The Cardiovascular System Cardiac Function

Transcription

1 The Cardiovascular System Cardiac Function Session Objectives. What you will cover The circulatory system Anatomy of the heart Flow of blood through the heart Conduction systems of the heart Cardiac cycle Structure and function of blood vessels Haemodynamics Blood pressure Your objectives are State the function of the chambers, valves and blood vessels associated with the heart in terms of blood flow State what happens to the chambers, valves and blood vessels of the heart during one cardiac cycle Identify structures associated with the conduction system of the heart State the role of the SA node, AV node, bundle of His and Purkinje fibres Identify the waves generated in an ECG and relate them to electrical activity in the heart Define tachycardia, bradycardia and sinus arrhythmia and ventricular fibrillation Define stroke volume, cardiac output. Explain the Frank-Starling Law of the Heart Identify blood vessels by their structures Describe the structure and function of a capillary bed Explain the function of anastomoses Identify factors affecting blood flow Define blood pressure as a product of cardiac output and total peripheral resistance Explain why blood pressure fluctuates in arteries Explain the blood pressure changes across the circulatory system Describe factors that influence blood pressure State normal ranges for blood pressure and pulse rates State abnormal ranges for blood pressure and pulse rates Suggested reading: Tortora, Ch 21 Marieb Ch 20; Gould Ch18 24

2 The Cardiovascular System Cardiac Function All tissues need a steady supply of blood to provide them with nutrients and oxygen. This continuous blood supply helps to maintain tissue homeostasis in terms of delivering nutrients and taking away toxins and wastes. This homeostasis can only be maintained if blood keeps moving. The heart drives blood through blood vessels ensuring adequate oxygenation and nutrient delivery. This is known as the cardiovascular or circulatory system. Basic Anatomy of the Heart - External and Internal. You need to know the basic anatomy of the heart so practice, practice, practice. 25

3 The direction of blood flow through the heart is from right to left. The flow is unidirectional. Atria have much thinner walls than the ventricles. Give reasons for the atria having thin walls and the ventricles having thicker walls. Atria Ventricles 26

4 Maintenance of unidirectional blood flow heart valves. There are two sets of valves to consider in the heart. One set are called the atrioventricular valves, and the other are the semilunar valves. Where are the atrioventricular valves located? Where are the semilunar valves located? There are two types of atrioventricular valves. Below are two simple representations of these. Identify the valves. When the ventricles are relaxed which valves are closed? What does closure of these valves encourage or prevent? 27

5 The Sinoatrial node. Also known as the SA node. This contains cells that establish the heart beat, called the sinus rhythm. It is a natural pacemaker. If your heart was taken out of your chest, it could beat independently at a resting rate of around 100 beats per minute. A normal range for resting heart rate is beats per minute. This is because other nervous and hormonal controls influence heart rate [more later]. When the SA node is active/fires/depolarises, which structures of the heart contract first? The atrioventricular [AV] node is located in the septum between the atria. When electrical impulses reach the AV node there is a delay of 100 milliseconds [0.1sec]. Why do you think there is a delay in the conduction of the impulses across the heart at this time? What does this delay allow? The apex is the tip of the heart. Ventricular contractions start from there. Why is this physiologically sensible? 28

6 Recording the electrical activity of the heart. An electrocardiogram [ECG] is a recording of the electrical activity of the heart. Reading ECGs is a highly skilled activity, but there are some simple rhythms and characteristic waveform shapes that can tell us a lot about the activity of the heart. ECGs are recorded by placing electrodes across the chest wall in a characteristic positions. These are shown below. Each electrode picks up electrical impulses from the heart. These are then converted into a linear trace that makes up the ECG. The trace is recorded onto graph paper that moves through the machine at 25 mm per second. The squares on the graph paper are 5mm large squares, equal to 0.2 seconds and 1mm small squares equal to 0.04 seconds. Here is a typical diagram of an ECG trace for one heartbeat. There are characteristic blips or waves. Count the number of 1mm boxes from the start of the P wave through to the end of the T wave. boxes. Calculate the time interval between the start of the P wave and the end of the T wave. 29

