The Heart

Heart

The Heart

Heart Slightly larger than the size of a fist Hollow, muscular organ Contains 4 chambers found in chest between lungs surrounded by membrane called Pericardium Pericardial space is fluid-filled to nourish and protect the heart.

Serous membrane Continuous with blood vessels

The Heart Wall Pericardium –outer layer surrounding heart 3 layers of the heart wall are: Pericardium –outer layer surrounding heart Myocardium – Middle layer Endocardium – Inner layer Between Pericardium and Myocardium Pericardial cavity made up of 2 layers of serous membrane Parietal layer Visceral layer Pericardial space is fluid-filled to nourish and protect the heart.

Myocardial tissue Structure Under the microscope, cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). Like skeletal muscle it is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. However, cardiac myocytes are narrower and much shorter than skeletal muscle cells, being about 0.02 mm wide and 0.1 mm long, and are more rectangular than smooth muscle cells, which are normally spindle-shaped. They are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction. A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between myocytes. These are known as intercalated discs, and are due to areas where the membranes of adjacent myocytes come very close together. The intercalated discs have two important functions: one is to ‘glue’ the myocytes together so that they do not pull apart when the heart contracts; the other is to allow an electrical connection between the cells, which, as we will see, is vital to the function of the heart as a whole. The electrical connection is made via special junctions (gap junctions) between adjoining myocytes, containing pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium (continuous cellular material).

Cardiac Muscle Cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). It is striated due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. They contain one nucleus but many mitochondria, which provide the energy required for contraction. Structure Under the microscope, cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). Like skeletal muscle it is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. However, cardiac myocytes are narrower and much shorter than skeletal muscle cells, being about 0.02 mm wide and 0.1 mm long, and are more rectangular than smooth muscle cells, which are normally spindle-shaped. They are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction. A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between myocytes. These are known as intercalated discs, and are due to areas where the membranes of adjacent myocytes come very close together. The intercalated discs have two important functions: one is to ‘glue’ the myocytes together so that they do not pull apart when the heart contracts; the other is to allow an electrical connection between the cells, which, as we will see, is vital to the function of the heart as a whole. The electrical connection is made via special junctions (gap junctions) between adjoining myocytes, containing pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium (continuous cellular material).

Myocardial Cells Interconnected by Gap Junctions

Cardiac Muscle Cell The intercalated discs have two important functions to: glue’ the myocytes together so that they do not pull apart when the heart contracts allow an electrical connection between the cells, a vital to the function of the heart as a whole. The electrical connection is made via special gap junctions between adjoining myocytes. These are pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium.

Cardiac Muscle Cell A unique feature of cardiac muscle is the presence of intercalated discs. The intercalated discs have two important functions to: ‘glue’ the myocytes together so that they do not pull apart when the heart contracts allow an electrical connection between the cells, a vital to the function of the heart as a whole. The electrical connection is made via special gap junctions between adjoining myocytes. These are pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium. Structure Under the microscope, cardiac muscle is seen to consist of interlacing bundles of cardiac myocytes (muscle cells). Like skeletal muscle it is striated with narrow dark and light bands, due to the parallel arrangement of actin and myosin filaments that extend from end to end of each myocyte. However, cardiac myocytes are narrower and much shorter than skeletal muscle cells, being about 0.02 mm wide and 0.1 mm long, and are more rectangular than smooth muscle cells, which are normally spindle-shaped. They are often branched, and contain one nucleus but many mitochondria, which provide the energy required for contraction. A prominent and unique feature of cardiac muscle is the presence of irregularly-spaced dark bands between myocytes. These are known as intercalated discs, and are due to areas where the membranes of adjacent myocytes come very close together. The intercalated discs have two important functions: one is to ‘glue’ the myocytes together so that they do not pull apart when the heart contracts; the other is to allow an electrical connection between the cells, which, as we will see, is vital to the function of the heart as a whole. The electrical connection is made via special junctions (gap junctions) between adjoining myocytes, containing pores through which small ions and therefore electrical current can pass. As the myocytes are electrically connected, cardiac muscle is often referred to as a functional syncytium (continuous cellular material).

The Heart

Valves The Heart contains four valves The Tricuspid valve opens from the R atrium into the R ventricle The Pulmonary (semilunar) valve opens from the R ventricle into the pulmonary artery The Mitral valve opens from the L atrium into the L ventricle The Aortic (semilunar) valve opens from the L ventricle into the Aorta

The Heart: Valves Function of the valves Situated at the entrance and exit of the ventricles Ensure that blood moves only in one direction…Forward Blood flows though the valves as a result of pressure changes

The Cardiac Cycle Refers to the repeated pattern of contraction and relaxation of the heart Phase of contraction is called systole, Phase of relaxation is called diastole The heart has a two step pumping action: the atria contract simultaneously, followed approx 0.1-0.2 seconds later by the ventricles.

