Cardiovascular System

The endocardium lines the inner heart chambers and covers heart valves and is continuous with the inner lining of blood vessels. Purkinje fibers are located in the endocardium. They participate in the contraction of the heart muscle. The endocardium also allows the blood to flow freely

Myocardium is the middle layer of the heart wall. It is composed mainly of cardiac muscle and forms the bulk of the heart. The myocardium is the layer that contracts and is surrounded by the endocardium and the epicardium. Myocardium stimulates heart contractions to pump blood from the ventricles and relaxes the heart to allow the atria to receive blood. These contractions produce a heartbeat. The beating of the heart drives the cardiac cycle which pumps blood to cells and tissues of the body.

The epicardium is a superficial layer of the serous pericardium, a double-layered envelope surrounding the heart it is an outer protective layer that prevents over extension during heart beats

Bicuspid valve is situated between the left atrium and the left ventricle. It allows blood to flow one way only, from the left atrium into the left ventricle. The Tricuspid valve is situated between the right atrium and the right ventricle. It allows blood to flow from the right atrium into the right ventricle. The job of the bicuspid and tricuspid valves is to prevent backflow into the atria when the ventricles are contracting and forcing blood into the circulatory system.

Chordae tendineae are chord-like tendons that connect to the bicuspid and tricuspid valves. They prevent the valves from turning inside out.

The pulmonary vein carries oxygen rich blood from the lungs to the left atrium of the heart. There are four pulmonary veins which extend from the left atrium to the lungs. They are the right superior, right inferior, left superior, and left inferior pulmonary veins.

The pulmonary artery carries deoxygenated blood from the heart to the lungs it is the only artery in the body that carries deoxygenated blood.  The main pulmonary artery extends from the right ventricle of the heart and branches into left and right pulmonary arteries. The left and right pulmonary arteries extend to the left lung and right lungs.

Vena cava are the two largest veins in the body. These blood vessels carry de-oxygenated blood from various regions of the body to the right atrium of the heart. As the de-oxygenated blood is returned to the heart and continues to flow through the cardiac cycle, it is transported to the lungs where it becomes oxygenated. The blood then travels back to the heart and is pumped out to the rest of the body by the aorta. Superior vena cava is a large vein that carries deoxygenated blood from the upper half of the body to the right atrium. Inferior vena cava is the large vein that carries deoxygenated blood from the lower half of the body into the heart. It enters the right atrium at a lower right posterior side of the heart.

The aorta is the largest artery in the human body originating from the left ventricle of the heart down to the abdomen where it branches off into smaller arteries. The aorta transports oxygenated blood to all parts of the body.

The heart is divided into four chambers that are connected by heart valves. The upper two heart chambers are called atria. Atria receive blood returning to the heart from the body and ventricles pump blood from the heart to the body. The atria receive and collect the blood coming to the heart. They then deliver blood to the lower left and right ventricles which pump blood away from the heart through powerful, rhythmic contractions. The atria are the receiving chambers for blood returning to the heart from the body. They need to contract only minimally to push blood into ventricles so the atria are relatively small. The right atrium receives blood returning to the heart from the superior and inferior vena cava. The superior vena cava returns de-oxygenated blood the upper part of the body to the right atrium. The inferior vena cava returns de-oxygenated blood from the lower body regions to the right atrium. The left atrium receives blood returning to the heart from the pulmonary veins. The pulmonary veins extend from the left atrium to the lungs and bring oxygen-rich blood back to the heart.

The lower two chambers of the heart are called ventricles. The right ventricle receives blood from the right atrium and pumps blood into the pulmonary artery which routes the blood to the lungs where gas exchange occurs. The left ventricle receives blood from the left atrium and pumps blood into the aorta the largest artery which takes oxygenated blood away from the heart and around the body. This ventricle has a thick wall because it has to pump blood around the body    

The aortic valve is one of the four valves in the heart, this valve is situated at exit of the left ventricle of the heart where the aorta begins. The aortic valve lets blood from the left ventricle be pumped up into the aorta but prevents blood once it is in the aorta from returning to the heart. The pulmonary valve stands at the opening from the right ventricle. It lets blood head in the right direction and keeps it from sloshing back from the pulmonary artery into the heart. The aortic and pulmonary valves guard the bases of the lager arteries attached to the ventricles and prevent backflow into ventricles. The aortic and pulmonary valves are known as semilunar valves. Theses valves are forced open as the blood rushes past them. When the ventricles relax and the blood flows backwards toward the heart the valves close.  

