The Cardiovascular System:

1
Chapter 18

• The Cardiovascular System
• The Heart

2
Heart Anatomy

• Approximately the size of your fist
• Location
• Superior surface of diaphragm
• Left of the midline
• Anterior to the vertebral column, posterior to
  the sternum

3
Coverings of the Heart Anatomy

• Pericardium a double-walled sac around the
  heart composed of
• A superficial fibrous pericardium
• A deep two-layer serous pericardium
• The parietal layer lines the internal surface of
  the fibrous pericardium
• The visceral layer or epicardium lines the
  surface of the heart
• They are separated by the fluid-filled
  pericardial cavity

Coverings of the Heart Physiology

• The pericardium
• Protects and anchors the heart
• Prevents overfilling of the heart with blood
• Allows for the heart to work in a relatively
  friction-free environment

4
Pericardial Layers of the Heart
5
Heart Wall

• Epicardium visceral layer of the serous
  pericardium
• Myocardium cardiac muscle layer forming the
  bulk of the heart
• Fibrous skeleton of the heart crisscrossing,
  interlacing layer of connective tissue
• Endocardium endothelial layer of the inner
  myocardial surface

6
External Heart Major Vessels of the Heart
(Anterior View)

• Vessels returning blood to the heart include
• Superior and inferior venae cavae
• Right and left pulmonary veins
• Vessels conveying blood away from the heart
  include
• Pulmonary trunk, which splits into right and left
  pulmonary arteries
• Ascending aorta (three branches)
  brachiocephalic, left common carotid, and
  subclavian arteries

Vessels that Supply/Drain the Heart

• Arteries right and left coronary (in
  atrioventricular groove), marginal, circumflex,
  and anterior interventricular arteries
• Veins small cardiac, anterior cardiac, and
  great cardiac veins

7
External Heart Anterior View
8
External Heart Major Vessels of the Heart
(Posterior View)

• Vessels returning blood to the heart include
• Right and left pulmonary veins
• Superior and inferior venae cavae
• Vessels conveying blood away from the heart
  include
• Aorta
• Right and left pulmonary arteries

External Heart Vessels that Supply/Drain the
Heart (Posterior View)

• Arteries right coronary artery (in
  atrioventricular groove) and the posterior
  interventricular artery (in interventricular
  groove)
• Veins great cardiac vein, posterior vein to
  left ventricle, coronary sinus, and middle
  cardiac vein

9
External Heart Posterior View
10
Gross Anatomy of Heart Frontal Section
11
Atria of the Heart

• Atria are the receiving chambers of the heart
• Each atrium has a protruding auricle
• Pectinate muscles mark atrial walls
• Blood enters right atria from superior and
  inferior venae cavae and coronary sinus
• Blood enters left atria from pulmonary veins

Ventricles of the Heart

• Ventricles are the discharging chambers of the
  heart
• Papillary muscles and trabeculae carneae muscles
  mark ventricular walls
• Right ventricle pumps blood into the pulmonary
  trunk
• Left ventricle pumps blood into the aorta

12
Pathway of Blood Through the Heart and Lungs

• RA ? tricuspid valve ? RV
• RV ? pulmonary semilunar valve ? pulmonary
  arteries ? lungs
• Lungs ? pulmonary veins ? LA
• LA ? bicuspid valve ? LV
• LV ? aortic semilunar valve ? aorta
• Aorta ? systemic circulation

For simplicity, the actual number of 2 pulmonary
arteries and 4 pulmonary veins has been reduced
to one each
13
Coronary Circulation

• Coronary circulation is the blood supply to the
  heart muscle itself
• Ensures blood delivery to heart even if major
  vessels are occluded

Arterial Supply
Venous Supply
14
Heart Valves

• Heart valves ensure unidirectional blood flow
  through the heart
• Atrioventricular (AV) valves lie between the
  atria and the ventricles
• AV valves prevent backflow into the atria when
  ventricles contract
• Chordae tendineae anchor AV valves to papillary
  muscles
• Aortic semilunar valve lies between the left
  ventricle and the aorta
• Pulmonary semilunar valve lies between the right
  ventricle and pulmonary trunk
• Semilunar valves prevent backflow of blood into
  the ventricles

15
Heart Valves
16
Atrioventricular Valve Function
17
Semilunar Valve Function
18
Microscopic Anatomy of Heart Muscle

• Cardiac muscle is striated, short, fat, branched,
  and interconnected
• The connective tissue endomysium acts as both
  tendon and insertion
• Intercalated discs anchor cardiac cells together
  and allow free passage of ions
• Heart muscle behaves as a functional syncytium

