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Explore the importance of the circulatory system in transporting chemicals, maintaining homeostasis, and preventing diseases. Learn about its components, blood flow patterns, the role of the heart, and the exchange of materials through capillaries.
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The Circulatory System Chapter 42
Why have a circulatory system? • Diffusion is too slow a process to transport chemicals through the body of an animal. • The presence of a circulatory system reduces the distance a substance must diffuse to enter or leave a cell. • The circulatory system does more than move gases, it is a critical component to maintaining homeostasis of the body.
Components of the cardiovascular system • Heart • Four chambers in humans • Arteries • Carry blood away from heart • Branch into smaller arterioles • Veins • Carry blood toward the heart • Capillaries • The site of chemical exchange between blood and interstitial fluid • Connect arteries to veins
Blood flow pattern in humans • ‘Double-loop’ circulation • body vena cava right atrium right ventricle pulmonary artery lung pulmonary vein left atrium left ventricle aorta arteries arterioles capillaries venules veins vena cava • The blood that is low in oxygen is completely separated from the blood that is rich in oxygen
The heart • Located beneath the sternum • Cone-shaped and about the size of a clenched fist • Surrounded by a double layered sac • Comprised mostly of cardiac muscle tissue • The 2 atria are thin-walled and collect blood • The ventricles are thick-walled and pump blood • There are 4 valves which prevent backflow of blood during contraction
The heartbeat • A complete sequence of contraction and relaxation is the cardiac cycle • Systole – the heart muscle contracts and the chambers pump blood • Diastole – the heart muscle relaxes and the chambers fill with blood • The tempo of contraction is controlled by the sinoatrial (SA) node, sometimes called the pacemaker • The SA node can be influenced by hormones, body temperature, exercise
Blood pressure • The force that blood exerts against a vessel wall • Pressure is greatest in arteries during ventricular systole – this is the main force propelling blood from the heart through the vessels • Stress may trigger responses that cause the smooth muscles of vessel walls to contract, increasing blood flow and pressure • Blood in the veins is under very little pressure – it is aided in returning to the heart by skeletal muscles • Veins have valves that prevent backflow of blood
Blood flow to different body parts is regulated • All tissues and organs receive enough blood even though only 5% - 10% of the capillaries are carrying blood at any given time • The brain, heart, kidneys, and liver usually carry a full load of blood • Blood flow to capillaries in other areas is controlled by smooth muscles in the artery walls • After a meal increased flow to digestive tract • Exercise increased flow to skeletal muscles
Exchange of materials • Occurs across the thin walls of the capillaries • The capillary wall is a single ‘leaky’ layer of endothelial cells • About 85% of the fluid that leaves the blood at the arteriole end of the capillary re-enters from the interstitial fluid at the venous end • The remaining 15% of the fluid is lymph
The lymphatic system • Lymph is similar in composition to interstitial fluid • It enters the lymphatic system by diffusing into lymph capillaries • It then drains back into the circulatory system at two locations near the shoulders • Lymph nodes are specialized swellings along the system that filter the lymph and attack viruses and bacteria
Blood • Connective tissue – cells suspended in liquid plasma • Average person has 4-6 L of whole blood • Plasma • 90% water • Also contains electrolytes and plasma proteins • Erythrocytes (RBCs = red blood cells) • Transport oxygen • No nuclei • Contain hemoglobin • Leukocytes (white blood cells) • Function in defense and immunity • Number will increase during times of infection • Platelets • Function in blood clotting
Blood clotting • Platelets clump together and form a temporary plug • Clotting factors are released that convert fibrinogen to fibrin • Fibrin forms threads to form the clot • Hemophilia – caused by a defect in the genes that control this process
Cardiovascular disease • Accounts for more than 50% of all U.S. deaths • Heart attack – blockage of coronary arteries which leads to cardiac muscle damage • Stroke – blockage of arteries in the brain • Atherosclerosis – artery walls develop plaques which narrow the space and increase chance of clot formation and heart attack • Hypertension – chronic high blood pressure which can promote atherosclerosis