The human body contains around 60% of the water, which is very important for survival. Various fluids present in the body such as blood, lymph are composed of water. The blood is composed of red blood cells, proteins, and plasma, the plasma in turn contains 90% of water. A healthy body maintains a balance between the fluid intake and the fluid that leaves the body. The body loses water through breathing, sweating, and urinating.
Loss of blood which results in the loss of water from the body, as 90% of the blood plasma is composed of water. This traumatic condition of an individual results in a decrease in the blood volume as well as blood pressure. A decrease in blood pressure stimulates the kidney to secrete renin into the bloodstream. This release of renin promotes the synthesis of angiotensin II from angiotensinogen by the kidneys. The angiotensin II excites the neurons in the subfornical organ, which stimulates the cells present in the lateral areas of the hypothalamus. This results in an increase in the production of antidiuretic hormone (ADH), also known as vasopressin. ADH regulates the level of water in the body. An increase in the levels of ADH increases the feeling of thirst. Therefore, the individuals who have lost large volumes of blood feel thirsty.
Decrease in blood volume due to loss of blood causes a decrease in blood pressure. This decline in blood pressure activates the renin-angiotensin mechanism which in turn stimulates the secretion of antidiuretic hormone that increases thirst.