19
Microscopic Anatomy of Heart Muscle
20
Cardiac Muscle Contraction

• Heart muscle
• Is stimulated by nerves and is self-excitable
  (automaticity)
• Contracts as a unit
• Has a long (250 ms) absolute refractory period
• Cardiac muscle contraction is similar to skeletal
  muscle contraction

21
Heart Physiology Intrinsic Conduction System

• Autorhythmic cells
• Initiate action potentials
• Have unstable resting potentials called pacemaker
  potentials
• Use calcium influx (rather than sodium) for
  rising phase of the action potential

22
Pacemaker and Action Potentials of the Heart
23
Heart Physiology Sequence of Excitation

• Sinoatrial (SA) node generates impulses about 75
  times/minute
• Atrioventricular (AV) node delays the impulse
  approximately 0.1 second
• Impulse passes from atria to ventricles via the
  atrioventricular bundle (bundle of His)
• AV bundle splits into two pathways in the
  interventricular septum (bundle branches)
• Bundle branches carry the impulse toward the apex
  of the heart
• Purkinje fibers carry the impulse to the heart
  apex and ventricular walls

24
Heart Physiology Sequence of Excitation
25
Heart Excitation Related to ECG
26
Extrinsic Innervation of the Heart

• Heart is stimulated by the sympathetic
  cardioacceleratory center
• Heart is inhibited by the parasympathetic
  cardioinhibitory center

27
Electrocardiography

• Electrical activity is recorded by
  electrocardiogram (ECG)
• P wave corresponds to depolarization of SA node
• QRS complex corresponds to ventricular
  depolarization
• T wave corresponds to ventricular repolarization
• Atrial repolarization record is masked by the
  larger QRS complex

28
Heart Sounds

• Heart sounds (lub-dup) are associated with
  closing of heart valves
• First sound occurs as AV valves close and
  signifies beginning of systole
• Second sound occurs when SL valves close at the
  beginning of ventricular diastole

Cardiac Cycle

• Cardiac cycle refers to all events associated
  with blood flow through the heart
• Systole contraction of heart muscle
• Diastole relaxation of heart muscle

29
Phases of the Cardiac Cycle

• Ventricular filling mid-to-late diastole
• Heart blood pressure is low as blood enters atria
  and flows into ventricles
• AV valves are open, then atrial systole occurs
• Ventricular systole
• Atria relax
• Rising ventricular pressure results in closing of
  AV valves
• Isovolumetric contraction phase
• Ventricular ejection phase opens semilunar valves
• Isovolumetric relaxation early diastole
• Ventricles relax
• Backflow of blood in aorta and pulmonary trunk
  closes semilunar valves
• Dicrotic notch brief rise in aortic pressure
  caused by backflow of blood rebounding off
  semilunar valves

30
Cardiac Output (CO) and Reserve

• CO is the amount of blood pumped by each
  ventricle / min
• CO is the product of heart rate (HR) and stroke
  volume (SV)
• HR is the number of heart beats per minute
• SV is the amount of blood pumped out by a
  ventricle per beat
• Cardiac reserve is the difference between resting
  and maximal CO

Cardiac Output Example

• CO (ml/min) HR (75 beats/min) x SV (70 ml/beat)
• CO 5250 ml/min (5.25 L/min)

31
Factors Affecting Stroke Volume
Regulation of Stroke Volume

• SV end diastolic volume (EDV) minus end
  systolic volume (ESV)
• EDV amount of blood collected in a ventricle
  during diastole
• ESV amount of blood remaining in a ventricle
  after contraction

• Preload amount ventricles are stretched by
  contained blood
• Contractility contractile force due to factors
  other than EDV
• Afterload back pressure exerted by blood in the
  large arteries leaving the heart

Frank-Starling Law of the Heart

• Preload, or degree of stretch, of cardiac muscle
  cells before they contract is the critical factor
  controlling stroke volume
• Slow heartbeat and exercise increase venous
  return to the heart, increasing SV
• Blood loss and extremely rapid heartbeat decrease
  SV

32
Extrinsic Factors Influencing Stroke Volume

• Contractility is the increase in contractile
  strength, independent of stretch and EDV
• Increase in contractility comes from
• Increased sympathetic stimuli
• Certain hormones
• Ca2 and some drugs
• Agents/factors that decrease contractility
  include
• Acidosis
• Increased extracellular K
• Calcium channel blockers

