Red Blood Cell Production

Haematocrit

The share of blood volume made of red cells. Thicker blood carries more oxygen and clots more easily, and this is the single most reliably moved number on cycle.

Haemoglobin

Oxygen-carrying protein in red cells. Moves with haematocrit and is the second half of the same reading.

Androgen Receptor

The receptor every anabolic-androgenic steroid is designed to hit. Binding moves it into the nucleus where it switches on genes for muscle protein synthesis, and in other tissues for hair loss, prostate growth and oil production.

Bone Marrow

Where red cells are built. Androgens push it harder, which is why they were once a treatment for anaemia and why haematocrit climbs on cycle.

Erythropoietin

The kidney's signal telling marrow to build red cells. Androgens raise it, and it is the upstream cause of the haematocrit rise rather than a separate effect.