water moves into the cell, causing the cell to swell and potentially burst (lysis)

What is blood osmolality?

A measure of the concentration of dissolved particles in the blood relative to water. (mainly influenced by Na+)

What happens to blood osmolality when the body loses water?

It increases because there is less water relative to the dissolved particles

What are osmoreceptors?

Receptors in the hypothalamus that detect changes in the blood osmolality

What happens whenn hypothalamic osmoreceptors detect increased blood osmolality?

They stimulate the thirst center in the hypothalamus, causing increased thrist

How does insufficient water affect blood volume?

It decreases it

How does decreased blood pressure contribute to thirst?

Decreased BP leads to increased angiotensin II, which stimulate the thirst center in the hypothalamus

What are the two major pathways that stimulate thirst when the body needs water?

1. ↓ Blood volume → ↓ BP → ↑ angiotensin II → ↑ thirst

2. ↑ Blood osmolality → hypothalamic osmoreceptors → thirst

How does dry mouth affect thirst?

It contributes to stimulation of the thirst center in the hypothalamus

What happens after the thirst center is stimulated?

Thirst increases → person drinks water → water is absorbed → blood osmolality decreases toward normal

What is normal serum/blood osmolality?

285-295 mOsm/kg

What serum electrolytes are mainly extracellular?

Na⁺, Cl, HCO₃, Ca

What serum electrolytes are mainly intracellular?

K⁺, Mg²⁺, PO₄^3-

What do serum electrolytes values represent?

Electrolyte concentrations in the extracellular space, not total-body electrolyte stores; they can look normal despite intracellular deficits

Which electrolyte is especially important for ECF osmolality?

Na+

Which electrolytes are important for electrical excitability of nerves and muscles?

K⁺, Ca²⁺ , Mg²⁺

Which electrolytes are important for acid-base buffering?

HCO₃^- and PO₄^3-

Which electrolytes are important for enzyme and ATP function?

Mg²⁺ and PO₄^3-

What two major concepts affect transcellular fluid balance?

Osmolality and tonicity, with Na⁺ regulation playing a major role

What are oncotic forces?

Forces created by proteins, especially albumin, that pull water toward them and influence fluid movement between blood and tissues.

Which organ is the predominant organ for regulating electrolyte and fluid homeostasis?

The kidneys