Cardiovascular System PDF

The heart is a pump. It fills with blood then the cardiac muscle contracts and squeezes the blood out of the chambers and into the vessels of the body. The blood carrying oxygen to the tissues and removing waste products of cellular respiration are carried through the body, propelled by the heart th...

The heart is a pump. It fills with blood then the cardiac muscle contracts and squeezes the blood out of the chambers and into the vessels of the body. The blood carrying oxygen to the tissues and removing waste products of cellular respiration are carried through the body, propelled by the heart through a closed system of blood vessels. The heart is surrounded by the lungs and protected by the ribs. **Mediastinum** in veterinary species the heart is located in a space between the pleural cavities of the lungs called the mediastinum. The trachea, esophagus, vessels and lymph nodes also run through the mediastinum. The Heart Wall ============== - The **pericardium** is the "sac" that suspends the heart within the mediastinum - The pericardium has an outer fibrous layer - This layer protects the heart and loosely attaches it to the diaphragm - The inner portion of the pericardium is made up of two layers of serous pericardium - The outer, parietal layer between the fibrous pericardium and the epicardium - The inner, visceral layer is closely adherent to the heart muscle and is called the **epicardium** - **Myocardium** is the thickest layer of heart tissue, made up of cardiac muscle - Between the myocardium and the chambers of the heart is the thin membranous lining called the **endocardium** Hardware Disease ================ - **Septic pericarditis** is a bacterial infection and inflammation of the fibrous pericardium - Septic pericarditis can occur in any species, but in cattle it is frequently caused by hardwire - Hardware disease is also known as **traumatic reticulopericarditis** - Cattle may consume pieces of wire in the farm yard that are then pushed through the wall of the first chamber of the stomach, the **reticulum,** by its muscular contractions - As the reticulum contracts and the infection spreads it may penetrate the diaphragm into the pericardium - The disease may be prevented by bolusing cattle with a magnet to prevent migration of any ingested hardware. Pericardial Effusion/ Cardiac Tamponade ======================================= - When the space between the fibrous pericardium and the epicardium fills with fluid, **pericardial effusion**, causing the heart to be unable to fill between contractions - The fibrous pericardium cannot stretch and as the sac fills there is less and less space for the heart to expand between contractions - When the heart is unable to expand it cannot fill with blood causing blood pressure to drop and heart rate to go up The Flow of Blood ================= - Deoxygenated blood is received from the body via the cranial and caudal vena cava (e) into the **right atrium** - The blood then passes through the **right atrioventricular valve (tricuspid valve)** into the **right ventricle** - During **systole** the right AV valve closes and the right ventricle pushes the deoxygenated blood out the **pulmonic valve** into the pulmonary arteries and finally to the **alveoli** where the blood is oxygenated - The blood then returns to the heart via the **pulmonary veins** to the **left atrium** - Blood from the left atrium flows through the **left atrioventricular valve (mitral /bicuspid valve)** into the **left ventricle** - During systole the mitral valve closes and the left ventricle contracts pushing blood through the **aortic valve** into the coronary arteries and the **aorta** and then the arteries of the body External Structures of the Heart ================================ - The heart is roughly triangle shaped, the **base** is the portion with the atria, located on the top of the heart - The **apex** is the point of the triangle, in this case the bottom, made up of the two ventricles - The right and left sides of the heart can be identified by the orientation of the **auricles** (auricle means ear, and they look like ears on the base of the heart). The right auricle is above the right atrium and the left is over the left atrium. When examining the heart grossly, you should be able to "shake hands" with the right atrium- try it in lab! - The **right auricle** is also a common place to find a vascular tumor known as **hemangiosarcoma.** This tumor may hemorrhage and result in cardiac tamponade. - The borders of the ventricles can be seen as grooves, or **sulci** filled with fat on the surface of the heart - The vessels returning to the heart from the body are under low pressure and have thinner walls than the aorta and arteries carrying blood away from the heart under high pressure that have thick, muscular walls. Internal Structures of the Heart ================================ - The **[tri]**cuspid valve has three flaps, or leaflets, that arise from a fibrous ring called the **annulus** of the valve - **Chordae tendineae** (heart strings) allow the valve to function in one direction only, so blood must flow in the direction previously described - The chordae tendineae connect the free edges of the valve leaflets to the **papillary muscles** which attach to the wall between the two ventricles and the **interventricular septum** - In the right ventricle there is a band of tissue that runs between the interventricular septum and the right ventricular free wall called the **moderator band** which lends structural support to the right ventricle - The **pulmonary valve** prevents backflow from the pulmonary arteries into