Fluid and Electrolyte Balance

Total Body Water (Infants)

$$75\%$$

Total Body Water (Adults)

$$50-60\%$$

Total Body Water (Older Adults)

$$45\%$$

Water Content: Brain and Kidneys

$$80-85\%$$

Water Content: Heart and Lungs

$$75-80\%$$

Water Content: Liver, Muscles, and Skin

$$70-75\%$$

Water Content: Blood

$$50\%$$

Water Content: Bones

$$20-25\%$$

Water Content: Teeth

$$8-10\%$$

Fluid Compartments

Consist of Intracellular fluid (ICF) and Extracellular fluid (ECF), which include interstitial spaces, plasma (vascular), and the transcellular compartment.

Transcellular Compartment

A sub-compartment of extracellular fluid including cerebral spinal fluid and fluid in body cavities such as peritoneal, pleural, and joints.

Primary Extracellular Fluid (ECF) Electrolytes

Sodium ($Na+$) and Chloride ($Cl-$) are found in large quantities.

Primary Intracellular Fluid (ICF) Electrolyte

Potassium ($K+$) is found in large quantities.

Potassium Concentration Ratio

$K+$ is approximately $28$ times more concentrated in Intracellular fluid (ICF) than in Extracellular fluid (ECF).

Osmosis

Movement of water down its concentration gradient across a semipermeable membrane.

Hydrostatic Pressure

The force exerted by a fluid against a wall, causing movement of fluid between compartments.

Osmotic Pressure

The hydrostatic pressure needed to oppose the movement of water across a membrane.

Tonicity

The tension or effect a solution with impermeable solutes exerts on cell size due to water movement across the cell membrane.

Isotonic Solution

A solution with the same solute and water concentration compared to body fluids; results in no change in cell size.

Hypotonic Solution

A solution with low solute concentration and high water concentration; causes cellular swelling ($\uparrow$ cell size).

Hypertonic Solution

A solution with high solute concentration and low water concentration; causes cellular dehydration ($\downarrow$ cell size).

Capillary Filtration Pressure (Hydrostatic)

Force that pushes water out of the capillary and into the tissue.

Capillary Colloidal/Osmotic Pressure

Force that pulls water into the capillary from the tissue; primarily due to plasma proteins like Albumin.

Interstitial Hydrostatic Pressure

Force that pushes water out of the tissue and into the capillary.

Interstitial Colloidal Osmotic Pressure

Force that pulls water into the tissue and out of the capillary.

Capillary Hydrostatic Pressure: Arterial vs. Venous

$30\,mmHg$ at the arterial end and $10\,mmHg$ at the venous end.

Interstitial Fluid Pressure

$$-3\,mmHg$$

Net Filtration

Occurs near the arterial end of the capillary because capillary hydrostatic pressure is greater than blood colloidal osmotic pressure.

Net Reabsorption

Occurs near the venous end of the capillary because blood colloidal osmotic pressure is greater than capillary hydrostatic pressure.

Osmolality

The ratio of solutes in a solution to a volume of solvent in a solution.

Antidiuretic Hormone (ADH)

Also known as Vasopressin; produced by the hypothalamus and released by the posterior pituitary to increase water reabsorption in the kidneys and constrict arterioles.

Systemic Response to High Osmolarity

Triggers thirst (increasing water intake) and ADH release (reabsorbing water from urine) to dilute the blood.

Systemic Response to Low Osmolarity

Triggers a lack of thirst (decreasing water intake) and decreased ADH release (losing water in urine) to concentrate the blood.

Electrolytes

Ions that perform functions in the body including nerve excitability, endocrine secretion, membrane permeability, buffering body fluids, and the movement of fluids between compartments.

Sodium ($Na^+$) Normal Range

$$139-145\,mEq/L$$

Potassium ($K^+$) Normal Range

$$3.5-5.0\,mEq/L$$

Chloride ($Cl^-$) Normal Range

$$98-106\,mEq/L$$

Calcium ($Ca^{2+}$) Normal Range

$$9-10.5\,mg/dL$$

Magnesium ($Mg^{2+}$) Normal Range

$$1.3-2.1\,mg/dL$$

Bicarbonate ($HCO_3^-$) Normal Range

$$21-28\,mEq/L$$

Calcium and Phosphate Storage

Greater than $90\%$ of these electrolytes are located in the bones and teeth.

