Ch.15 The Cardiovascular System/Ch.16 Cardiovascular Regulation and…
Ch.15 The Cardiovascular System/Ch.16 Cardiovascular Regulation and Integration/Ch.17 Functional Capacity of the Cardiovascular System
Cardiovascular System Components
The system consists of four components; 1) A pump that provides continuous linkage with the other three components, 2) A high-pressure distribution circuit, 3) Exchange vessels, 4) A low-pressure collection and return circuit
The heart provides the impetus for blood flow.
The heart muscle, or myocardium, represents a homogenous (or homologous) form of striated muscle similar to the slow-twitch fibers in skeletal muscle with high capillary density and numerous mitochondria.
One can view the heart as two separate pumps:
Right side hollow chambers with two crucial functions; 1) Receives blood (deoxygenated) returning from throughout the body, 2) pumps blood to the lungs for aeration through the pulmonary circulation
Left side with its two crucial functions; 1) receives oxygenated blood from the lungs, 2) pumps blood into the thick-walled, muscular aorta for distribution throughout the body in the systemic circulation
At rest, the myocardium requires considerable oxygen relative to its blood flow; it extracts about 70-80% of the oxygen from the blood in the coronary vessels.
The magnitude of myocardial oxygen extraction differs considerably from most other tissues, which use only about one-fourth of their available oxygen at rest.
Consequently, a proportionate increase in coronary blood flow in physical activity essentially provides the sole mechanism to increase myocardial oxygen supply.
The myocardium relies almost exclusively on energy released in aerobic reactions. It has limited anaerobic energy-generating capacity.
Its muscle fibers contain the greatest mitochondrial concentration of all tissues, with exceptional capacity for long-chain fatty acid catabolism as a primary means for ATP resynthesis.
Glucose, fatty acids and lactate formed from glycolysis in skeletal muscle provide the energy for myocardial functioning.
At rest, these three substrates contribute to ATP resynthesis, with most energy coming from fatty acid breakdown (60-70%).
Following a meal, glucose becomes the preferred energy substrate. In essence, the heart uses for energy whatever substrate it "sees" on a physiologic level.
1 more item...
A study followed nine patients over a seven year training period, RPP (rate-pressure product) increased 11.5% before ischemic symptoms appeared during graded exercise testing.
These findings provide indirect evidence for improved myocardial oxygenation, probably from greater coronary vascularization or reduced obstruction from the training adaptation.
The Arterial System
The arteries compose the high pressure tubing that propels oxygen-rich blood to the tissues.
Arterioles are smaller arterial branches that together with arteries form a highly efficient network to distribute blood throughout the body.
The walls of the arterioles contain smooth muscle that will constrict or relax to regulate blood flow to the periphery.
Arterial blood pressure reflects the combined effects of arterial blood flow each minute (cardiac output) and resistance to that flow in the peripheral vasculature.
The relationship can be expressed as: Blood pressure = Cardiac output x Total peripheral resistance
Systolic blood pressure is the highest pressure generated during left ventricular contraction (termed systole).
Systolic blood pressure provides an estimate of the work of the heart and the force that blood exerts against the arterial walls during ventricular systole.
Diastolic blood pressure refers to arterial blood pressure during the cardiac cycle's relaxation phase (termed diastole).
Diastolic blood pressure indicates peripheral resistance or the ease with which blood flows from the arterioles into the capillaries.
With high peripheral resistance, pressure within the arteries after systole does not rapidly dissipate.
Resistance to peripheral blood flow decreases dramatically during strenuous activity, when systolic pressure increases considerably more than diastolic pressure and cardiac output increases six to seven times the resting value in an elite endurance athlete.
Effective prevention strategies against a chronic rise in blood pressure include daily physical activity for at least 30min a day at a moderate to vigorous level.
Regular aerobic physical activity lowers systolic and diastolic blood pressure while more vigorous activity produces a greater lowering effect on diastolic pressure than more moderate physical activity.
Adequate potassium, calcium and magnesium intake can help as well.
Straining muscle actions, particularly the concentric (shortening) and/or static phase of muscle actions, mechanically compresses the peripheral arterial vessels that supply active muscles.
This dramatically increases total peripheral resistance and reduces muscle perfusion.
