This chapter focuses on homeostasis and derangements of homeostasis and haemodynamics. The mechanism by which the constancy of the internal environment is maintained and ensured is called the homeostasis. The normal composition of internal environment consists of water and electrolytes. The pH of blood and acid-base balance is regulated in the body by buffer system, pulmonary mechanism and renal mechanism. Disturbances of body fluid result in oedema which is defined as abnormal and excessive accumulation of “free fluid” in the interstitial tissue spaces and serous cavities. Oedema is caused by mechanisms that interfere with normal fluid balance of plasma, interstitial fluid and lymph flow. Normally, the interstitial fluid in the tissue spaces escapes by way of lymphatics. Obstruction to outflow of these channels causes localised oedema, known as lymphoedema. Some of the types of oedema include renal oedema, cardiac oedema, pulmonary oedema, cerebral oedema, hepatic oedema, nutritional oedema and myxoedema. Hyperaemia and congestion are the terms used for localised increase in the volume of blood within dilated vessels of an organ or tissue. The increased volume from arterial and arteriolar dilatation being referred to as hyperaemia or active hyperaemia, whereas the impaired venous drainage is called venous congestion or passive hyperaemia. Haemorrhage is the escape of blood from a blood vessel. The bleeding may occur externally or internally into the serous cavities or into a hollow viscus.