7 Complete the missing words. Each word is used only once. The small P wave accompanies of the. These chambers contract 100 msec after the P wave. The QRS complex appear as the depolarise. This is a relatively strong electrical signal because the muscle is larger than that of the upper chambers. The of these upper chambers is hidden by the depolarisation of the lower chambers. The T wave represents the repolarisation of the ventricles. Analysis of the ECG can help problems with the conduction systems of the heart. The time elapsed between the can be used to determine heart rate. If the time elapsed between this wave decreases, it indicates, where resting heart rate is greater than 100 beats per minute. If the resting heart rate is less than 60 bpm, this represents. Cardiac arrhythmias are patterns of cardiac electrical activity. Ventricular bradycardia atria R spikes repolarisation diagnose ventricles tachycardia depolarisation abnormal 30

8 Blood Vessels and Blood Pressure Basic Anatomy of Blood Vessels You need to know the basic anatomy of arteries, veins and capillaries so practice, practice, practice. Complete the following passage about blood vessels. The central cavity of a blood vessel is called the. Reduction of the diameter of this cavity is called. If the internal diameter of blood vessels increases, this is called. Blood is carried to the heart by and away from the heart by. Capillary beds are supplied by smaller versions of the blood vessels called and are drained by. Venules lumen arterioles vasodilation Arteries veins vasoconstriction 31

9 Capillary Beds These are formed from single arterioles. The arterioles subdivide into capillaries. What is the function of pre-capillary sphincters? What is a metarteriole? What is an anastomosis? What is the function of anastomoses? Veins, valves and venous return. Infolds of the tunica interna of veins form valves that stick out into the lumen of the vein. These curl in the direction of the blood flow. Many veins pass through muscles, or between muscles and bones. This is shown in the diagram on the right. What is the effect of muscular contraction on the return of blood to the heart [venous return]? Other than muscular pumps, what other organ system can assist with venous return? 32

10 Resistance to fluid flow This is a measure of how easy it is for a fluid to flow through a tube. In the circulatory system it is called vascular resistance or total peripheral resistance. The following factors affect total peripheral resistance. Describe what their effects are. Factor Effect Tube/blood vessel radius Tube/blood vessel length Fluid/blood viscosity List as many factors as you can that influence blood pressure 33

11 Measurement of blood pressure. This can be done using a manual sphygmomanometer. You will measure your own blood pressures using a digital sphygmomanometer. They operate on similar principles. Traditionally they detect the systolic and diastolic blood pressures of the brachial artery. To write your blood pressure out, you record the systolic pressure over the diastolic pressure, so that it looks like a fraction e.g. 120/80 mm Hg. This means that the systolic blood pressure was 120 mm Hg and the diastolic was 80 mm Hg. Find the ranges of blood pressures for the normotensive state, and mild, moderate and severe hypertension. Normotensive Mild Hypertension Moderate Hypertension Severe Hypertension Cardiac Output This is the volume of blood pumped by the heart in one minute. It can be calculated from the stroke volume and heart rate of an individual. Cardiac Output = Heart rate x Stroke volume CO = HR x SV Calculate the cardiac outputs in the table below. Heart Rate [beats per min] Stroke Volume [ml/beat] A B C D Cardiac Output [ml/min] 34

12 Stroke Volume Stroke volume is the volume of blood ejected by the ventricles per beat of the heart. It is controlled by three factors. These are pre-load, contractility and afterload. What is pre-load? What can increase pre-load? What is contractility? What generates afterload? Neural Control and Heart Rate The medulla oblongata [brain stem] is the origin of control of heart rate. What is the name of the regulatory sector for the heart in the medulla oblongata? Which part of the nervous system stimulates heart rate? Which large nerve innervates [goes into] the SA node? What effect does activity of this nerve have on heart rate? Web page links [accessed August 2004] ml