Regulation Site for Sodium and Potassium

Regulated in the renal collecting tubule, where sodium is reabsorbed from and potassium is excreted into the renal filtrate.

Primary Regulators of Sodium and Potassium Balance

Angiotensin II and aldosterone.

Renin

A substance released by the kidney into the blood to initiate the process of producing Angiotensin II.

Angiotensinogen

A substance released by the liver into the blood as a precursor in the angiotensin pathway.

Angiotensin-converting enzyme (ACE)

An enzyme located in pulmonary blood that converts Angiotensin I to Angiotensin II.

Aldosterone

A hormone secreted by the adrenal cortex that stimulates $Na^+$ and $H_2O$ reabsorption in the nephrons and increases the excretion of potassium in the distal tubule.

Aldosterone Release Stimuli

Released in response to increased potassium ($K^+$) or decreased sodium ($Na^+$).

Parathyroid Hormone (PTH)

A hormone that maintains blood calcium and phosphorus by stimulating their release from bone, increasing renal resorption of calcium, and increasing GI calcium absorption.

PTH Activation Requirements

Requires the activation of Vitamin D and magnesium.

Vitamin D

A vitamin that increases intestinal absorption of calcium and phosphorus and is necessary for normal bone formation.

Normal Blood pH Range

The normal range of human blood pH, which is between $7.35$ and $7.45$.

Acid

A compound that can dissociate and release a hydrogen ion ($H^+$); for example, $HCl \rightarrow H^+ + Cl^-$.

Base

A compound that can accept or combine with a hydrogen ion ($H^+$); for example, $HCO_3^- + H^+ \rightarrow H_2CO_3$.

Volatile Acid

Carbonic acid ($H_2CO_3$) produced from $CO_2$; it is in equilibrium with dissolved $CO_2$, which leaves the body via the lungs.

Nonvolatile (Fixed) Acids

Acids such as lactic acid, hydrochloric acid, and ketoacids produced by metabolic processes that are not eliminated by the lungs.

Chemical Buffer Systems

Body fluid mechanisms that combine with excess acids or bases to prevent large changes in pH; they provide a fast response.

Bicarbonate Buffer System

The principal extracellular fluid (ECF) buffer, consisting of a weak acid ($H_2CO_3$) and a weak base ($NaHCO_3$).

Transcellular $H^+/K^+$ exchange system

A buffer system where hydrogen ($H^+$) and potassium ($K^+$) ions move freely between the intracellular fluid (ICF) and extracellular fluid (ECF).

Protein Buffer System

The largest buffer system in the body, utilizing albumin and plasma globulins; it is amphoteric.

Amphoteric

The ability of a substance, such as proteins, to function as either an acid or a base.

Bone Buffer System

A mechanism where $H^+$ is exchanged for $Na^+$ and $K^+$; the dissolution of bone results in the release of $NaHCO_3$ and $CaCO_3$.

Hyperventilation

Increased ventilation that leads to increased $CO_2$ elimination, causing the body to "blow off" $CO_2$.

Hypoventilation

Decreased ventilation that results in decreased $CO_2$ elimination, causing the body to retain $CO_2$.

Renal $K^+/H^+$ exchange in Acidosis

A renal mechanism that increases $H^+$ elimination and retains $K^+$, leading to increased potassium levels.

Renal $K^+/H^+$ exchange in Alkalosis

A renal mechanism that retains $H^+$ and increases $K^+$ elimination, leading to decreased potassium levels.

Ammonia Buffer System

A renal control mechanism involved in the generation of new bicarbonate ($HCO_3^-$).

Phosphate Buffer System

A renal control mechanism located in the tubular lumen that assists in pH regulation.

Carbonic Acid Equilibrium Equation

$$CO_2 + H_2O \rightleftharpoons H_2CO_3 \rightleftharpoons H^+ + HCO_3^-$$