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Lecture 1 Notes.pdf

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Stephen Reid

1    LECTURE 1 1. Introduction to the Course These courses are going to cover four major topics: cardiovascular physiology, respiratory physiology, renal physiology, and then digestive physiology. 1A. Cardiovascular Physiology We're going to start out with lectures looking at the function of the cardiovascular system, so today, we'll begin by looking at electrical activity in the heart: we'll look at how electrical activity is conducted through the heart from the endogenous pacemaker through to contractile cells - you've all heard of implantable pacemakers, but there is an endogenous group of cells in the heart that function as the normal pacemaker in normal, non-pathological situations. In the next lecture, we're going to look at the ECG (electrocardiogram), which produces that generally familiar heartbeat pattern, and we'll see what the various deflections, and periods between deflections, represent; as well, we'll see how you can use an ECG to determine whether a heart is functioning normally, or to diagnose disease or abnormal states. We'll then look at what's called the electrical axis of the heart, which is the general, overall direction that electrical activity flows in the heart; again, this is a very useful diagnostic tool that can tell you whether there are various disease states in the heart. The axis can deviate from its normal direction, either to the left or the right, and which direction it goes and the extent to which it deviates can assist you in diagnosing problems. The cardiac cycle, the focus of the next lecture, tells about the various pressure and volume changes, the opening and closing of valves, and the contraction of atria and ventricles that occur during a single beat of the heart (or cardiac cycle). We're then going to look at regulation of cardiac output (cardiac output refers to the amount of blood the heart puts out per unit of time), and regulation of blood flow, and then heart failure and blood pressure. 1B. Respiratory Physiology When we come to respiratory physiology, we'll begin by looking at pulmonary mechanics, i.e., the pressure and volume changes, muscle contraction, and other factors that are associated with a single inspiration and expiration. We'll look at a technique of spirometry, which is used to measure lung volumes and capacities. Again, these are important diagnostic tests, and can be used to diagnose, for instance, whether someone has an obstructive lung disease, a restrictive lung disease, or whether the lung is functioning normally. Alveolar ventilation looks at how the air that we breathe in actually gets into the gas exchange site, the alveoli, and how this can be modified by changing breathing rate and tidal volume and how they are egulated. 2    Blood-gas transport looks at the linkage between oxygen and carbon dioxide transport in the red blood cell; everyone knows that hemoglobin in the red blood cells carries oxygen, but it is also critical for CO t2ansport in the blood as well - and you cannot separate oxygen and CO transpor2. Ventilation-perfusion matching looks at how we keep the volume of air that we breathe in, and the volume of blood pumped to our lungs relatively constant in order to allow for optimal gas transfer. Control of breathing looks at mechanisms in the brain that produce breathing: just as there is a pacemaker in the heart to control heart rate, there is what could be called a pacemaker in the brainstem that controls breathing without any conscious thought. And then, we'll look in one lecture at various sleep-related breathing disorders. 1C. Renal Physiology In the renal system, we're going to look at kidney function, the regulation of various ions, the role of different hormones in regulating kidney function, and then at how we have an osmotic gradient going from the outside to the inside of our kidney, which allows us to form a concentrated urine (without this gradient, we can't produce anything but the most dilute of urine). We'll spend about a lecture and a half looking at acid-base balance, which brings into play the renal, respiratory and cardiovascular systems - so it is a good integrated topic. 1D. Digestive Physiology And then finally, we'll spend the last four lectures looking at the digestive system. We'll look at the functions of the various components of the GI tract, as well as the various accessory organs that provide enzymes and other substances to facilitate gut function. Then we'll look at the digestion and absorption of various nutrients, and the neural and hormonal regulation of the digestive system. 2. The Cardiovascular System I've already alluded to the various topics that we're going to cover; and throughout our study of the cardiovascular system, we're always going to come back to some basic principles and equations that can really help in your understanding of cardiovascular control. The amount of blood pumped out of the heart per unit of time is called cardiac output, and it is a function of heart rate, and stroke volume. Stroke volume is the volume of blood pumped per beat. Thus, cardiac output is produced by the equation CO = HR x SV. All of the cardiovascular regulatory systems we're going to look at have, at their core, one basic, critical function: and that is to prevent blood pressure from falling too low. High blood pressure will kill, but it will take years; low blood pressure will kill in minutes. So every adjustment we see in the heart and blood vessels throughout cardiovascular function ultimately boils down to keeping blood pressure up so blood flow can occur to organs that are critically sensitive to low oxygen levels. 