Cardiovascular Physiology PDF

CARDIOVASCULAR PHYSIOLOGY Block: Foundations Block Director: James Proffitt, PhD Session Date: Tuesday, August 06, 2024 Time: 9:30 – 10:30 am Instructor: Zoe Cohen, PhD Department: Cellular & Molecular Medicine Email: zcohen@ema...

CARDIOVASCULAR PHYSIOLOGY Block: Foundations Block Director: James Proffitt, PhD Session Date: Tuesday, August 06, 2024 Time: 9:30 – 10:30 am Instructor: Zoe Cohen, PhD Department: Cellular & Molecular Medicine Email: [email protected] INSTRUCTIONAL METHODS Primary Method: IM13: Lecture ☐ Flipped Session ☐ Clinical Correlation Resource Types: RE18: Written or Visual Media (or Digital Equivalent) INSTRUCTIONS Please read lecture objectives, notes and watch the video associated with this material. READINGS N/A LEARNING OBJECTIVES 1. Identify the 3 basic components of the cardiovascular system. 2. Describe the heart as a single pump, then as 2 separate pumps. 3. Define: Heart Rate, Stroke Volume, Cardiac Output, Inotropy, Preload, Afterload. 4. Identify p-wave, QRS complex, T-wave, intervals and segments on a basic ECG trace. 5. Describe the major classes of vessels and classify them by their major “job.” 6. Describe how Cardiac Output and Systemic Vascular Resistance control blood pressure and how changes in either (or both) can lead to hypertension. CURRICULAR CONNECTIONS Below are the competencies, educational program objectives (EPOs), disciplines and threads that most accurately describe the connection of this session to the curriculum. Related Related Competency\EPO Disciplines Threads COs LOs CO-01 LO #1 MK-02: The normal structure and Physiology N/A function of the body as a whole and of each of the major organ systems CO-01 LO #2 MK-02: The normal structure and Physiology N/A function of the body as a whole and of each of the major organ systems Block: Foundations | COHEN [1 of 7] CARDIOVASCULAR PHYSIOLOGY Related Related Competency\EPO Disciplines Threads COs LOs CO-01 LO #3 MK-02: The normal structure and Physiology N/A function of the body as a whole and of each of the major organ systems CO-01 LO #4 MK-02: The normal structure and Physiology N/A function of the body as a whole and of each of the major organ systems CO-01 LO #5 MK-02: The normal structure and Physiology N/A function of the body as a whole and of each of the major organ systems CO-01 LO #6 MK-05: The altered structure and Physiology N/A function (pathology & pathophysiology) of the body/organs in disease NOTES: Throughout an average human life span, the heart contracts about 3 billion times. Without the heart beating, we die (sudden cardiac death)…but it’s not the only component of the cardiovascular system. The heart is the pump, imparting pressure on blood, which moves through a series of tubes (vessels), bringing nutrients and oxygen to all the cells in the body. Thus, the term cardio-(heart) vascular (vessels). Since all the body systems work together to create homeostasis, when a system such as the cardiovascular system fails to do its job (if any of the 3 basic components are not meeting the needs of the body), we see pathophysiology, not only of this system, but all systems in the body. The blood travels continuously through the circulatory system to and from the heart through 2 different vascular loops, both originating and terminating at the heart. The pulmonary circulation consists of a closed loop of vessels carrying blood between the heart and lungs, and the systemic Block: Foundations | COHEN [2 of 7] CARDIOVASCULAR PHYSIOLOGY circulation consists of a circuit of vessels carrying blood between the heart and other body systems. The figure to the right (Tortora and Derrickson, Principles of Anatomy and Physiology, 12th edition), depicts the entire cardiovascular system. The heart consists of 4 chambers, 2 atria (right and left) and 2 ventricles (right and left). The right atrium collects blood from the systemic circulation. This blood enters the right ventricle through the tricuspid valve (right AV valve) and from here enters the pulmonary circulation (via the pulmonary semilunar valve). Following the passage through the lungs, the blood enters the left atrium and then, through the mitral valve (left AV valve) and into the left ventricle. The blood in the left ventricle then enters the systemic circulation again (through the semilunar aortic valve). The loop between the heart and lungs (pulmonary circuit) is what we call “in series”, meaning that blood moves from one structure (the heart-particularly the right ventricle) to