What is the effect of a low hemoglobin level, and what is the body's compensatory response?

Hemoglobin is responsible for transporting oxygen from the lungs to the body's tissues. A low hemoglobin level means that less oxygen can be carried by the blood. This leads to reduced oxygen delivery to cells and tissues, causing symptoms like tiredness and shortness of breath, as seen in the patient's case.

When the body detects low oxygen levels (hypoxia) due to low hemoglobin, the kidneys release a hormone called erythropoietin (EPO). Erythropoietin stimulates the red bone marrow to increase the production of red blood cells. This is a compensatory mechanism to try and increase the oxygen-carrying capacity of the blood.

The student's answer, "Red bone marrow," refers to the site where the body attempts to compensate for low hemoglobin by producing more red blood cells. Therefore, an effect of low hemoglobin is the stimulation of the red bone marrow to increase red blood cell production.

The effect of a low hemoglobin level is that the body's tissues receive less oxygen. To compensate for this, the red bone marrow is stimulated to produce more red blood cells.

The final answer is $\boxed{\text{Red bone marrow is stimulated to produce more red blood cells}}$.