Muscle blood flow decreases proportionally to the percentage of maximum force capacity exerted.
The magnitude of the hypertensive response relates directly to the intensity of effort and quantity of muscle mass activated.
1 more item...
During rhythmic muscular activity (eg, jogging, swimming, bicycling), vasodilation in the active muscles reduces total peripheral resistance to enhance blood flow through large portions of the peripheral vasculature.
Alternated muscle contraction and relaxation also provide an effective force to propel blood through the vascular circuit and return it to the heart.
Increased blood flow during rhythmic, stead-rate activities rapidly increases systolic pressure during the first few minutes.
Blood pressure then levels off at 140-160 mm Hg for healthy men and women. As activity continues systolic pressure gradually declines because the arterioles in the active muscles continue to dilate, further reducing peripheral resistance to blood flow.
1 more item...
After an initial rapid rise from the resting level, systolic blood pressure increases linearly with exercise intensity, while diastolic pressure remains stable or decreases slightly at the higher activity levels.
1 more item...
Physical activity with the arms produces considerably higher systolic and diastolic blood pressures and consequently greater cardiovascular strain than leg activity performed at a given percentage of VO2max in each form of exertion.
Upon completion of a single bout of sub maximal physical activity, blood pressure temporarily falls below pre-exercise levels for normotensive and hypertensive individuals from an unexplained peripheral vasodilation.
The hypotensive response to activity can last up to 12hr. It occurs in response to either low or moderate intensity aerobic activity or resistance exercise.
One explanation for post exercise hypotension proposes that a considerable quantity of blood remains pooled in the visceral organs and/or skeletal muscle vascular beds during recovery.
The venous pooling effect reduces central blood volume, which in turn decreases atrial filling pressure and lowers systemic arterial blood pressure.
1 more item...
At rest, the skin receives approximately 5% of the 5L of blood pumped by the heart each minute.
During physical activity in a hot, humid climate, up to 20% of the total blood flow diverts to the body's surface for one major purpose; to dissipate heat.
The arterioles branch and form smaller and less muscular vessels called metarterioles. These vessels end in a meshwork of microscopically small blood vessels called capillaries, which generally contain 6% of the total blood volume
Capillary density varies depending on a particular tissue's location and function.
The heart has much greater capillary density than skeletal muscle.
The big difference between blood flow during activity compared to rest is that "unused" capillaries open to increase blood flow during activity.
The precapillary sphincter, a ring of smooth muscle that encircles the vessel at its origin, controls capillary diameter.
Two factors trigger relaxation of precapillary sphincters to open more capillaries: 1) driving force of increased local blood pressure plus intrinsic neural control, 2) local metabolites produced in physical activity
Blood flow in active muscles increases almost linearly with exercise intensity up to maximal exertion.
Blood flow velocity relates inversely to the vasculature's cross section.
A huge surface area with a slow rate of blood flow of approximately 0.5 to 1.0 mm x sec at rest provides a highly effective means of exchange between the blood and neighboring tissues.
The Venous System
The continuity of the vascular system progresses as the capillaries feed deoxygenated blood at almost a trickle into the small veins or venules with which they merge.
Blood flow velocity then increases because the cross-sectional area of the venous system is smaller than for capillaries.
The venous system has a low pressure of blood.
There are flap-like valves spaced at short intervals within veins allow blood to flow in only one direction toward the heart.
The low pressure in the venous circuit means that the smallest muscular contractions or even minor pressure changes within the thoracic cavity with breathing, readily compress the veins.
The alternate compression and relaxation of veins, including the one-way action of their valves, provides a "milking" or wringing action that propels blood back to the heart.
2 more items...
Stimulation of the sympathetic cardioaccelerator nerves releases the catecholamines epinephrine and norepinephrine.
These neurohormones accelerate SA node depolarization, causing the heart to beat faster (the chronotropic effect).
Catecholamines also increase myocardial contractility (the inotropic effect) to augment how much blood the heart pumps with each beat.
In general, sympathetic stimulation produces catabolic effects that prepare the body to "fight or flight"
Preganglionic parasympathetic neurons lie within brainstem tissue and the lower spinal cord. They leave brainstem and spinal cord to affect diverse body areas.
