Chapter 26

FLUID, ELECTROLYTE, AND ACID-BASE BALANCE

BODY FLUIDS AND FLUID COMPARTMENTS

Section 26.1
BODY WATER CONTENT

  • Varies based on: weight, age, and sex

    • Early embryo: 97%

    • Newborn: 77%

    • Adult male: 60%

    • Adult female: 50%

    • Elderly: 45%

  • Differences are largely determined by adipose tissue vs. skeletal muscle.

Water Content of Body Organs and Tissues

  • Water content varies among organs and tissues:

    • Teeth: 8%

    • Brain: 85%

FLUID COMPARTMENTS

Fluid Compartments in the Human Body

  • Intracellular fluid (ICF): Fluid within cells

  • Interstitial fluid (IF): Part of extracellular fluid (ECF), found between cells

  • Blood plasma: Second component of ECF

  • Materials travel between cells and plasma in capillaries through IF.

Proportion of Total Body Fluid

  • Most body water is intracellular fluid.

  • Interstitial fluid: second largest volume, surrounding non-blood cells.

COMPOSITION OF BODY FLUIDS

Electrolytes

  • Cations: sodium (Na+), potassium (K+), hydrogen (H+), magnesium (Mg2+), calcium (Ca2+)

  • Anions: chloride (Cl-), bicarbonate (HCO3-), phosphate (PO4^3-), sulfate (SO4^2-)

Non-electrolytes

  • Glucose

  • Urea

  • Protein

ELECTROLYTE MINERAL ABSORPTION Mechanisms

  • Na+: Channel-mediated diffusion, cotransport, or active transport

  • Ca2+: Active transport

  • K+: Channel-mediated diffusion

  • Mg2+: Active transport

  • Fe2+: Active transport

  • Cl-: Channel-mediated diffusion or carrier-mediated transport

  • I-: Channel-mediated diffusion or carrier-mediated transport

  • HCO3-: Channel-mediated diffusion or carrier-mediated transport

  • NO3-: Channel-mediated diffusion or carrier-mediated transport

  • PO4^3-: Active transport

  • SO4^2-: Active transport

  • Lipids and Creatinine are also includes.

ELEMENTS IN KEY BODILY FLUIDS

  • Composition varies among ICF, IF, and plasma:

    • Plasma and IF compositions are similar; both differ significantly from ICF.

CAPILLARY EXCHANGE

Capillary Exchange

  • Net filtration occurs near the arterial end of capillaries:

    • Capillary hydrostatic pressure (CHP) is greater than blood colloidal osmotic pressure (BCOP).

  • Midpoint of capillary: No net movement of fluid (CHP = BCOP).

  • Net reabsorption: Near venous end; BCOP exceeds CHP.

FACILITATED DIFFUSION

Facilitated Diffusion

  • Example: Glucose utilizes facilitated diffusion to move down its concentration gradient via carrier protein channels in cell membranes.

SODIUM-POTASSIUM PUMP

  • The sodium-potassium pump is fueled by ATP and transfers sodium (Na+) out of the cytoplasm into the ECF while moving potassium (K+) into the cytoplasm from ECF.

WATER BALANCE

Section 26.2
ABSORPTION OF WATER

  • Dietary input:

    • Food and drink: 2000 mL

    • Digestive secretions:

    • Saliva: 1500 mL

    • Gastric secretions: 1500 mL

    • Liver (bile): 1000 mL

    • Pancreas (pancreatic juice): 1000 mL

    • Intestinal secretions: 2000 mL

    • Total Water Intake: 9000 mL

    • Reabsorption by small intestine: 7800 mL

    • Colon: 1250 mL reabsorbed, with 150 mL lost in feces.

THIRST RESPONSE

  • Triggered by:

    • Insufficient water in the body

    • Decreased blood volume

    • Increased blood osmolality

    • Decreased blood pressure

  • Osmoreceptors in hypothalamus, dry mouth, and response of thirst center triggered by angiotensin II.

ANTIDIURETIC HORMONE (ADH)

  • ADH produced in hypothalamus, released by posterior pituitary.

  • Functions include:

    • Causing kidneys to retain water

    • Constricting arterioles in peripheral circulation

    • Affecting social behaviors in mammals.

AQUAPORINS

  • ADH binds to receptors on collecting tubule cells, resulting in aquaporins insertion into the plasma membrane, greatly increasing water flow into the bloodstream.

DISTURBANCES IN WATER BALANCE

  • Conditions include:

    • Dehydration

    • Diabetes insipidus

    • Hypotonic hydration.

ELECTROLYTE BALANCE

Section 26.3
ELECTROLYTE BALANCE Components

  • Regulation of:

    • Sodium

    • Potassium

    • Chloride

    • Calcium

    • Phosphate

REGULATION OF SODIUM

  • Involves:

    • Aldosterone

    • Atrial Natriuretic Peptide (ANP)

    • Hormonal influences:

    • Estrogen enhances Na+ reabsorption

    • Progesterone decreases Na+ reabsorption

    • Glucocorticoids enhance Na+ reabsorption.

ALDOSTERONE FEEDBACK LOOP

  • Aldosterone, released by adrenal gland, facilitates Na+ reabsorption and water retention.

