ch10. circulatory system

• Human heart


  About 300 g, located between lungs, left of center, rests on diaphragm
• Heart + circulatory system

  Pumps blood and oxygen all around body
• Heart (cardiovascular disease) – leading cause of death worldwide
• Increasing complexity of CVS in vertebrates

  • Fish: Simple, 2 chambers – one atrium and one ventricle
  • Amphibians and some reptiles: 3 chambers – two atria and one ventricle – problem is that oxygenated and deoxygenated blood is mixed so not very efficient
  • Birds and mammals: 4 chambers – two atria and two ventricles – most efficient
• Human heart


  4 chambers. The atria are receiving chambers for blood entering heart. The ventricles are pumps that force blood to lungs or around body
• Heart composed of three main layers

  • Endocardium (innermost layer)
  • Myocardium (middle)
  • Epicardium (outermost layer)
• Heart protected by


  The epicardium + a fluid-filled sac that surround the heart, called the pericardium
• Myocardium


  • Contains cardiac muscle + connective tissue + blood vessels. This is the thickest of the three layers. The cardiac muscle fibers are what allow the heart to contract
  • Myocardium of left ventricle thicker than that of right ventricle as more pumping power is required by left ventricle to send blood around body when compared to the lower pumping ability required to send blood to the lungs, which are located nearby
• Heart equivalent to two pumps
• William Harvey – British physician - First person to describe the circulation of blood. Published famous book describing this


  1628
• Cardiac cycle


  1. Circuit (pump) 1: Right atrium receives venous (or deoxygenated) blood from the inferior and superior vena cava. Blood passes from right atrium to right ventricle via tricuspid valve. Right ventricle contracts and sends blood through pulmonary semilunar valve and pulmonary artery to lungs. Blood picks up oxygen and loses carbon dioxide in lungs, returns to heart left atrium via pulmonary veins

  2. Circuit (pump) 2: Oxygenated blood passes from the left atrium to left ventricle via bicuspid or mitral valve. Left ventricle contracts and sends blood through the aortic semi-lunar valve and aorta to all the tissues of the body. From the tissues, blood returns to the right atrium via the inferior and superior vena cava
• Pulmonary circulation


  Low pressure circuit as blood only being sent to lungs, which are nearby
• Systemic circulation


  High pressure circuit as blood has to be pumped a long way (i.e., from the heart to all parts of the body)
• Heart valves


  • Atrioventricular valves – between the atria and ventricles
  • Tricuspid valve – between the right atrium and ventricle – 3 flaps
  • Bicuspid/mitral valve - between the left atrium and ventricle – 2 flaps
  • Semi-lunar valves - between the ventricles and big arteries leading away from heart
  • Aortic semi-lunar valve – between left ventricle and aorta
  • Pulmonary semi-lunar valve – between right ventricle and pulmonary artery
• Heartbeat rate


  Inversely proportional to size of animal. Big animals have lower heartbeat rate / smaller animals have higher heartbeat rate
• Heartbeat rate in humans

  Relatively high at birth, then drops to adolescents, then increases slightly in old age. Typically heartbeat rate in healthy adults - ~72 bpm but this is higher with exercise and lower during sleep
• Heartbeat


  Measured with stethoscope. Typically, two noises – 'lub' indicates ventricles starting to contract and is due to bicuspid and tricuspid valves closing, 'dup' indicates end of ventricular contraction when semi-lunar valves close
• Heart 'murmur'


  Doctor hears a swishing/hissing noise rather than a clear lub/dup, which indicates that blood is leaking through the valves. This might indicate a serious heart problem
• Cardiac electrical conduction system

  1. Cardiac muscle fibers (in myocardium) contract via excitation-contraction coupling whereby an electrical stimulus (action potential; AP) is converted into a mechanical response (muscle contraction)

  2. The AP is generated in pacemaker cells (specialized cardiomyocytes) located in the sinoatrial and atrioventricular nodes of the heart. The AP is then conducted to contractile cardiomyocytes via gap junctions