the lungs, to the heart (left ventricle). The loop between the heart and body (systemic circulation) is “in parallel”, meaning that blood moves to many organs at the same time. In order for blood to move through the chambers of the heart (and as we’ll learn, throughout the vascular system) we need to generate pressure. In the heart, pressure needs to be high enough to open valves (to move to the next chamber or the pulmonary/systemic systems). This pressure is generated by contraction of the muscle cells that make up the bulk of the heart (the cardiomyocytes). If certain cardiomyocytes contract as a single unit (called a syncytium), they are able to generate the needed pressure to move blood throughout the vascular system. Block: Foundations | COHEN [3 of 7] CARDIOVASCULAR PHYSIOLOGY The reason the heart can beat autorhythmically (without outside influence…although in most cases, nervous and hormonal input play an important role), is due to specialized cells in the heart termed autorhythmic cells. These cells form the sino-atrial node (SA node), the atrio- ventricular node (AV node), the Bundle of His and the Purkinjie fibers. The figure to the right outlines the location of the autorhythmic cells. Because these cells create action potentials (electrical signals), placement of electrodes on the surface of a person’s chest (and some other locations that you’ll learn about when covering ECGs) can pick up the overall electrical characteristics of the heart, termed the Electrocardiogram (ECG). A representative ECG tracing has identifiable wave-forms as well as segments and intervals. For right now, you should understand that depolarization of a portion of the heart leads to contraction and that repolarization of a portion of the heart leads to relaxation. This will become much clearer as the cardio portion of the block continues! The P-wave represents atrial depolarization. Therefore, after the P-wave, we would expect to see atrial contraction. The QRS complex represents ventricular depolarization, which will be followed by ventricular contraction. (Hidden within the QRS complex is atrial repolarization and so atrial relaxation). The T wave is ventricular repolarization, and thus signals ventricular relaxation. The other components of the ECG that you can see in the image above are sections called segments and those called intervals. Segments are isoelectric (no waveforms!) and so are periods of time when atria/ventricles are either completely depolarized or repolarized). The S-T segment shown above signifies total ventricular depolarization and total atrial repolarization. Intervals include waveforms and are what either atria or ventricles do. So, the P-Q interval depicts atrial depolarization (P-wave) and repolarization (hidden in the QRS), and the Q-T interval depicts ventricular depolarization and repolarization. Block: Foundations | COHEN [4 of 7] CARDIOVASCULAR PHYSIOLOGY Vessels: Blood Vessel are the tubes that carry blood from the heart to the body (and lungs) and return the blood back to the heart. There are 4 distinct types of vessels that will be mentioned here and will be described in detail in the CPR block. Arteries are sometimes called the “pressure” vessel, because they deal with the greatest stress on the wall, being the vessels that follow the contracting heart. Arteries set up a pressure- gradient (from high to low) that allow blood to move through the body. This pressure is called Mean Arterial Pressure (MAP). Arterioles are called “resistance” vessels. These vessels have thick layers of smooth muscle surrounding them. When the muscles contract (constrict), flow decreases and when the smooth muscle relaxes (dilate) flow increases. Capillaries are where exchange takes place. Exchange of gases, nutrients and plasma. These constituents flow down their concentration gradient. Veins are very easy to stretch (called compliance). At rest, more than half of a person’s blood volume is found in the veins. In fact, due to the ability to stretch out, there needs to be assistance from things such as skeletal muscle in order for blood to return to the heart. Blood Pressure: The arterial blood pressure is perhaps the most important regulated cardiovascular variable. The blood pressure on the arterial side of the circulation must be high enough to effectively drive the blood through the various organs. However, it’s also very important to keep the pressure from getting too high! If the pressure exceeds normal limits, there will be an increased workload, ‘Afterload’ imposed upon the heart. Also, chronically increased blood pressure causes stiffening of the blood vessels. Thus, it is important to keep arterial pressure within strict limits and to do so requires sophisticated regulatory mechanisms. To understand how arterial blood pressure is regulated, it is necessary to first understand the physiological factors that determine the arterial blood pressure. The arterial blood pressure depends upon: 1.) the contractile properties of the heart, 2.) the properties of the vasculature (compliance and vasculature tone) and 3.) the blood volume. Let’s first discuss the determinants of the arterial blood pressure. Then, we will see how these determinants are regulated to keep the arterial blood pressure within strict, normal limits. Block: Foundations | COHEN [5 of 7] CARDIOVASCULAR PHYSIOLOGY The relationship of the cardiac output (CO), arterial blood pressure and total peripheral resistance is: CO = PA - PV / TPR where: PA = Mean Arterial Blood Pressure PV = Venous Blood Pressure TPR = Total Peripheral Resistance (SVR = Systemic Vascular Resistance) Because PV ~ 0 mm Hg, we can substitute Mean Arterial Pressure (MAP) for the pressure difference: CO = MAP /TPR That is, the cardiac output equals the mean arterial blood pressure (MAP) divided by the total peripheral resistance (TPR). The equation above can be rearranged in terms of MAP: MAP = CO x TPR Equation 3 simply states that the Mean Arterial Pressure is a function of both the CO and the TPR. If the TPR remains constant and the cardiac output increases, the mean arterial blood pressure will increase. If the CO remains constant and the total peripheral resistance increases, the mean arterial blood pressure will increase. Let’s focus on each factor, first the CO. The Determinants of the Cardiac Output. The determinants of cardiac output (CO) are heart rate (HR) and stroke volume (SV), so: CO = HR x SV Determinants of the Total Peripheral Resistance (TPR). The relationship for vascular resistance in a single tube or vessel: R = 8ηl / π r4 The components are: Ŋ = viscosity (the “thickness” of the blood) l = length π = pi r4 = radius to the forth power (so small changes in radius lead to BIG changes in resistance) Block: Foundations | COHEN [6 of 7] CARDIOVASCULAR PHYSIOLOGY Before we get going in the CPR block, here are some terms and definitions that will hopefully be useful. Heart Rate (HR): How often the heart contracts each minute (beats per minute, bpm) Stroke Volume (SV): The volume of blood ejected from the left ventricle with each contraction (mL) Cardiac Output (CO): The volume of blood ejected from the left ventricle each minute (mL/min or L/min) Preload: The pressure in the left ventricle prior to contraction. It is closely related to the volume of blood in the left ventricle prior to contraction. Afterload: The pressure that the ventricle has to overcome for blood to be able to be ejected Contractility: The ability of the heart to contract (generate force) Inotropy: Force of contraction Lusitropy: Force of relaxation Ejection Fraction: The percentage of blood that leaves the left ventricle with each beat Vasodilation: Increasing of vascular radius (which decreases resistance and increases flow) Vasoconstriction: Decreasing of vascular radius (which increases resistance and decreases flow) Systole: Contraction of a chamber in the heart (atrial or ventricular) Diastole: Relaxation of a chamber in the heart (atrial or ventricular) Compliance: Ability of a chamber (or vessel) to stretch Elasticity: Ability of a chamber (or vessel) to return to its original shape Perfusion: Movement of blood into and out of a vessel Filtration: Movement of fluid out of a capillary based on pressures Reabsorption: Movement of fluid into a capillary based on pressures Edema: Swelling. Increased fluid within the tissues Vascular Tone: The amount of vasoconstriction of a vessel under basal conditions Extrinsic: From outside Intrinsic: From inside Viscosity: The “thickness” of a fluid End Systolic Volume: The amount of blood left in a chamber (usually the left ventricle) after contraction End Diastolic Volume: The amount of blood left in a chamber (usually the left ventricle) at the end of relaxation (prior to contraction) Segments: sections on an ECG that are isoelectric (structures are either completely depolarized or repolarized) Intervals: sections on an ECG that include either the atria or the ventricles (and include both depolarization and repolarization) Block: Foundations | COHEN [7 of 7]