HI09 - Blood and Fluid Compartments

What are the main components of blood?

Cells, ions, proteins, and platelets.

What is hematocrit?

The percentage of blood volume composed of erythrocytes.

What is the average hematocrit in men vs women?

45% in men, 40% in women.

What is the most abundant plasma protein?

Albumin.

What are the main plasma proteins?

Albumin, globulins, and fibrinogen.

What is the primary function of albumin?

Maintains oncotic pressure and binds various substances for transport.

What are globulins?

Carrier proteins and immunoglobulins (antibodies).

What is fibrinogen's function?

A precursor to fibrin, essential for blood clotting.

How is total body water (TBW) measured?

Using tritiated water (³H₂O).

How is extracellular fluid (ECF) measured?

Using inulin or ²²Na⁺.

How is intracellular fluid (ICF) determined?

By subtracting ECF from TBW (ICF = TBW - ECF).

How is plasma volume measured?

Using ¹²⁵I-albumin or Evan's blue dye.

How is interstitial fluid volume determined?

ISF = ECF - plasma volume.

What are the other fluid compartments besides ECF and ICF?

Lymph, bone and dense connective tissue fluid, and transcellular fluid.

What is transcellular fluid?

Fluid within epithelial-lined spaces like synovial, pleural, peritoneal, and CSF spaces.

What are the three 'fluid spaces'?

First (intravascular), second (interstitial/intracellular), and third (non-exchangeable spaces like peritoneum).

What causes third-spacing of fluid?

Fluid accumulation in non-exchangeable compartments, such as in severe burns or ascites.

What is the major cation in the ECF?

Sodium (Na⁺).

What anions balance sodium in the ECF?

Chloride and bicarbonate.

What is the major cation in the ICF?

Potassium (K⁺).

What balances potassium in the ICF?

Organic anions and proteins.

What are the main routes of fluid loss?

Sweat, insensible loss (skin and respiration), and feces.

What is insensible water loss?

Water loss through skin and respiration (~800 mL/day), cannot be prevented.

What percentage of GI secreted water is reabsorbed?

About 98%.

What are the key factors in fluid balance?

Osmolarity, hydrostatic pressure, oncotic pressure, sodium balance, and water balance.

Define osmolarity.

Osmoles per liter of solution.

Define osmolality.

Osmoles per kilogram of water.

Where does most fluid exchange occur?

At the capillaries.

What type of cells make up capillary walls?

A single layer of endothelial cells with a basement membrane.

What are interendothelial junctions?

Tight or adherens junctions between endothelial cells controlling permeability.

What increases capillary leakiness during inflammation?

Histamine and cytokines that widen endothelial gaps (100-400 nm).

How do gases and small nonpolar molecules cross capillaries?

By diffusion through endothelial cells (transcellular).

How do small polar molecules cross capillaries?

Through water-filled pores or clefts between cells.

How do macromolecules like proteins cross capillaries?

Through large pores, fenestrations, or by vesicular transcytosis (caveolae).

What drives capillary water movement?

Hydrostatic and osmotic (oncotic) pressures.

What is capillary hydrostatic pressure at the arterial end compared to the venous end?

About 35 mmHg at the arterial end and only 15 mmHg at the venous end.

What is the function of plasma osmotic pressure?

Pulls fluid back into capillaries due to plasma proteins.

What is the result of higher hydrostatic pressure at the arterial end?

Filtration.

What is the result of higher oncotic pressure at the venous end?

Reabsorption.

What are the two ways ECF homeostasis is maintained?

By regulating osmolarity (water) and volume (sodium).

Which receptors sense changes in blood volume?

Baroreceptors.

Which receptors sense changes in osmolarity?

Osmoreceptors.

Where are primary osmoreceptors located?

In the hypothalamus.

What does the Renin-Angiotensin-Aldosterone System (RAAS) do?

Increases sodium reabsorption and raises blood volume/pressure.

What triggers RAAS activation?

Low blood pressure or low sodium.

What is aldosterone's main effect?

Reabsorbs sodium in the kidney.

What hormone has the opposite effect of aldosterone?

Atrial Natriuretic Peptide (ANP).

What does ANP do?

Promotes sodium and water excretion.

What is ADH also called?

Arginine vasopressin.

Where is ADH produced and released?

Produced in the hypothalamus; released by the posterior pituitary.

What stimulates ADH release?

High plasma osmolarity or low blood volume.

What are ADH's effects?

Vasoconstriction and increased water reabsorption.

What are isotonic alterations?

Gain or loss of ECF without changing osmolarity.

What causes isotonic fluid loss?

Hemorrhage, sweating, or inadequate intake.

What causes isotonic fluid excess?

Excess IV fluids or aldosterone secretion.

What are hypertonic alterations?

ECF osmolarity greater than normal.

What causes hypernatremia?

Water loss (dehydration) or Na⁺ gain.

What is a consequence of hypernatremia?

Cell shrinkage.

What are hypotonic alterations?

ECF osmolarity below normal.

What causes hyponatremia?

Na⁺ loss or water gain (SIADH, adrenal insufficiency, water intoxication).

What is a consequence of hyponatremia?

Cell swelling.

What is the function of the lymphatic system?

Removes excess fluid and proteins from interstitial spaces and returns them to circulation.

How much fluid is filtered but not reabsorbed by capillaries daily?

About 2 liters.

Where does lymph drain back into the bloodstream?

At the inferior vena cava (via thoracic duct).

How do lymphatic capillaries differ from systemic capillaries?

They have larger openings, thin walls, and one-way valves.

What structural feature allows lymphatics to reabsorb large molecules like proteins?

Overlapping endothelial flaps that act as one-way valves.

What determines lymph flow?

Interstitial fluid pressure—higher pressure increases lymph flow.

What is edema?

Excess fluid accumulation in body tissues.

What are the two types of edema?

Intracellular and extracellular.

What causes intracellular edema?

Hyponatremia, reduced metabolism, or inflammation.

Why is intracellular edema dangerous in the CNS?

It increases intracranial pressure.

What causes extracellular edema?

Increased capillary filtration or lymphatic obstruction.

What are the main causes of extracellular edema?

High capillary pressure, low plasma proteins, increased permeability, or blocked lymph return.

What causes increased capillary pressure?

• High blood volume (hypernatremia)

• High venous pressure (from heart failure, venous obstruction)

• Decreased arteriolar resistance (from impaired sympathetics or vasodilator drugs)

What causes decreased plasma proteins?

Protein loss in urine or liver failure.

What increases capillary permeability?

Inflammation, infection, or immune reactions.

What causes lymphatic obstruction?

Cancer, infection, or surgical removal of lymph nodes.

What is pitting edema?

Edema where pressure leaves a persistent indentation. Caused by extracellular edema.