Homeostasis and Fluid Balance 1

Homeostasis and Fluid Balance

The Goal: Homeostasis

• The primary goal in patient care, especially in the ICU, is to restore homeostasis.

• Patients require care because they've lost homeostasis.

• All interventions aim to bring patients back to a state of balance.

Universal Solvent: Water

• Fluid balance equates to water balance.

• Assess if the patient has adequate water and if it's correctly distributed.

Water Distribution

• Most water resides inside cells (intracellular).

• Administering sterile water intravenously can be fatal due to osmosis, which could lead to cell lysis.

• The aim is to hydrate cells without altering red blood cell shape.

• Approximately two-thirds of body water is intracellular.

• The remaining one-third is extracellular fluid (ECF).

Extracellular Fluid (ECF)

• ECF includes intravascular fluid (blood).

• Intravascular means inside blood vessels.

• One-third of water volume is in the blood; two-thirds is in interstitial fluid around cells.

Hydration and Blood Pressure

• Volume correlates with blood pressure.

• Low blood pressure often indicates low blood volume (insufficient water in the blood).

• Kidneys and lungs play critical roles in correcting fluid balance.

• Arterial blood gases (ABGs) reflect lung and kidney function.

Isotonic Solutions

• Ideally, administer isotonic solutions to maintain blood cell shape and volume.

• Minimum: 0.9% sodium chloride.

• Rehydration involves hypotonic solutions to drive water into cells.

Dehydration

• In dehydration, cells shrink.

• Infusing fluids aims to hydrate cells, not just blood.

• Hypotonic solutions (e.g., lactated Ringer's, 0.9% sodium chloride) facilitate water movement into cells via osmosis.

• Osmosis is passive diffusion to hydrate cells.

Blood Composition

• When blood sits, it separates into components.

• Red blood cells and other formed elements sink to the bottom.

• Plasma (yellowish fluid) sits at the top.

Plasma

• Water-based and contains hormones, nutrients, and electrolytes.

• Nonliving.

• Doctors specify serum, whole blood, or plasma based on testing needs.

• Plasma is water-based; dehydration reduces plasma volume.

Barriers and Perfusion

• Systemic circulation occurs at capillaries.

• Oxygen exits blood to enter tissues, exchanging for carbon dioxide.

• Capillary and tissue linings are thin to facilitate gas exchange.

• Simple squamous epithelium lines blood vessels (endothelium).

• Interstitial space lies between tissues.

• Thin linings allow water retention in compartments while enabling gas exchange.

Hydrostatic vs. Osmotic Pressure

• Hydrostatic pressure is a "push" pressure, determined by systolic blood pressure.

• Blood pressure indicates how much oxygen is pushed into cells.

• Osmotic pressure is a "pull" pressure.

• Solutes, especially plasma proteins, dictate osmotic pressure.

Plasma Proteins

• Plasma proteins (albumin, fibrinogen, globulins) are key determinants of osmotic pressure.

• Albumin is a major protein influencing water pull.

• Liver synthesizes plasma proteins.

• Liver failure reduces protein production, impeding water reabsorption into blood vessels, causing edema.

Electrolytes

• Electrolytes are charged particles (ions) in water.

• Charges are positive (cations) or negative (anions).

• Cations are positively charged (e.g., sodium, potassium, calcium).

• Anions are negatively charged.

• Anion gap indicates electrolyte imbalance with excess anions.

Action Potentials

• Action potentials are driven by sodium and potassium (cations).

• Sodium and potassium are abundant cations vital for survival.

• Chloride is the most abundant anion.

• 0.9% sodium chloride provides key positive and negative ions.

Phosphates

• Phosphate groups are essential for ATP (energy currency) production.

• ATP: adenosine triphosphate.

• Phosphates are concentrated intracellularly.

Cation Distribution

• Sodium is abundant outside cells.

• Potassium is abundant inside cells.

• This distribution is essential for action potentials.

• Calcium is tightly regulated and readily mobilized when needed.

Water Gain and Loss

• Water balance involves gains and losses.

Water Gain

• Sources: drinking and eating.

• Watermelon, cucumbers, and citrus fruits contribute to water intake.

