Study sheet: Physiology of Body Fluid Compartments

📋 Course Outline

1. Physiology and Biological Organization
2. Body Water and Homeostasis
3. Measuring Body Fluid Volumes
4. Chemical Composition and Concentrations
5. Water Exchange and Osmosis
6. Tonicity and Red Blood Cells
7. Fluid Shifts Between Compartments
8. Plasma and Interstitial Exchange

📖 1. Physiology and Biological Organization

🔑 Key Concepts & Definitions

• Physiology : the study of the roles, functions, and interactions of living organisms, their organs, tissues, cells, and organelles with one another and with their environment.
• Physiological variable : a biological measurement that can change over time.

📝 Essential Points

• 🔄 The levels of organization are:
  1. chemical
  2. cellular
  3. tissue
  4. organ
  5. systemic
  6. whole organism

📌 A tissue is a group of similar cells performing the same specialized function, an organ is a coordinated group of tissues performing a defined function, and a system is a group of organs or tissues forming a functional unit.

💡 Memory Hook

Chemical → cellular → tissue → organ → system → organism

📖 2. Body Water and Homeostasis

🔑 Key Concepts & Definitions

• Homeostasis : Claude Bernard — the capacity of an open or closed system to preserve functional equilibrium despite imposed internal or external constraints.

★ Must-know

• In adults, approximately 50% to 70% of the body consists of fluid, and the course assumes a value of 60%.

• The extracellular fluid volume consists of plasma and interstitial fluid and represents 45% of total body water.

• A plasma sodium concentration below 120 mmol/L is hyponatremia, which makes plasma hypotonic and can cause dangerous cerebral cellular swelling.

• A plasma potassium concentration above 5 mmol/L is hyperkalemia, which depolarizes cells and can cause fatal arrhythmias.

Further detail

• The percentage of body water decreases with age and is lower in women than in men because women have less muscle mass.

💡 Memory Hook

Perturbation → regulatory response → return toward dynamic equilibrium

📖 3. Measuring Body Fluid Volumes

🔑 Key Concepts & Definitions

• Hematocrit : the percentage of total blood volume occupied by red blood cells.

★ Must-know

• Body fluid volumes are measured indirectly by allowing a marker to dilute within the compartment of interest.

• A suitable marker must: distribute homogeneously in the studied compartment, not enter other compartments, not be metabolized or synthesized, be non-toxic, be measured rapidly, simply, and reproducibly

• Blood volume is the sum of plasma volume and blood-cell volume, with average values of 3 L for plasma volume, 40% to 45% for hematocrit, and 5 L for total blood volume.

Further detail

• In the marker example, injecting 20 mL of a 1% solution provides 200 mg of dye, and a plasma concentration of 0.060 mg/mL gives a plasma volume of 3.33 L and, with a hematocrit of 70%, a blood volume of 11.1 L.

💡 Memory Hook

A marker spreads through one compartment like dye in a measured container

📖 4. Chemical Composition and Concentrations

🔑 Key Concepts & Definitions

• Electrolyte : an electrically charged compound present among the solutes of body fluids.

★ Must-know

📌 Mass concentration is mass per volume, molar concentration is amount of substance per solution volume, molal concentration is amount of substance per solvent mass, and equivalent concentration counts electrical charge per volume.

• Osmolar concentration counts particles per solution volume, whereas osmolal concentration counts particles per solvent mass.

• One mole of glucose corresponds to one osmole because it remains one particle, whereas one mole of NaCl corresponds to two osmoles because it produces Na+ and Cl− particles.

📌 Plasma and interstitial fluid have similar electrolyte composition and osmolarity, but plasma contains much more protein: 72 g/L versus 2 g/L in interstitial fluid.

Further detail

• Examples of cations are Na+, K+, Mg2+, and H+, whereas examples of anions are Cl−, HCO3−, and proteins.

💡 Memory Hook

Moles count matter; osmoles count particles

📖 5. Water Exchange and Osmosis

🔑 Key Concepts & Definitions

• Osmosis : the passive movement of water toward the compartment with the higher osmole concentration to dilute it and equalize concentrations.
• Aquaporin : a tetrameric membrane protein that provides a pathway for water movement.

📝 Essential Points

• Across a semipermeable membrane that allows water but not solutes to pass, water moves passively from the medium with fewer osmoles toward the medium with more osmoles.

• Water crosses cell membranes and capillary walls through the lipid bilayer and mainly through specific membrane channels called aquaporins.

💡 Memory Hook

Osmole concentration gradient → passive water movement → concentration equalization

📖 6. Tonicity and Red Blood Cells

★ Must-know

• In a hypotonic medium, a red blood cell takes up water, swells, and may burst because the cell is more concentrated than its environment.

• In an isotonic medium, water movements into and out of a red blood cell balance and the cell remains normal.

• In a hypertonic medium, water leaves a red blood cell toward the external medium, causing the cell to shrink and become wrinkled.

Further detail

• Plasma osmolarity is used as the reference osmolarity in medicine.

💡 Memory Hook

Hypotonic: cells swell; isotonic: cells remain stable; hypertonic: cells shrink

📖 7. Fluid Shifts Between Compartments

★ Must-know

• After drinking pure water, extracellular volume increases, extracellular osmolarity decreases, water shifts from plasma to interstitial fluid and then into cells, and the final state is global overhydration with increased body water and weight.

• Urinary loss of a hypertonic fluid decreases plasma and extracellular volume, lowers extracellular osmolarity, shifts water from plasma to extracellular fluid and then into cells, and produces extracellular dehydration with intracellular overhydration.

• Gain of isotonic fluid increases extracellular volume without changing osmolarity or osmole quantity because the fluid remains in the plasma.

Further detail

• Pure-water gain can cause nausea, aversion to water, hypertension, and increased diuresis to restore volume and osmolarity.

• Urinary loss of a hypertonic fluid decreases total body water, body weight, and osmole quantity and can cause a fall in blood pressure.

• Excessive isotonic-fluid gain can cause edema when interstitial fluid volume increases by more than 30%.

💡 Memory Hook

Initial extracellular change → water redistribution → final volume and osmolarity

📖 8. Plasma and Interstitial Exchange

📝 Essential Points

• The skin removes water through sweat, which is a hypotonic fluid richer in water than salt, whereas the kidneys remove water through urine, which is described here as hypertonic and richer in salt than water.

📊 Synthesis Tables

Tonicity Effects on Red Blood Cells

External medium	Water movement	Cell outcome
Hypotonic	Water enters the cell	Swelling and possible rupture
Isotonic	Balanced movement	Normal volume
Hypertonic	Water leaves the cell	Shrinking and wrinkling

Fluid Gain and Loss Effects

Situation	Osmolarity	Main distribution
Pure-water gain	Decreases	Water moves into cells
Hypertonic-fluid urinary loss	Extracellular osmolarity decreases	Water moves into cells
Isotonic-fluid gain	Unchanged	Fluid remains in plasma