33
Contractility and Norepinephrine

• Sympathetic stimulation releases norepinephrine
  and initiates a cyclic AMP second-messenger system

34
Regulation of Heart Rate

• Positive chronotropic factors increase heart rate
• Negative chronotropic factors decrease heart rate

Regulation of Heart Rate Autonomic Nervous
System

• Sympathetic nervous system (SNS) stimulation is
  activated by stress, anxiety, excitement, or
  exercise
• Parasympathetic nervous system (PNS) stimulation
  is mediated by acetylcholine and opposes the SNS
• PNS dominates the autonomic stimulation, slowing
  heart rate and causing vagal tone

35
Chapter 19

• The Cardiovascular System Blood Vessels

36
Blood Vessels

• Blood is carried in a closed system of vessels
  that begins and ends at the heart
• The three major types of vessels are arteries,
  capillaries, and veins
• Arteries carry blood away from the heart,
• Veins carry blood toward the heart
• Capillaries contact tissue cells and directly
  serve cellular needs

Generalized Structure of Blood Vessels

• Arteries and veins are composed of three tunics
  tunica interna, tunica media, and tunica
  externa
• Lumen central blood-containing space surrounded
  by tunics
• Capillaries are composed of endothelium with
  sparse basal lamina

37
Generalized Structure of Blood Vessels
38
Tunics

• Tunica interna (tunica intima)
• Endothelial layer that lines the lumen of all
  vessels
• In vessels larger than 1 mm, a subendothelial
  connective tissue basement membrane is present
• Tunica media
• Smooth muscle and elastic fiber layer, regulated
  by sympathetic nervous system
• Controls vasoconstriction/vasodilation of vessels
• Tunica externa (tunica adventitia)
• Collagen fibers that protect and reinforce
  vessels
• Larger vessels contain vasa vasorum

39
Elastic (Conducting) Arteries

• Thick-walled arteries near the heart e.g. the
  aorta
• Large lumen allow low-resistance conduction of
  blood
• Contain elastin in all three tunics
• Withstand large blood pressure fluctuations
• Allow blood to flow fairly continuously through
  the body

Muscular (Distributing) Arteries and Arterioles

• Muscular arteries distal to elastic arteries
• More smooth muscle and less elastic tissue
• Active in vasoconstriction
• Arterioles smallest arteries lead to capillary
  beds
• Control flow into capillary beds via vasodilation
  and constriction

40
Capillaries

• Capillaries are the smallest blood vessels
• Walls consisting of a thin tunica interna, one
  cell thick
• Allow only a single RBC to pass at a time
• Pericytes on the outer surface stabilize their
  walls
• There are three structural types of capillaries
• continuous,
• fenestrated, and
• sinusoids

41
Continuous Capillaries

• Continuous capillaries are abundant in the skin
  and muscles, and have
• Endothelial cells that provide an uninterrupted
  lining
• Adjacent cells that are held together with tight
  junctions
• Intercellular clefts of unjoined membranes that
  allow the passage of fluids
• Continuous capillaries of the brain
• Have tight junctions completely around the
  endothelium
• Constitute the blood-brain barrier

42
Fenestrated Capillaries

• Found wherever active capillary absorption or
  filtrate formation occurs (e.g., small
  intestines, endocrine glands, and kidneys)
• Characterized by
• An endothelium riddled with pores (fenestrations)
• Greater permeability to solutes and fluids than
  other capillaries

43
Sinusoids

• Highly modified, leaky, fenestrated capillaries
  with large lumens
• Found in the liver, bone marrow, lymphoid tissue,
  and in some endocrine organs
• Allow large molecules (proteins and blood cells)
  to pass between the blood and surrounding tissues
• Blood flows sluggishly, allowing for modification
  in various ways

44
Capillary Beds

• A microcirculation of interwoven networks of
  capillaries
• Vascular shunts metarteriole thoroughfare
  channel connecting an arteriole directly with a
  postcapillary venule
• True capillaries 10 to 100 per capillary bed,
  capillaries branch off the metarteriole and
  return to the thoroughfare channel at the distal
  end of the bed

45
Blood Flow Through Capillary Beds

• Precapillary sphincter
• Cuff of smooth muscle that surrounds each true
  capillary
• Regulates blood flow into the capillary
• Blood flow is regulated by vasomotor nerves and
  local chemical conditions, so it can either
  bypass or flood the capillary bed