the right ventricle - The pulmonary valve also has three leaflets that attach to a fibrous ring, it ensures the blood flows only out of the right ventricle into the pulmonary arteries during contraction - After the blood becomes oxygenated in the lungs it reenters the heart through the **pulmonary vein** into the **left atrium** - The **mitral valve** has two leaflets (also known as the **[bi]cuspid valve**) that work in one direction (because of their chordae tendineae) that prevent backflow of blood into the left atrium when the ventricle contracts - The thick walled left ventricle does not have a moderator band - Finally, blood leaves the left ventricle through the **aortic valve** - The **aortic** and **pulmonic valves** are both **semilunar valves** that lack chordae tendineae The Cardiac Cycle ================= - Each complete contraction and relaxation of the heart is called the cardiac cycle - **Systole** is when the heart muscle contracts and blood is ejected from the atria to the ventricles and from the ventricles to the arteries - **Diastole** is when the heart muscle relaxes and fills with blood - The impulse for the heartbeat comes from the **sinoatrial node (SA node)** - The SA node is an area of specialized cardiac muscle cells capable of automatically generating the electrical impulses that trigger the beating of the heart - The SA node generates an electrical current by the movement of the positively charged cations Na+, K+, Ca++ and negatively charged anions Cl- - Cations are pumped across the cell membrane in a process called **polarization** - When the gates of the cell membrane are opened, cations flow out of the cell to equalize the charge in a process called **depolarization** - **Depolarization generates an electrical current that causes the heart muscle to contract** - The electrical current generated by the SA node travels by two routes on its way from the base to the apex of the heart - The speedy route is through the specialized cardiac muscle cells of the **SA node** to the **AV node, bundle of His** and the **Purkinje fibers** - The slower, local route through the rest of the cardiac muscle fibers - Unlike other muscle cells, the cardiac cells can transmit an electrical impulse from one cell to the next (skeletal muscle requires an electrical message from a nerve) - After the electrical impulse leaves the SA node it travels quickly to the AV node - There is a slight delay in conduction through the AV node - The AV node is the **only route of conduction of the electrical impulse from the atria to the ventricles** - This delay allows the atria to complete their systolic contraction before ventricular systole begins - If the atrial and ventricular systole occurred at the same time the pressure of the contracting ventricles would too high for the blood to flow from the atria into the ventricles - After the delay at the AV node the electrical impulse travels through specialized fibers in the ventricles called the **bundle of His** and the **Purkinje fibers** - The fibers of the bundle of His travel down the interventricular septum to the apex of the right and left ventricles - The Purkinje fibers carry the impulses up the from the bundle of His into the ventricular myocardium - **Even though the impulse starts at the heart base, the contraction of the ventricles begins at the apex, facilitating the ejection of blood from the ventricles** - During ventricular systole the atria have entered diastole and are filling with blood in preparation for the next contraction Fetal Circulation ================= - The fetus receives oxygen from the blood of its mother - Since gas exchange does not occur in the lungs of the fetus, only enough blood to support the growing lung tissue is required - Fetal circulation allows most of the blood to bypass the pulmonary circulation - Oxygenated blood from the placenta flows through the **umbilical vein** - Oxygenated blood from the umbilical vein flows through the **ductus venosus** which allows some of the blood to bypass the liver into the caudal vena cava - In the fetus the blood can bypass the pulmonic circulation by two routes: - **Foramen ovale** directly connects the right and left atria - The blood that does not flow directly into the left atrium, enters the pulmonary artery and may flow into the lungs or bypass the lungs into the aorta via the **ductus arteriosus** - During the first breath the lungs inflate and the neonate begins to oxygenate its own blood - Normally, the ductus venosus constricts so that blood no longer bypasses the liver and the foramen ovale and ductus arteriosus close so that blood may no longer bypass the lungs - **PDA- Patent Ductus Arteriosus** is when the ductus arteriosus does not close at birth and continues to allow blood to bypass the lungs, resulting in **cyanosis** "blue baby" and a pronounced "washing machine" murmur. The PDA must be surgically ligated to establish normal circulation through the lungs. Heart Sounds ============ - Listening to the heart, or to lungs or GI sounds, is called **auscultation** - The normal cardiac cycle results in two sounds: **lub (S1)** and the **dub (S2)** - S1 is generated by the simultaneous closure of the right AV (tricuspid) and left AV (mitral) valves at the beginning of ventricular systole - S2 is generated by the closure of the aortic and pulmonic (semilunar) valves at the beginning of ventricular diastole - The heart sounds should be evaluated from both sides of the chest, but are typically loudest on the left (or along the right sternal border in cats) - In large animals the stethoscope must be placed against the chest wall near the level of the