Human Circulatory System

Human Circulatory System • Interacts with other body systems to maintain homeostatsis • Delivers nutrients, oxygen to cells • Carries away waste products and CO2 • Transports chemical messengers through out body • Helps maintain a constant body temperature • Plays role in blood pressure control

Structure of Circulatory System • Is made up of three basic parts: • Pump (heart) • Circulating fluid (blood) • Blood vessels to carry circulating fluid

Blood • Average adult has 4 to 6 L of blood • Is made up of four basic components • Plasma • Erythrocytes (red blood cells) • Leucocytes (white blood cells) • Platelets

Plasma • Liquid component of blood • Accounts for 55% of blood volume • Is 90% water • Contains dissolved salts, proteins, hormones, nutrients, waste products and gases

Erythrocytes • Are the most abundant cells in blood • Are manufactured in the bone marrow and stored in the spleen • Primary function is to carry oxygen • Contain no nuclei or mitochondria but do contain hemoglobin molecules • Iron is a key component of RBC giving blood its bright red colour when exposed to oxygen • RBC have a biconcave shape making them flexible and allows them to move through vessels of different shapes and sizes

Platelets • Are important agents in the blood for clotting • Are cell fragments broken from special cells in bone marrow and have no nucleus • Release chemicals (clotting factors) when they encounter damaged blood vessels • Strand-like molecules called fibrin form a mesh or clot

Leucocytes • Are out numbered by RBC’s 700 to 1 • Are responsible for helping defend the body from disease and infection • Amoeboid-shaped cells destroy and consume invading bacteria and damaged cells • There are two types both have a nucleus and granulocytes have granules in the cytoplasm while agranulocytes do not • Both are manufactured in the bone marrow

Blood Types • Proteins on the surface of the RBC determine blood type (agglutinogens A and B) • Type A has agglutinogen A • Type B has agglutinogen B • Type AB has agglutinogens A and B • Type O has no proteins on surface

Each blood type is also associated with specific proteins in the plasma called agglutinins • Type A has agglutinin B • Type B has agglutinin A • Type AB has neither • Type O has agglutinin A and B • When the same agglutinin and agglutinogen combine the blood thickens and will not flow which is lethal

Pulmonary and Systemic Circuits • The pulmonary circuit is a low-pressure system • Deoxygenated blood from the right side of the heart travels to the lungs where gas exchange occurs and then oxygenated blood returns to the left side of the heart • The systemic circuit is a high pressure system • Oxygenated blood from the left side of the heart and travels to all parts of the body

Coronary Circulation • The heart has a special circulation which provides a fresh supply of oxygen to the muscles of the heart • Blockage of a coronary artery can result in lack of oxygen to heart muscle tissue resulting in tissue death • This is a heart attack, a heart attack can also cause irregular heartbeat and prevent the heart from filling and contracting normally

Heart Structure and Function

Heart Cycle • Heartbeat is result of coordinated contraction of heart muscle • A region of muscle in right atrium called sinoatrial node or pacemaker maintains the heart’s intrinsic pumping rhythm • Contractions in the SA node travel to the atrioventricular node and then travel along the Purkinje fibres in the septum toward the ventricles

Heart Muscle • Cardiac muscle is composed of striated like other muscle tissue but has a unique branching pattern • Cardiac muscle is myogenic muscle because it has the ability to contract without being stimulated by external nerves which is why it can continue to beat for a short time after being removed from the body

Heart Sounds • The typical lubb-dubb heart sounds are caused by the closing of the heart valves • The heart works in a continuous cycle of relaxation and contraction • During distole the heart is relaxed and blood flows into all four chambers • Distole ends with the contraction of the atria • During this stage blood pressure is reduced and is called diastolic pressure

Systole begins with the contraction of the ventricles which forcefully expels blood from the heart • Blood passes through the semi-lunar valves into the pulmonary artery and the aorta • The tricuspid and bicuspid valves are closed to prevent blood from flowing back into the atria • Blood pressure is increased and is called the systolic pressure

Blood Vessels • Transport the blood from one part of the body to another • Blood is carried away from the heart in arteries which branch out into arterioles and then into capillaries • Arteries are thick walled into order to withstand the pressure of the blood within them • The expansion and contraction of the arteries can be felt as your pulse

Capillaries are the narrowest of all blood vessels • RBC must squeeze through in single file and so they slow down allowing them to pick up CO2 and release O2 • Every tissue of the body is within approx. 0.1 mm of a capillary • Capillaries are the bridge between the arterial and venous systems

Capillaries merge to form venules which in turn merge to form veins • Veins are reservoirs that hold about half of the total blood volume • Veins have thinner walls, larger diameters and less muscle than arteries • Veins transport blood back to the heart and must work against gravity • Blood pressure is low in the veins and so one way valves and the contraction of skeletal muscle move blood through veins

Human Circulatory System