46
Venous System Venules

• Are formed when capillary beds unite
• Allow fluids and WBCs to pass from the
  bloodstream to tissues
• Postcapillary venules smallest venules,
  composed of endothelium and a few pericytes
• Large venules have one or two layers of smooth
  muscle (tunica media)

47
Venous System Veins

• Veins are
• Formed when venules converge
• Composed of three tunics, with a thin tunica
  media and a thick tunica externa consisting of
  collagen fibers and elastic networks
• Capacitance vessels (blood reservoirs) that
  contain 65 of the blood supply
• Veins have much lower blood pressure and thinner
  walls than arteries
• To return blood to the heart, veins have special
  adaptations
• Large-diameter lumens, which offer little
  resistance to flow
• Valves (resembling semilunar heart valves), which
  prevent backflow of blood
• Venous sinuses specialized, flattened veins
  with extremely thin walls (e.g., coronary sinus
  of the heart and dural sinuses of the brain)

48
Vascular Anastomoses

• Merging blood vessels, more common in veins than
  arteries
• Arterial anastomoses provide alternate pathways
  (collateral channels) for blood to reach a given
  body region
• If one branch is blocked, the collateral channel
  can supply the area with adequate blood supply
• Thoroughfare channels are examples of
  arteriovenous anastomoses

49
Blood Flow

• Actual volume of blood flowing through a vessel,
  an organ, or the entire circulation in a given
  period
• Is measured in ml per min.
• Is equivalent to cardiac output (CO), considering
  the entire vascular system
• Is relatively constant when at rest
• Varies widely through individual organs,
  according to immediate needs

Blood Pressure (BP)

• Force per unit area exerted on the wall of a
  blood vessel by its contained blood
• Expressed in millimeters of mercury (mm Hg)
• Measured in reference to systemic arterial BP in
  large arteries near the heart
• The differences in BP within the vascular system
  provide the driving force that keeps blood moving
  from higher to lower pressure areas

50
Resistance

• Resistance opposition to flow
• Measure of the amount of friction blood
  encounters as it passes through vessels
• Generally encountered in the systemic circulation
• Referred to as peripheral resistance (PR)
• The three important sources of resistance are
  blood viscosity, total blood vessel length, and
  blood vessel diameter

Resistance Factors Viscosity and Vessel Length

• Resistance factors that remain relatively
  constant are
• Blood viscosity thickness or stickiness of
  the blood
• Blood vessel length the longer the vessel, the
  greater the resistance encountered

51
Resistance Factors Blood Vessel Diameter

• Changes in vessel diameter are frequent and
  significantly alter peripheral resistance
• Resistance varies inversely with the fourth power
  of vessel radius (one-half the diameter)
• For example, if the radius is doubled, the
  resistance is 1/16 as much
• Small-diameter arterioles are the major
  determinants of peripheral resistance
• Fatty plaques from atherosclerosis
• Cause turbulent blood flow
• Dramatically increase resistance due to turbulence

52
Blood Flow, Blood Pressure, and Resistance

• Blood flow (F) is directly proportional to the
  difference in blood pressure (?P) between two
  points in the circulation
• If ?P increases, blood flow speeds up if ?P
  decreases, blood flow declines
• Blood flow is inversely proportional to
  resistance (R)
• If R increases, blood flow decreases
• R is more important than ?P in influencing local
  blood pressure

53
Systemic Blood Pressure

• The pumping action of the heart generates blood
  flow through the vessels along a pressure
  gradient, always moving from higher- to
  lower-pressure areas
• Pressure results when flow is opposed by
  resistance
• Systemic pressure
• Is highest in the aorta
• Declines throughout the length of the pathway
• Is 0 mm Hg in the right atrium
• The steepest change in blood pressure occurs in
  the arterioles

54
Arterial Blood Pressure

• Arterial BP reflects two factors of the arteries
  close to the heart
• Their elasticity (compliance or distensibility)
• The amount of blood forced into them at any given
  time
• Blood pressure in elastic arteries near the heart
  is pulsatile (BP rises and falls)
• Systolic pressure pressure exerted on arterial
  walls during ventricular contraction
• Diastolic pressure lowest level of arterial
  pressure during a ventricular cycle
• Pulse pressure the difference between systolic
  and diastolic pressure
• Mean arterial pressure (MAP) pressure that
  propels the blood to the tissues
• MAP diastolic pressure 1/3 pulse pressure

