Trace blood through the chambers, valves and vessels of the heart and the pulmonary and systemic circuits, explain the cardiac cycle, conduction pathway, ECG, cardiac output and blood pressure, and analyze cardiovascular imbalances

Learning goal: Identify the functions of the cardiovascular system; describe the heart's cardiac muscle, coronary blood supply, four chambers, four valves and major vessels; trace the pathway of blood through the heart and the pulmonary and systemic circuits; differentiate arteries, arterioles, capillaries, venules and veins by structure and function; explain the cardiac cycle, the conduction pathway and how an electrocardiogram and blood pressure measure them; calculate cardiac output from heart rate and stroke volume; compare vertebrate hearts; and describe homeostatic imbalances such as myocardial infarction, aneurysm, atherosclerosis, hypertrophic cardiomyopathy, hypotension, hypertension and arrhythmias.

What the system does. The cardiovascular system consists of the heart and blood vessels. Its primary function is transport: it moves oxygen, carbon dioxide, hormones, nutrients, waste products and other chemical messengers between the places they enter or are made and the places they are used or removed. The heart is the pump; the vessels are the pipes and, at the capillaries, the exchange surfaces.

The heart as an organ. The heart is mostly cardiac muscle (the myocardium), lined inside by endocardium and covered by the pericardium. Cardiac muscle cells are striated like skeletal muscle, but they are short, branched, usually have one nucleus, and are joined end to end by intercalated discs. The discs contain gap junctions that let the electrical signal pass directly from cell to cell, so the muscle of each pair of chambers contracts almost as one unit. The cells are packed with mitochondria because the heart never rests and depends on a constant oxygen supply. That supply does not come from the blood inside the chambers, because oxygen cannot diffuse fast enough from the chambers through the whole thickness of the wall; the heart muscle is supplied with oxygenated blood by the coronary arteries, which branch from the aorta just above the aortic valve.

Four chambers, four valves. The two upper chambers, the right and left atria, receive blood returning to the heart. The two lower chambers, the right and left ventricles, pump blood out. A wall, the septum, separates right from left. Between each atrium and its ventricle is an atrioventricular valve: the tricuspid on the right and the mitral (bicuspid) on the left. At the exit of each ventricle is a semilunar valve: the pulmonary valve on the right and the aortic valve on the left. Valves open when pressure behind them is higher than pressure in front and snap shut when the pressure reverses, so blood moves one way only.

Tracing the pathway, worked fully. Follow one red blood cell that has just delivered oxygen to a muscle in the leg. It returns through veins to the inferior vena cava (blood from the head and arms returns through the superior vena cava) and enters the right atrium. It passes through the tricuspid valve into the right ventricle, which contracts and pushes it through the pulmonary valve into the pulmonary trunk and pulmonary arteries to the lungs. In lung capillaries it releases carbon dioxide and picks up oxygen. It returns through the pulmonary veins to the left atrium, passes through the mitral valve into the left ventricle, and is pushed through the aortic valve into the aorta, which branches to every organ. The loop from right ventricle to lungs to left atrium is the pulmonary circuit; the loop from left ventricle to body to right atrium is the systemic circuit. Notice that the pulmonary arteries carry oxygen-poor blood and the pulmonary veins carry oxygen-rich blood: arteries and veins are defined by direction (away from or toward the heart), not by oxygen content. The left ventricle's wall is much thicker than the right's because it must push blood through the whole systemic circuit, a far longer, higher-resistance path than the lungs.

The vessels in order. Blood flows from arteries, to arterioles, to capillaries, to venules, then to veins. Arteries have thick walls with elastic tissue and smooth muscle that stretch with each beat and recoil to keep blood moving. Arterioles have relatively more smooth muscle and control how much blood each tissue receives by narrowing or widening. Capillaries have walls one cell thick (endothelium) and are where exchange happens: in the body's tissues, oxygen, nutrients and chemical messengers diffuse out of the blood into the tissue fluid, and carbon dioxide and other wastes diffuse in (in the lungs the gradients run the other way). Venules collect blood from capillaries, and veins return it to the heart. Veins have thinner walls, wider lumens and low pressure, and many, especially in the limbs, have valves that keep blood flowing toward the heart; squeezing by nearby skeletal muscles pushes blood from valve to valve.

The cardiac cycle and its electrical control. One heartbeat is a cardiac cycle: systole is contraction and diastole is relaxation and filling. The atria contract first, topping up the ventricles; then the ventricles contract, the atrioventricular valves close (the first heart sound) and the semilunar valves open; then the ventricles relax, the semilunar valves close (the second heart sound), and the chambers refill. The beat starts itself: the sinoatrial (SA) node in the right atrium fires spontaneously and sets the rhythm. The signal spreads across both atria, reaches the atrioventricular (AV) node, is delayed briefly so the atria finish emptying, then travels down the AV bundle, the bundle branches in the septum and the Purkinje fibers into the ventricle walls, which contract from the bottom up. An electrocardiogram (ECG or EKG) records this electrical activity from the skin: the P wave is atrial depolarization, the QRS complex is ventricular depolarization (atrial repolarization is hidden within it) and the T wave is ventricular repolarization. Irregular spacing or odd wave shapes reveal arrhythmias and damage.

