Unit 1: Fluids and pH - Study Notes (NDMU/PAS504)
Fluid Compartments and Water Movement
Fluid is essential: about 60\% of body weight as body water; for a 70\text{ kg} adult male, Total Body Water (TBW) ≈ 42\ \text{L}.
TBW sources: \text{drinking fluids},\; ext{in ingestion of water in food},\; ext{oxidative metabolism}.
Water losses: \text{urine},\; ext{stool},\; ext{vaporized from skin and lungs}.
Two major fluid compartments
Intracellular fluid (ICF): about \frac{2}{3}\text{ of total fluid} (≈25\ \text{L}; sometimes listed as 25–28 L depending on source)
Extracellular fluid (ECF): about \frac{1}{3}\text{ of total fluid} (≈15\ \text{L})
Interstitial fluid (IF): ≈12\ \text{L} (≈80% of ECF)
Intravascular fluid (plasma): ≈3\ \text{L} (≈20% of ECF)
Other ECF compartments include cerebrospinal fluid (CSF), sweat, saliva, urine, lymphatic, synovial, intestinal, biliary, hepatic, pancreatic, pleural, peritoneal, pericardial, intraocular fluids, etc.
Major figures (typical values)
TBW ≈ 42\,\text{L}; ICF ≈ 25\,\text{L}; ECF ≈ 15\,\text{L}
Plasma volume ≈ 3\,\text{L} (≈20% of ECF)
IF ≈ 12\,\text{L} (≈80% of ECF)
Water movement between compartments
Water moves by osmosis through aquaporins (water channels in cell membranes)
Osmosis: movement of water across a membrane to reach solute concentration equilibrium
Aquaporins: specialized water channels enabling rapid water flux
Osmotic balance
ECF osmotic balance largely governed by Na+ (sodium)
ICF osmotic balance largely governed by K+ (potassium)
Concept to remember
Each fluid compartment has a distinct pattern of electrolytes; the key idea is that osmosis and active solute distribution drive water shifts to maintain homeostasis
Water movement between plasma and tissue (capillary beds)
Plasma and tissue are both ECF compartments; water and solutes move between plasma and interstitial tissue via capillary beds
Four forces determine fluid movement (net filtration/reabsorption):
Capillary hydrostatic pressure, P_c: pushes fluid OUT of capillaries
Capillary oncotic pressure, π_c: pulls fluid INTO capillaries (due to plasma proteins)
Interstitial hydrostatic pressure, P_i: pushes fluid INTO the capillary space
Interstitial oncotic pressure, π_i: pulls fluid OUT of capillaries (due to proteins/cells in tissue)
Net filtration pressure (NFP) formula (conceptual):
\text{NFP} = (Pc - \pic) - (Pi - \pii)
Typical forces (visualization on a normal capillary bed)
Blood pressure in capillary pushes fluid out at the arterial end; proteins pull fluid back in
Oncotic pressure by plasma proteins pulls fluid into capillaries; interstitial oncotic pressure pulls fluid out of capillaries
Interstitial hydrostatic pressure and capillary hydrostatic pressure are dynamic and vary by tissue and local conditions
Everyday capillary exchange in numbers
Normal daily filtration: about 20\ \text{L} of fluid filters from capillaries at the arteriolar end and flows through the interstitial space
Reabsorption: about 17\ \text{L} is reabsorbed at the venous end
Lymphatics: about 3\ \text{L/day} of fluid (and any leaked proteins) are removed by the lymphatic system
Almost 7× the total plasma volume filters daily; lymphatics help prevent edema by clearing remaining fluid
Filtration/reabsorption details (normal values)
Arteriolar end: capillary hydrostatic pressure ≈ 35\ \text{mmHg}; capillary oncotic pressure ≈ 25\ \text{mmHg}
Venous end: capillary hydrostatic pressure ≈ 15\ \text{mmHg}; capillary oncotic pressure ≈ 25\ \text{mmHg}
Net effect is reabsorption predominating at venule end; any surplus interstitial fluid is handled by lymphatics
Edema: accumulation of fluid in the interstitial space
Edema definition: accumulation of fluid in interstitial space
Common causes
Increased capillary hydrostatic pressure (e.g., venous obstruction)
Decreased capillary oncotic pressure (e.g., hypoalbuminemia from malnutrition or liver disease)
Increased capillary permeability (inflammation/immune response)
Lymphatic obstruction (lymphedema)
Sodium retention (increased fluid retention)
Edema in disease states
Congestive heart failure (CHF): ineffective pumping leads to venous congestion
↑ venule blood pressure impedes reabsorption at venous end; lymphatics overwhelmed; accumulation in tissues, especially legs
Nutritional edema (starvation): inadequate plasma proteins → ↓ oncotic pressure → fluid leaks out and is not reabsorbed
Concept summary for edema
Edema reflects imbalance between filtration and reabsorption; when filtration exceeds reabsorption and lymphatics cannot compensate, interstitial fluid accumulates
pH balance and buffers
pH basics
Normal arterial pH: 7.35-7.45