55
Capillary Blood Pressure

• Capillary BP ranges from 20 to 40 mm Hg
• Low capillary pressure is desirable because high
  BP would rupture fragile, thin-walled capillaries
• Low BP is sufficient to force filtrate out into
  interstitial space and distribute nutrients,
  gases, and hormones between blood and tissues

Venous Blood Pressure

• Venous BP is steady and changes little during the
  cardiac cycle
• The pressure gradient in the venous system is
  only about 20 mm Hg
• A cut vein has even blood flow a lacerated
  artery flows in spurts

56
Factors Aiding Venous Return

• Venous BP alone is too low to promote adequate
  blood return and is aided by the
• Respiratory pump pressure changes created
  during breathing suck blood toward the heart by
  squeezing local veins
• Muscular pump contraction of skeletal muscles
  milk blood toward the heart
• Valves prevent backflow during venous return

57
Maintaining Blood Pressure

• Maintaining blood pressure requires
• Cooperation of the heart, blood vessels, and
  kidneys
• Supervision of the brain
• The main factors influencing blood pressure are
• Cardiac output (CO)
• Peripheral resistance (PR)
• Blood volume
• Blood pressure CO x PR
• Blood pressure varies directly with CO, PR, and
  blood volume

58
Controls of Blood Pressure

• Short-term controls
• Are mediated by the nervous system and bloodborne
  chemicals
• Counteract moment-to-moment fluctuations in blood
  pressure by altering peripheral resistance
• Long-term controls regulate blood volume

59
Short-Term Mechanisms Neural Controls

• Neural controls of peripheral resistance
• Alter blood distribution to respond to specific
  demands
• Maintain MAP by altering blood vessel diameter
• Neural controls operate via reflex arcs
  involving
• Baroreceptors
• Vasomotor centers of the medulla and vasomotor
  fibers
• Vascular smooth muscle

60
Short-Term Mechanisms Vasomotor Center

• Vasomotor center a cluster of sympathetic
  neurons in the medulla that oversees changes in
  blood vessel diameter
• Maintains blood vessel tone by innervating smooth
  muscles of blood vessels, especially arterioles
• Cardiovascular center vasomotor center plus the
  cardiac centers that integrate blood pressure
  control by altering cardiac output and blood
  vessel diameter

Short-Term Mechanisms Vasomotor Activity

• Sympathetic activity causes
• Vasoconstriction and a rise in blood pressure if
  increased
• Blood pressure to decline to basal levels if
  decreased
• Vasomotor activity is modified by
• Baroreceptors (pressure-sensitive),
  chemoreceptors (O2, CO2, and H sensitive),
  higher brain centers, bloodborne chemicals, and
  hormones

61
Short-Term Mechanisms Baroreceptor-Initiated
Reflexes

• Increased blood pressure stimulates the
  cardioinhibitory center to
• Increase vessel diameter
• Decrease heart rate, cardiac output, peripheral
  resistance, and blood pressure
• Declining blood pressure stimulates the
  cardioacceleratory center to
• Increase cardiac output and peripheral resistance
• Low blood pressure also stimulates the vasomotor
  center to constrict blood vessels

62
Short-Term Mechanisms Chemical Controls

• Blood pressure is regulated by chemoreceptor
  reflexes sensitive to oxygen and carbon dioxide
• Prominent chemoreceptors are the carotid and
  aortic bodies
• Reflexes that regulate blood pressure are
  integrated in the medulla
• Higher brain centers (cortex and hypothalamus)
  can modify BP via relays to medullary centers

63
Chemicals that Increase Blood Pressure

• Adrenal medulla hormones norepinephrine and
  epinephrine increase blood pressure
• Antidiuretic hormone (ADH) causes intense
  vasoconstriction in cases of extremely low BP
• Angiotensin II kidney release of renin
  generates angiotensin II, which causes intense
  vasoconstriction
• Endothelium-derived factors endothelin and
  prostaglandin-derived growth factor (PDGF) are
  both vasoconstrictors

64
Chemicals that Increase Blood Pressure

• Atrial natriuretic peptide (ANP) causes blood
  volume and pressure to decline
• Nitric oxide (NO) has brief but potent
  vasodilator effects
• Inflammatory chemicals histamine, prostacyclin,
  and kinins are potent vasodilators
• Alcohol causes BP to drop by inhibiting ADH