The colloid osmotic pressure in the capillary is caused by __________.

• A. plasma proteins such as albumin
• B. dissolved salts like sodium chloride
• C. red blood cells
• D. glucose molecules

Answer: (A)

Verified step by step guidance

1. Understand that colloid osmotic pressure (also called oncotic pressure) is a form of osmotic pressure exerted by proteins, especially large molecules, in a blood vessel's plasma that tends to pull water into the circulatory system.
2. Recognize that dissolved salts like sodium chloride and small molecules such as glucose do not significantly contribute to colloid osmotic pressure because they can freely pass through capillary walls and do not create a sustained osmotic gradient.
3. Recall that red blood cells are cellular components and do not directly contribute to osmotic pressure; instead, osmotic pressure is related to solutes dissolved in plasma.
4. Identify that plasma proteins, particularly albumin, are large molecules that cannot easily cross capillary membranes, thus they remain in the blood and exert an osmotic pull that draws water into the capillaries.
5. Conclude that the colloid osmotic pressure in capillaries is primarily caused by plasma proteins such as albumin.