The Physiology of the The Physiology of the Cardiovascular SystemCardiovascular System

The Circulatory System, The Heart, Cardiac The Circulatory System, The Heart, Cardiac Action PotentialsAction Potentials

Lectured by Bien Nillos, MDLectured by Bien Nillos, MD

Ten unique characteristics Ten unique characteristics

The system is a closed circle rather than The system is a closed circle rather than being open-ended and linear.being open-ended and linear.

The system is elastic rather than rigid.The system is elastic rather than rigid. The system is filled with liquid at a positive The system is filled with liquid at a positive

mean pressure ("mean cardiovascular mean pressure ("mean cardiovascular pressure"), which exists independent of pressure"), which exists independent of the pumping action of the heart. the pumping action of the heart.

The right and left ventricles, which pump into The right and left ventricles, which pump into the same system that they pump out of, are in the same system that they pump out of, are in series with two interposed vascular beds series with two interposed vascular beds (systemic and pulmonary).(systemic and pulmonary).

The heart fills passively, rather than by The heart fills passively, rather than by actively sucking.actively sucking.

As a consequence of the heart's passive As a consequence of the heart's passive filling, the circulation rate is normally filling, the circulation rate is normally regulated by peripheral-vascular factors, regulated by peripheral-vascular factors, rather than by cardiac variables.rather than by cardiac variables.

The flow from the heart is intermittent, while The flow from the heart is intermittent, while the flow to it is continuous. the flow to it is continuous.

Normally, there is an excess expenditure of Normally, there is an excess expenditure of energy by the heart needed for the circulation energy by the heart needed for the circulation rate imposed by peripheral vascular regulators rate imposed by peripheral vascular regulators ("pump energy excess").("pump energy excess").

Normally, ventricular capacity is in excess of the Normally, ventricular capacity is in excess of the diastolic filling volume ("pump capacity excess").diastolic filling volume ("pump capacity excess").

The slowing effect of any vascular resistance on The slowing effect of any vascular resistance on flow rate depends on its location, with reference flow rate depends on its location, with reference to upstream compliance, as well as its to upstream compliance, as well as its magnitude. magnitude.

three major types of pumps three major types of pumps

PUMP TYPE #1PUMP TYPE #1: : Both sucks and forcibly ejects Both sucks and forcibly ejects

fluid. fluid. This pump uses energy both to This pump uses energy both to

actively fill at its inlet and to empty actively fill at its inlet and to empty its contents at its outlet. its contents at its outlet.

The output in a hydraulic system is The output in a hydraulic system is determined exclusively by two determined exclusively by two pump variables: the stroke rate pump variables: the stroke rate and the stroke volume.. and the stroke volume..

PUMP TYPE #2PUMP TYPE #2: : sucks and blows but, instead of sucks and blows but, instead of

producing a specific flow rate, producing a specific flow rate, creates a specific pressure creates a specific pressure gradient between its inlet and gradient between its inlet and outlet. outlet.

Centrifugal pumps fall into this Centrifugal pumps fall into this category. category.

With this type, two pump factors With this type, two pump factors (rate and power), as well as two (rate and power), as well as two non-pump factors (pressure non-pump factors (pressure and resistance in the system), and resistance in the system), effect output. effect output.

PUMP TYPE #3PUMP TYPE #3: : is passive filling, and does not suck at its is passive filling, and does not suck at its

inlet.inlet. It expends no energy to fill, it only It expends no energy to fill, it only

expends energy to empty. expends energy to empty. This type of pump, even though it does the This type of pump, even though it does the

work and thus produces the flow, is totally work and thus produces the flow, is totally dependent upon external factors to dependent upon external factors to determine the output. determine the output.

THE RIGHT AND LEFT VENTRICLES THE RIGHT AND LEFT VENTRICLES ARE TWO TYPE #3 PUMPSARE TWO TYPE #3 PUMPS::

The heart, like the urinary bladder, is a The heart, like the urinary bladder, is a hollow muscular organ that does not suck hollow muscular organ that does not suck to fill, but produces circulation by ejecting to fill, but produces circulation by ejecting whatever fluid enters it at diastole. whatever fluid enters it at diastole.

