Cardiovascular System: Heart

The heart is a vital organ in the human body that plays a central role in the circulatory system. It is a muscular pump responsible for circulating blood throughout the body, supplying oxygen and nutrients to various tissues and organs while removing waste products.

The heart is located in the thoracic cavity, between the lungs, and slightly to the left of the midline. It is roughly the size of a closed fist and is enclosed within a protective double-walled sac called the pericardium. The pericardium consists of an outer fibrous layer and an inner serous layer, which secretes a fluid that reduces friction during heartbeats.

The heart consists of four chambers: two atria and two ventricles. The atria are the upper chambers and receive blood returning from the body (right atrium) and lungs (left atrium). The ventricles, located below the atria, are responsible for pumping blood out of the heart. The right ventricle pumps blood to the lungs for oxygenation, while the left ventricle pumps oxygenated blood to the rest of the body. The heart is divided into two halves by a muscular wall called the septum. The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs via the pulmonary circulation. The left side receives oxygenated blood from the lungs and pumps it to the systemic circulation, supplying oxygen and nutrients to all body tissues.

The heart functions as a muscular pump that contracts and relaxes rhythmically, allowing for the continuous circulation of blood throughout the body. This rhythmic contraction is known as the cardiac cycle, which is coordinated by electrical signals generated within the heart.

The heartbeat is initiated by the sinoatrial (SA) node, often called the natural pacemaker of the heart. The SA node generates electrical impulses that cause the atria to contract, forcing blood into the ventricles. The electrical signal then travels to the atrioventricular (AV) node, which delays the impulse slightly, allowing the ventricles to fill completely before contracting. The AV node then transmits the signal to the ventricles, causing them to contract and pump blood out of the heart.

The heart valves play a crucial role in ensuring one-way blood flow. The atrioventricular valves, including the tricuspid valve on the right side and the mitral (bicuspid) valve on the left side, prevent the backflow of blood from the ventricles into the atria during ventricular contraction. The semilunar valves, including the pulmonary valve and aortic valve, prevent the backflow of blood from the arteries into the ventricles during relaxation.

Blood vessels called arteries carry oxygenated blood away from the heart, while veins carry deoxygenated blood back to the heart. The largest artery in the body, the aorta, originates from the left ventricle and branches out to supply oxygenated blood to all body tissues. The veins, including the superior and inferior vena cava, return deoxygenated blood from the body to the right atrium.

Anatomy of the Heart

The heart is made up of four chambers: two atria and two ventricles. The heart also contains two atrioventricular (AV) valves between the artia and ventricles anchored to the heart using chordae tendineae and papillary muscles from the myocardium, and two semilunar valves between the ventricles and arteries leaving the heart.  These valves control the flow of blood through the heart and prevent blood from flowing backward.

·         The atria are the top two chambers of the heart and receive blood from the body (right atrium) and the lungs (left atrium).

·         The ventricles are the bottom two chambers of the heart and are responsible for pumping blood out of the heart. The right ventricle pumps blood to the lungs for oxygenation, while the left ventricle pumps oxygen-rich blood to the rest of the body.

·         The tricuspid valve is an AV valve between the right atria and right ventricle, made up of three leaf shaped membranes arranged in a circle.

·         The pulmonary semilunar valve is between the right ventricle and the pulmonary trunk in three small cusped shapes.

·         The bicuspid (mitral) valve is an AV valve between the left artia and left ventricle, made up of two leaf shaped membranes arranged in a circle.

·         The aortic semilunar valve is between the left ventricle and the aorta in three small cusped shapes.

The heart is surrounded by a sac called the pericardium, which helps protect the heart and keeps it in place. The walls of the heart are made up of three layers: the epicardium, the myocardium, and the endocardium. The epicardium is the outermost layer of the heart and is made up of a thin layer of connective tissue. The myocardium is the middle layer of the heart and is made up of muscle tissue that contracts to pump blood. The endocardium is the innermost layer of the heart and is made up of a thin layer of cells that line the heart chambers and valves.

The pericardium is a fibrous sac that surrounds and protects the heart. It is a double-layered membrane that consists of the fibrous pericardium and the serous pericardium.

The fibrous pericardium is the outermost layer of the pericardium. It is made up of dense connective tissue and is anchored to the diaphragm below and the great vessels above. The fibrous pericardium provides support and protection to the heart and helps to prevent over-expansion of the heart during diastole.

The serous pericardium is the innermost layer of the pericardium and is further divided into two layers: the parietal layer and the visceral layer.

·         The parietal layer lines the fibrous pericardium and is separated from it by a potential space called the pericardial cavity.

·          The visceral layer, also known as the epicardium, covers the surface of the heart and is continuous with the parietal layer at the base of the heart.

Between the parietal and visceral layers of the serous pericardium is a thin layer of fluid called pericardial fluid. This fluid acts as a lubricant, allowing the layers to slide smoothly over each other during the contraction and relaxation of the heart.

The pericardium plays an important role in maintaining the structure and function of the heart. It helps to prevent excessive movement of the heart within the chest cavity, and it also provides a barrier against infection and inflammation.