RENIN-ANGIOTENSIN SYSTEM for ALDOSTERONE PRODUCTION

  1. Kidney releases renin into blood.

  2. Liver releases angiotensinogen into blood.

  3. Angiotensin I be converted to Angiotensin II via angiotensin-converting enzyme (ACE) in pulmonary blood.

  4. Angiotensin II stimulates aldosterone secretion from adrenal cortex.

  5. Aldosterone promotes Na+ and H2O reabsorption in nephrons.

REGULATION OF POTASSIUM

  • K+ levels move opposite of Na+ and are also regulated by:

    • Aldosterone

    • ANP

REGULATION OF CHLORIDE

  • Chloride regulation resembles Sodium:

    • Aldosterone increases blood chloride levels

    • ANP decreases blood chloride levels.

REGULATION OF CALCIUM

  • Parathyroid Hormone (PTH): Raises blood calcium levels.

    • Increased osteoclast activity, absorption in intestines, and renal reabsorption.

  • Calcitonin: Lowers blood calcium levels via decreased osteoclast activity and absorption in intestines.

REGULATION OF PHOSPHATE

  • Regulates in opposition to Calcium:

    • PTH: Decreases plasma phosphate while increasing calcium

    • Calcitonin: Increases plasma phosphate while decreasing calcium.

PTH AND BLOOD CALCIUM

  • PTH ensures calcium homeostasis by raising calcium when low and vice versa for calcitonin.

ACID-BASE BALANCE

Section 26.4
pH SCALE

  • Normal ECF pH range: 7.35 to 7.45.

  • Conditions:

    • pH < 7.35: Acidemia (acidosis)

    • pH > 7.45: Alkalemia (alkalosis).

    • Severe acidosis (pH < 7.0) may result in:

    • Central nervous system deterioration (coma)

    • Irregular cardiac contractions leading to heart failure

    • Peripheral vasodilation and reduced blood pressure, which may cause circulatory collapse.

IMPORTANT TERMS

  1. Buffer: Substance opposing pH changes; maintains normal pH limits in body fluids (7.35-7.45).

  2. Strong acids: Completely ionize (e.g., HCl).

  3. Weak acids: Partially ionize (e.g., H2CO3).

  4. Acids: Increase hydrogen ion concentration.

  5. Bases: Increase hydroxide ion concentration.

  6. Salts: Ionic compounds excluding H+ and OH-.

RESPIRATORY REGULATION OF BLOOD pH

  • The respiratory system can reduce blood pH by removing CO2, affecting acid-base balance.

BICARBONATE CONSERVATION

  • Tubular cells do not reabsorb bicarbonate; it is conserved instead.

CHEMICAL BUFFER SYSTEMS

  1. Phosphate buffer system: Important for ICF and urine buffering.

  2. Protein buffer system: Buffers pH in both ECF and ICF; includes hemoglobin and plasma protein buffers.

  3. Carbonic acid-bicarbonate system: Most important for extracellular buffering; involves HCO3- as weak base and H2CO3 as weak acid.

    • Example of chemical reaction:
      HCl + NaHCO3 → H2CO3 + NaCl.

PHOSPHATE BUFFERS

  • NaH2PO4 acts as weak acid, Na2HPO4 as weak base; Example:
    HCl + Na2HPO4 → NaH2PO4 + NaCl.

  • Hemoglobin buffers CO2 from cells while preventing significant pH changes.

PROTEIN BUFFERS

  • Powerfully abundant buffering system in both compartments.

  • Carboxyl groups = weak acids; amine groups = weak bases.

PHYSIOLOGICAL BUFFERING SYSTEMS

  • Regulatory mechanisms provided by:

    • Lungs

    • Kidneys.

DISORDERS

Section 26.5
EDEMA

  • Allergic reactions causing capillaries to leak excess fluid leading to tissue accumulation.

SYMPTOMS OF ACIDOSIS AND ALKALOSIS

  • Both can affect multiple organ systems and can be diagnosed via blood tests.

HOMEOSTATIC IMBALANCES

  • Includes various electrolyte imbalances:

    • Hypernatremia, Hyponatremia

    • Hyperkalemia, Hypokalemia

    • Hyperphosphatemia, Hypophosphatemia

    • Hyperchloremia, Hypochloremia

    • Hypercalcemia, Hypocalcemia

    • Hypermagnesemia, Hypomagnesemia

    • Hyperproteinemia, Hypoproteinemia.

MINERAL FUNCTION AND IMBALANCES

  • Calcium: Crucial for function; deficiencies lead to conditions such as Osteoporosis while excess leads to kidney stones.

  • Chlorine: Important for HCl production; deficiencies cause muscle cramps; excess causes vomiting.

  • Copper: Rare deficiency but important for enzyme functionality. Excess conditions seen in Wilson’s disease.

  • Magnesium: Needed for ATP formation; deficiency leads to muscle spasms. Excess causes diarrhea.

  • Potassium: Vital for muscle and nerve function; deficiency affects heart rhythm. Excess often due to renal failure.

  • Phosphorus: Important for bones; deficiency leads to weakness; excess can affect iron absorption.

  • Sodium: Crucial electrolyte for various body functions; deficiency leads to fatigue; excess can result in hypertension and edema.

ABNORMALITIES IN ACID-BASE BALANCE

  1. Metabolic acidosis:

    • Decreased HCO3-, increased H+, decreased pH

    • Causes: Diarrhea, ketosis, renal dysfunction

  2. Metabolic alkalosis:

    • Increased HCO3-, decreased H+, increased pH

    • Causes: Vomiting, diuretics, alkaline drug use

  3. Respiratory acidosis:

    • Increased CO2 leading to increased H+, decreased pH

    • Causes: Hypoventilation

  4. Respiratory alkalosis:

    • Decreased CO2 leading to decreased H+, increased pH

    • Causes: Hyperventilation.