Ch 3 · 8.3 The heart · AS & A Level Biology 9700
Topic 3 of 3 in Transport in mammals — AS & A Level Biology 9700 — 48 lessons.
Introduction
The mammalian heart is a muscular double pump that drives blood around the pulmonary and systemic circuits. Its structure, the electrical events that coordinate each heartbeat, and the pressure changes of the cardiac cycle are closely linked and are often tested together through pressure graphs of the cardiac cycle. (Syllabus section 8.3.)
Key ideas — External and internal structure of the heart
Externally, the heart has four chambers: two thin-walled atria above two thicker-walled ventricles, supplied with blood by the coronary arteries and covered by a protective pericardium. Internally, the right atrium receives deoxygenated blood from the venae cavae and the left atrium receives oxygenated blood from the pulmonary veins; each atrium opens into the ventricle below through an atrioventricular valve, tricuspid on the right, bicuspid on the left, held closed under ventricular pressure by tendinous cords attached to papillary muscles, preventing backflow into the atria. The right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary artery; the left ventricle, with a much thicker muscular wall because it must generate enough pressure to pump blood around the entire systemic circulation, pumps blood through the aortic semilunar valve into the aorta.
🧑🏫 Teacher's explanation
Think of the heart as a dual-pump system in a house. You have two small receiving rooms (atria) and two powerful pumping rooms (ventricles). The right side is like a small pump for a single room (the lungs), while the left side is like a heavy-duty pump for the entire skyscraper (the whole body). Because the left side has to push blood much further against higher resistance, its walls must be much thicker and more muscular.
📖 Definitions
🔍 Worked example — Easy
A student observes a heart diagram. They note that the left ventricle wall is 3 imes thicker than the right ventricle wall. Explain why this structural difference is necessary for mammalian circulation.
🔍 Worked example — Standard
During the cardiac cycle, the pressure in the left ventricle rises above the pressure in the aorta. Describe the effect this has on the aortic semilunar valve and the direction of blood flow.
🔍 Worked example — Exam-style
Calculate the percentage increase in wall thickness if the right ventricular wall is 0.5 cm and the left ventricular wall is 1.5 cm.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
Key ideas — The cardiac cycle
The cardiac cycle is the sequence of pressure and volume changes in one heartbeat. During atrial systole, the atria contract, raising atrial pressure slightly and completing ventricular filling. During ventricular systole, the ventricles contract; pressure in the ventricles rapidly exceeds atrial pressure, closing the atrioventricular valves, and once ventricular pressure exceeds that in the aorta and pulmonary artery, the semilunar valves open and blood is ejected, so ventricular volume falls sharply. During diastole, the ventricles relax, pressure falls below that in the arteries, the semilunar valves close, and once ventricular pressure falls below atrial pressure the atrioventricular valves open and the ventricles begin to refill.
🧑🏫 Teacher's explanation
Think of the heart as a dual-action pump system consisting of two rooms (atria) and two powerful pistons (ventricles). To move fluid through a pipe, you must create a pressure difference. If the pressure in the room is higher than the pressure in the pipe, the door (valve) opens and fluid flows out. If the pressure in the room drops below the pipe, the door slams shut to prevent backflow.
📖 Definitions
🔍 Worked example — Easy
Compare the thickness of the walls of the left ventricle and the right ventricle.
🔍 Worked example — Standard
Explain why the atrioventricular valves close during ventricular systole.
🔍 Worked example — Exam-style
Calculate the percentage change in ventricular volume if the initial volume is 120 ml and the volume after ejection is 50 ml.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Initiation and coordination of the heartbeat
The heart is myogenic: it initiates its own contractions rather than relying on nerve impulses. The sinoatrial node, a group of specialised cells in the wall of the right atrium, acts as the pacemaker, generating a wave of electrical excitation that spreads through the walls of both atria, causing them to contract together. This excitation reaches the atrioventricular node, which briefly delays the impulse, allowing the atria to finish emptying into the ventricles, before passing it down specialised conducting fibres, the Purkyne tissue, which run through the septum to the apex of the heart and spread excitation up through the ventricle walls from the base, causing coordinated contraction from the apex upwards, which squeezes blood up and out through the arteries. A layer of non-conducting tissue between the atria and ventricles stops the excitation passing straight down the walls, so the AV node is the only route.
🧑🏫 Teacher's explanation
Think of the heart as a highly coordinated pump that doesn't need a 'brain' to tell it to beat; it has its own built-in electrical wiring. Imagine a large building where the electricity starts in a basement control room (the SA node), travels through the upper floors (atria), pauses briefly at a security checkpoint (the AV node) to make sure everyone has moved downstairs, and then rushes through specialized cables (Purkyne tissue) to trigger a massive, coordinated squeeze from the bottom up.
📖 Definitions
🔍 Worked example — Easy
A student observes that the left ventricle wall is significantly thicker than the right ventricle wall. Explain why this structural difference is necessary for efficient blood circulation.
🔍 Worked example — Standard
Describe the sequence of electrical excitation through the heart during one cardiac cycle.
🔍 Worked example — Exam-style
Calculate the percentage increase in heart rate if a person's heart rate increases from 70 bpm to 105 bpm.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Predict and explain the effect this would have on the efficiency of the cardiac cycle. [3] [3 marks]
Key ideas — Why the walls differ in thickness
Both ventricles still pump the same volume of blood per beat; only the pressure differs.
🧑🏫 Teacher's explanation
Think of the heart as a dual-pump system in a house. The atria are like small hand-held sprayers that just move water from a bucket into a funnel; they don't need much muscle because the distance is tiny. The ventricles are like heavy-duty industrial pumps. The right ventricle is a medium-sized pump that just sends water through a short hose to a nearby filter (the lungs). The left ventricle, however, is a massive, high-pressure pump that has to push water through miles of plumbing to reach every corner of a skyscraper (the whole body).
📖 Definitions
🔍 Worked example — Easy
A student observes a cross-section of a mammalian heart. They measure the thickness of the right ventricular wall as 3 mm and the left ventricular wall as 9 mm. Calculate the ratio of the thickness of the left ventricle to the right ventricle.
🔍 Worked example — Standard
Explain why the walls of the atria are thinner than the walls of the ventricles.
🔍 Worked example — Exam-style
Compare the thickness of the left and right ventricular walls and explain the physiological necessity for this difference.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the difference in the thickness of the muscular walls of the atria and the ventricles. [2] (b) Explain why the left ventricle has a thicker muscular wall than the right ventricle. [3] [5 marks]
Worked example
Question. The graph of pressure in the left atrium, left ventricle and aorta shows that at 0.10 s ventricular pressure rises above atrial pressure, and at 0.15 s it rises above aortic pressure. State what happens to the valves at each time, and explain why. [4]
🔍 Worked example — Easy
Compare the thickness of the walls of the left ventricle and the right ventricle. Explain why this difference exists.
🔍 Worked example — Standard
A patient's heart rate is 75 beats per minute. Calculate the duration of one complete cardiac cycle in seconds.
🔍 Worked example — Exam-style
During the cardiac cycle, the pressure in the left ventricle rises above the pressure in the aorta. Describe the effect on the semilunar valve and explain why this occurs.
⚠️ Common errors
Common mistakes
Practice
Answers