Cardiovascular system

General pattern of blood flow

Heart Atria- upper chambers Ventricles- lower chambers Receive blood returning to the heart Ventricles- lower chambers Receive blood from the atria and contract to force blood out of the heart into the arteries Right ventricle is thinner than the left Right pumps short distance to the lungs, little resistance Left must force blood to all other parts of the body, high resistance Septum- solid, wall like separation between the atrium and the ventricle on the right with those on the left Blood never mixes with blood from the other side

valves Atrioventricular valve-right tricuspid and left bicuspid ensure one-way blood flow Tricuspid valve- blood flows from right atrium to right ventricle, prevents back flow Right ventricle contracts, pressure increases, valve closes Pulmonary valve- allows blood to leave the right ventricle,prevents back flow Bicuspid valve- blood passes from left atrium to left ventricle, prevents back flow Aortic valve-opens when ventricle contracts allows blood to leave When closed prevents blood from backing up in ventricle

Path of blood Blood low in O2 enters right atrium through vena cavae and coronary sinus Right atria wall contracts, blood enters right ventricle Right ventricle wall contracts and blood moves into pulmonary trunk and its branches From pulmonary arteries blood enters capillaries associated with lungs Oxygenated blood returns to heart through pulmonary veins through left atrium Left atrial wall contracts and blood moves into left ventricle, left ventricle contracts blood moves to aorta

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Heart blood supply Coronary arteries supply blood to the heart’s tissue Cardiac veins drain blood that has passed through the myocardial capillaries Join coronary sinus on the heart’s posterior surface which empties into the right atrium

Cardiac cycle When atria relax blood flows into them Increases pressure Most of the blood goes directly to ventricles Atrial systole– atrial contraction Forces the rest of the blood into ventricles Atrial diastole– atrial relaxation

Cardiac conduction system Sinoatrial node (S-A node)- Can reach threshold on their own Spread to surrounding myocardium and stimulates fibers Rhythmic Referred to as pacemaker Left and right contact almost simultaneously Atrioventricular node (A-V node) Provides normal conduction Fibers have small diameters that slows the impulse Allows more time for atria to empty and ventricle fill with blood Muscles are whorls which cause twisting motion when contract

Cardiac conduction system During ventricular contraction A-V valves remain closed, atrial pressure increases Ventricles relax- vemtricular diastole A-V valves open Blood flows to ventricles Pressure decreases while filling Atria contracts increasing pressure in ventricle When pressure exceeds atrial pressure A-V closes Pressure increases until valves of pulmonary trunk open and blood flows to arteries

Electrocardiogram (ECG) Recording of the electrical changes in the myocardium during a cardiac cycle 1st change– P wave Depolarization of atrial fibers will lead to contraction Fibers reach ventricular fibers and depolarize rapidly QRS complex– Q wave, R wave, and S wave Corresponds to depolarization of ventricular fibers just prior to contraction of ventricular walls T wave Electrical change as ventricular muscle fibers repolarize Atrial repolarization is at the same as ventricular fibers depolarize

Arteries and arterioles Artery Innermost layer helps prevent blood clotting by providing a smooth surface and secreting biochemicals that inhibit platelet aggregation May also regulate local flow by secreting substances that dilate or constrict blood vessels Arteries progressively divide into thinner tubes and eventually give rise to arterioles Walls thin as they near the capillaries

Capillaries The smallest diameter of blood vessels Connect the arterioles and venules Extensions of the inner linings of arterioles Area where exchange of materials occurs Exchange in capillaries Gas, nutrients, and metabolic by products The substances exchanged move through capillary walls through diffusion, filtration, and osmosis Filtration forces molecules through a membrane with hydrostatic pressure

Capillary exchange cont. Blood pressure moves blood through arteries and arterioles Pressure decreases as the distance from the heart increases Greater in the arteries than arterioles, arterioles greater than capillaries, etc. Filtration effect primarily at arteriolar end

Veins and Venules Venules are microscopic vessels that continue from the capillaries and merge to form veins Veins contain flaplike valves Valves close if blood begins to back yup in a vein Aid in returning blood to the heart because they open if blood flow is toward the heart and close if it is in the opposite direction Also function as blood reservoirs If a hemorrhage causes a drop in arterial blood pressure, veins are stimulated to constrict and help maintain blood pressure by returning more blood to the heart Ensures a nearly normal blood flow even when as much as 25% is lost

Blood pressure Blood pressure is the force blood exerts against the inner walls of the blood vessels Arterial pressure rises and falls in a pattern corresponding to the cardiac cycle The maximum pressure during ventricular contraction is called the systolic pressure The lowest pressure that remains in the arteries until the next venticular contraction is diastolic pressure The alternating expanding and recoiling of the artery wall is felt as the pulse

Blood pressure factors Heart action Determines how much blood enters the arterial system Stroke volume-the amount of blood that is discharged from the left ventricle with each contraction Cardiac output- volume discharged/minute Stroke volume x heart rate Blood volume Equals the sum of formed elements and plasma volumes in vascular system Varies with age and sex and directly with body size Normally directly proportional to blood volume Can fall if fluid balance is upset

Factors cont. Peripheral resistance Produced from friction between blood and vessel walls Slows blood flow Contraction of smooth muscles in arteriolar walls increases pressure Dilation lessens resistance and drops the pressure in response Blood viscosity The greater the viscosity, the greater the resistance Blood cells and plasma proteins increase blood viscosity Blood pressure increases as viscosity increases and drops as viscosity decreases

Control of blood pressure Two important mechanisms for maintaining normal arterial pressure are: Regulation of cardiac output Regulation of peripheral resistance

Cardiac output Volume of blood discharged from the left ventricle and heart rate Starling’s law of the heart--Greater the length of myocardial fibers the greater the contractional force Baroreceptors sense changes in pressure Increases nerve impulse travels to brain, heart rate decreases Triggers cardioinhibitor reflex—cardiac output falls and blood pressure moves to normal level Cardioaccelerator reflex triggered by decreasing arterial pressure Causes heart to beat faster, increases heart rate, and pressure

Peripheral resistance Changes in the arteriole diameters regulate peripheral resistance Small diameters offer greater resistance to blood flow Vasomotor center controls impulses sent to arteriole walls Certain chemicals also influence peripheral resistance by affecting precapillary sphincters and smooth muscle in arteriole walls

Venous blood flow Blood pressure decreases as blood moves through the arterial system and into the capillary networks Depends on skeletal muscle contraction, breathing movements, and vasoconstriction of veins Contracting skeletal muscles- thicken and press on nearby vessels squeezing blood inside Respiratory movements- causes pressure changes in thoracic cavity Inspiration- pressure decreases in thoracic cavity and rises in abdominal cavity which squeezes blood out of veins

Venous blood cont. Vasoconstriction returns venous blood to the heart Venous pressure is low, sympathetic reflexes stimulate smooth muscle in veins to contract veins provide blood reservoir that can adapt its capacity to changes in blood volume If blood is lost and blood pressure falls, venoconstriction can force blood out of reservoir

Paths of circulation Two major pathways Pulmonary circuit Consist of vessels that carry blood from the heart to the lungs and back to the heart Systemic circuit Carries blood from the heart to all parts of the body and back again