Cardiovascular system:
increased heart rate, blood pressure, risks of arrhythmia. The heart has to work harder and faster and this stress can have deleterious effects on an older animal.

Gastrointestinal system:
slowing of gastric transit time and increased intestinal secretions lead to vomiting, anorexia, risk of gastric ulceration, constipation.

Immune system: impaired.
This leads to increased risk of infection, enhances metastatic tumour spread, increased risk of tumour occurrence in animals in remission.

Metabolism:
increased metabolism and oxygen consumption. Breakdown of muscle, fat and glucose stores. AS a result delayed wound healing, weight loss and increased tissue breakdown. Weak old dogs become weaker!

Respiration:
increased respiratory rate and depth of breathing increases. Increased risk of pneumonia.

Urinary:
urine retention, water and sodium retention. Electrolyte imbalances.