5-Cardiac output cardiovascular system.pptx

All about cardiovascular system.
Cardiac out means blood flowing from heart to body.

1. Cardiac output Dr. Mahreen Siddique

2. Cardiac output Cardiac output is the quantity of blood pumped into the aorta each minute by the heart. Or The volume of blood each ventricle pumps as a function of time, Usually expressed in liters per minute, is called the cardiac output.

3. The cardiac output is calculated by multiplying the heart rate (HR)—the number of beats per minute—and the stroke volume (SV)—the blood volume ejected by each ventricle with each beat: CO = HR X SV For example, if each ventricle has a rate of 72 beats/min and ejects 70 mL of blood with each beat, the cardiac output is CO 5.0 L/min.

4. For young, healthy men, resting cardiac output averages about 5.6 L/min. For women, this value is about 4.9 L/min. When one considers the factor of age as well— because with increasing age, body activity and mass of some tissues (e.g., skeletal muscle) diminish—the average cardiac output for the resting adult, in round numbers, is often stated to be about 5 L/min.

5. NORMAL VALUES FOR CARDIAC OUTPUT AT REST AND DURING ACTIVITY Cardiac output varies widely with the level of activity of the body. The following factors, among others, directly affect cardiac output: ( 1) the basic level of body metabolism, ( 2) whether the person is exercising, ( 3) the person’s age, and ( 4) the size of the body

6. FACTORS controlling CARDIAC OUTPUT Cardiac output is maintained (determined) by four factors: 1. Venous return 2 . Force of contraction 3 . Heart rate 4 . Peripheral resistance

7. VENOUS RETURN Venous return is the amount of blood which is returned to heart from different parts of the body. When it increases, the ventricular filling and cardiac output are increased. Thus, cardiac output is directly proportional to venous return, provided the other factors (force of contraction, heart rate and peripheral resistance) remain constant.

8. When increased quantities of blood flow into the heart T he increased blood stretches the walls of the heart chambers As a result of the stretch, the cardiac muscle contracts with increased force T his action empties the extra blood that has entered from the systemic circulation Therefore, the blood that flows into the heart is automatically pumped without delay into the aorta and flows again through the circulation.

9. Stretching the heart causes the heart to pump faster, resulting in an increased heart rate. That is, stretch of the sinus node in the wall of the right atrium has a direct effect on the rhythmicity of the node to increase the heart rate as much as 10 to 15 percent. In addition, the stretched right atrium initiates a nervous reflex called the Bainbridge reflex , passing first to the vasomotor center of the brain and then back to the heart by way of the sympathetic nerves and vagi , also to increase the heart rate.

10. Cardiac output is usually determined by the sum of all the various factors throughout the body that control local blood flow. All the local blood flows summate to form the venous return, and the heart automatically pumps this returning blood back into the arteries to flow around the system again . Cardiac output = Total tissue blood flow

11. FORCE OF CONTRACTION Cardiac output is directly proportional to the force of contraction, provided the other three factors remain constant. According to Frank-Starling law, force of contraction of heart is directly proportional to the initial length of muscle fibers, before the onset of contraction. Force of contraction depends upon preload and afterload.

12. Preload Preload is the stretching of the cardiac muscle fibers at the end of diastole, just before contraction. It is due to increase in ventricular pressure caused by filling of blood during diastole . Stretching of muscle fibers increases their length, which increases the force of contraction and cardiac output. Thus , force of contraction of heart and cardiac output are directly proportional to preload.

13. Afterload Afterload is the force against which ventricles must contract and eject the blood. Force is determined by the arterial pressure. At the end of isometric contraction period, semilunar valves are opened and blood is ejected into the aorta and pulmonary artery. So, the pressure increases in these two vessels. Now , the ventricles have to work against this pressure for further ejection. Thus, the afterload for left ventricle is determined by aortic pressure and afterload for right ventricular pressure is determined by pressure in pulmonary artery. Force of contraction of heart and cardiac output are inversely proportional to afterload.

14. HEART RATE Cardiac output is directly proportional to heart rate provided, the other three factors remain constant. Moderate change in heart rate does not alter the cardiac output. If there is a marked increase in heart rate, cardiac output is increased. If there is marked decrease in heart rate, cardiac output is decreased.

15. Rhythmic beating of the heart at a rate of approximately 100 beats/ min will occur in the complete absence of any nervous or hormonal influences on the SA node. This is the inherent autonomous discharge rate of the SA node. A large number of parasympathetic and sympathetic postganglionic neurons end on the SA node. Activity in the parasympathetic neurons (which travel within the vagus nerves) causes the heart rate to decrease, whereas activity in the sympathetic neurons causes an increase. In the resting state, there is considerably more parasympathetic activity to the heart than sympathetic, so the normal resting heart rate of about 70–75 beats/min is well below the inherent rate of 100 beats/min.

16. PERIPHERAL RESISTANCE Peripheral resistance is the resistance offered to blood flow at the peripheral blood vessels. Peripheral resistance is the resistance or load against which the heart has to pump the blood. So, the cardiac output is inversely proportional to peripheral resistance . Resistance is offered at arterioles so, the arterioles are called resistant vessels.

17. Under many conditions, the long-term cardiac output level varies reciprocally with changes in total peripheral vascular resistance, as long as the arterial pressure is unchanged. peripheral resistance increases above normal, the cardiac output falls; conversely, when the total peripheral resistance decreases, the cardiac output increases . Cardiac output = Arterial pressure_______ Total peripheral resistance

18. PATHOLOGICAL VARIATIONS Increase in Cardiac Output Cardiac output increases in the following conditions: 1. Fever : Due to increased oxidative processes 2 . Anemia: Due to hypoxia 3 . Hyperthyroidism: Due to increased basal metabolic rate.

19. Decrease in Cardiac Output Cardiac output decreases in the following conditions: 1. Hypothyroidism : Due to decreased basal metabolic rate 2 . Atrial fibrillation: Because of incomplete filling of ventricles 3 . Incomplete heart block with coronary sclerosis or myocardial degeneration: Due to defective pumping action of the heart 4 . Congestive cardiac failure: Because of weak contractions of heart 5 . Shock: Due to poor pumping and circulation 6 . Hemorrhage: Because of decreased blood volume.