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Circulatory System Topic 6.3 6.2 6.2 • Basic human heart facts – Resting heart beat is about 60 beats per minute with each beat ejecting 75 ml of blood – Maximal heart rate can be greater than 200 bpm each with a volume of 120+ ml – Coronary arteries supply the heart with oxygen and nutrients – The heart is asymmetrical with the left side being slightly larger than the right – Over time aerobic exercise can increase all of the above 6.2 • Parts of the heart – Atria = collecting chambers • collect blood that is pumped into them by the veins – Ventricles = pumping chambers • Pump blood into arteries at high pressure – Valves prevent blood from flowing in the wrong direction – Veins carry blood towards the heart – Arteries carry blood away from the heart 6.2 • Flow of blood through the heart – Deoxygenated blood flows from the superior vena cava (vein carrying blood from the body) into the right atrium – At the same time, oxygenated blood is flowing from the Pulmonary veins, which come from the lungs, into the left atrium – The walls of the atria contract and blood is pushed into the ventricles – During this the atrioventricular are open and the semilunar valves are closed 6.2 – After the ventricles have filled with blood, the atrioventricular valves close and the semi-lunar valves – The walls of the ventricles contract – Blood is pushed from the right ventricle into the Pulmonary Artery (carries blood to the lungs) – Blood is pushed from the left ventricle into the Aorta (carries blood to the body) 6.2 – Once the ventricles are emptied the semi-lunar valves close, preventing backflow of blood from the arteries – The whole process begins again as another heartbeat 6.2 • Control of the heartbeat – Myogenic muscle contraction • Signal for heartbeat comes from the heart itself and not from the brain • Signal originates in the Sinoatrial Node • SAN is a specialized set of cardiac cells that generate an electric impulse • The impuse spreads across the atria at a speed of approx 1m per sec., causing the atria to contract 6.2 • The impulse is “insulated” from the ventricular muscle so only the atria contract • The atrioventricular node picks up and conducts the impulse • The impulse is carried by muscle fibers down the center of the heart (fibers are located in the septum that separates the left and right ventricles) • Once the impulse reaches the end of these fibers (very bottem of the heart) the ventricles will contract 6.2 • The heart is now in full systole (contraction) • The pressure increase closes the atrioventricular valves and closes the semilunar valves • The contraction stops and the heart is now in full diastole (relaxation) • Pressure in the ventricles falls and the backflow of blood in the arteries closes the semi-lunar valves 6.2 – Control of the heartbeat • The myogenic rythym of the heart can be controlled by signals to the heart from either the brain or hormones • These signals will increase or decrease the heartrate depending on the demand for oxygen • There are nerves that come from the brainstem (medulla) which have involuntary control over heart rate 6.2 • The adrenal glands produce the hormone adrenalin • The target tissue for adrenalin is the AV node • Adrenalin travels through the blood from the adrenal glands (located just above the kidney) to the SAN • The adrenalin stimulates the node to increase its electrical output, which therefore results in an increase in heart rate • Adrenalin is usually produced in response to stressful situations, which might often require extra oxygen (e.g. “fight or flight” response) Blood Vessels 6.2 • Structure and function of blood vessels – Arteries • Thick outer layer of longitudinal collagen and elastic fibers to avoid bulges and leaks • Thick wall to withstand high pressure (bp is at its highest just after leaving the heart) • Thick layers of circular elastic muscle fibers to help pump blood on after each heart beat • Narrow lumen (empty inner space that blood flows through) to help maintain high pressures – High pressure is needed to help get blood through the body 6.2 – Veins • Thin wall with little collagen and elastic fibers – Blood is at a much lower pressure after it has passed through the body tissues (little danger of bursting) – Thin wall allows for muscle fibers to squeeze it in order to help move blood back to the heart • Wide lumen – Needed to accumulate the slow flowing blood • Have valves to prevent backflow of blood 6.2 – Capillaries • Wall consists of a single layer of thin cells – Allows for the distance for diffusion in or out to the surrounding tissues to be small – Pores between cells in the wall allow some of the plasma (fluid portion of blood) to leak out and form tissue fluid – Very narrow lumen • Only about 20 micrometers across • Allows capillaries to fit into tight spaces • Many small capillaries together will have a larger surface area than fewer wider ones 6.2 6.2 • Blood – Components • Plasma (fluid portion) – Composed of salts (e.g. sodium and chloride), gases (oxygen and carbon dioxide), proteins (e.g. albumin and fibrinogen), nutrients (e.g. glucose) and waste (e.g. urea) • Erythrocytes (red blood cells) – transport oxygen • Leucocytes (white blood cells) – Two types: phagocytes and lymphocytes – Fight disease • Platelets (cells without nucleus) – help with blood clotting 6.2 6.2 – Blood transports: • Nutrients – Glucose, amino acids • Gases – Carbon dioxide, oxygen • Hormones • Antibodies • Urea • Heat