Bio 6.5 Circulatory System PDF

# Unit 6: The Circulatory System ## The Heart The heart is a large, muscular organ that pumps blood around the body. It contains many different layers made of different types of tissue, but we will mainly focus on the pericardium and cardiac muscle. The pericardium is made of inflexible connective...

# Unit 6: The Circulatory System ## The Heart The heart is a large, muscular organ that pumps blood around the body. It contains many different layers made of different types of tissue, but we will mainly focus on the pericardium and cardiac muscle. The pericardium is made of inflexible connective tissue and a type of epithelium called Serosa, which secretes lubricating fluids to allow the heart to slide around the thoracic cavity with little friction. The heart itself is made of cardiac muscle, a special type of muscle that has striations one nucleus per cell, and contracts involuntarily. The source of heart contractions and their rhythm are from electrical impulses which are conducted in and around the heart muscle. We will begin by exploring the chambers of the heart before moving onto electrical impulse control. ## Heart Structures 1. Left and right atria: Act as collecting chambers. - Right atrium: collects deoxygenated blood from the vena cava. - Left atrium: collects oxygenated blood from the pulmonary veins. 2. Left and right ventricles: Act as pumps. - Right ventricle: sends blood to the lungs via the pulmonary trunk. - Left ventricle: sends blood to the entire body via the aorta. - The left ventricle has a thicker muscular wall than the right ventricle. Why might this be the case? - Generate more force to push blood a greater distance. 3. Atrioventricular valves: valves placed between the atria and ventricles of the heart. These act to prevent the backflow of blood from the ventricles back into the atria, as they only open one way. Why is this important? - Right AV valve: tricuspid valve (3 flaps) - Left AV valve: bicuspid/ mitral valve (2 flaps) 4. Chordae tendineae: fibrous strings that support the valves and allow them to only open one way. In other words, they keep the valves from inverting. 5. Semilunar valves: valves placed between the ventricles and the arteries exiting the heart. They also prevent backflow of blood. There are two sets: - Pulmonary semilunar valve: From right ventricle to pulmonary trunk. - Aortic semilunar valve: From left ventricle to aorta. 6. Pulmonary trunk: Branches off the right ventricle to form the pulmonary arteries that go to the lungs. 7. Aorta: Branches off the left ventricle and eventually branches off to the whole body. 8. Septum: A thick wall of muscle that separates left and right sides of heart. Why might we want to separate both sides of the heart? - Separate oxygenated & deoxygenated blood to prevent mixing. ## The Cardiac Cycle The cardiac cycle refers to one heartbeat. In every heartbeat, there are two steps: 1. Systole (contraction) 2. Diastole (relaxation) | TIME | ATRIA | VENTRICLES | | :-------- | :-------- | :------------- | | 0.15 Sec | Systole | Diastole | | 0.30 Sec | Diastole | Systole | | 0.40 Sec | Diastole | Diastole | | =0.85 Sec | 70 Bpm | reset | During one heartbeat, both atria will contract, which stimulates both ventricles to contract. Once the ventricles contract, the atria relax. When the atria are contracting, the ventricles are relaxed. Therefore, we can have atrial systoles and diastoles, and ventricular systoles and diastoles. Ventricles have longer + slower contractions than atria. Why might this be? (Because blood must be pumped all over body!) When the heart valves close, they make a characteristic sound. When using a stethoscope, a doctor is listening for the opening & closing of the heart valves, which make the "lub dub" sound. The average heart rate is about 70 beats per minute (BPM). The heartbeat is said to be intrinsic, which means it can still beat without input from the central nervous system. The cardiac muscle relies on inputs from nodes, which are small, independent clumps of nervous tissue inside the heart. You can detect your resting heart rate using your carotid pulse or your radial pulse. ## What causes a heart to beat? 1. **Sinoatrial (SA) node** - This node is found in the upper wall of the right atrium. It receives input from parts of the nervous system to control heart rate. It sends an electrical impulse that causes the atria to contract, which starts the entire cascade of the cardiac cycle. It sends out an impulse every 0.85 seconds. In this way, it is called the pacemaker. 2. **Atrioventricular (AV) node** - Found at the bottom of the right atrium near the Septum. This receives the electrical impulse from the SA node and helps conduct it from the atria to the ventricles. It also delays the received signal by about 0.12 seconds to allow the atria to refill with blood before the ventricles contract. 3. **Bundle of His** - This connects to the AV node and conducts the signal to the ventricles to allow them to contract. It passes into the ventricular septum and divides into two branches, the left and right bundles of His. 4. **Purkinje fibres** - The Purkinje fibres are nestled in the outer walls of the ventricles and septum. They send the impulse to the ventricles to force contraction. They stimulate both ventricles to contract from the bottom upwards. ## What is a Premature Ventricular Contraction? A premature ventricular contraction (PVC) occurs when the Purkinje fibres fire without input from the rest of the heart. This stimulates the ventricles to contract randomly in between normal heartbeats. The SA node does not have any initial input, and the result is an arrhythmia. It feels like the heart "skipping a beat (palpitation)". They are relatively common but can be life threatening in certain circumstances.