Venous Return - University of the West Indies

Venous return Karen Thaxter Nesbeth University of the West Indies Department of Basic Medical Sciences Cardiac Output, Venous Return and their Regulation Cardiac output is precisely controlled Maintain the optimal amount of flow to tissues Prevent undue stress on...

Venous return Karen Thaxter Nesbeth University of the West Indies Department of Basic Medical Sciences Cardiac Output, Venous Return and their Regulation Cardiac output is precisely controlled Maintain the optimal amount of flow to tissues Prevent undue stress on the heart.  THE FRANK-STARLING LAW OF THE HEART: Control of Stroke volume Strength of ventricular contraction varies directly with EDV – Is an intrinsic property of myocardium – As EDV increases, myocardium is stretched more, causing greater contraction & SV Preload = End diastolic volume  Preload (End Diastolic Volume) Critical factor controlling stroke volume – ↑preload → ↑stretch of muscle → ↑force of contraction → ↑SV – ↑EDV leads to ↑stretch of myocardium. Afterload = End Systolic Pressure Pressure against which ventricles contract Wall tension in ventricles during systole Determined by peripheral resistance Affected by – Pleural Pressure – Vascular compliance – Vascular resistance Regulation of Cardiac Output volume  Cardiac Output Depends on venous return, which, in turn, depends on the rate of flow to the tissues Venous return: the rate of flow of blood back to the heart from the tissues. Rate of flow to tissues depends on tissue needs cardiac output is proportional to the energy requirements of the tissues.  Factors affecting Cardiac Output Cardiac output: Indicator of function of the heart CO = HR x SV Influencing Cardiac output (CO) Cardiovascular controller: ANS Hormones Mechanical factors: preload (Frank-Starling); afterload  Autonomic Influences on CO 1. Sympathetic stimulation: Increases contractility of the heart (SV) – therefore decreases End systolic volume Increases HR (positive chronotropic effect) – MOST IMPORTANT in CO adjustment – Can go to triple resting HR – Reduced filling time 2. Parasympathetic stimulation: Heart primarily under influence of PNS at rest – reduced HR Returns to baseline HR after fright / exercise  Factors affecting Cardiac Output Slow Heart Rate – allows for more time for ventricular filling, increasing EDV increases stroke volume Extremely Fast Heart Rate – low venous return decreased stroke volume.  Mechanical Factors affecting Cardiac Output Increased Venous Return – Cardiac muscle fibers are stretched by increased blood volume returning to the heart – (increased venous return and EDV). Increased stretch results in greater force of contraction, Increases stroke volume.  Factors affecting Cardiac Output Exercise – activates the sympathetic nervous system increases heart rate Increases contractility Increases stroke volume Both the higher heart rate and squeezing action of skeletal muscles on veins increase venous return, contributing to increased stroke volume.  Cardiac reserve: – the difference between cardiac output at rest & the maximum volume of blood the heart is capable of pumping per minute – i.e. the difference between resting and maximal CO – permits cardiac output to increase dramatically during periods of physical activity  Factors affecting Cardiac Output Sudden Drop in Blood Pressure – results in low venous return decreases stroke volume **heart rate increased due to sympathetic activity, and normal cardiac output is maintained. Rising Blood Pressure – reduces sympathetic activity, decreasing heart rate. increases arterial pressure ventricles must overcome arterial pressure before semilunar valves open, increasing ESV and decreasing stroke volume. Reduced cardiac output helps bring blood pressure down to normal levels.  Factors affecting Cardiac Output Sudden Drop in Blood Volume - (eg. due to severe blood loss) low venous return and therefore decreased stroke volume. Sympathetic activity increases heart rate Maintains cardiac output. Excess Calcium – Increases stroke volume by enhancing contractility.  Venous Return & Cardiac Output Cardiac output increases with atrial pressure. Normal atrial pressure is about 0 mm Hg. Venous return decreases with atrial pressure. Venous return curve Determinants of Venous Return Mean systemic Right filling Resistance to Flow Atrial pressure Pressure Pressure change is slight. Thus, small increase in RA Pressure causes dramatic reduction in venous return. (mean systemic filling pressure).  Venous Return Volume of blood flowing back to the heart through the veins. There are 5 main mechanisms that enable blood to flow back to the heart: 1. Venous valves 2. Muscle pump 3. Respiratory pump 4. Venous smooth muscle 5. Gravity  Venous Valves One-way valves, (also called pocket valves) in the veins prevent backflow of blood direct blood flow back towards the heart.  Muscle Pump Veins are situated between skeletal muscle groups when contracting and relaxing muscle helps push/squeeze blood through the veins back towards the heart  Respiratory Pump Breathing in and out produces pressure changes within the chest cavity Inhalation – diaphragm flattens, pushes on internal abdominal organs – They in turn push on veins and this forces blood up into the thorax Exhalation – increased intrathoracic pressure – Compression of veins in thorax – blood is forced through valves and into heart Smooth Muscle Contraction and relaxation of smooth muscle within the middle layer of the veins’ walls helps to push blood through the veins and back towards the heart. Gravity Blood from the upper body, above the heart, is aided by gravity, which helps blood flow back to the heart Hydrostatic pressure increases BP below the heart and decreases BP above the heart Blood flows from high pressure to low pressure regions.  Quiz time! Define cardiac output. What two factors does cardiac output depend on? What is the mathematical relationship between cardiac output, heart rate, and stroke volume.  Define heart rate. What is the average heart rate in an adult at rest? Define stroke volume. What is the average stroke volume in an adult at rest?  Define end diastolic volume. Define end systolic volume. What is the mathematical relationship between end diastolic volume, end systolic volume, & stroke volume? If the ESV is 50 ml and the EDV is 120 ml, what is the stroke volume? If the heart rate is 75 beats per minute and the stroke volume is 70 ml per beat, then what is the cardiac output?  What is the relationship between cardiac muscle stretch and force of contraction? What effect does this have on stroke volume?  What's the relationship between venous return and stroke volume? What is the effect of increased sympathetic activity on heart rate and stroke volume? How does this affect cardiac output?  What effect does a slow heart rate have on stroke volume? What effect does a fast heart rate have on stroke volume? What is the effect of exercise on heart rate and stroke volume? – How does this affect cardiac output?  http://www.wiley.com/college/jenkins/04702 27583/animations/ch19/index_19_08_02.htm l