IntroductionAccording to Foex and Sear (2004), hypertension refers to the sustainable increase in blood pressure in systemic arteries which often leads to increased long-term morbidity and mortality . A patient with hypertension is said to have a systolic blood pressure greater than 140 mmHg or a diastolic blood pressure greater than 90 mmHg (Stedman, 2005). According to statistics obtained from Morbidity and Mortality Weekly Report (2012), approximately 31% of the total 67 million adults in America suffer from hypertension. This astonishing figure indicates that hypertension has become one of the most prevalent health problems in society today. There are a myriad of possible factors that can contribute to hypertension, such as the cardiovascular system, the renin-angiotensin-aldosterone system, and the sympathetic nervous system (Foex & Sear, 2004). This research paper aims to study the dysfunction of the renin-angiotensin-aldosterone system in the pathophysiology of hypertension and the effects of an antihypertensive drug, losartan, on angiotensin II type 1 receptors in response to the disease. Pathophysiology of Hypertension Renin is a type of protein enzyme secreted by the juxtaglomerular system of the kidneys when blood pressure in the arteriole is low. It is produced and stored as an inactive form known as prorenin in juxtaglomerular cells (Hall, 2010). Abnormalities in the renin-angiotensin-aldosterone system pathway such as overproduction of renin or excessive release of angiotensin II can lead to hypertension (Williams & Lemke, 2008). For example, when renin secretion is abnormally high, a large amount of angiotensinogen (renin substrate) secreted by the liver will be converted into angiotensin I by renin. ...... half of the sheet ......-S6. doi: 10.1016/j.annemergmed.2007.11.004Kaplan, New Mexico (2006). Kaplan clinical hypertension. (9th ed.). Columbia: Lippincott Williams and Wilkins. Lemke, T. L., & Williams, D. A. (2008). Foye's principles of medicinal chemistry. (6th ed.). United States: Lippincott Williams & Wilkins. Weekly morbidity and mortality report. (2012, September 7). Vital signs: awareness and treatment of uncontrolled hypertension among adults - United States, 2003–2010. Centers for Disease Control and Prevention. Retrieved from http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6135a3.htmStedman, T. L. (2005). Stedman's Medical Dictionary. (5th ed.). Philadelphia: LippincottWilliams & Wilkins. Shankie, S. (2001). Hypertension in the spotlight. London: Pharmaceutical Press.Silverthorn, DU (2009). Human physiology: an integrated approach. (5th ed.). United States: Benjamin Cummings.