A drop in oxygen supply triggers the kidney and bone marrow to increase production of red blood cells.
A protein called erythropoietin (Epo), primarily produced by the kidney and liver, helps stimulate red blood cell (RBC) volume at the rate of 2.4 million RBCs per second.  Between 1983 and 1985, commercial production of Epo was introduced, using recombinant DNA technology. Later synthetic enhancements were made to this hormone produced from natural blood fractions, allowing Epo to last longer.
Common Medical Uses
Epo is primarily used for the treatment of anemia (low RBC count) in human dialysis and cancer patients. Erythropoietin-stimulating agents (ESAs) stimulate the bone marrow to generate more Epo and restore hemoglobin levels. Breast cancer patients experience a drop in hemoglobin following chemotherapy. The use of ESAs in multiple disease states has been well established and is now considered first-line treatment for many forms of anemia,  including critical illnesses such as heart failure. 
Erythropoietin Use for Depression and Bipolar Symptoms
Depression and bipolar disorder are associated with reduced neural plasticity and deficits in memory, attention and executive function. Epo is involved in neuroplasticity and is a candidate for future treatment of affective disorders. In an 8-week, double-blind, placebo-controlled, parallel-group clinical research study of 40 patients each, stratified for age and gender, the investigators have demonstrated that a single dose of Epo improves cognitive function and reduces neurocognitive processing of negative emotional information in healthy and depressed individuals similar to effects seen with conventional antidepressants. 
Take-away tip: Epo for mental health.
- Red blood cell. Wikipedia. Erich Sackmann, Bilogical Membranes Architecture and Function, 1995
- Use of Erythropoietin-stimulating Agents in Breast Cancer Patients: A Risk Review. Medscape, WebMD
- Erythropoietin. Wikipedia
- Miskowiak K, Vinberg Maj, Harmer C, et al. Effects of erythropoietin on depressive symptoms and neurocognitive deficits in depression and bipolar disorder. Trials Journal, 11:97; October 13, 2010