The Circulatory System

1. The Circulatory System Chapter 42

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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