At high altitude, the RBC in human blood
A. Increase in size
B. Decrease in size
C. Increase in number
D. Decrease in number

Hint: Due to a decrease in oxygen hemoglobin saturation at high altitudes, a hypoxic condition occurs that causes hypoxia-inducible factor 1 (HIF- 1) to become stable and stimulates the production of erythropoietin (EPO), which is a hormone secreted by the kidneys. EPO stimulates red blood cell production from bone marrow. This increases hemoglobin saturation and oxygen delivery.

Complete answer:
Option A- Increase in size
An increase in the size of RBC is known as Macrocytosis. It describes red blood cells that are larger than normal. They are also known as megalocytosis or microcythemia. This condition commonly causes no signs or symptoms and is usually detected incidentally on routine blood tests.

Option B- Decrease in size
RBCs are smaller than the usual size of RBCs and give rise to the condition known as Microcytic, hypochromic anemia. As the name suggests, it is the type of anemia which is circulating (microcytic) and has decreased red color (hypochromic).

Option C- Increase in number
To compensate for the decrease in Normal physiological changes, increases in the RBC count occur at high altitudes or after strenuous physical training. At high altitudes, the less atmospheric weight pushes air into the lungs which causes a decrease in the partial pressure of oxygen and hypoxia.

Option D- Decrease in number
A decrease in red blood cells causes condition anemia. It can occur from either a decrease in the number of red blood cells, a decrease in the hemoglobin content, or both.

Thus, the correct option is C- Increase in number.

Note: Red blood cells are also known as red cells, red blood corpuscles, RBCs, haematodes, erythroid cells, or erythrocytes. They are the most common type of blood cell. They are the vertebrate's principal means of delivering oxygen to the body tissues via blood flow through the circulatory system.