Cardiac output and blood pressure. Cardiac output is the volume pumped by one ventricle per minute: cardiac output = heart rate x stroke volume. For a resting adult with a heart rate of 70 beats per minute and a stroke volume of 70 milliliters, cardiac output = 70 x 70 = 4,900 mL per minute, about 4.9 liters. Either factor can change it: during exercise both rise. Blood pressure is the force of blood on artery walls, measured in millimeters of mercury and written as systolic over diastolic, for example 120/80: the peak pressure during ventricular systole over the lowest pressure during diastole.

Comparing vertebrate hearts. A fish heart has two chambers, one atrium and one ventricle, in a single circuit: blood passes through the gills, where it loses much of its pressure in the narrow gill capillaries, and then on to the body without returning to the heart. An amphibian heart has three chambers, two atria and one ventricle, so some mixing of oxygen-rich and oxygen-poor blood occurs. Birds and mammals have four chambers with a complete septum, so the two circuits are fully separate, oxygen-rich blood is never diluted, and the systemic circuit can run at high pressure. That supports the high metabolic rate of endothermic animals, which generate their own body heat.

Homeostatic imbalances. Atherosclerosis is the build-up of fatty plaque in artery walls, narrowing them. When a coronary artery is blocked, the heart muscle it supplies is starved of oxygen and dies: myocardial infarction, a heart attack. An aneurysm is a weakened, bulging section of an artery wall that can rupture. Hypertrophic cardiomyopathy is abnormal thickening of the heart muscle, often inherited, that can obstruct outflow and disturb rhythm. Hypertension (persistently high blood pressure) strains the heart and damages vessels; hypotension (low blood pressure) can reduce blood flow to the brain and cause fainting. Arrhythmias are abnormal rhythms, seen on an ECG. Long periods in microgravity also decondition the system: fluid shifts toward the head, and astronauts can feel faint when standing after landing.

Investigation: pulse and cardiac output. Find the radial pulse at the wrist, count beats for 30 seconds and double it. Repeat after one minute of steady stepping, and at one and two minutes of recovery. Using an assumed resting stroke volume of 70 mL, calculate cardiac output at each time. Anyone with a medical reason not to exercise acts as timekeeper. The model cannot measure stroke volume, which also rises during exercise, so the calculation underestimates the true increase. If your teacher provides a preserved sheep heart, a supervised dissection lets you trace the same pathway through real chambers and valves: wear gloves and eye protection, cut only with the tools and technique your teacher demonstrates, cutting away from yourself, and wash your hands afterward.

Misconception. "Arteries carry oxygenated blood and veins carry deoxygenated blood" fails for the pulmonary vessels; direction relative to the heart is the definition.

Worked example 1 (pathway): Trace a molecule of oxygen from the air to a cell in the big toe. Alveolus to lung capillary; pulmonary vein; left atrium; mitral valve; left ventricle; aortic valve; aorta; arteries of the leg; arteriole; capillary in the toe, where oxygen diffuses into the tissue fluid and then into the cell. The molecule never passes through the right side of the heart.

Worked example 2 (cardiac output): A student's resting heart rate is 64 beats per minute with an assumed stroke volume of 75 mL: cardiac output = 64 x 75 = 4,800 mL per minute, 4.8 L. After exercise, heart rate is 120 and stroke volume rises to 100 mL: 120 x 100 = 12,000 mL per minute, 12 L. The ratio 12,000 / 4,800 = 2.5, so output increased 2.5 times, with both factors contributing.

Worked example 3 (ECG to cycle): An ECG shows a P wave, then a QRS complex, then a T wave, repeating every 0.8 seconds. The heart rate is 60 / 0.8 = 75 beats per minute. The P wave marks atrial depolarization just before the atria contract; the QRS marks ventricular depolarization just before ventricular systole, when the atrioventricular valves close; the T wave marks ventricular repolarization as the ventricles move into diastole.

Worked example 4 (vessel identification): A micrograph shows vessel X with a thick muscular, elastic wall and a small round lumen, and vessel Y with a thin wall, a large collapsed lumen and a valve. X is an artery, built to withstand and smooth high pressure pulses; Y is a vein, at low pressure, relying on valves and muscle squeezing to return blood to the heart.

Representation task: Draw a labeled front view of the heart with four chambers, four valves, the venae cavae, pulmonary trunk and arteries, pulmonary veins, aorta and coronary arteries. Use one color for oxygen-poor and one for oxygen-rich blood, add arrows for the pathway, and mark the pulmonary and systemic circuits. Beside it, draw the five vessel types in order with one structural feature and one function each.

Independent performance: Carry out the pulse investigation and write a report. Success criteria: (a) heart rate is recorded at rest, immediately after a controlled exercise and at two recovery times, with the method stated; (b) cardiac output is calculated at each time using the formula and the assumed stroke volume, with units; (c) the report explains why the calculation underestimates the change, naming stroke volume; (d) the pathway of blood through the heart and both circuits is written as a numbered sequence including all four valves; (e) one paragraph explains why the four-chambered heart supports endothermy, comparing it with fish and amphibian hearts; (f) one cardiovascular imbalance is described with its cause and its effect on blood flow.