Cardiovascular system

• The Heart

  • Four-chambered organ that provides the drive for blood flow
  • Weighs 11 oz. for average male, 9 oz. for female
  • Pumps ~70 mL for each beat (stroke volume) at rest
  • At rest in 1 day, ~1900 gallons pumped through the heart, or 52 million gallons for a 75-y life span
  • Heart muscle called myocardium; myocardial fibers interconnect in latticework fashion to allow the heart to function as a unit
• The Heart


  1. Right side receives blood returning from throughout the body and pumps blood to the lungs for aeration through the pulmonary circulation

  2. Left side receives oxygenated blood from the lungs and pumps blood into the thick-walled, muscular aorta for distribution throughout the body in the systemic circulation

  3. Two sides separated by interventricular septum
• The Heart


  • Atrioventricular valves: Tricuspid provides one-way blood flow from the right atrium to right ventricle, Bicuspid/mitral provides one-way blood flow from left atrium to left ventricle
  • Semilunar valves located in arterial wall just outside heart prevent blood from flowing back into the heart between contractions
• The Arterial System


  • High-pressure tubing that propels oxygen-rich blood to tissues
  • Comprised of layers of connective tissue and smooth muscle
  • Blood pumped from left ventricle enters aorta and is distributed throughout the body through a network of arteries and arterioles
  • Smooth muscle in arteriole walls either constrict or relax to regulate blood flow to periphery
  • Blood flowing through capillaries moves slowly (0.05 to 0.1 cm · s^-1) compared with any main arteries or veins
• Blood Pressure


  Force of blood against arterial walls during cardiac cycle
• Blood Pressure

  
  

  4. Blood pressure = cardiac output × total peripheral resistance
• Systolic Blood Pressure (SBP)


  Contraction phase of cardiac cycle (systole), estimate of work of heart and force blood exerts against arterial walls during systole
• Diastolic Blood Pressure (DBP)


  Relaxation phase of cardiac cycle (diastole), indicates peripheral resistance or ease that blood flows from arterioles into capillaries
• Mean Arterial Pressure (MAP)


  Average force exerted by blood against arterial wall during cardiac cycle, MAP = diastolic BP + [0.33 × (Systolic BP − Diastolic BP)]
• Cardiac Output (Q)


  Cardiac output (Q) = Mean arterial pressure (MAP) ÷ Total peripheral resistance (TPR)
• Total Peripheral Resistance (TPR)


  Total peripheral resistance (TPR) = Mean arterial pressure (MAP) ÷ Cardiac output (Q)
• MAP and cardiac output


  Estimate change in total resistance to blood flow in the transition from rest to exercise
• Resistance to peripheral blood flow decreases dramatically during strenuous physical activity
• The Capillaries


  • Arterioles branch and form smaller and less muscular vessels called metarterioles
  • Metarterioles end in microscopically small blood vessels called capillaries that contain 6% of total blood volume
  • Capillary wall consists of a single layer of rolled up endothelial cells
  • Some capillaries are so narrow that only one blood cell at a time can squeeze through
• Precapillary sphincter


  A ring of smooth muscle that encircles the capillary at its origin and controls its diameter
• Precapillary sphincter


  1. Constriction and relaxation provide a means for blood flow regulation within a specific tissue to meet metabolic requirements

  2. Two factors trigger precapillary sphincter relaxation to open more capillaries: Driving force of increased local BP plus intrinsic neural control, Local metabolites produced in exercise
• The Venous System


  1. Capillaries feed deoxygenated blood into small veins or venules

  2. Veins in lower body eventually empty into inferior vena cava, the body's largest vein

  3. Vena cava returns blood to right atrium from abdomen, pelvis, and lower extremities

  4. Venous blood from vessels in head, neck, shoulder region, thorax, and abdominal wall flows into superior vena cava to join inferior vena cava at the heart (mixed-venous blood)

  5. Mixed-venous blood enters right atrium
• Venous Return


  • Valves within veins allow blood to flow in only one direction toward the heart
  • Smallest muscular contractions or minor pressure changes within the thoracic cavity with breathing readily compressing veins
  • Alternate compression and relaxation of veins and one-way valve action provide a "milking" action to propel blood back to the heart
  • Without valves, a person would faint from reduced venous return and resulting diminished brain blood flow every time they stood up
• Valves in Veins Ensures that Blood Flows Toward the Heart
• Varicose Veins