Cardiac Cycle

The heart is a muscular organ that pumps blood throughout the body. The process of blood flow through the heart begins with the right atrium, the upper chamber of the heart, receiving deoxygenated blood from the superior and inferior vena cava, the two largest veins in the body. The blood then passes through the tricuspid valve, which separates the right atrium from the right ventricle, the lower chamber of the heart. Once the right ventricle is filled with blood, it contracts, pumping the blood through the pulmonary semilunar valve and into the pulmonary trunk, which splits into left and right pulmonary arteries that carries the blood to the lungs. In the lungs, carbon dioxide is removed from the blood, and oxygen is added. The newly oxygenated blood then returns to the heart via the pulmonary veins, entering the left atrium, which completes the pulmonary circuit of the cardiovascular system.

The left atrium then pumps the oxygenated blood through the mitral valve, also known as the bicuspid valve, into the left ventricle. The left ventricle, being the strongest chamber of the heart, contracts and pumps the oxygenated blood through the aortic semilunar valve and into the aorta, the largest artery in the body. The aorta then carries the oxygenated blood to all parts of the body, providing the necessary oxygen and nutrients to the tissues and organs.  The blood then returns to the heart through the superior and inferior vena cava completing the systemic circuit.

Throughout the blood flow process, the heart is aided by a series of electrical impulses that cause the heart muscles to contract and relax in a coordinated fashion, known as the cardiac cycle. This rhythmic beating of the heart is essential for maintaining healthy blood flow and overall cardiovascular function.

The cardiac cycle is a series of events that occur in the heart as it pumps blood. It consists of three main stages: atrial systole, ventricular systole, and diastole. Here's a detailed explanation:

·         Atrial systole begins with the contraction of the atria (the heart's upper chambers). The AV valves (tricuspid and mitral) are open, allowing blood to flow from the atria into the ventricles (the heart's lower chambers). The SA node, known as the heart's natural pacemaker, initiates the electrical impulse that causes atrial contraction.

·         Ventricular systole starts as the atria finish contracting and start to relax, the ventricles begin to contract. The contraction of the ventricles causes the AV valves to close, preventing blood from flowing back into the atria. This closure produces the first heart sound, "lub." The semilunar valves (pulmonary and aortic) open, allowing blood to be pumped out of the ventricles. The right ventricle sends blood to the lungs via the pulmonary artery, while the left ventricle sends blood to the rest of the body via the aorta.

·         During atrial diastole, the atria (upper chambers of the heart) are in a relaxed state. Blood flows passively from the veins into the atria and then through the open AV valves into the ventricles. Atrial diastole occurs simultaneously with ventricular systole, as the ventricles are contracting and pumping blood out of the heart.

·         Ventricular diastole is the relaxation phase of the ventricles (lower chambers of the heart). After the ventricles have contracted and pumped blood out, they relax to allow blood to flow in from the atria. The semilunar valves (pulmonary and aortic) close to prevent blood from flowing back into the ventricles, producing the second heart sound, "dub." The AV valves open to facilitate blood flow from the atria to the ventricles. 

The blood flow through the heart involves the right atrium receiving deoxygenated blood from the vena cava, which then passes through the tricuspid valve into the right ventricle. The right ventricle then pumps the blood through the pulmonary valve into the pulmonary artery, carrying the blood to the lungs for oxygenation. The oxygenated blood returns to the heart via the pulmonary vein, entering the left atrium, which pumps the blood through the mitral valve into the left ventricle. The left ventricle then pumps the oxygenated blood through the aortic valve into the aorta, which carries the blood to all parts of the body.

Function of the Heart

The heart's primary function is to pump blood throughout the body to deliver oxygen and nutrients to the cells and remove waste products. Blood enters the heart through the superior and inferior vena cava and flows into the right atrium. From there, it passes through the tricuspid valve and enters the right ventricle. The right ventricle then pumps the blood through the pulmonary valve and into the pulmonary artery, which carries it to the lungs for oxygenation.

After the blood is oxygenated in the lungs, it returns to the heart through the pulmonary veins and enters the left atrium. From there, it passes through the mitral valve and enters the left ventricle. The left ventricle then pumps the blood through the aortic valve and into the aorta, which carries it to the rest of the body.

The heart beats approximately 100,000 times per day, pumping about 5 liters of blood per minute. The rate and strength of the heart's contractions are controlled by a complex system of electrical signals, hormones, and neural input.

Electrocardiogram 

The electrocardiogram (ECG or EKG) is a diagnostic test that records the electrical activity of the heart. It is a non-invasive test that is commonly used to diagnose various heart conditions, such as arrhythmias, ischemia, and heart attack. In this article, we will discuss the anatomy and physiology of the heart and how it relates to the ECG.

The heart beats due to electrical impulses that are generated within the heart muscle by the subendocardial (intrinsic) nervous system, the electrical system of the heart. These impulses are initiated by the sinoatrial (SA) node, which is located in the right atrium. The SA node acts as the natural pacemaker of the heart and sets the heart rate.  The SA node sets a rhythm of 100 beats per minute, which is slowed by innovation from cranial nerve X the vagus nerve during parasympathetic times or vagal tones.

