When the concentration of oxygen (O2) drops in the tissues, several physiological responses occur to help meet the oxygen demand of cells. This condition is known as hypoxia. Here's what happens as O2 concentration decreases in the tissues:
1. **Vasodilation:** Blood vessels in the affected tissues dilate (widen). This allows for increased blood flow to the area, bringing more oxygenated blood to the tissues.
2. **Increased Heart Rate:** The heart rate may increase to pump more oxygen-rich blood to the tissues.
3. **Release of Oxygen from Hemoglobin:** Hemoglobin, the oxygen-carrying protein in red blood cells, releases oxygen more readily in tissues with low oxygen levels. This is due to the Bohr effect, where hemoglobin's affinity for oxygen decreases in an acidic, low-oxygen environment.
4. **Production of Erythropoietin:** In response to chronic hypoxia, the kidneys release a hormone called erythropoietin (EPO). EPO stimulates the bone marrow to produce more red blood cells, increasing the oxygen-carrying capacity of the blood over time.
5. **Shift to Anaerobic Metabolism:** In extreme cases of tissue hypoxia, cells may switch to anaerobic metabolism, which doesn't rely on oxygen. This can lead to the production of lactic acid and can cause tissue damage if prolonged.
6. **Hypoxia Signaling Pathways:** Cells have specific signaling pathways that activate under low-oxygen conditions (hypoxia-inducible factors or HIFs). These pathways can lead to changes in gene expression, promoting adaptive responses to low oxygen levels.
Overall, the body's responses to decreasing O2 concentration in the tissues aim to ensure that cells receive the necessary oxygen for their survival and function. However, prolonged or severe hypoxia can have detrimental effects on tissues and organs.