When the body’s metabolism has a problem , producing of enough energy is decreased which leads to exhaustion.
Metabolism is a complicated mechanism in cells leads to provide enough ATP ( source of energy ) and maintenance of tissues. For normal metabolism having enough oxygen, glucose, minerals, proteins , essential fatty acids and water are necessary. Therefore deficiency of one of them or any interaction with the process of metabolism cause exhaustion.
Reasons for tiredness:
1-Cardiovascular disease: any cardiac or vascular problem which interferes with enough blood circulation causes fatigue.Blood circulation is necessary to send oxygen to cells and return waste materials from cells to lung, liver or kidneys.
2-Respiratory system disease: lung absorbs the oxygen and sends out carbon dioxide from the blood.Every disease of respiratory system including nasal cavity, larynx, pharynx, trachea and lung can causes low oxygen in blood which can affect the cell metabolism.
3-Anemia : low red blood cells or hemoglobin which carry oxygen from respiratory system to cells causes exhaustion of your pet.Internal or external parasites, deficiencies like iron , chronic blood loss ( like gastric ulcer) and bone marrow diseases are in this category.
4- Low oxygen: living in high altitude, air pollution in cities can lead to fatigue not only in humans but also in animals.
5- Starvation: deprivation from food for long term.