Cardiac out put (C.O.):
* It is the amount of blood pumped from the heart per minute, it is C.O = heart rate X stroke volume, i.e. when one of them increase it will increase the cardiac out put
* Heart rate increased by sympathetic stimulation and decreased by parasympathetic stimulation
* When the force of the contraction of the heart increase it will increase the stroke volume, and the force of contraction increased either by (1) extrinsic control (sympathetic stimulation) or by (2) intrinsic control (Frank-Starling law), i.e. when the venous return increase which may be caused by vasoconstriction of veins by sympathetic stimulation it will increase the amount of blood filling in the ventricle in the end of the diastole (End diastolic volume) and this will result in more force of contraction and thus more stroke volume
* Don’t forget that increased Ca level will also increase the force of contraction and stroke volume
* In a summary; sympathetic stimulation will increase heart rate (increase C.O.) and force of contraction (increased stroke volume and C.O.) and increase venous return, which also increase the stroke volume
* Keep in your mind that when CO. is increased blood pressure will increase usually
Q. What we need in order to make blood flow?
A. To make blood flow between two points there must be a pressure difference between these points (pressure gradient) and blood will flow from high to low pressure
Example the pressure in aorta and large vessels is greatest, while in the venous end (vena cavae and right atrium) is lowest, that’s why blood flow in this forward direction
Amount of blood volume passing through the vessel per unit time, is directly proportional to pressure gradient in that vessel and inversely proportional to the resistance
Blood flow = pressure gradient / resistance (the blood flow could be the cardiac out put)
Velocity of the blood flow:
* Amount of blood flow per a distance of the blood vessel
* It is inversely proportional to cross sectional area i.e. when the diameter of vessel increase for a given blood flow (constant)
* It is the opposition of a vessel to blood flow, so when the resistance increase blood flow will decrease for a given pressure gradient, and this resistance is resulted from friction of the flowing blood with the wall of the vessel
Resistance depend on 4 factors :
1- viscosity of blood
2- length of blood vessel
3- radius or diameter of the vessel
4- Distensibility of the blood vessel, when increase it decrease the resistance (Distensibility decrease with aging)
* When the viscosity of the blood increase the resistance increase also, viscosity is determined by the number of RBCs in the blood (hematocrit)
* Usually viscosity is constant in normal persons so, it doesn’t affect on resistance
* When the length of the vessel increase the resistance will increase because more friction will happen between blood and the wall but also this factor is constant (not changed), so it doesn’t affect resistance
* When the radius of the vessel decreased it will increase resistance (inversely proportional) this factor is the most important in determining resistance (changeable)
* Resistance is inversely proportional to the fourth power of the radius (i.e. the radius multiplied by itself 4 times)
* So any change in radius will produce significant change in resistance
Q. What is the relationship between resistance and cardiac out put?
A. When the diameter of the vessel increase the resistance will decrease and vice versa, so if there is arterial vasodilatation it will decrease the resistance and thus cardiac out put increase
Q. How vasodilatation occurs?
A. By the following factors; (1) increased CO2 (2) NO (nitric oxide) (3) certain substances and hormones
* CO2 increase when tissue metabolism increase as to say they will dilate vessels (decreased resistance) and make more blood flow to that tissue of particular metabolism
Q. What happen if the muscle is in exercise?
A. CO2 will increase which will make vasodilatation and increase blood flow; also there is sympathetic stimulation, which will affect heart (the rate and stroke volume) and thus increasing cardiac out put to whole body including the muscles, also sympathetic stimulation make venous constriction and by this increasing venous return and then stroke volume and C.O.
