Blood Composition and Plasma Functions

Blood Composition

• Blood is a living tissue, part of the cardiovascular system
• It transports nutrients, respiratory gases, waste, hormones, and regulates temperature
• Blood accounts for ~7% of body mass
• Whole blood contains plasma, white blood cells, platelets, and red blood cells

Plasma

• Plasma accounts for ~55% of blood volume
• Plasma is similar to interstitial fluid (ISF)
• Plasma is mostly water (~90%) with ions, nutrients, respiratory gases, and waste products
• Plasma contains proteins, including albumins, globulins, and fibrinogen
• Albumin is the most abundant plasma protein and contributes the most to the colloidal osmotic pressure (C.O.P.)

Colloidal Osmotic Pressure

• Important in fluid balance between plasma and ISF
• Plasma proteins do not diffuse across the capillary wall, they contribute to the C.O.P.
• C.O.P. is ~25 mm Hg

Starling Forces

• Hydrostatic pressure and C.O.P. determine the movement of fluid across the capillary wall
• Filtration is the movement of fluid from capillaries to ISF, driven by hydrostatic pressure
• Osmotic flow is the movement of fluid from ISF to capillaries, driven by C.O.P.
• Excess filtered fluid is drained by the lymphatic system

Lymphatic System

• Network of vessels that collect excess fluid from ISF and return it to the bloodstream
• Lymphatic vessels are highly permeable to all ISF constituents, including proteins
• Lymphatic drainage is important for maintaining fluid balance and removing proteins from the ISF

Capillary Exchanges

• Nutrients and wastes move by diffusion
• Starling forces determine the distribution of fluid between plasma and ISF
• Filtration pushes fluid out of capillaries into ISF
• Osmotic forces pull water back into the capillaries
• Lymphatic system drains excess filtered fluid

Blood Functions

• Transportation of nutrients, respiratory gases, wastes, hormones
• Regulation of temperature
• Acid-base balance (normal pH range: 7.35 - 7.45)
• Protection through white blood cells and plasma proteins

Blood Composition

• Blood is a highly dynamic tissue, part of the cardiovascular system.
• About 7% of body mass.
• Composed of plasma, a type of extracellular fluid (ECF).
• Also contains intracellular fluid (ICF) within blood cells.

Blood Volume

• Normal blood volume is normovolemia.
• Hypovolemia describes lower than normal blood volume.
• Hypervolemia describes higher than normal blood volume.

Hematocrit

• Hematocrit (Ht) is the percentage of blood volume occupied by red blood cells.
• Normal value is approximately 45%.
• It is a useful clinical index.

Plasma Composition

• Approximately 90% water.
• Contains ions: Na+, K+, Ca++, Mg++, Cl-, HCO3-, PO4--
• Can be approximated by 0.9% NaCl physiological saline solution.
• Contains nutrients, respiratory gases, and wastes: glucose, amino acids, lipids, O2, CO2, urea, lactic acid.
• Contains proteins (colloids): Albumin, Globulins, Fibrinogen.

Plasma Protein Groups

• Albumin, globulins, and fibrinogen
• Synthesized in the liver, except for gamma globulin which originates in lymphoid tissues.
• Contribute to the colloidal osmotic pressure (C.O.P.) – also known as oncotic pressure.
• Disease of the liver results in decreased plasma protein levels.

Electrophoresis

• A fractionation method based on the movement of charged particles along a voltage gradient.
• Rate of migration is influenced by the number and distribution of charges as well as molecular weight (MW) of each protein.
• Each protein migrates at a characteristic rate.
• Serum electrophoresis is a pattern showing the presence of proteins in serum, which is plasma with fibrinogen removed.

Transcapillary Dynamics

• Refers to the exchange of fluids between capillaries and the interstitial fluid (ISF).
• The movement of molecules across the capillary wall is significantly influenced by pressure differences and the osmotic pressure.
• Pressure differences drive the movement of fluid through filtration.
• Osmotic pressure, particularly due to proteins in the blood, leads to the movement of fluid into the blood.

Starling Forces

• The major forces determining fluid movement between plasma and ISF.
• These forces are filtration and osmotic flow.

Hydrostatic Pressure

• The pressure exerted by fluids inside the capillaries.
• Tends to “push out” fluid from inside the capillaries into the ISF.

Osmotic Flow

• Generated by plasma proteins which tend to “pull in” fluid into capillaries.

Lymphatic System

• A network of vessels responsible for draining excess fluid from the ISF back into the bloodstream.
• Walls are highly permeable to all ISF constituents, including proteins.

Colloidal Osmotic Pressure (C.O.P.)

• The osmotic pressure exerted by proteins in plasma, pulling fluid into the capillaries.
• Each protein fraction exerts an osmotic pressure directly related to its concentration in the plasma and inversely related to its molecular weight.
• Albumin contributes the most to the C.O.P., due to its high concentration and relatively small size.
• The osmotic pressure of a solution depends on the number of osmotically active particles per unit volume, not their configuration, size, or charge.