During normal function, the heart not only During normal function, the heart not only doesn't develop a pressure negative to the doesn't develop a pressure negative to the intrathoracic pressure, but it offers an intrathoracic pressure, but it offers an impediment to filling because of its limited impediment to filling because of its limited volume-pressure compliance. volume-pressure compliance.

With pumps that cannot suck to With pumps that cannot suck to fill, there must be a positive fill, there must be a positive pressure at the inlets for any pressure at the inlets for any blood to run into the ventricles, in blood to run into the ventricles, in order for there to be any pump order for there to be any pump output output

If there is no pressure in the cardiovascular If there is no pressure in the cardiovascular system, no blood can run into the ventricles and system, no blood can run into the ventricles and there can be no flow. there can be no flow.

Normally, there is a mean cardiovascular Normally, there is a mean cardiovascular pressure above zero, which the heart distributes. pressure above zero, which the heart distributes.

The heart, rather than being responsible for the The heart, rather than being responsible for the pressure in the vascular system, is a circulating pressure in the vascular system, is a circulating device. device.

It lowers the pressure at the ventricular inlets It lowers the pressure at the ventricular inlets and raises it at the ventricular outlets. and raises it at the ventricular outlets.

With a positive pressure in the cardiovascular With a positive pressure in the cardiovascular system, when blood is ejected into the arterial system, when blood is ejected into the arterial side of the circle, a pressure gradient is created side of the circle, a pressure gradient is created between the arteries and veins. between the arteries and veins.

This gradient causes blood to flow around This gradient causes blood to flow around the circle back to the ventricular inlets. the circle back to the ventricular inlets. Therefore, the output rate varies directly Therefore, the output rate varies directly with the magnitude of that mean with the magnitude of that mean cardiovascular pressure. cardiovascular pressure. The higher the The higher the pressure, the higher the gradient, the pressure, the higher the gradient, the greater the flow rategreater the flow rate. .

passive filling of the ventricle occurs from passive filling of the ventricle occurs from point "4" to point "1“point "4" to point "1“

there is an isovolumetric increase in there is an isovolumetric increase in pressure between 1 and 2pressure between 1 and 2

blood is ejected between 2 and 3 blood is ejected between 2 and 3 then isovolumetric relaxation occurs back then isovolumetric relaxation occurs back

down to point 4.down to point 4.

MEAN CARDIOVASCULAR MEAN CARDIOVASCULAR PRESSUREPRESSURE

the pressure in the cardiovascular system with the pressure in the cardiovascular system with the circulation stopped, after the pressure has the circulation stopped, after the pressure has equalized between the arteries, capillaries, equalized between the arteries, capillaries, veins, and cardiac chambers. Do not confuse veins, and cardiac chambers. Do not confuse this pressure with central venous pressure, this pressure with central venous pressure, venous filling pressure, or mean-arterial venous filling pressure, or mean-arterial pressure. Mean cardiovascular pressure is the pressure. Mean cardiovascular pressure is the pressure related to the blood volume and the pressure related to the blood volume and the compliance of the entire elastic cardiovascular compliance of the entire elastic cardiovascular compartment. compartment.

Without mean cardiovascular pressure there Without mean cardiovascular pressure there would be would be no circulationno circulation. The heart doesn't . The heart doesn't generate the pressure in the vascular system, it generate the pressure in the vascular system, it merely distributes the mean cardiovascular merely distributes the mean cardiovascular pressure. The cardiac ventricles take the mean pressure. The cardiac ventricles take the mean cardiovascular pressure and distribute it by cardiovascular pressure and distribute it by raising the pressure on the arterial sides while raising the pressure on the arterial sides while lowering it on the venous sides. The two lowering it on the venous sides. The two ventricles, being passively filling pumps, cannot ventricles, being passively filling pumps, cannot suck, so they lower the inlet pressures toward — suck, so they lower the inlet pressures toward — but never below — zero, in relation to the but never below — zero, in relation to the ambient pressure in the chest. ambient pressure in the chest.