Blood vessels

Arteries

Arteries take oxygenated blood away from the heart to be delivered around the body The arteries deliver the oxygen rich blood to the capillaries where the actual exchange of oxygen and carbon dioxide occurs. The capillaries then deliver the waste  blood to the veins for transport back to the lungs and heart. The large artery that leaves the right ventricle is called the pulmonary artery and the artery that leaved the left ventricle is the aorta. Pulmonary arteries carry blood from the heart to the lungs where the blood picks up oxygen. The oxygenated blood is then returned to the heart via the pulmonary veins. The aorta is the main systemic artery and the largest artery of the body. It originates from the heart and branches out into smaller arteries which supply blood to the head region, the heart itself and the lower regions of the body. The muscular wall of the artery helps the heart pump the blood. When the heart beats the artery expands as it fills with blood. When the heart relaxes the artery contracts which has a force that is strong enough to push the blood along. This rhythm between the heart and the artery results in an efficient circulation system.The blood moves under pressure into smaller arteries, finally reaching the smallest branches known as arterioles. As the arteries devide further they become smaller and smaller, until they are classed as arterioles.

Arterioles

These are smaller versions of arteries and they connect arteries to capillaries. Major arterioles are thick walled with small diameters. Arterioles are responsible for blood flow and blood pressure. They contain muscles that allow the vessel to constrict and stop blood flow to certain areas if it is not required. 

Capillaries

Capillaries are the smallest blood vessels in the body. They are microscopic. They are just one cell thick to allow capillary exchange. A capillary bed is the capillary structure found in a body organ or skeletal muscle. Capillary beds contain thousands and millions of capillaries for each muscle structure or body organ. As blood passes through the muscle or organ capillary system, it gives up oxygen and nutrients and takes in carbon dioxide and other waste products. Each capillary connects to a vein and an arterial end which connects to an artery. Capillaries are also involved in the body's release of excess heat. During exercise, for example, your body and blood temperature rises. To help release this excess heat, the blood delivers the heat to the capillaries which then rapidly release it to the tissue.

Veins

Veins are similar to arteries but they transport blood at a lower pressure so they are not as strong as arteries. Veins also differ in that they are supported by valves. Valves prevent a backflow of blood and ensure that the blood in veins is not under pressure. The vein valves are necessary to keep blood flowing toward the heart, but they are also necessary to allow blood to flow against the force of gravity. For example, blood that is returning to the heart from the foot has to be able to flow up the leg. Veins act as low pressure reservoirs and move stored blood into general circulation during exercise. Pulmonary veins  carry oxygenated blood from the lungs to the left atrium of the heart. Systemic veins return deoxygenated blood from the rest of the body to the right atrium of the heart. Superficial veins are located close to the surface of the skin and are not located near a corresponding artery. Deep veins are located deep within muscle tissue and are typically located near a corresponding artery

Venules

The smallest veins in the body are called venules. They receive blood from the arteries via the arterioles and capillaries. But unlike capillaries venules have some connective tissue in their walls. Venules collect the outflow of blood from the capillary bed at low pressure. The venules branch into larger veins which eventually carry the blood to the largest veins in the body, the vena cava. The blood is then transported from the superior vena cava and inferior vena cava to the right atrium of the heart.

Vasodilatation and Vasoconstriction

Blood flow is controlled by pressure, this is achieved by pressure by the vasoconstriction and vasodilatation

Vasodilatation is the widening of blood vessels due to the relaxation of smooth muscular vessel walls, particularly in the large and small arterioles and large veins. Vasodilatation involves an increase in the diameter of the blood vessels resulting in an increased blood flow to the muscle area supplied by the vessel. Opening the vessels to the skin allows the heat to be carried to the surface of the body where it escapes into the atmosphere. This is why you go red when you exercise.  

Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the smooth muscular wall of the vessels, particularly the large arteries, small arterioles and veins. Vasoconstriction involves a decrease in the diameter of a blood vessel walls resulting in the reduction of blood flow. In cold weather blood flow to the skin is decreased through vasoconstriction meaning that less heat is lost to the atmosphere.

How the heart works

Cardiac cycle

The cardiac cycle is the sequence of events that take place during one complete heart beat. There are 4 stages and each stage depends on whether the heart chambers are filling with blood while the heart is relaxing.

The four stages of the cardiac cycle are

1.     Atrial diastole. This is where the atrium is relaxed and receives the blood

2.     Atrial systole. This is where the atrium contracts and pushes blood down into the ventricles

3.     Ventricular diastole. This is where the ventricle is relaxed and where it receives the blood from the atrium

4.     Ventricular systole. This is where the ventricle contracts and pushes blood into the aorta and pulmonary vein

During the diastole phase, the atria and ventricles are relaxed. Blood flows into the right and left atria. The sino atrial node contracts which makes the atria contract. The valves located between the atria and ventricles are open, allowing blood to flow through to the ventricles the semilunar valves close to prevent blood from back flowing back into the atria 

During the systole phase, the ventricles contract pumping blood into the arteries. Atrioventricular valves close and semilunar valves open. The right ventricle sends blood to the lungs via the pulmonary artery. The left ventricle pumps blood to the aorta.

SAN sino atrial node

SA Node is a pacemaker in the atrial wall below the opening of the superior vena cava. When the sinoatrial node contracts it generates nerve impulses that travel throughout the heart wall It sends electrical impulse that triggers each heart beat. The impulse spreads through the atria. This causes both atria to contract. The SA node is located in the upper wall of the right atrium. It is composed of nodal tissue that has characteristics of both muscle and nervous tissue.

AVN atrio ventricular node

The atrio ventricular bundle is a bundle of specialised fibres in the heart that transmit the cardiac impulses form the atria to the ventricles. The atrio ventricular node is found in the right atria. The atrio ventricular node in turn sends an impulse through the nerve network to the ventricles. When the impulses from the sino atrial node reach the atrio ventricular node they are delayed for a sight second. This delay allows the atria to contract and empty their contents first.  

Bundle of his

The impulses are then sent down the atrioventricular bundle. This bundle of fibres branches off into two bundles and the impulses are carried down the centre of the heart to the left and right ventricles.

Purkinje fibres

These are found in the inner ventricular walls of the heart beneath the endocardium. At the base of the heart the atrioventricular bundles start to divide further into Purkinje fibres. When the impulses reach these fibres they trigger the muscle fibres in the ventricles to contract. These fibres are specialised myocardial fibres that conduct an electrical stimulus, which make the heart contract in a rhythmical way.

Autonomic nervous system

Sympathetic

When the sympathetic nervous system is activated by emotional or physical stressors such as exercise or anxiety, sympathetic fibres release chemical called norepinephrine which is a chemical transmitter substance realised at nerve endings to increase the heart rate. The chemoreceptor’s detect carbon dioxide in the blood and sends a signal to the medulla which then sends of a signal to the sympathetic nervous system that controls adrenaline. The sympathetic nervous systems then release adrenaline into the heart which makes it increase its flow in blood and increases in cardiac output/stroke volume which means there’s an increase in heart rate.

Parasympathetic

The parasympathetic system opposes sympathetic effects and effectively reduces heart rate when a stressful situation has passed. Parasympathetic responses are managed by a chemical called acetylcholine, acetylcholine is a chemical transmitter substance released at nerve endings to relax the heart rate.  Barorecpetor’s detect change in blood pressure and sends a signal to the parasympathetic nervous system that controls acetylcholine. The parasympathetic nervous system then releases acetylcholine into the heart which makes the heart rate decrease.