Acidosis: pH < 7.35
Alkalosis: pH > 7.45
Extremes: pH below 6.8 or above 7.8 are life-threatening
Key ions for acid-base balance
Chemically: hydrogen ions H^+ and hydroxide OH^-
Physiologically: hydrogen ions H^+ and bicarbonate HCO_3^-
Major chemical reaction relevant to physiology
\ce{CO2 + H2O <=> H2CO3 <=> HCO3^- + H^+}
Buffer systems: two lines of defense
First line: chemical buffers
Second line: physiologic buffers (organ systems that adjust pH by excreting/retaining acids/bases)
Chemical buffers
Bicarbonate system: \mathrm{HCO3^- \rightleftharpoons H2CO_3}
Phosphate system: \mathrm{HPO4^{2-} \rightleftharpoons H2PO_4^-}
Ammonia system: \mathrm{NH3 \rightleftharpoons NH4^+}
Proteins (e.g., hemoglobin, albumin, intracellular proteins) contribute buffering capacity
Major intracellular buffer: phosphate
Physiological buffers
Respiratory mechanism (short-term): lungs regulate by exhaling CO2
Lower pH stimulates respiration; higher pH suppresses respiration
Blood pH sensed by medulla; signals to diaphragm to adjust ventilation
Renal mechanism (long-term): kidneys adjust by excreting H+ and reabsorbing HCO3-
Also uses phosphate and ammonia buffers (NaHPO4- ⇄ NaH2PO4-, NH3 ⇄ NH4+)
Not required to memorize detailed mechanisms; understand these processes occur renally
pH imbalances and compensations
Four main categories (arterial pH):
Respiratory acidosis: pH < 7.35; elevated PaCO_2 due to ventilation depression
Respiratory alkalosis: pH > 7.45; decreased PaCO_2 due to hyperventilation
Metabolic acidosis: pH < 7.35; decreased HCO_3^- or increased noncarbonic acids
Metabolic alkalosis: pH > 7.45; increased HCO_3^-, often from loss of metabolic acids
Typical acid-base compensation (summary from slide)
Respiratory acidosis
Primary disturbance: ↑ PaCO_2
Compensation: kidneys conserve HCO_3^- and excrete H^+
Respiratory alkalosis
Primary disturbance: ↓ PaCO_2
Compensation: kidneys excrete HCO_3^- and retain H^+
Metabolic acidosis
Primary disturbance: ↓ HCO_3^- (or rise of noncarbonic acids)
Compensation: hyperventilation (↓PaCO_2)
Metabolic alkalosis
Primary disturbance: ↑ HCO_3^-
Compensation: hypoventilation (↑PaCO_2) and renal adjustments to conserve H^+ and excrete bicarbonate
Practical shorthand (from the chart):
If pH is low and PaCO2 is high: respiratory acidosis
If pH is high and PaCO2 is low: respiratory alkalosis
If pH is low with low HCO3-: metabolic acidosis (renal/respiratory compensation in play)
If pH is high with high HCO3-: metabolic alkalosis (respiratory compensation in play)
Common clinical examples (from slides)
Respiratory acidosis causes: hypoventilation due to brainstem suppression, chest wall dysfunction, parenchymal lung disease
Respiratory alkalosis causes: hyperventilation due to high altitude, fever, anxiety
Metabolic acidosis causes: lactic acidosis, diarrhea, renal failure, starvation, diabetic ketoacidosis
Metabolic alkalosis causes: vomiting, gastric suctioning, excessive bicarbonate intake, diuretic therapy
Review and practice questions (key takeaways)
Review objectives (concepts you should be able to explain):
Major ECF and ICF compartments and water movement between them
How edema arises from changes in the capillary forces and lymphatic drainage
The components and roles of buffer systems in pH regulation
Differences between chemical and physiological buffers; how the respiratory and renal systems contribute
The four main acid-base disorders and how the body compensates
Practice questions (with answers)
Question 1: A person with heart failure has edema in the lower legs and sacral area. The nurse suspects this condition is due to a(n):
Right answer: Increase in capillary hydrostatic pressure
Question 2: A person with chronic lung disease, productive cough, hypoventilation, headache, and muscle twitching presents; the nurse suspects:
Right answer: Respiratory acidosis
Question 3: A patient arrives with loss of consciousness, Kussmaul respirations, history of diabetes, and two days of vomiting/diarrhea; suspect:
Right answer: Metabolic acidosis
Question 4: The main regulator of osmotic balance in extracellular fluid (ECF) is:
Right answer: Na^+
Fluid Compartments and Water Movement
Fluid is essential: about 60\% of body weight as body water; for a 70\text{ kg} adult male, Total Body Water (TBW)
(\approx 42\ \text{L}). This percentage can vary with age (higher in infants, lower in elderly) and body fat (lower in obese individuals).TBW sources: drinking fluids (primary source), ingestion of water in food, and metabolic water produced through oxidative metabolism (cellular respiration, approximately 300-400\ \text{mL/day}).
