The Heart & Circulatory System
The mammalian heart is a muscular double pump that maintains separate pulmonary and systemic circuits, ensuring oxygenated and deoxygenated blood do not mix. This guide covers the pressure and valve changes of the cardiac cycle, the structure of arteries, veins, and capillaries, and how haemoglobin loads and unloads oxygen.
- Cardiac Cycle
- The sequence of events in one heartbeat, comprising atrial systole, ventricular systole, and diastole.
- Bohr Effect
- The decrease in the oxygen affinity of hemoglobin in response to a lowered blood pH resulting from an increased concentration of carbon dioxide.
- Tissue Fluid
- The fluid surrounding cells in tissues, formed from blood plasma forced out of capillaries by hydrostatic pressure.
Interactive Human Body & Cell Structure Explorer
Click any internal organ or organelle to inspect its structure, toggle organs on/off to see what happens, or test yourself in Quiz Mode.
Heart (Double Circulatory Pump)
Structure & Function:
Pumps blood in a double circulatory system: the right ventricle pumps deoxygenated blood to the lungs, and the thick muscular left ventricle pumps oxygenated blood at high pressure around the whole body.
Key Exam Mark-Scheme Point:
Exam tip: The left ventricle has a much thicker muscular wall than the right ventricle because it must pump blood at high pressure all the way around the entire body, not just to the lungs.
What happens if this organ fails or is removed?
Without pressure from the heart, blood flow stops immediately. Oxygen and glucose cannot reach tissues, and waste carbon dioxide cannot be removed.
Tip: Use the “Simulate Organ Failure” button above to see how each organ system depends on the others to maintain homeostasis and cellular respiration.
Key Concepts & Exam Definitions (5)
- Cardiac Cycle
- The sequence of events in one heartbeat, comprising atrial systole, ventricular systole, and diastole.
- Bohr Effect
- The decrease in the oxygen affinity of hemoglobin in response to a lowered blood pH resulting from an increased concentration of carbon dioxide.
- Tissue Fluid
- The fluid surrounding cells in tissues, formed from blood plasma forced out of capillaries by hydrostatic pressure.
- Myogenic
- Describes muscle tissue (like cardiac muscle) that can initiate its own contractions without nervous stimulation.
- Atheroma
- A fatty deposit in the inner lining of an artery, restricting blood flow.
Quick-Fire Flashcards (5)
Click any card to check the answer, then mark it as Known to track your progress.
Which blood vessel carries oxygenated blood from the lungs to the heart?
What is the function of the Purkyne tissue?
How is the structure of a capillary adapted for its function?
What prevents the AV valves from turning inside out during ventricular systole?
What does an electrocardiogram (ECG) measure?
Exam-Style Questions & Mark Schemes (3)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Describe how tissue fluid is formed and how it is returned to the circulatory system.
Click “Check Mark Scheme” to compare your answer, At the arterial end of a capillary, hydrostatic pressure is greater than the osmotic pressure, forcing fluid (water, small molecules) out of the blood plasma to form tissue fluid. Proteins remain in the blood. At the venule end, the loss of fluid and presence of proteins means the water potential of the blood is lower than the tissue fluid. Water re-enters the capillary by osmosis. Excess tissue fluid is drained into the lymphatic system.
- Q2
Explain the Bohr effect and its significance to actively respiring tissues.
Click “Check Mark Scheme” to compare your answer, Actively respiring tissues release lots of carbon dioxide, lowering blood pH. This alters the tertiary structure of hemoglobin, reducing its affinity for oxygen. Consequently, the oxyhemoglobin dissociation curve shifts to the right, meaning oxygen unloads more readily at a given partial pressure of oxygen. This ensures more oxygen is delivered to the tissues that need it most.
- Q3
Outline the events of the cardiac cycle.
Click “Check Mark Scheme” to compare your answer, 1) Diastole: Heart muscle relaxes, atria and ventricles fill with blood. AV valves open as atrial pressure > ventricular pressure. 2) Atrial Systole: Atria contract, pushing remaining blood into ventricles. 3) Ventricular Systole: Ventricles contract from the apex upwards. Ventricular pressure rises above atrial pressure, closing AV valves. When ventricular pressure exceeds aortic/pulmonary pressure, semi-lunar valves open, and blood is forced out.
Common Exam Questions (3)
What is meant by a double circulatory system?
In a double circulatory system, blood passes through the heart twice during each complete circuit of the body: the right side pumps deoxygenated blood to the lungs at lower pressure, and the left side pumps oxygenated blood to the rest of the body at high pressure.
What initiates the heartbeat?
The heartbeat is myogenic and begins at the sinoatrial node (SAN) in the wall of the right atrium, which acts as the natural pacemaker by generating regular waves of electrical excitation.
What causes the atrioventricular valves to close?
When the ventricles contract during ventricular systole, blood pressure inside the ventricles rises above the pressure in the atria, forcing the atrioventricular (bicuspid and tricuspid) valves shut to prevent backflow.
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