Chapter 7

Cardiovascular System:

• Delivers O₂ nutrients 

• Removes CO₂, other waste

• Transports hormones, other molecules

• Temperature balance and fluid regulation

• Acid-base balance 

• Immune function 

Three major circulatory elements

1. A pump (heart)

2. Channels or tubes (blood vessels)

3. A fluid medium (blood vessels)

• Heart generates pressure to drive blood through vessels

• Blood flow must meet metabolic demands

Blood Flow Through Heart

Right heart: Pulmonary circulation

• Pumps deoxygenated blood from body to lungs

• Superior, inferior vena cava→ RA→ tricuspid valve → RV→ pulmonary valve→ pulmonary arteries→ lungs

Left heart: systemic circulation

• Pumps oxygenated blood from lungs to body 

• Lungs→ pulmonary veins→ LA→ mitral valve→ LV → aortic valve→ aorta

Myocardium: cardiac muscle

• LV has most myocardium

  • Must pump blood through systemic circulation

  • Thicker wall than right ventricle

  • LV hypertrophies with exercise and with disease

  • Exercise adaptations are different than disease adaptations

• Only one fiber type (similar to type I)

  • High capillary density

  • High number of mitochondria

  • Striated

• Cardiac muscle fibers connected by intercalated discs

  • Desmosomes: hold cells together

  • Gap junctions rapidly conduct action potentials  

Cardiac muscle fibers

• Single nucleus

• located in

Skeletal muscle fibers

• Multiple nuclei

• Three different types of fibers

  • type I, IIa, and type IIx

• Fibers are long, cylindrical and unbranched. Muscles striated with fibers arranged in parallel bundles

• Activity is that it contracts as needed to produce movement or resist gravity through excitation-contraction coupling

Calcium-induced calcium release

• Skeletal muscle cells

  • Ca2+ released from SR

• Myocardial cells

  • Calcium-induced calcium release 

Intrinsic Control of Heart Activity: Cardiac Control System

• Spontaneous rhythmically: special heart cells generate and spread electrical signal

• Sinoatrial (SA) node

• Atrioventricular (AV) node

• Purkinje fibers

Electrical signal spreads via gap junctions

• Intrinsic HR: 100 beats/min

• Observed in heart transplant patients (no neural innervation)

SA node: initiates contraction signal 

• Pacemaker cells in upper posterior RA wall

• Signal spreads from SA node via RA/LA to AV node

• Stimulates RA, LA contraction

AV node: delays, relays signal to ventricles

• In RA wall near center of heart

• Delay allows RA, LA to contract before RV, LV

• Relays signal to AV bundle after delay 

AV bundle: relays signals to RV, LV

• Travels along interventricular septum

• Divides into right and left bundle branches

• Spread throughout entire ventricle wall

• Stimulate RV, LV contraction

Extrinsic Control of Heart Activity: Autonomic Nervous System

• Parasympathetic- reaches heart via vagus nerve

• Carries impulse to SA, AV nodes

  • Releases acetylcholine, hyperpolarizes cells

  • Decreases HR, force of contraction 

Decreases HR below intrinsic HR

• Intrinsic HR: 100 beats/min

• Normal resting HR (RHR): 60 to 100 beats/min

• Elite endurance athlete: 35 beats/min    

Sympathetic has opposite effects of parasympathetic

• Carries impulses to SA, AV nodes

  • Release norepinephrine, facilitates depolarization

  • Increases HR, force of contraction

  • Endocrine system can have similar effect (epinephrine, norepinephrine)

• Increases HR above intrinsic HR

  • Determines HR during physical, emotional stress 

ECG has 10 electrodes, 12 leads

• Different electrical views

• Diagnostic tool for coronary artery disease

Three basic phases 

• P wave: atrial depolarization

• QRS complex: ventricular depolarization

• T wave: ventricular repolarization

Stroke Volume (SV): volume of blood pumped in one heartbeat

• During systole, most (not all) blood ejected

• EDV-ESV=SV

• 100mL-40mL= 60 mL

Ejection fraction (EDF): percent of EDV pumped

• SV/EDV=EF

• 60 mL/100mL=0.6 or 60%

• Clinical index of heart contraction failure

Cardiac Output (Q.)

