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Structure of the heart and Cardiac Cycle Structure of the heart DEFINITIONS/EXPLANAT IONS SYSTOLE • Period of contraction of heart DIASTOLE • Period of relaxation of heart SINO-ATRIAL NODE (SAN) • Pace maker of the heart, initiates cardiac impulse • Found in region of right atrium • A specialised area of cardiac muscle fibres. DEFINITIONS/EXPLANAT IONS ATRIO-VENTRICULAR NODE (AVN) • Second node in right atrium responsible for passing on cardiac impulse BUNDLE OF HIS • Found in the septum and conducts the cardiac impulse to the apex (tip) of the heart PURKINJE FIBRES • Send impulses to ventricles to contract SEQUENCE OF EVENTS 1. Originates in the SA Node (sinoatrial node). 2. Sends a wave of excitation through the atria causing them to contract. 3. This then stimulates the AV node (atrioventricular node). 4. Sends a wave through the bundle of his and the purkingjie fibres causing the ventricles to contract. SEQUENCE OF EVENTS 5. This is called VENTRICULAR SYSTOLE. 6. Once systole has finished DIASTOLE follows • All this takes place in a fraction of a second Cardiac Cycle • Refers to the electrical and mechanical movements of the heart. • Rest one complete heartbeat takes 0.8 seconds. CONTROL OF HEART RATE • The timing of the cardiac contractions is altered by two extrinsic factors: 1. Neural control 2. Hormonal control • One intrinsic factor can also alter HR • Neural control is the most important control mechanism NEURAL CONTROL • The SA node is controlled by the autonomic nervous system • Two nerves stimulate this node; 1. Sympathetic cardiac accelerator nerve (speeds up HR) 2. Parasympathetic vagus nerve (slows down HR) • Overall control of both nerves is with the cardiac control centre in the medulla of the brain. STIMULATION OF CARDIAC CONTROL CENTRE Stimulated by: 1. Muscle receptors in muscle and joints at the start of exercise 2. Chemoreceptors in the muscle that sense chemical changes in muscle and blood (changes in O2, CO2 and pH levels) 3. Emotional excitement 4. Changes in blood pressure detected by baroreceptors in aortic and carotid arteries HORMONAL CONTROL • • • • Adrenaline (secreted from adrenal glands in the kidney) stimulates the SA node This causes an increase in HR Prepares body for the ‘flight or fight’ scenario Adrenaline also increases the strength of contraction produced by the myocardium (heart muscle) INTRINSIC CONTROL • Warm muscle conducts nerve impulses faster • Thus HR of a warm heart increases • A drop in temperature will reduce HR • Increased venous return stretches heart which stimulates SA node and thus increases HR and SV • This is called ‘Starling’s Law of the heart’ Changes in Heart Rate, Stroke Volume, and Cardiac Output Activity Heart rate Stroke volume Cardiac output (beats/min) (ml/beat) (L/min) Resting (supine) 55 95 5.2 Resting (standing and sitting) 60 70 4.2 Running 190 130 24.7 Cycling 185 120 22.2 Swimming 170 135 22.9 Changes of Cardiac output • Increases why? • What does this enable to happen? • What happens to heart rate why? • What happens to stroke volume why? Did You Know…? Arteries always carry blood away from the heart; veins always carry blood back to the heart with the help of breathing, the muscle pump, and valves. THE CARDIOVASCULAR RESPONSE TO EXERCISE BLOOD DISTRIBUTION AT REST THE MUSCLE PUMP Blood Distribution w Matched to overall metabolic demands w Autoregulation—arterioles within organs or tissues dilate or constrict w Extrinsic neural control—sympathetic nerves within walls of vessels are stimulated w Determined by the balance between mean arterial pressure and total peripheral resistance Key Points The Vascular System w Blood returns to the heart with the help of breathing, the muscle pump, and valves in the veins. w Blood is distributed throughout the body based on the needs of tissues; the most active tissues receive the most blood. w Autoregulation controls blood flow by vasodilation in response to local chemical changes in an area. (continued) Key Points The Vascular System w Extrinsic neural factors control blood flow Key Points primarily by vasoconstriction. The Vascular System w flow factors control blood neural pressure w Extrinsic Systolic blood (SBP) is the primarily by vasoconstriction. highest pressure within the vascular (SBP) is the blood pressure w Systolic pressure within the vascular systemhighest while diastolic blood pressure blood pressure while diastolic system is the lowest. (DBP) (DBP) is the lowest. w w Mean arterial pressure (MAP) is the average pressure on the arterial walls. Mean arterial pressure (MAP) is the average pressure on the arterial walls.