• Water is a byproduct of metabolism.

• Glucose + Oxygen \rightarrow ATP + CarbonDioxide + Water

Water Loss

• Primary loss: urination.

• Evaporation from skin (sweat) and exhalation.

• Exhaled breath contains humidity (visible in cold conditions).

• Homeostasis requires balanced water intake and loss.

Dehydration Awareness

• Mild, moderate, and severe cases exist.

• Intervention varies based on severity.

• Prevent dehydration by drinking enough water, especially in hot weather.

• Replenish minerals when consuming reverse osmosis water.

Thirst Center: Hypothalamus

• The hypothalamus regulates body temperature and thirst.

• It stores ADH (antidiuretic hormone) and oxytocin in the posterior pituitary.

Antidiuretic Hormone (ADH)

• ADH conserves water by reducing urination.

• When thirsty, the hypothalamus releases more ADH, signaling collecting ducts to retain water.

• Decreased ADH release increases urine output.

Receptors and Thirst

• Oral cavity osmoreceptors detect high solute concentration, signaling ADH release.

• Baroreceptors in blood vessels detect low blood pressure (low water volume), stimulating thirst.

• Atrial stretch receptors (Frank-Starling mechanism) detect blood volume returning to the heart; low stretch signals thirst.

Frank-Starling Mechanism

• Whatever blood leaves the heart must return to the heart.

• The cardiovascular system is a closed loop.

Alcohol and ADH

• Alcohol impairs ADH production, leading to increased urination and thirst.

Aldosterone and Sodium

• Aldosterone regulates sodium levels.

• Water follows sodium.

• Aldosterone comes from the adrenal gland (cortex), not medulla.

Atrial Natriuretic Peptide (ANP)

• The heart is an endocrine gland producing peptide hormones.

• ANP (atrial natriuretic peptide) and BNP (brain natriuretic peptide) regulate sodium.

• ANP is antagonistic to aldosterone.

• ANP increases sodium excretion, leading to water loss and decreased blood pressure.

Fluid Intoxication

• Caused by giving hypotonic solution to patients who don't need it. Can lead to cell lysis.

ABG Analysis and Blood pH

• Blood pH normal range: 7.35 to 7.45.

• pH < 7.35: Acidosis.

• pH > 7.45: Alkalosis.

• Lungs and kidneys regulate blood pH; lungs act faster.

Metabolic vs. Respiratory

• Metabolic: relates to kidney function.

• Respiratory: relates to lung function.

Bicarbonate Buffer System

• Buffers decrease pH change (not neutralize).

• Buffers maintain blood pH between 7.35 and 7.45.

• pH measures circulating hydrogen ions; more hydrogen ions indicate acidity.

Metabolic Acidosis

• High bicarbonate ion concentrations lead to lower blood pH.

• The more bicarbonate you have the more it functions as a base.

Carbon Dioxide and pH

• Low CO2 often results from hyperventilation because CO2 is exhaled with each breath.

• CO2 converts to carbonic acid in the blood, decreasing blood pH.

• High CO2 = more Carbonic Acid

Breathing and pH Control

• Respiratory rate impacts blood CO2 levels and pH.

• During panic attacks, holding breath increases CO2 and lowers pH. Regulate breathing.

Importance of Blood Draws

• If the patient cannot maintain their blood pH, they should be readmitted to the hospital.

Buffer Systems

• Protein-based systems are major buffers.

• Proteins are amino acid chains with amino groups (NH2) and acid groups (COOH).

• Proteins contribute to urea production and pH buffering.

Hemoglobin

• Remember that H2CO3 is carbonic acid.

• Remember glucose is C6H{12}O_6

• Carbonic anhydrase converts carbon dioxide to carbonic acid.

Case Study: Compensation

• Lungs and kidneys regulate pH together.

• If lungs have an issue, the kidneys fix it. If kidneys have an issue, the lungs fix it.

Metabolic Alkalosis

• Metabolic alkalosis means the kidneys are the problem. In alkalossi, pH > 7.45.

• Lungs compensate by altering ventilation rate.

• Goal: decrease pH.

• Change respiratory rate to change the carbonic acid in the blood.