Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 42.2 Coordinated Cycles of Heart Contraction Drive Double Circulation in Mammals
1. Mammalian Circulation Overview
- Right ventricle pumps oxygen-poor blood to lungs via pulmonary arteries
- Gas exchange occurs in lung capillaries: O₂ absorbed, CO₂ released
- Oxygen-rich blood returns to left atrium, moves to left ventricle
- Left ventricle pumps blood to the body through the aorta
- Blood delivers nutrients and collects waste, then returns to right atrium
2. Heart Structure and Function
- Four chambers: two atria (thin) and two ventricles (thick, especially left)
- Cardiac cycle = systole (contraction) + diastole (relaxation)
- Typical output: 70 mL/beat × 72 bpm = ~5 L/min
- Four valves: AV valves prevent backflow into atria; semilunar valves prevent backflow from arteries
3. Electrical Control of Heartbeat
- SA node (pacemaker) initiates impulses for contraction
- Impulse spreads through atria → delayed at AV node → sent to ventricles
- This timing ensures atria empty before ventricles contract
- Autonomic nervous system adjusts heart rate (sympathetic speeds, parasympathetic slows)
- Hormones (e.g., epinephrine) and temperature also influence rate
4. Heart Sounds and Murmurs
- “Lub-dup” comes from valve closures: AV (lub), semilunar (dup)
- Defective valves can cause heart murmurs; some may need surgical repair
- Not all murmurs are harmful
In a Nutshell
The heart drives double circulation in mammals with a precise cycle of contraction and relaxation. Four chambers and valves ensure one-way flow. Electrical signals coordinate timing, and the nervous and endocrine systems regulate rhythm. This system supports the oxygen and nutrient demands of complex organisms.