The electrical impulses generated by the SA node spread through the atria, causing them to contract and pump blood into the ventricles. The impulses then reach the atrioventricular (AV) node, which is located between the atria and ventricles. The AV node acts as a gatekeeper, delaying the impulses before allowing them to pass into the ventricles.  The AV node sets a rhythm of approximately 60 beats per minute and will take over if the SA node stops functioning.

The impulses then travel through the bundle of His, which divides into the left and right bundle branches. These branches then spread the impulses throughout the ventricles using purkinje fibers, causing them to contract and pump blood out of the heart.

The ECG records the electrical activity of the heart by measuring the voltage changes on the skin surface that result from the electrical activity of the heart. The ECG is typically recorded using electrodes that are placed on the chest, arms, and legs.

The ECG waveform consists of several different components, each of which corresponds to a different stage of the cardiac cycle.

·         The P wave represents the depolarization of the atria.

·         The QRS complex represents the depolarization of the ventricles and repolarization of the atria.

·         The T wave represents the repolarization of the ventricles.

The P wave is typically small and rounded, and it corresponds to the atrial contraction. The QRS complex is a larger waveform that corresponds to the ventricular contraction. The T wave is typically larger than the P wave and is usually rounded or pointed.

Abnormalities in the ECG waveform can indicate various heart conditions. For example, an enlarged QRS complex may indicate ventricular hypertrophy, while an inverted T wave may indicate ischemia or injury to the heart muscle.

The electrocardiogram is a valuable diagnostic tool that allows clinicians to assess the electrical activity of the heart. By understanding the anatomy and physiology of the heart, we can better interpret the ECG waveform and identify abnormalities that may indicate underlying heart conditions.

Cardiac Output

Cardiac output is a critical physiological parameter that describes the amount of blood pumped by the heart in a given period, usually one minute. It is a fundamental measure of cardiovascular function and can provide important diagnostic information in various cardiovascular diseases. In this explanation, we will explore the factors affecting cardiac output and how it can be calculated.

Cardiac output (CO) can be calculated by multiplying the stroke volume (SV) by the heart rate (HR). The stroke volume is the volume of blood ejected by the left ventricle during each contraction, while the heart rate is the number of times the heart contracts per minute.

·         Therefore, the formula for cardiac output is CO = SV x HR.

The stroke volume is affected by several factors, including preload, afterload, and contractility. Preload is the degree of stretch of the cardiac muscle fibers before contraction, and it is determined by the amount of blood returning to the heart. A higher preload increases the stroke volume due to increased stretch and subsequent contraction of the cardiac muscle fibers. Afterload, on the other hand, is the resistance that the heart must overcome to pump blood out of the ventricles. An increase in afterload reduces the stroke volume, while a decrease in afterload increases the stroke volume. Contractility refers to the force of contraction of the cardiac muscle fibers and is influenced by factors such as calcium ion concentration, sympathetic nervous system stimulation, and certain medications.

The heart rate is regulated by the autonomic nervous system, which comprises the sympathetic and parasympathetic nervous systems. The sympathetic nervous system stimulates the heart to increase heart rate, while the parasympathetic nervous system decreases heart rate. Factors such as stress, exercise, and certain medications can also affect heart rate. The normal resting cardiac output for a healthy adult is typically around 5 liters per minute. During exercise or stress, cardiac output can increase to meet the increased oxygen and nutrient demands of the body. However, in certain cardiovascular diseases such as heart failure or shock, the cardiac output may be reduced, leading to inadequate tissue perfusion and organ dysfunction.

Overview

The cardiovascular system, specifically the heart, plays a crucial role in pumping blood throughout the body. The heart consists of four chambers: two atria and two ventricles. It also contains valves that control blood flow and prevent backflow. The heart is surrounded by the pericardium, which provides protection and support.

Blood flow through the heart starts with the right atrium receiving deoxygenated blood from the body. It passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve into the pulmonary artery, leading to the lungs for oxygenation. Oxygenated blood returns to the heart through the pulmonary veins, entering the left atrium and passing through the mitral valve into the left ventricle. The left ventricle then pumps the oxygenated blood through the aortic valve into the aorta, supplying it to the rest of the body.

The heart's primary function is to pump blood to deliver oxygen and nutrients to cells while removing waste products. It beats approximately 100,000 times per day and is controlled by electrical impulses, hormones, and neural input.

The electrocardiogram (ECG) is a diagnostic test that records the heart's electrical activity. It measures voltage changes on the skin surface using electrodes placed on the chest, arms, and legs. The ECG waveform represents different stages of the cardiac cycle and can indicate various heart conditions.

Cardiac output, a crucial physiological parameter, describes the amount of blood pumped by the heart in one minute. It can be calculated by multiplying stroke volume (the volume of blood ejected by the left ventricle per contraction) by heart rate. Factors affecting cardiac output include preload, afterload, contractility, and heart rate regulation by the autonomic nervous system.

Understanding the structure, function, and physiology of the heart, as well as cardiac output, is vital for studying human anatomy, diagnosing heart conditions, and managing cardiovascular diseases.