  3. SA node cells spontaneously depolarize at ~100 contractions/min

  4. Impulses from SA node controls contraction of atria

  5. Contraction of ventricles regulated by impulses from atrioventricular (AV) node >> Bundle of His >> Left and right bundle branches >> Purkinje fibers

  6. The two atria contract together due to the Bachmann's bundle (or interatrial band), which runs from the SA node in right atrium into left atrium

  7. There are motor neurons in the heart but these ONLY regulate the rate and strength of heartbeat….they don't initiate the heartbeat
• Cardiac cycle


  Includes ventricular contraction (called systole) and relaxation (diastole). Diastole is longer than systole to give the heart chance to relax between contractions and give time for the ventricles to fill with blood
• The sequence of electrical conductance can be seen on a ECG (electrocardiogram), producing a graph comprising the P wave, QRS complex and T wave


  P wave: spread of excitation over both atria

  P-R interval: time for impulse conduction from atria to the ventricles

  QRS complex: spread of excitation over ventricles (bigger deflection and shorter duration than P wave)

  S-T interval (0): all ventricular muscle is in the same depolarised state

  T wave: ventricular repolarization of different parts of the ventricle at different times
• Circulatory system


  • Arteries – arterioles – capillaries – venules – veins
  • Arteries and arterioles – carry oxygenated blood AWAY from the heart to tissues and organs
  • Capillaries – smallest, most delicate blood vessels found in tissues and organs
  • Venules and veins - carry deoxygenated blood from the tissues/organs TO the heart
• Blood vessels


  • Arteries have thicker walls and smaller lumens than veins, but veins have valves (2-flap)
  • In general, blood vessels nearer to the heart have wider diameter and thicker walls than those nearer to the organs and tissues. Capillaries have the thinnest walls, which are required for gas exchange
• Steady state blood flow is the same through any cross section of the circulation. The amount of blood flowing out of the right heart equals the amount of blood flowing into the left heart. Otherwise, blood would accumulate in the lungs →drown in your own blood!
• Blood flow rate


  At rest, the blood flow rate of an average adult, amounts to about 5 L/min
• Blood velocity


  How far a blood cell moves along a blood vessel in one minute (measured in cm/min)
• Blood flow


  The volume of blood that passes a given cross-section in one minute (measured in ml/min)
• Total cross-sectional area of the "VASCULAR TREE" is greatest in the CAPILLARIES
• While the total cross-sectional area of the vascular tree is greatest in the CAPILLARIES, the blood velocity is slowest in the CAPILLARIES. This is to provide time for gas (O2, CO2) and nutrient exchange (glucose etc)
• Blood flow (ml/min) is CONSTANT throughout the vascular tree because: TOTAL BLOOD FLOW = BLOOD VELOCITY x CROSS SECTIONAL AREA
• Blood pressure (BP)


  Measured with sphygmomanometer (manometer) in mmHg. Systolic pressure - peak pressure in the arteries when the ventricles contract. Diastolic pressure - minimum pressure in the arteries when the ventricles are filling with blood
• Blood pressure of healthy adult human

  120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg)
• Hypotension


  <90/60 mmHg - dizziness, fainting, or circulatory shock (as blood flow to organs too low)

  causes of low blood pressure: sepsis (blood poisoning), haemorrhage (blood loss), toxins (severe allergic reaction, anaphylactic shock), hormonal abnormalities (lack of cortisol or aldosterone), eating disorder (anorexia nervosa and bulimia)
• Hypertension


  Typically anything above 140/90 mmHg - mechanical stress on the arterial walls. Over time, is a risk factor for heart attacks and strokes
• Heart (cardiovascular) disease


  Various disorders of the heart and blood vessels
• Heart attacks/strokes


  Mainly caused by a blockage preventing blood from flowing to the heart or brain. Commonly due to atherosclerosis - build-up of fats, cholesterol, and white blood cells (called plaque) in and on the artery walls. Plaque can cause arteries to narrow, blocking blood flow. Might also burst, leading to a blood clot
• Heart disease risk factors

  • Age: (men > 45 yr / woman > 55 yr)
  • Family history
  • Gender: men> women
  • Lifestyle: diet, smoking, alcohol, lack of exercise, obesity