When stimulated parasympathetic neurons release acetylcholine, which retards the rate of sinus discharge to slow the heart rate.
A reduced heart rate, or bradycardia, results largely from stimulation of the pair of vagus nerves whose cell bodies originate in the medulla's cardioinhibitory center.
The vagus nerves descend to the thorax and abdominal regions. They carry about 80% of all parasympathetic fibers.
Parasympathetic stimulation excites some tissues including muscles of the iris, gallbladder and bile ducts, bronchi and coronary arteries. Inhibits muscles of gut sphincters, intestines, and skin vasculature.
Parasympathetic stimulation induces all glandular secretions except for those of the sweat glands.
In general, parasympathetic stimulation produces anabolic responses that promote normal function and conserve energy.
At the start of and during low-moderate activity, heart rate increases by inhibition of parasympathetic stimulation.
In strenuous activity heart increases by additional parasympathetic inhibition and direct activation of sympathetic carioaccelerator nerves.
Heart rate acceleration relates directly to activity intensity and duration.
The much slower contribution to heart rate increase from the sympathetic nervous system (triggered by reflex activity and not central command) does not occur until achieving moderate intensity.
Heart Rate Variability
HRV refers to the variation in time intervals between heartbeats.
A wide variation in time intervals generally reflects a "healthy" balance between sympathetic and parasympathetic input to the myocardium, while little variation may reflect a dysfunctional autonomic input.
Low heart rate variability relates to increased risk for heart failure, myocardial infarction and sudden cardiac death.
On the brighter side, regular physical activity promotes an increase in heart rate variability.
Central Command Input
Central command provides the greatest control over exercise heart rate.
Impulses originating in the brain's higher somatomotor central command center continually modulate medullary activity.
Impulses form the "feed forward" central command descend via small afferent nerves through the cardiovascular center in the medulla.
This type of neural control operates during the pre-exercise anticipatory period and during the early stage of exercise.
Motro cortex stimulation of the medulla increases with the size of the muscle mass activated during activity.
Muscle blood flow also increases in anticipation of activity
That response demonstrates training specificity because the magnitude of the pre-exercise increases in mean arterial pressure and decreases in skeletal muscle vascular resistance varies with physical activity intensity, duration, and specific mode of prior training.
Activation of receptors in active joints and muscles also contributes to accelerator input when activity begins.
Friction between the blood and internal vascular wall creates resistance or force that impedes blood flow.
Three factors determine resistance: 1) blood thickness or viscosity, 2) length of the conducting tube, 3) blood vessel radius (probably the most important)
In the body, the transport vessel length remains constant, while blood viscosity varies only slightly under most conditions.
With the pressure differential within the vascular circuit remaining constant, a small change in vessel radius dramatically alters blood flow.
Physiologically, constriction (ie nonactive tissue) and dilation (ie active tissue) of the smaller arterial blood vessels provide the crucial mechanism to regulate regional blood flow.
Skeletal muscle blood flow closely couples to metabolic demands.
At rest, only one of every 30 to 40 capillaries in muscle tissue remain open.
1 more item...
Racial differences in resting blood pressure relate to a lower sensitivity to nitric oxide's dilating action in blacks than in whites.
Cardiac output (Q meaning quantity) expresses the amount of blood pumped by the heart during a 1-min period.
Output from the heart, as with any pump, depends on its rate of pumping (heart rate, HR) and quantity of blood ejected with each stroke (stroke volume, SV).
Cardiac output computes as follows: Cardiac output = heart rate x stroke volume
There are three common methods of measurement to assess the cardiac output of a closed circulatory system in humans:
1) Direct Fick Method; basically divides oxygen consumption by the a-vO2 difference. Requires heart catheterization.
2) Indicator dilution; Uses dye and tracts the dye over a certain duration
3) CO2 rebreathing; Substitutes CO2 consumption for O2 consumption in the Fick equation.
Cardiac Output At Rest
An individual's cardiac output can vary considerably during rest. Influencing factors can include emotional conditions that alter cortical outflow (central command) to the cardioaccelerator nerves and nerves that modulate arterial resistance vessels.
Stroke volume and cardiac output for women (sedentary) average about 25% below values for men; in women, the stroke volume at rest averages 50 to 60 mL.