MEAN CARDIOVASCULAR PRESSURE MEAN CARDIOVASCULAR PRESSURE = =

energy forcing fluid into the bodyenergy forcing fluid into the body resistance to fluid loss from the bodyresistance to fluid loss from the body

Slow feedback mechanismSlow feedback mechanism

Elevation of the mean cardiovascular pressure Elevation of the mean cardiovascular pressure above normal —› causes increase in cardiac above normal —› causes increase in cardiac output —› causes increased renal blood flow —› output —› causes increased renal blood flow —› results in increased renal output —› thereby results in increased renal output —› thereby lowering blood volume and mean cardiovascular lowering blood volume and mean cardiovascular pressure back to normal. Conversely, low mean pressure back to normal. Conversely, low mean cardiovascular pressure —› low cardiac output cardiovascular pressure —› low cardiac output —› low renal blood flow —› decreased renal —› low renal blood flow —› decreased renal output until the mean cardiovascular pressure is output until the mean cardiovascular pressure is restored to normal by continuing fluid intake. restored to normal by continuing fluid intake.

Clinical evidence of the Clinical evidence of the homeostatic mechanism: homeostatic mechanism:

Rapid mean cardiovascular Rapid mean cardiovascular pressure buffer mechanismspressure buffer mechanisms

Elasticity: The elasticity of the vascular Elasticity: The elasticity of the vascular system prevents sudden blood volume system prevents sudden blood volume loss or gain from causing a linear, loss or gain from causing a linear, temporary change in mean cardiovascular temporary change in mean cardiovascular pressure. pressure.

Vascular/extravascular equilibrium: There Vascular/extravascular equilibrium: There is a pressure equilibrium between the is a pressure equilibrium between the various extravascular compartments of the various extravascular compartments of the body and the cardiovascular spacebody and the cardiovascular space

Humeral and neuro-muscular-Humeral and neuro-muscular-vascular reflexesvascular reflexes

These responses from stimuli, which alter These responses from stimuli, which alter vascular compliance, act as buffer vascular compliance, act as buffer systems. They prevent sudden changes in systems. They prevent sudden changes in mean cardiovascular pressure from mean cardiovascular pressure from sudden position changes, such as going sudden position changes, such as going from lying to standing. They also buffer the from lying to standing. They also buffer the effect of sudden loss of blood volume from effect of sudden loss of blood volume from hemorrhage. hemorrhage.

IMPEDANCE TO THE FLOW OF BLOOD FROM IMPEDANCE TO THE FLOW OF BLOOD FROM THE OUTLETS AROUND TO THE INLETS OF THE OUTLETS AROUND TO THE INLETS OF

THE VENTRICLESTHE VENTRICLES

(1) resistance (1) resistance (2) elasticity(2) elasticity

(3) limited compliance of the ventricles to filling(3) limited compliance of the ventricles to filling (4) inertia of intermittent blood flow to the (4) inertia of intermittent blood flow to the

ventricles. ventricles. The interrelationship of these four factors makes The interrelationship of these four factors makes

flow determination much more complicated than flow determination much more complicated than plain resistance, which is the single impediment plain resistance, which is the single impediment in rigid, open ended, linear hydraulic systems. in rigid, open ended, linear hydraulic systems.

All All resistance factorsresistance factors — including blood — including blood viscosity, cross-section area of any viscosity, cross-section area of any vascular bed, margination of blood vascular bed, margination of blood constituents, — play roles in flow rate constituents, — play roles in flow rate determination only when linked with their determination only when linked with their location to compliance upstream. location to compliance upstream.

SUMMARYSUMMARY: The cardiovascular system is a : The cardiovascular system is a closed elastic circle, containing two passive closed elastic circle, containing two passive filling pumps in series with two vascular filling pumps in series with two vascular beds, systemic and pulmonary. Normally, the beds, systemic and pulmonary. Normally, the circulation rate made by the two pumps is circulation rate made by the two pumps is controlled by mean cardiovascular pressure controlled by mean cardiovascular pressure and inlet impedance. It is only during heart and inlet impedance. It is only during heart failure, when heart function is limiting the failure, when heart function is limiting the cardiac output, that the heart is regulating cardiac output, that the heart is regulating circulation rate.circulation rate.

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Psych StudentsPsych Students

Four groups:Four groups: Group A: discuss systole and diastole: Group A: discuss systole and diastole:

definitions and differencesdefinitions and differences Group B: discuss the parts and functions of a Group B: discuss the parts and functions of a

BP apparatusBP apparatus Group C: how to measure Blood Pressure Group C: how to measure Blood Pressure

using BP apparatususing BP apparatus Group D: conditions which increase or Group D: conditions which increase or

decrease Blood Pressuredecrease Blood Pressure