Water losses: urine (regulated by kidneys, primary excretion route for solutes), stool (minor loss, but significant in diarrhea), and insensible losses (vaporized from skin via diffusion and sweating, and from lungs during breathing, which are not consciously perceived).
Two major fluid compartments, separated by the cell membrane:
Intracellular fluid (ICF): about \frac{2}{3}\text{ of total fluid} ((\approx 25-28\ \text{L})). This fluid is found within all body cells. It is rich in potassium (K^+), magnesium (Mg^{2+}), and phosphate (PO$_4^{3-}$). Proteins are also in higher concentration here than in ECF.
Extracellular fluid (ECF): about \frac{1}{3}\text{ of total fluid} ((\approx 14-15\ \text{L})). This fluid is outside the cells.
Interstitial fluid (IF): (\approx 12\ \text{L}) (approx. 80% of ECF). This fluid surrounds the cells, bathing them directly. Its composition is similar to plasma but with very little protein.
Intravascular fluid (plasma): (\approx 3\ \text{L}) (approx. 20% of ECF). This is the fluid component of blood. It is rich in sodium (Na^+), chloride (Cl^-), and bicarbonate (HCO$_3^-$), and contains a high concentration of proteins (albumins, globulins).
Other ECF compartments (transcellular fluids) include cerebrospinal fluid (CSF), synovial fluid in joints, peritoneal and pleural fluids, sweat, saliva, urine, lymphatic fluid, intestinal, biliary, hepatic, pancreatic, and intraocular fluids. These are typically small in volume but serve specialized functions.
Major figures (typical values for a 70\text{ kg} adult male):
TBW (\approx 42\ \text{L})
ICF (\approx 25-28\ \text{L})
ECF (\approx 14-15\ \text{L})
Plasma volume (\approx 3\ \text{L}) ((\approx 20\%) of ECF)
IF (\approx 11-12\ \text{L}) ((\approx 80\%) of ECF)
Water movement between compartments is primarily driven by osmotic gradients:
Water moves by osmosis through aquaporins (specialized water channels in cell membranes) from an area of lower solute concentration to an area of higher solute concentration.
Osmosis: The passive movement of water across a semipermeable membrane to equalize solute concentrations on both sides. This movement continues until osmotic equilibrium is reached.
Aquaporins: Integral membrane proteins that facilitate rapid water flux across cell membranes. Their presence allows cells to quickly adjust to osmotic changes.
Osmotic balance: The state where the solute concentration (and thus water activity) is equal across a semipermeable membrane.
ECF osmotic balance is largely governed by Na^+ (sodium) due to its high concentration and the Na^+/K^+-ATPase pump actively maintaining its gradient.
ICF osmotic balance is largely governed by K^+ (potassium), which is the most abundant intracellular cation, maintained by the same Na^+/K^+-ATPase pump.
Concept to remember: Each fluid compartment has a distinct pattern of electrolytes and proteins. The key idea is that active solute distribution (e.g., via ion pumps) and passive water shifts (osmosis) work in concert to maintain dynamic homeostasis and cell volume.
Water movement between plasma and tissue (capillary beds)
Plasma and interstitial fluid are both ECF compartments; water and solutes move freely between plasma and interstitial tissue via capillary beds. This exchange is crucial for delivering nutrients and removing waste products from cells.
