Biology 212 Anatomy & Physiology I 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. Water balance – Keeping the right amounts of water in the right places 2. Electrolyte balance – Keeping the right amounts of electrolytes/ions/minerals in

Terms to know: Solvent = a fluid in which things are dissolved (only important one in human physiology = water) Solutes = the things dissolved in that solvent Electrolytes or Ions = solutes that have an electrical charge Cations have positive electrical charges (K+, Ca++, etc) Anions have negative electrical charges (Cl-, HPO3-, etc) Osmolarity = measurement of the concentration of all solutes together within a body fluid

Water Balance: Water = more than half of total body mass 3 major compartments: Plasma ~8% of body water Interstitial ~30% of body water Intracellular ~60% of body water

Water Balance: Water = more than half of total body mass 3 major compartments: Plasma ~8% of body water Interstitial ~30% of body water Intracellular ~60% of body water Many smaller compartments: ~ 2% Urine, Lymphatic fluid, Cerebrospinal fluid, Saliva, Synovial fluid, Digestive system contents, Semen, Respiratory system fluids, Peritoneal fluid, Tears, Occular fluids, Pericardial fluid, Pleural fluid, etc.

These fluid compartments are in equilibrium with each other as both water (“solvent”) and all of the molecules dissolved in that water (“solutes”) move from one compartment to another.

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. o

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. o

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: About 2.5 liters/day with “normal” activity.

We also lose water in a number of ways: About 2.5 liters/day with “normal” activity.

In order to maintain homeostasis, the body must maintain water balance: Water gained by body must equal Water lost by body o

In order to maintain homeostasis, the body must maintain water balance: Water gained by body must equal Water lost by body Physical activity, high temperatures, and other things may increase water loss, requiring an increase in water gain. Similarly: Increased water gain must be offset by increased water loss.

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

Even a modest amount of dehydration 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, it is primarily regulated by increasing or decreasing our sense of thirst. Even a modest amount of dehydration a) decreases the blood volume/pressure and b) increases the concentration of solutes (“osmolarity”) in the blood. This stimulates regions of the brain called “thirst centers” which cause you to increase your intake of water.

Obviously, the human body must have mechanisms to regulate both water intake and water output: Most of our water loss is urine, so this is primarily regulated through the action of antidiuretic hormone on the kidneys. Even a modest amount of dehydration increases the concentration of solutes (“osmolarity”) in the blood. Through receptors in the hypothalamus, this stimulates increased production of antidiuretic hormone by the posterior pituitary, which increases reabsorption of water by the kidney, this decreasing the amount lost into the urine.

Obviously, the regulation of both water intake and water loss have the same objective: water water intake output = Anything that increases water intake must be balanced by increased output. Anything that decreases water input must be balanced by decreased output. Anything that increases water output must be balanced by increased input. Anything that decreases water output must be balanced by decreased input.

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 the concentration of all of the atoms and molecules (the solutes) dissolved in that water. Many of these solutes carry electrical charges, so we call them electrolytes 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 by the hormone aldosterone, which is secreted by the adrenal cortex and acts on kidney nephrons. Either a decrease in sodium or an increase in potassium in the blood stimulates secretion of aldosterone. This causes the kidneys to retain sodium in the blood, and to eliminate potassium into the urine.

If sodium increases or If potassium decreases, then the secretion of aldosterone is inhibited This causes the kidneys to eliminate sodium into the urine, and to retain potassium in the blood.

Calcium is primarily regulated by calcitonin and parathyroid hormone Blood Ca++ level returns to normal Calcium is primarily regulated by calcitonin and parathyroid hormone Ca++ moved from blood to bone Blood Ca++ level decreases Thyroid secretes calcitonin Parathyroid glands secrete parathyroid hormone Blood Ca++ level continues to increase Ca++ moved from bone to blood Blood Ca++ level returns to normal

Chloride is very strongly attracted to Na+, so it is normally regulated at the same time as that cation. If the kidneys eliminates Na+, Cl- is “pulled along” and also eliminated. If the kidneys retain Na+ in the blood, Cl- is also retained.

Bicarbonate is regulated by the lungs. Bicarbonate is formed when carbon dioxide dissolves in water so When you exhale more carbon dioxide you eliminate bicarbonate from the blood. When you exhale less carbon dioxide, 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) have very different concentrations in different compartments: For example: 1. In discussing the urinary system, we discussed how the concentrations of many solutes are different in the urine than in the blood.

(electrolytes and other solutes have different concentrations in different fluid compartments): For example: 2. The concentration of Na+ is approximately equal in plasma and the interstitial fluid surrounding most cells but This is more than 10x its concentration in intracellular fluid

(electrolytes and other solutes have different concentrations in different fluid compartments): For example: 3. The concentration of K+ is approximately equal in plasma and the interstitial fluid surrounding most cells but This is only 1/10th its concentration in intracellular fluid

(electrolytes and other solutes have different concentrations in different fluid compartments): For example: 4. The concentration of proteins in the plasma is 30x their concentration in the interstitial fluid surrounding cells but Their concentration in the intracellular fluid compartment is more than 3x as high as in plasma

Remember: this regulation maintains homeostasis: Solutes are maintained within narrow limits in body fluids: Even when additional amounts are being added or removed from the body, and Even when their concentrations may be much higher or much lower in a different fluid compartment.

Let’s stop here: Your textbook also discusses the homeostasis of acid/base balance, or pH. This is certainly important but We will not be discussing it in lecture