3    We can calculate blood pressure by multiplying cardiac output by what is called peripheral resistance. This refers to the resistance to blood flow within the circulatory system. We'll see that this resistance occurs primarily in small vessels, particularly small arteries called arterioles. So, cardiovascular regulatory mechanisms are going to impinge on these three factors: heart rate, stroke volume, and peripheral resistance. Whether it’s a nervous mechanism or a hormonal mechanism, all adjustments that occur in the cardiovascular system are going to affect one or more of these three variables with the ultimate goal of keeping blood pressure levels up. 2A. Heart Anatomy Before we start looking at electrical activity, I just want to cover some basic heart anatomy to place everything into perspective, and then look at some basic patterns of blood flow. The human heart, like the hearts of other mammals, is a four-chambered heart. There is a left side and a right side; with both an atria and ventricle on either side. Blood comes back from circulation via the vena cava, with the inferior vena cava coming from the lower body and a superior vena cava coming from the upper body: they empty into the right atria, and then blood flows through an atrioventricular valve, in this case the tricuspid valve to the right ventricle. When the right ventricle contracts, it pumps blood up through the pulmonary arteries, up to the lungs. Oxygenated blood returns to the heart via the pulmonary veins and enters the left atria, and moves from there to the left ventricle, and then up to the aorta, into systemic circulation. The atria and the ventricles are separated by several septa - there is an interatrial septum that separates the atria, and an interventricular septum that separates the two ventricles. The left ventricle is the ventricle that pumps blood to the systemic circulation, which has rather high resistance; thus the left ventricle's muscles are substantially greater than those of the right. When we look at blood flow, we'll see how the pulmonary circuit (which is supplied by the right ventricle) has substantially less pressure in it: and so the right ventricle doesn't need to pump with such force, as it faces far less resistance. To reiterate the pattern of blood flow, then, we have deoxygenated blood from the systemic circulation coming into the right side of the heart, and then being pumped through the pulmonary arteries to the 4    lungs. The blood is oxygenated, returns to the heart via the pulmonary veins, goes into the left ventricle and then is pumped via the aorta back to the systemic circulation. The pulmonary circuit is the only place where we see an artery carrying deoxygenated blood and a vein carrying oxygenated blood; the opposite is true throughout the rest of the body. This is because the designation of artery/vein has nothing to do with oxygenation, but rather whether it is carrying blood towards or away from the heart. Red and blue colours are generally used to designate oxygenated and deoxygenated blood, respectively. 2B. The Conduction System of the Heart So next, let's look at the electrical conduction system of the heart. The electrical activity that will eventually cause the heart to polarize and contract is generated in a little region in the right atria called the sinoatrial (SA) node. It consists of a group of cells that have no resting membrane potential, and will spontaneously depolarise, thereby sending a wave of electrical activity through this conduction system; where it will go to the contractile muscle cells, and cause the heart to contract. Everything, then, begins in the sinoatrial node. We have action potentials fired in the sinoatrial node cells, in the pacemaker cells, and this electrical activity spreads throughout the atria, through what are termed internodal pathways. The pathways travel out from the nodes throughout the atria, carrying waves of depolarisation. These waves of depolarisation cause the heart to contract; they travel through the atria, to the ventricle, through a single pathway: the AV (atrioventricular) node. The septa, between the atria and the ventricles, prevent any electrical activity from simply going anywhere in the heart - instead, all electrical activity must, at some point, go through the AV node. So we have these waves of depolarisation, going from the SA node, through the atria, to the AV node, where they pass on to the ventricles. From there, these waves of depolarisation travel through an AV bundle, or what is called the Bundle of His, and then through two branch bundles, a left and a right, which go through the interventricular septum. From there, the activity goes through what are known as Purkinje fibres, which originate at the bottom (apex) of the heart and move upward into the contractile muscle. This pattern of activity has the effect of causing the heart to contract starting at the bottom, easing the flow of blood up into the aorta or the pulmonary artery. The cells in this internodal pathway system are modified muscle cells, not contractile cells - the job of these cells is instead to pass waves of depolarisation onto the contractile cells, thereby triggering their contraction. 5    Cells within the heart are electrically coupled via gap junctions. From the S
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