Cardiac Cycle Systole Contractile phase of heart Electrical and mechanical changes E.g. blood pressure changes E.g. blood volume changes Diastole Relaxation phase of heart Takes twice as long as systole E.g. resting HR = 60 Systole = 0.3 s Diastole = 0.6 s

The heart beat begins when the heart muscles relax and blood Cardiac cycle STEP ONE blood from the body blood from the lungs The heart beat begins when the heart muscles relax and blood flows into the atria.

the valves open to allow blood into the ventricles. Cardiac cycle STEP TWO The atria then contract and the valves open to allow blood into the ventricles.

The cycle then repeats itself. Cardiac cycle STEP THREE The valves close to stop blood flowing backwards. The ventricles contract forcing the blood to leave the heart. At the same time, the atria are relaxing and once again filling with blood. The cycle then repeats itself.

5. Ventricles contract, about two thirds of the volume they contain is ejected, leaving one third called the end systolic volume cycle 1. Atria fill with blood 4. Atria contract Contributing approx 20% To end diastolic volume 2.AV valve opens when pressure in atria exceeds ventricle 3. Blood flows from atria to ventricles through open AV valve. This contributes approx 80% of end diastolic volume The Cardiac cycle

Heart conduction

Electrical Activity of the Heart Contraction of heart depends on electrical stimulation of myocardium Impulse is initiated on right atrium and spreads throughout the heart May be recorded on an ECG

Electrical Conduction System Myocardial Cells Characteristics automaticity: cells can depolarize without any impulse from outside source (self-excitation) excitability: cells can respond to an electrical stimulus conductivity: cells can propagate the electrical impulse from cell to another contractility: the specialized ability of the cardiac muscle cells to contract

The Hearts conduction pathway

Depolarization and Impulse Conduction Heart is autorhythmic Depolarization begins in sinoatrial (SA) node Spread through atrial myocardium Delay in atrioventricular (AV) node 24 Sept. 2008 EKG-Lab.ppt

Depolarization and Impulse Conduction Spread from atrioventricular (AV) node AV bundle Bundle branches Purkinje fibers 24 Sept. 2008 EKG-Lab.ppt

Electrocardiogram Records electrical activity of the heart P wave Atrial depolarization QRS complex Ventricular depolarization T wave Ventricular repolarization

Electrical conduction P Wave represents atrial depolarisation QRS Wave represents ventricular depolarisation T Wave represents Ventricular repolarisation

Cardiac blood flow Cardiac blood flow is different from blood flow in other organs. Blood flows around coronary vessels during diastole. Myoglobin in myocardial cells stores oxygen. This ensures that the heart muscle has a constant supply.

Cardiac output = stroke volume x heart rate Refers to the amount of blood ejected from the left ventricle of the heart per minute Determined by stroke volume and heart Rate Cardiac output = stroke volume x heart rate    

Factors which influence blood pressure Cardiac output (CO) Total Peripheral Resistance (TPR) Or BP =CO x TPR  

Cardiac Output Range of normal at rest is 4 – 6 L.min During aerobic activity the increase in cardiac output is roughly proportional to intensity. Max. Q is in range of 20 – 40 L.min, depending on size, heredity, and conditioning.

Heart Rate Range of normal at rest is 50 – 100 b.m Increases in proportion to exercise intensity Max. HR is 220 – age Medications or upper body exercise may change normal response

Factors affecting Heart Rate Gender Autonomic nerve activity Age Circulating hormones e.g adrenaline, thyroxine Activity and exercise Temperature The baroreceptor reflex Emotional states Waugh and Grant (2006)

Stroke Volume Range of normal at rest is 60 – 100 ml.b During exercise, SV increases quickly, reaching max. around 40% of VO2 max. Max. SV is 120 – 200 ml.b, depending on size, heredity, and conditioning. Increased SV during rhythmic aerobic exercise is due to complete filling of ventricles during diastole and/or complete emptying of ventricles during systole.

Cardiac blood supply The heart receives its blood supply via the coronary arteries. These arteries supply a huge network of capillaries Ensures that myocardial cells are close to their blood supply Diffusion of gases between the myocardial cells and capillaries occurs very quickly

Sympathetic parasympathetic The heart is influenced by autonomic nerves originating in the cardiovascular centre in the medulla oblongata Consisting of sympathetic and parasympathetic nerves with antagonistic autonomic nerves Sympathetic parasympathetic Cardiac control center in medulla oblongata maintains balance between the two

Parasympathetic: from medulla oblongata (vagus nerve) Nerve branches to S-A and A-V nodes, and slows rate Parasympathetic activity : Can increase to slow heart rate Can decrease to increase heart rate Sympathetic nervous system (celiac plexus) SA node, AV node and the myocardium of the atria and ventricles. Sympathetic activity : increases HR force of contraction

Summary The heart is a four chambered pump, which is effectively two pumps working together The heart contains valves which function to ensure no backflow of blood The wall of the heart is made up of the following layers: pericardium, myocardium, endocardium The heart has ‘autorhythmicity’ it will beat without outside nervous input The heart has a specialist conduction pathway- ensuring a smooth coordinated contraction, Cardiac muscle contains large numbers of mitochondria, and the heart has an excellent blood supply