Fluid, Electrolyte, and Acid-Base Balance Study Guide

How do you gain and lose fluid? Where is fluid found in your body?

Gain:

• Metabolic water: Produced as a by-product of dehydration synthesis reactions and aerobic respiration

• Performed water: Ingested in food and drinks

Loss:

• Insensible water: Not aware of the loss (breath and cutaneous respiration)

• Sensible water: Noticeable output (urine and sufficient sweating)

• Obligatory water loss: Output that is unavoidable (expires air, cutaneous respiration, sweat, fecal moisture, minimum urine output)

How is water balance regulated?

• Water balance largely depends on the sensations of thirst when the osmolarity in your blood increases.

  • When you are dehydrated either your body will increase the amount of solutes in your blood or decrease blood pressure. When decreasing blood pressure your body will release renin to convert Angiotensin I -> Angiotensin II to release Aldosterone for water retention. 

  • Both mechanisms (osmolarity and BP) will activate the osmoreceptors in your hypothalamus that conserves saliva and makes your mouth dry. This will ultimately cause the feelings of thirst which will motivate you to drink water. 

  • Drinking water can cause short term and long term effects. Short term effects are rehydrating your mouth and slightly enlargens your stomach or intestines to inhibit urges to drink. For long term effects your blood will become hydrated again. 

Be able to describe or recognize disorders of water balance.

Fluid Deficiency:

• Volume depletion (hypovolemia): proportionate amounts of water in sodium are lost without replacement

• Dehydration: Body eliminates significantly more water than sodium so the ECF osmolarity rises

Fluid Excess:

• Volume excess: Both sodium and water are retained and ECF remains isotonic

• Hypotonic hydration (hypovolemia): More water than sodium is retained or ingested and ECF becomes hypotonic

Fluid Sequestration: Condition in which excess fluid accumulates in a particular location

What are the key electrolytes in your body? Why are each of these important (what are their roles)?

• Major cations:

  • Sodium: Important for resting membrane potential, when potassium flows out of a cell and makes the cell more negative. As well as depolarization of muscles, when your cells become positive.

  • Potassium: Similar to sodium they are responsible for returning a cell to its resting membrane potential (becomes more negative). Potassium is also part of the sodium-potassium pump that is also responsible for resting membrane potentials and fluid in cells.

  • Calcium: Calcium makes your bones healthier and more durable, it is also responsible for muscle contraction. In your heart calcium helps with muscle contraction in order for your blood to pump through and out of the heart.

  • Magnesium: Second most abundant intracellular ion after potassium

• Major anions:

  • Chloride: Follows sodium everywhere so maintained the same way

  • Phosphate: Every process that depends on ATP depends on phosphates

Describe how sodium, potassium, and calcium are regulated.

Sodium: Ingesting minimum 0.5 sodium each day, however the typical values people eat exceed this. When blood pressure is low it will stimulate renin and angiotensin to create aldosterone, responsible for storing salt in the kidneys. When there is too much potassium (hyperkalemia) it will automatically stimulate aldosterone production, both will activate the renal tubules. 

• For low BP more salt will be absorbed so water will follow and increase fluid volume

• For hyperkalemia, more potassium is forced to be transported to urine for excretion. 

Potassium: Similar to sodium when dealing with hyperkalemia. The glomerulus is responsible for excreting as much potassium as it can. For hypokalemia (low K+ levels), the renal tubules hold on to the potassium. Aldosterone will also secrete more sodium so that less potassium is excreted from the urine. 

Calcium: PTH, calcitriol and calcitonin controls the amount of calcium that should or shouldn't be secreted. 

• High levels of calcium in the blood activates calcitonin to inhibit osteoclast activity and stimulate osteoblast activity

• Low levels of calcium activates PTH to stimulate osteoclast activity and inhibit osteoblast activity. 

Be able to describe or recognize disorders of sodium, potassium, and calcium.

Sodium:

• Hypernatremia: water retention, hypertension, and edema

• Hyponatremia: similar to hypervolemia

Potassium:

• Hyperkalemia: less concentration gradient between intracellular fluid and extracellular fluid so diffusion of potassium is reduced and cell is closer to the threshold for setting off action potential

• Hypokalemia: more potassium moves from intracellular fluid to extracellular fluid, making the cells hyperpolarized

Calcium:

• Hypercalcemia: reduces sodium permeability of plasma membranes and inhibits depolarization of nerve and muscle cells

• Hypocalcemia: increases sodium permeability of plasma membranes, causing muscular and nervous systems to be overly excitable

Define acids and bases and describe acid base balance.

Acid Base Balance: State in which the pH of the body fluids is homeostatically regulated within its range

Acids: Release hydrogen in a solution

• Strong acids ionize freely, marked lower pH

• Weak acids ionize only slightly

Bases: Chemicals that accept a hydrogen ion

• Strong bases ionize freely, markedly raise pH

• Weak bases ionize only slightly

Define buffer and describe the two types of buffer systems.

Buffer: Any mechanism that resist pH changes by converting a strong acid or base to a weak acid or base

• Physiological buffer: Maintains pH by determining excretion of acids, bases, or carbon dioxide. Is present in the respiratory and urinary system, will be removed either in the form of urine (takes longer), and the bicarbonate ions and carbon dioxide transport system (happens within minutes).

• Chemical buffer: Will attach itself to hydrogen and remove it, the concentration will either increase or decrease from the absence of hydrogen. 

Describe the three types of chemical buffer systems.

Bicarbonate: Reaction occurs in reverse, when shifting to the right of the reaction the carbonic acids become weak and give up a hydrogen ion to decrease pH, when moving to the left the bicarbonate ion will become weak by attaching itself to hydrogen and increasing pH. 

Phosphate: Similar to the bicarbonate system but has a much stronger reaction. When moving to the right, hydrogen will be released and decreases pH. When moving to the left it will attach to a hydrogen to increase pH. 

Protein: Makes up a quarter of the buffering power of body fluids that are in the cell. Within the amino group carboxyl will give up a hydrogen to decrease pH, and another amino group -NH2 will attach to a hydrogen to increase pH back to its normal value. 

Be able to describe or recognize disorders of acid-base balance.

Respiratory:

• Acidosis: Hypoventilation, apnea, or respiratory arrest; asthma; emphysema; cystic fibrosis; chronic bronchitis; narcotic overdose

• Alkalosis: Hyperventilation due to pain or emotions such as anxiety; oxygen deficiency (as at high elevation)

Metabolic:

• Acidosis: Excess production of organic acids in diabetes mellitus and starvation; long-term anaerobic fermentation; hyperkalemia; chronic diarrhea; excessive alcohol consumption; drugs such as aspirin and laxatives

• Alkalosis: Rare but can result from chronic vomitting; overuse of bicarbonates (antacids); aldosterone hypersecretion