* NO (nitric oxide) is produced intrinsically by the endothelium of the blood vessel it causes vasodilatation some times called (endothelial derived factor)
Pharmacological point: there is a drug called Viagra that contain synthetic nitric oxide will cause vasodilatation in blood vessels of penis used to treat conditions of impotence (difficult errection), those patients shouldn’t take this drug when they are taking antihypertensive drugs because both of the drugs will reduce blood pressure
Q. Why old persons develop hypertension?
A. Because the blood vessels will become more resistant (atherosclerosis) and for that the blood flow will be reduced, but the heart try to pump more forcefully to keep the blood flow to organs constant
* Veins are more distensible than arteries so that they capacitate more blood than them and for that they are considered as blood reservoir
* For the reason above, the veins has low resistance and therefore the blood pressure inside them is low also
Note: never forget that blood pressure is directly proportional to resistance
Case discussion: if some one lost blood by bleeding lets say 1 liter his blood volume and blood pressure will decrease, how the body will compensate for that?
There will be sympathetic stimulation which will result in; (1) vasoconstriction for veins to increase the venous return and so to increase the cardiac out put (2) stimulation of the heart to increase heart rate and stroke volume and both will increase the C.O. (3) vasoconstriction to arteries to increase resistance and so increasing blood pressure, all these factors will increase the blood pressure back to normal
Don’t forget that …… Pressure = cardiac out put X resistance
Q. How venous return is controlled?
A. By the following:
1) Venous valves: they allow the blood to flow in one direction toward heart only
* When the skeletal muscles contract in normal body activity they compress on the neighboring veins that lie near by them and thus squeezing them to push blood toward heart only because of the presence of venous valves
2) Sympathetic stimulation, which causes vasoconstriction to veins
3) Respiration: as we know that there is negative intrathoracic pressure (intra pleural) which is changing in respiratory cycle and that help in pumping blood through the veins that lie inside thoracic cage
Regulation of blood pressure: controlled by:
1) Baroreceptors; in aortic arch and carotid sinus, if blood pressure decreased the baroreceptor will send information to brain stem where the cardiovascular centers, these centers will make reflex increase in heart rate, force of contraction venous and arterial vasoconstriction (all these mediated by sympathetic), all these factors will increase the blood pressure
2) Renin – angiotensin – aldosteron system:
* As you know that renin is released from kidneys, which will result in production of angiotensin II and secretion of aldosteron, angiotensin make vasoconstriction and aldosteron increase blood volume by increasing Na reabsorption and both effects will increase the blood pressure
1- when the blood volume increase this will increase the venous return and by this increasing C.O. and blood pressure
2- keep in your mind that vasoconstriction in veins will cause increased venous return, while in arteries will cause increased resistance and thus blood pressure
Q. How to measure blood flow?
A. (1) By magnetic device around blood vessel, which will detect moving particles in blood
(2) By ultrasound device (sonar) which use physical phenomenon called Doppler shift
Q. How to measure blood pressure (BP)?
A. (1) directly by inserting a needle (a canula) to blood vessel, which is linked to a device measure the blood pressure
(2) By sphygmomanometer as follow:
a) When we apply a pressure by a cuff on large artery (like Brachial artery) and when this pressure is more than systolic pressure then there is no blood flow
b) Then we decrease the pressure of the cuff gradually until it become like systolic pressure at this point we will hear a turbulent blood flow by the stethoscope applied on the artery then we know that this level of pressure in the cuff is equal to systolic pressure
c) And then we continue decreasing the pressure inside the cuff until the turbulent flow sound disappear , and at this point of pressure present inside the cuff when the sound disappear it is the value of the diastolic pressure
Note :there are two types of blood flow:
1- Laminar flow: when the fluid molecules slide smoothly in one direction and there is no sound associated with the flow
2- Turbulent flow: when the molecules of the fluid move in more than one direction and not smoothly and there is a sound associated with the flow, and this occur as a result of narrowing or stenosis of the vessel (example in atherosclerosis plaque )
* When we examine an old man patient we suspect that he has atherosclerosis and that’s why we put the stethoscope on the neck (carotid arteries) to hear if there is any turbulent flow
1- pulse pressure = systolic pressure – diastolic pressure
2- mean arterial pressure = diastolic pressure + 1/3 (pulse pressure)