Four Starling forces determine net fluid movement (net filtration or reabsorption) across the capillary wall:
Capillary hydrostatic pressure (P$_c$): The pressure exerted by blood within the capillaries. It pushes fluid OUT of the capillaries into the interstitial space. It is highest at the arterial end and decreases along the capillary.
Capillary oncotic pressure ((\pi_c)): The osmotic pressure exerted by plasma proteins (primarily albumin) within the capillaries. It pulls fluid INTO the capillaries from the interstitial space. Plasma proteins are too large to easily cross the capillary membrane.
Interstitial hydrostatic pressure (P$_i$): The pressure exerted by the fluid in the interstitial space. It pushes fluid INTO the capillary space. This pressure is normally very low or even slightly negative.
Interstitial oncotic pressure ((\pii)): The osmotic pressure exerted by proteins/cells in the interstitial tissue. It pulls fluid OUT of capillaries. Normally, the interstitial space contains very few proteins, so (\pii) is typically low.
Net Filtration Pressure (NFP) formula (Starling Equation): The balance of these four forces determines the direction and magnitude of fluid movement.
\text{NFP} = (Pc - Pi) - (\pic - \pii)
A positive NFP indicates net filtration (fluid moving out of the capillary).
A negative NFP indicates net reabsorption (fluid moving into the capillary).
Typical forces and their visualization on a normal capillary bed:
At the arterial end of the capillary, P$c$ is higher than (\pic), leading to net filtration of fluid out of the capillary.
At the venous end of the capillary, P$c$ has decreased significantly, becoming lower than (\pic), leading to net reabsorption of fluid into the capillary.
Oncotic pressure by plasma proteins ((\pi_c)) remains relatively constant along the capillary, pulling fluid into the capillaries.
Interstitial hydrostatic pressure (P$i$) and interstitial oncotic pressure ((\pii)) are generally low and relatively stable under normal conditions, but can vary by tissue and local conditions.
Everyday capillary exchange in numbers:
Normal daily filtration: Approximately 20\ \text{L} of fluid filters from capillaries at the arteriolar end and flows through the interstitial space, delivering nutrients and oxygen.
Reabsorption: Approximately 17\ \text{L} of this fluid (along with metabolic waste products) is reabsorbed back into the capillaries at the venous end.
Lymphatics: About 3\ \text{L/day} of the remaining excess interstitial fluid (and any leaked proteins that cannot be reabsorbed directly into the capillaries) are removed by the lymphatic system. This fluid, now called lymph, is eventually returned to the venous circulation.
The fact that almost 7\times the total plasma volume filters daily highlights the dynamic nature of capillary exchange. The lymphatic system is vital in preventing edema by clearing any remaining interstitial fluid and proteins.
Filtration/reabsorption details (normal values):
Arteriolar end: Capillary hydrostatic pressure (P$c$) (\approx 35\ \text{mmHg}); capillary oncotic pressure ((\pic)) (\approx 25\ \text{mmHg}). Net filtration pressure is positive, leading to fluid movement out.
Venous end: Capillary hydrostatic pressure (P$c$) (\approx 15\ \text{mmHg}); capillary oncotic pressure ((\pic)) (\approx 25\ \text{mmHg}). Net filtration pressure is negative, leading to net reabsorption of fluid into the capillary.
The net effect is that reabsorption predominates at the venule end, but since filtration slightly exceeds reabsorption, any surplus interstitial fluid is efficiently handled and returned to circulation by the lymphatics.
Edema: accumulation of fluid in the interstitial space
Edema definition: The abnormal accumulation of excess fluid in the interstitial space, leading to swelling of tissues.
Common causes (reflecting imbalances in Starling forces or lymphatic function):
Increased capillary hydrostatic pressure (P$_c$): This pushes more fluid out of the capillaries.
Examples: Venous obstruction (e.g., deep vein thrombosis), right-sided heart failure (leading to increased venous pressure in systemic circulation), left-sided heart failure (leading to pulmonary edema), kidney disease (fluid overload), or prolonged standing.
Decreased capillary oncotic pressure ((\pi_c)): This reduces the pull of fluid into the capillaries.
Examples: Hypoalbuminemia (low plasma protein levels) due to malnutrition, liver disease (impaired albumin synthesis), kidney disease (nephrotic syndrome, where proteins are abnormally excreted in urine), or severe burns (plasma protein loss from damaged skin).
Increased capillary permeability: Capillary walls become