Nursing Entrance Exam

Blood Flow to the Heart The coronary arteries arise at the base of the ascending aorta, where systemic pressures are highest. Each time the heart contracts, it squeezes the coronary vessels, reducing blood flow. In the left ventricle, systolic pressures are high enough that blood can flow into the myocardium only during diastole; over this period, elastic rebound helps drive blood along the coronary vessels. Normal cardiac muscle cells can tolerate these brief circulatory interruptions because they have substantial oxygen reserves. Epinephrine released during sympathetic stimulation promotes the vasodilation of coronary vessels while increasing the strength of cardiac contractions and heart rate. When blood flow is decreased to the myocardium, ischemia occurs and the tissue is damaged. If the oxygen deficit is severe enough, the tissue dies causing myocardial infarction (commonly called a heart attack). 7.5 The Immune System – Tour of the System The Lymphatic System and Body Defenses The human body has multiple defense mechanisms that provide resistance, which is the ability to fight illness disease, and infection. Body defenses are sorted into two general categories: nonspecific defenses , which do not distinguish one type of threat from another. Their response is the same, regardless of the type of invading agent. Present at birth, these defenses include physical barriers, fever, inflammation, phagocytic cells, immunological surveillance, complement, and interferons. They provide a defensive capability known as nonspecific resistance. Specific defenses, on the other hand, protect against particular threats. For example, a specific defensemay protect against infection by one type of bacterium, but be ineffective against other bacteria and viruses.Many specific defenses develop after birth as a result of accidental or deliberate exposure to environmental hazards. The body’s specific defenses produce a state of protection known as immunity, or specific resistance. Leukocytes (white blood cells) are the main players in the specific immune response. Nonspecific Defenses Nonspecific defenses prevent the approach, deny the entry, or limit the spread of microorganisms or other environmental hazards. Physical barriers, such as the skin, keep hazardous organisms and materials outside the body. Phagocytes engulf pathogens and cell debris. Examples of phagocytes are the macrophages of peripheral tissues and the microphages of blood. Immunological surveillance uses NK cells to destroy abnormal cells. Interferons are chemical messengers that coordinate the defenses against viral infections. Complement is a system of circulating proteins that assists antibodies in destroying pathogens. The inflammatory response is a localized, tissue-level response that tends to limit the spread of an injury or infection. Fever is an elevation of body temperature that accelerates tissue metabolism and the activity of defenses.

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