This "gender difference" generally relates to the average woman's smaller body size.
Endurance training brings the heart's sinus node under greater influence of acetylcholine, the parasympathetic hormone that slow heart rate.
At the same time, resting sympathetic activity decreases.
This longer-term training adaptation partially explains the low resting heart rates of many elite endurance athletes.
Relatively brief training periods exert only a minimal lowering effect on resting heart rte.
Two factors help to explain the large stroke volume and low heart rate (at rest) of endurance trained athletes;
1) Increased vagal (parasympathetic) tone and decreased sympathetic drive, both of which slow the heart
2) Increased blood volume, myocardial contractility and compliance (ability to distend in response to pressure; reduced cardiac stiffness) of the left ventricle, all of which augment the heart's stroke volume.
Cardiac Output During Physical Activity
Systemic blood flow increases directly with intensity of physical activity.
Trained persons generally achieves a slightly lower maximum (but have much higher cardiac outputs) heart rate than a sedentary person of a similar age.
The endurance athlete achieves a large maximal cardiac output solely through a large stroke volume.
in one study, the VO2max of athletes averaged 62.5% larger than the sedentary group.
This paralleled a 60% larger stroke volume.
The maximal heart rates of all groups were similar, making the differences in cardiac output (and VO2max) almost entirely due to differences in maximal stroke volume.
Three physiologic mechanisms increase the heart's stroke volume during physical activity;
1) The first, intrinsic to the myocardium, involves enhanced cardiac filling in diastole followed by a more forceful systolic contraction.
Any factor that increases venous return or slows the heart produces greater ventricular filling or preload during the cardiac cycles diastolic phase.
An increase in end-diastolic volume stretches myocardial fibers and initiates a powerful ejection stroke during contraction.
Enhanced diastolic filling also occurs in swimming because the body's horizontal position optimizes venous return.
2 more items...
2) Neurohormonal influence governs the second mechanism that involves normal ventricular filling with a subsequent forceful ejection and emptying during systole.
3) Training adaptations that expand blood volume and reduce resistance to blood flow in peripheral tissues provides the third mechanism.
Cardiac Output Distribution
Blood generally flows to tissues in proportion to their metabolic demands.
At rest in a thermoneutral environment, cardiac output flows to tissues as follows; Liver 27%, kidneys 22%, muscle 20%, Brain 14%, Skin 6%, Heart 4%, other 7%
During strenuous activity cardiac output goes from 5,000mL to 25,000mL
Muscle receives 84% of that cardiac output
While most of the other tissues decrease their received percentage of cardiac output, their absolute volume received goes up.
Increased oxygen extraction from the available blood supply generally maintains the oxygen needs of tissues with reduced blood flow.
During intense effort, the visceral organs sustain a substantially reduced blood supply for more than 1hr.
Regular aerobic training diminishes the typical vasoconstrictor response to splanchnic and renal issues during sustained exercise, an effect that probably contributes to improved endurance.
At rest, the myocardium normally uses approximately 75% of the oxygen in the blood flowing though the coronary circulation.
1 more item...
A rise in core temperature also redistributes blood to the periphery for body cooling.
Blood flow to the skin, the primary heat-exchange organ, increases during light and moderate activity in response to the rise in core temperature.
During near-maximal effort, the skin restricts its blood flow, redirecting it to active muscle, even in a hot environment.
Cardiac Output And Oxygen Transport
Arterial blood carries about 200mL of oxygen per liter.
If resting cardiac output each minute equals 5L, potentially 1000mL of oxygen becomes available to the body.
A healthy, young adult with a maximum heart rate of 200b per min and stroke volume of 80mL generates a maximum cardiac output of 16 L per min.
Even during maximal activity, hemoglobin saturation with oxygen remains nearly complete, so each liter of arterial blood carries about 200mL.
Consequently, 3200mL of oxygen circulates each minute via a 16L cardiac output.
The brain and skin tissues, for example, do not increase markedly with physical activity, yet they still require a substantial blood supply.
An unmistakable association exists; a low maximal oxygen consumption corresponds closely with a low maximum cardiac output whereas a 5 or 6 L VO2max invariably accompanies a 30-40 L cardiac output.
