Human Physiology: Summary Notes & Self-Test Questions (Essentials of Medical Physiology) for USMLE 2024/2025 – Tested & Approved
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Human Physiology: Summary Notes & Self-Test Questions (Essentials of Medical Physiology) for USMLE 2024/2025 – Tested & Approved" is an expertly crafted study aid designed specifically for students preparing for the exams.
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❖ Total body water makes up 50%-70% of body weight, varying with body fat.
Leaner individuals and males typically have higher water content.
❖ Water is distributed between Intracellular Fluid (ICF) and Extracellular
Fluid (ECF).
○ ICF: Makes up two-thirds of total body water and exists within cells.
○ ECF: Comprises one-third of total body water, divided into plasma (in blood
vessels) and interstitial fluid (surrounding cells).
Composition Differences:
❖ ICF and ECF have distinct solute compositions:
➢ ICF: High in potassium (K⁺) and magnesium (Mg²⁺) with proteins and organic
phosphates as anions.
➢ ECF: Dominated by sodium (Na⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻).
❖ Electroneutrality is maintained within each compartment, balancing positive and
negative charges.
❖ pH and osmolarity are regulated, with pH lower in ICF than ECF.
Concentration Maintenance:
❖ Concentration gradients across cell membranes are sustained by active transport
mechanisms, primarily:
○ Na⁺-K⁺ ATPase pump: Maintains Na⁺ outside and K⁺ inside cells.
○ Ca²⁺ ATPase: Regulates intracellular Ca²⁺.
❖ These gradients are vital for physiological functions, including nerve/muscle cell action
potentials and nutrient absorption.
Importance of Gradients:
❖ Concentration gradients across cell membranes are essential for critical functions:
○ Resting membrane potential relies on K⁺ gradient.
○ Action potential upstroke depends on Na⁺ gradient.
○ Excitation-contraction coupling in muscles requires Ca²⁺ gradient.
○ Nutrient absorption (e.g., glucose) depends on Na⁺ gradient across
membranes.
,Concentration Differences Between Plasma and Interstitial Fluids
1. Protein Distribution:
▪ Plasma contains proteins (e.g., albumin) that are mostly excluded from
interstitial fluid due to their large size, creating a key compositional difference
between the two.
2. Gibbs-Donnan Equilibrium:
▪ The presence of negatively charged plasma proteins leads to a slight
redistribution of small ions across the capillary wall to maintain
electroneutrality.
▪ This effect causes plasma to have a slightly lower concentration of small anions
(e.g., Cl⁻) and a slightly higher concentration of small cations (e.g., Na⁺, K⁺)
compared to interstitial fluid.
3. Gibbs-Donnan Ratio:
▪ This ratio quantifies the minor concentration differences for permeant ions: for
example, the Cl⁻ concentration in plasma is 95% of that in interstitial fluid.
▪ Though these differences are present, they are often negligible in clinical
considerations.
Test yourself
❖ How does the distribution of total body water relate to body composition, and what
implications does this have for hydration needs in individuals with varying body fat
percentages?
❖ In what ways do the unique solute compositions of intracellular fluid (ICF) and
extracellular fluid (ECF) contribute to critical physiological functions within the body?
❖ How does the presence of plasma proteins, such as albumin, affect the distribution of
ions between plasma and interstitial fluid, and what role does the Gibbs-Donnan
equilibrium play in this process?
❖ Why is the Na⁺-K⁺ ATPase pump essential for maintaining concentration gradients
across cell membranes, and how do these gradients support cellular functions like
nerve signaling and muscle contraction?
❖ How do concentration differences between ICF and ECF influence the osmolarity of
body fluids, and how does the body restore osmolarity balance when it shifts?
, ❖ What mechanisms allow the body to maintain macroscopic electroneutrality in each
fluid compartment, and why is this balance crucial for normal physiological processes?
❖ In what ways might the differences in Ca²⁺ concentrations between ICF and ECF
impact cellular processes, particularly in muscle and nerve cells?
❖ How do selective permeabilities of cell membranes prevent the dissipation of
concentration gradients, and what might happen if cell membranes were permeable
to all substances?
❖ How does pH regulation in body fluids differ between intracellular and extracellular
compartments, and what are the physiological consequences of these differences?
❖ What are some examples of physiological processes that depend on the
concentration gradients established by the Na⁺-K⁺ pump, and how might disruptions
in this pump affect overall body function?
Composition of Cell Membranes
❖ Cell membranes consist mainly of lipids and proteins.
❖ The lipid component includes phospholipids, cholesterol, and glycolipids, which
create a selective barrier.
❖ This barrier allows lipid-soluble substances (e.g., carbon dioxide, oxygen, fatty acids)
to pass freely, while water-soluble substances (e.g., ions, glucose) are restricted.
❖ The membrane proteins include transporters, enzymes, hormone receptors, ion
channels, and cell-surface antigens, which serve various roles in cellular function and
communication.
Phospholipid Structure
Phospholipids are amphipathic molecules, meaning they have both hydrophilic (water-
attracting) heads and hydrophobic (water-repelling) tails.
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