• Total volume of blood pumped per minute

• Q.=HR x SV

  • RHR -70 beats/min, standing SV -70 mL/beat

  • 70 beats/min x 70 mL/beat=4900 mL/min

  • Use L/min (4.9 L/min)

• Resting cardiac output 4.2 to 5.6 L/min

  • Average total blood volume 5L

  • Total blood volume circulates once every minute

The Vascular System

• Arteries: carry blood away from heart

• Arterioles: control blood flow, feed capillaries

• Capillaries: site of nutrient and waste exchange

• Venules: collect blood from capillaries 

• Veins: carry blood from venules back to heart

Blood Pressure

• Systolic Blood pressure (SBP)

  • Highest pressure in artery (during systole)

  • Top number 110 to 120 mmHg

• Diastolic Blood pressure (DBP)

  • Lowest pressure in artery (during diastole)

  • Bottom number 70 to 80 mmHg

• Mean arterial pressure (MAP)

  • Average pressure over entire cardiac cycle

  • MAP= ⅔ DBP + ⅓ SBP or DBP + ⅓ PP (Pulse Pressure)

General Hemodynamics

• Blood flow: required by all tissues

• Pressure: force that drives flow

  • Provided by heart contraction

  • Blood flows from region of high pressure (LV, arteries) to region of low pressure (veins, RA)

  • Pressure gradient =100 mmHg- 0 mmHg

• Resistance: force that opposes flow

  • Provide by physical properties of vessels

  • R= [nL/r^4] → radius most important factor 

Blood flow Q.= pressure/R

Pressure = Q. x R

R

• Small changes in arteriole radius affect R

• Vasoconstriction, vasodilation

Arterioles: resistance vessels

• Control systemic R

• Site of most potent VC and VD

• Responsible for 70 to 80% of P drop from LV to RA  

Vasoconstrictor (+) and vasodilator (-) influences acting upon arterioles to determine vascular tone. extrinsic factors are from outside of the tissue, whereas intrinsic factors are from within the tissue or vessel. Tone is determined by balance of all the factors 

Intrinsic Control of Blood Flow

• The ability of the local tissues to dilate or constrict the arterioles that serve them & alter regional blood flow depending on immediate needs of those tissues

• Metabolic 

  • In response to increased oxygen demand, strongest stimulus

  • Drop in pH, increase in hydrogen and lactic acid, decrease oxygen. Increase CO₂, increase temperature

  • Result in local vasodilation 

• Endothelium-mediated vasodilation

  • Innermost layer of artery (endothelial) 

  • Endothelial cells line inside of artery

  • Very sensitive to movement of red blood cells

• Myogenic contraction

  • Arteries and vessels sensitive to pressure changes

  • Can dictate whether vessel dilates or constricts

    • Decrease in pressure is dilation, increase in pressure is constriction

Myogenic mechanism

• How arteries & arterioles react to an increase or decrease of blood pressure to allow for constant blood flow within the vessel

• Increased BP= vasoconstriction= drop in blood flow

• Goal= consistent blood flow through smaller arterioles and capillaries

Local control of Muscle Blood

• Functional Sympatholysis- vasoactive molecules released from the active skeletal muscle & endothelium infinity local sympathetic vasoconstriction   

Extrinsic Neural Control of Blood Flow

• Upstream of local, intrinsic control 

• At rest vessels are moderately 

• Redistribution of flow at organ, system level

• Sympathetic nervous system innervates smooth muscle in arteries and arterioles

  • Baseline sympathetic activity→ vasomotor tone

  • Sympathetic activity → VC

  • Sympathetic activity → VC (passive VD)

Distribution of Venous Blood

• At rest, veins contain ⅔ blood volume

  • High capacity of blood

  • Elastic, balloonlike vessel walls

  • Serve as blood reservoir 

• Venous réservoir can be liberated sent back to heart and into arteries

  • Sympathetic stimulation

  • Venoconstriction 

Integrative Control of Blood Pressure

• Blood pressure maintained by autonomic      

• Baroreceptors  

  • Sensitive to changes in arterial pressure

  • Afferent signals from baroreceptor to brain

  • Efferent signals from brain to heart, vessels 

  • Adjust arterial pressure back to normal

•  Chemoreceptors, mechanoreceptors in muscle  

Return of Blood to Heart

• Upright posture makes venous return to heart more difficult 

• Three mechanisms assist venous return 

  • One-way venous valves

  • Muscle pump 

  • Respiratory pump  

Blood

• Three major functions

  • Transportation (O₂ nutrients, waste)

  • Temperature regulation

  • Acid-base (pH) balance

• Blood volume: 5 to 6 L in men, 4 to 5 L in women

• Whole blood= plasma + formed elements

• Made of 55% plasma and rest made up of >99% RBC and <1% WBC and platelets

Red Blood Cells

• No nucleus, cannot reproduce 

  • Replaced regularly via hematopoiesis

  • Life span 4 months

  • Produced and destroyed at equal rates

• Hemoglobin

  • Oxygen-transporting protein in red blood cells (4 O₂/hemoglobin) 

  • Heme (pigment, iron O₂) + globin (protein)

  • 250 million hemoglobin/red blood cells

  • Oxygen-carrying capacity: 20 mL O₂/100 mL blood

Blood Viscosity 

• Thickness of blood (due to red blood cells)

• Twice as viscous as water

• Viscosity as hematocrit

• Plasma volume must as red blood cells

  • Occurs in athletes after training, acclimation 

  • Hematocrit and viscosity remains stable

Otherwise, blood flow or O₂ transport may suffer