A 5-6 L increase in blood flow accompanies each 1L increase in oxygen consumption above the resting value, this relationship remains essentially unchanged regardless of activity mode over a broad range of dynamic exercises.
Maximal cardiac output relates to VO2max in the ratio of about 6:1
Teenage and adult females generally exercise at any level of submaximal oxygen consumption with a 5 to 10% larger cardiac output than males.
The 10% lower hemoglobin concentration in women than in men explains this apparent gender difference in sub maximal cardiac output.
Oxygen Extraction: The a-vO2 Difference
If only blood flow increased tissue oxygen supply, then increasing cardiac output from 5L per min at rest to 100 L min during maximum physical activity would achieve the 20 fold oxygen consumption increase common among endurance athletes.
Instead, hemoglobin releases a considerable quantity of its 'reserve" oxygen from blood that perfuses active tissues.
Oxygen consumption during physical activity increases by two mechanisms:
1) Increased total quantity of blood pumped by the heart (ie increased cardiac output)
2) Greater use of the already existing large quantity of oxygen carried by the blood (ie expanded a-vO2 difference)
Resting metabolism consumes about 5mL of oxygen from the 20mL of oxygen in each deciliter of arterial blood that passes through the tissue capillaries.
Thus, 15mL of oxygen or 75% of the blood's original oxygen load still remains bound to hemoglobin.
During Physical Activity
Arterial blood oxygen content varies little from its value of 20mL x dL at rest throughout the full exercise intensity range.
In contrast, mixed-venous oxygen content varies between 12 and 15 mL x dL during rest to a low of 2-4 mL x dL during maximal exertion.
The difference represents oxygen extraction from arterial blood as it circulates throughout the body.
The capacity of each deciliter of arterial blood to carry oxygen (yellow line) increases during physical activity from an increased concentration of red blood cells known as hemoconcentration.
Hemoconcentration results from the progressive moment of fluid from the plasma to the interstitial space by two mechanisms;
1) Increases in capillary hydrostatic pressure as blood pressure rises
2) Metabolic byproducts of exercise metabolism that osmotically draw fluid into tissue spaces from the plasma
Muscle biopsy specimens from the quadriceps femoris muscle show a relatively large ratio of capillaries to muscle fibers in individuals who exhibit large a-vO2 differences during intense activity.
An increased capillary to fiber ratio reflects a positive endurance training adaptation that enlarges the interfere for nutrient and metabolic gas exchange during exercise.
Individual muscle cells' ability to generate energy aerobically represents another important factor that governs oxygen extraction capacity.
1 more item...
Cardiovascular Adjustments To Upper-Body Exercise
Maximal Oxygen Consumption
The highest oxygen consumption during arm exercise averages 20-30% lower than consumption during leg exercise.
Similarly, arm exercise produces lower maximal values for heart rate and pulmonary ventilation.
Submaximal Oxygen Consumption
Submaximal physical activity reverses the pattern for oxygen consumption between upper and lower body exercise observed during maximal effort.
Arm exercise requires greater oxygen consumption than leg exercise at any submaximal power output throughout the comparison range.
Two factors produce this additional oxygen cost at higher intensities of arm cycling:
1) Lower mechanical efficiency in upper-body exercise from the additional energy requirement of static muscle actions that do not contribute to external work
2) Recruitment o additional musculature and hence energy requirement to stabilize the torso during arm exercise.
Any level of submaximal oxygen consumption (or percentage VO2max) or power output with upper-body exercise provides greater physiologic strain than lower-body exercise.
Specifically, submaximal arm exercise produces higher heart rates, pulmonary ventilations, and perceptions of effort than comparable intensities of leg exercise.
Upper-body physical activities places a greater strain (ie greater force per unit muscle, greater percentage of maximum capacity, and more metabolic byproducts) on the relatively smaller upper-body musculature for any submaximum exercise level.
Added strain augments peripheral feedback to the medulla, which increases heart rate and blood pressure.
The elevated heart rate response in submaximal arm exercise probably results from two factors:
1) greater feed-forward stimulation from the brain's central command to the medullary control center.
2) increased feedback stimulation to the medulla from peripheral receptors in active tissue.