  • Valves within a vein fail to maintain one-way blood flow; blood gathers in vein so they become excessively distended and painful
  • Usually occurs in surface veins of lower extremities
  • In severe cases, phlebitis occurs where the venous wall becomes inflamed and deteriorates
  • Sclerotherapy and laser ablation cause the vein to collapse, and body will reroute blood flow to deeper veins
  • People with varicose veins should avoid static, straining-type exercises like resistance training
  • Exercise does not prevent varicose veins, but regular exercise can minimize discomfort and complications
• Venous Pooling


  1. Because the muscle pump contributes to venous return, the resulting hydrostatic force can cause blood to pool in the lower extremities

  2. People may faint when forced to maintain upright posture without movement

  3. Fluid backs up in the capillary bed and seeps into the surrounding tissues causing swelling or edema

  4. Reduced venous return reduces both cardiac output and arterial blood pressure, increased heart rate results

  5. Support stockings for individuals with varicose veins or poor venous return can reduce the hydrostatic blood shifts to lower extremity veins in the upright position
• Hypertension


  Defined as systolic pressure >130 mm Hg; diastolic pressure >80 mm Hg
• Hypertension chronically strains the cardiovascular system, and, if left untreated, can damage arterial vessels and lead to arteriosclerosis, heart disease, stroke, and kidney failure
• Current estimates place nearly 50% of the current adult U.S. population in the hypertensive category
• Hypertension: Treatment Strategies


  • Increasing regular physical activity to at least 1 hr daily
  • Modest weight loss (especially in the overweight and obese)
  • Stress management
  • Smoking cessation
  • Reduced sodium and alcohol consumption
  • Increased dietary nitrate, and adequate calcium and magnesium intake
• Hypertension: Lifestyle Choices


  • Every 20-lb weight loss reduces SBP 5 to 20 mm Hg
  • Eating a lower fat diet rich in vegetables, fruits, and legumes (e.g., DASH diet, see Chapter 3) reduces SBP 8 to 14 mm Hg
  • Increase aerobic physical activity to 30 min · d^-1 reduces SBP 4 to 9 mm Hg
  • Limit sodium intake to ≤1500 mg · d^-1 reduces SBP 2 to 8 mm Hg
  • Limit alcohol intake to ≤1 drink · d^-1 lowers SBP 2 to 4 mm Hg
• Straining muscle actions, particularly the concentric (shortening) and/or static phase muscle actions


  Mechanically compress peripheral arterial vessels supplying the active muscles
• Arterial vascular compression


  Dramatically increases total peripheral resistance and reduces muscle perfusion
• Acute cardiovascular strain with heavy resistance exercise could prove harmful to individuals with heart and vascular disease
• During rhythmic muscle activity


  Vasodilation in active muscles reduces TPR to enhance blood flow through peripheral vasculature
• Alternate muscle contraction/relaxation


  Propels blood through the vascular circuit back to the heart
• Increased blood flow during steady-rate exercise


  Rapidly increases SBP during the first few minutes
• SBP often declines as steady-rate exercise continues

  Arterioles in active muscles continue to dilate, which reduces peripheral resistance to blood flow
• DBP generally remains unchanged throughout exercise
• After an initial rapid rise from resting level


  SBP increases linearly with exercise intensity
• SBP may increase to 200 mm Hg or higher in healthy, fit individuals during maximum exercise (despite reduced TPR)
• High SBP


  Likely reflects the heart's large cardiac output during maximal exercise by individuals with high aerobic capacity
• Exercise with arms


  Produces higher SBP and DBP than leg exercise performed at a given percentage of VO2max in each exercise mode
• Occurs because


  Smaller arm muscle mass and vasculature offer greater resistance to blood flow than activation of larger leg mass and blood supply
• Individuals with cardiovascular dysfunction should rhythmically exercise relatively large muscle groups in contrast to exercise that engages a limited muscle mass