elbow and as a far cranially as possible - In small animals the stethoscope is placed on the chest wall just caudal to the elbow - Also, in large animals a third heart sound (S3) may be ausculted due to rapid ventricular filling and a fourth sound (S4) due to contraction of the atria - S3 and S4 are not usually heard in dogs and cats Heart rate and Cardiac Output ============================= - The heart must move blood to the tissues to deliver oxygen and nutrients and to remove waste - In order to maintain **tissue perfusion** the heart must maintain **cardiac output** - **Cardiac output** is determined by two factors: **stroke volume** and **heart rate** - **Stroke volume** is the amount of blood ejected with each cardiac contraction - **Heart rate** is how often the heart contracts - **CO=SV x HR** - The fact that a large animal's stroke volume is so much larger explains why they can survive with a much lower heart rate than and small animal - Heart rate is determined, in part, by the rate of spontaneous SA node depolarization but is influenced by other factors - Exercise increases oxygen demand resulting in increased heart rate **(positive chronotropy)** and increased contractility **(positive inotropy)** which increases stroke volume - **Starling's Law:** states that increased filling of the heart (increased preload) results in increased cardiac contraction. Stretching of the ventricular muscle increases the force of its contraction - **Changes in blood pressure may affect stroke volume and heart rate** - Shock is a rapid drop in blood pressure from any of a number of reasons - Low blood pressure results in decreased preload and therefore decreased stroke volume, heart rate must increase to maintain cardiac output - The autonomic nervous system can influence cardiac output by releasing epinephrine during a fight or flight response, which results in increased stroke volume and therefore increased cardiac output - General anesthesia can stimulate the parasympathetic portion of the autonomic nervous system, releasing acetylcholine that decreases stroke volume, heart rate and thus decreases cardiac output - It is for this reason medications that block the parasympathetic response are given prior to general anesthesia Electrocardiogram ================= - An instrument called an electrocardiograph measures the electrical activity in the heart and produces an **electrocardiogram (ECG or EKG)** a graphical representation of the depolarization and repolarization of the heart muscle - The normal electrical activity of the heart is represented: Labeled ECG waveform - The **P wave** represents the depolarization of the atria, followed by the **QRS complex** representing ventricular depolarization and finally, the **T wave** represents repolarization of the ventricles Vascular Anatomy and Physiology =============================== - ***[A]*rteries carry blood *[A]*way from the heart** - **Veins carry blood towards the heart** - Blood going into the systemic circulation is under higher pressure than the blood in the pulmonary or coronary circulation - Because of the high pressure, the aorta is the largest diameter vessel with the thickest vessel wall - The aortic wall has a fibrous outer layer, a middle layer of smooth muscle and elastic connective tissue and a smooth inner layer of endothelium (simple squamous epithelial tissue) - As the aorta leaves the heart there are branches that form the carotids, subclavians, then caudal to the spine and abdominal organs and then branches again to the iliac and femoral arteries for the hind limbs - The arteries continue to branch into smaller **arterioles** so that the tissue may be supplied with oxygen and nutrients through the thin-walled capillaries - From the capillaries blood begins its route back to the heart through **venules** that merge to form veins - The blood in veins is under less pressure and **valves** in small and medium sized veins ensure blood travels only in the direction of the heart - Veins travel towards the heart next to an artery carrying blood away from the heart - As the veins merge, those in the forelimbs become the **brachiocephalic veins** and those in the hind limbs the **iliac veins** while the **jugulars** drain blood from the head - Most blood vessels have smooth muscle in their walls which allows constriction and dilation - This allows blood to be shunted to specific portions of the body - During fight or flight response the sympathetic nervous system constricts vessels that supply the extremities and digestive system and redirects blood to the vital organs: heart, brain and muscles - **Blood vessels may also constrict to maintain blood pressure when blood volume or cardiac output is decreased** Congestive Heart Failure (CHF) ============================== - Occurs in animals whose hearts have lost their pumping ability, reducing forward flow of blood, resulting in congestion - CHF can be right or left sided - **Right sided heart failure** results in congestion of the blood returning from the systemic circulation often associated with **ascites and limb edema** - **Left sided heart failure** decreases venous return from the heart and congestion of blood in the pulmonary circulation and **pulmonary edema** - The reduced cardiac output associated with heart failure may reduce perfusion to other organs, especially the kidneys - Medications used to treat heart failure include - Positive inotropes to increase the strength of cardiac contractions - Diuretics to eliminate edema - Vasodilators to enhance blood flow to organs - Congestive heart failure cannot be cured, but can be medically managed to improve quality of life