Cardiac Output

Cardiac output: Cardiac Output (CO) is the amount of blood pumped by the heart per unit of time and it depends on 2 important variables * Stroke volume (SV) = the amount of blood ejected per beat of the heart (Normally, 70 ml/beat) * Heart rate (HR) = heart beats per minute (normally, 70 beats/min) 	Therefore, the cardiac output (CO) is dependent on the stroke volume (SV) and the heart rate (HR): CO (L/min) = SV (ml/beat)Ⅹ HR (beats/min) = 4.9 L/min (normally)

Stroke volume can be determined by 3 important things

① Preload (ie, the end diastolic volume):The more cardiac muscle is stretched, the harder it contracts.

*Positive agents i. Slow heart rate ii. Exercise * Negative agents i. Blood loss ii. Rapid heart rate

② Contractility: Heart muscle fibers can contract at different strengths.

* Myocardial contractility is the intrinsic ability of the heart to contract independent of preload and afterload.

* Changes in the ability to produce force during contraction result from different degrees of binding between myosin and actin filaments; i. No relationship with the changes in the fiber length! * The degree of binding that occurs depends on concentration of calcium ions in the cell; i. in an intact heart, it is usually the action of the sympathetic nervous system (through catecholamines) that determines the concentration of calcium ions in the cytosol of cardiac muscle cells. * All factors that cause an increase in contractility work by causing an increase in intracellular [Ca2+] during contraction. * Positive agents i. ↑Calcium of ICM ii. Hormones iii. Sympathetic Nervous System iv. Drugs * Negative agents i. ↑Extracellular K+ (Hypercalemia) ii. Acidosis: H+ and K+ exchange across the membrane; - if one H+ is heading in, a K+ is heading out and vice versa. So, as a result, this effect can be the same as the EC K+    iii.Ca2+ channel blocker

③ Afterload (ie, the mean arterial pressure): Resistance to blood leaving the heart * In fact, afterload cannot be controlled by the cardiac function, but affects the cardiac out. So, these are the positive causes for afterload; i. High hematocrict ii. Stenosis of vessels iii. High blood pressure iv. Constricted blood vessels v. Lipids in vessels (atherosclerosis) vi. Lipids in blood vii. Too much fluid

Heart Rate can be determined by autonomic nervous system

①Autonomic Nervous System * Sympathetic NS: Fight or Flight → ↑HR (→always,↑contractility) * Parasympathetic NS: Rest and Digest → negative effect on HR

② Bainbridge reflex (↔ Preload): increases the HR when the cardiac muscle is over-stretched. * The Bainbridge reflex, also called the atrial reflex, is an increase in heart rate due to an increase in central venous pressure. Increased blood volume is detected by stretch receptors located in both atria at the venoatrial junctions. * ↑Venous return → ↑the pressure of vena cavae and then the right atrium → stimulation of the stretch receptor at the venoatrial junctions → signaling the medullary control centers to decrease parasympathetic tone via the vagus → ↑HR * Drawing more blood out of the right atrium → a decrease in atrial pressure and the great veins. * This continues until right atrial blood pressure returns to normal levels, upon which the heart rate decreases to its original level.