Biology 212Anatomy & Physiology I

Dr. Thompson

Fluid Balance

Recall: major concept of human physiology = Homeostasis:

A state of "dynamic (changing) equilibrium (balance)" in which the body's internal environment is maintained within narrow limits even when 1) the external environment changes significantly and 2) things are introduced into or removed from the body Food Urine Drinks Feces Drugs Sweat Air Bleeding Carbon dioxide

Two major components of fluid homeostasis:

1. Keeping the right amounts of water in the right places

2. Keeping the right amounts of electrolytes/ions/minerals in the right places

Terms to know:

Solvent

Solutes Electrolytes or Ions Cations Anions

Osmolarity

Water Balance:

3 major compartments:

That exchange of water and solutes among compartments is possible only because the cells and membranes lining each of those compartments are permeable to them.

For example:

Fluid from the lumen of the intestine crosses its epithelium to get into the extracellular space, then crosses the epithelium of the blood vessels and lymphatic vessels.

Fluid from the plasma of the blood crosses the epithelium of capillaries to enter the extracellular/interstitial compartment, and those fluids can cross the same epithelium to enter the plasma or lymph compartments.

Fluid from the extracellular/interstitial compartment crosses the plasma membranes of all cells to enter the intracellular compartment, and that fluid crosses the same membrane to enter the extracellular/interstitial compartment.

That equilibrium changes every time - water is added (food, water, etc); or - water is lost through sweat, urine, feces, respiration, etc.

We gain water in a number of ways:

We also lose water in a number of ways:

In order to maintain homeostasis, the body must maintain water balance:

Recall that various fluid compartments are all in equilibrium with each other, so any water gain or water loss will be distributed across these.

Obviously, the human body must have mechanisms to regulate both water intake and water output:

Since most of our water intake comes from liquids we drink,

Even a modest amount of dehydrationa)b)

This stimulates

Obviously, the human body must have mechanisms to regulate both water intake and water output:

Most of our water loss is urine,

Even a modest amount of dehydration

Through receptors in

Obviously, the regulation of both water intake and water loss have the same objective:

Anything that increases water intake

Anything that decreases water input

Anything that increases water output

Anything that decreases water output

However, it is not enough to just regulate the total volume of water in the body and in various fluid compartments.

We must also regulate

Many of these solutes carry electrical charges, so we call them when they are part of any body fluid.

While there are dozens of electrolytes, we will focus on five as examples of different ways in which electrolytes can be regulated:

Sodium (Na+) Chloride (Cl-)

Potassium (K+) Bicarbonate (HCO3-)

Calcium (Ca++)

Sodium and potassium are regulated together, primarily

Either a decrease an increase in the blood stimulates

This causes

If or If , then

This causes

Blood Ca++ level returns to normal

Blood Ca++ level continues to increase

Blood Ca++ level decreases

Blood Ca++ level returns to normal

Ca++ moved from blood to bone

Ca++ moved from bone to blood

Calcium is regulatedby

Chloride so it is normally regulated at the same time as that cation.

Bicarbonate is regulated

Bicarbonate is formed when carbon dioxide dissolves in water

so

When you exhale more carbon dioxide you eliminatebicarbonate from the blood.

When you exhale less carbondioxide, you retain bicarbonate in the blood

Remember: this regulation maintains homeostasis:

Electrolytes are being maintained within narrow limits in body fluids even when additional amounts are being added or removed from the body.

For example, in the blood: - Calcium must be between 8.8 and 10.3 mg/100 ml - Chloride must be between 95 and 107 mEq/liter - Potassium must be between 3.5 and 5.2 mEq/liter

Unlike water, which can easily cross most membranes to move from one compartment to another, electrolytes (and other solutes such as proteins) can have very different concentrations in different fluid departments:

For example:

1.

(electrolytes and other solutes can have very different concentrations in different fluid departments):

For example:

2.

(electrolytes and other solutes can have very different concentrations in different fluid departments):

For example:

3.

(electrolytes and other solutes can have very different concentrations in different fluid departments):

For example:

4.

Remember: this regulation maintains homeostasis:

Solutes are maintained within narrow limits in body fluids: Even when

and

Even when