The Heart and the Circulatory Pathways: Quick Revision

A brief note on The Heart and the Circulatory Pathways. Read it before a test. Try each question at the end before you open its answer.

Circulation in animals

• Open: arthropods and molluscs. Blood flows into sinuses.
• Closed: annelids and chordates. Blood stays in vessels. The flow can be regulated more precisely.
• Fish: 2 chambers, deoxygenated blood only, single circulation.
• Amphibians and reptiles (except crocodiles): 3 chambers, the blood mixes in the single ventricle, incomplete double circulation.
• Crocodiles, birds, mammals: 4 chambers, no mixing, complete double circulation.

The human heart

• Between the lungs, tilted to the left, in a double walled pericardium with pericardial fluid.
• Two thin atria and two thick ventricles. The left ventricle has the thickest wall.
• Valves: tricuspid on the right, bicuspid (mitral) on the left, semilunar at the pulmonary artery and the aorta.
• Nodal tissue: SAN in the upper right corner of the right atrium. It is the pacemaker, at 70 to 80 a minute.
• Then: AVN in the lower left corner of the right atrium, AV bundle, right and left bundles, Purkinje fibres.

The pathways

• Systemic: left ventricle, aorta, arteries, capillaries, veins, vena cavae, right atrium.
• Pulmonary: right ventricle, pulmonary artery, lungs, pulmonary veins, left atrium.
• Arteries carry blood away from the heart, and veins carry it towards the heart.
• Hepatic portal system: intestine, hepatic portal vein, liver. Coronary system: supplies the heart muscle.

A worked example

A red cell goes 4 times round the body of a mammal. How many times does it pass through the heart?

1. Each round passes through the heart twice.
2. So the count is $4 \times 2 = 8$.

Practice with new numbers

A program to try

The program prints the route of the blood through the chambers.

Mistakes to avoid

• Saying that a fish has a double circulation. It has a single one.
• Putting the tricuspid valve on the left. It is on the right.
• Saying that all veins carry deoxygenated blood. The pulmonary veins carry oxygenated blood.
• Placing the SAN near the septum. That is the AVN. The SAN is in the upper right corner.

Check yourself

1. How many chambers has the heart of a crocodile?

2. Which vessel takes blood from the right ventricle to the lungs?

3. Which part of the nodal tissue is the pacemaker?

4. Where does the hepatic portal vein carry the blood?

Sources

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 19, The Cardiovascular System: The Heart.
• NCERT (2023). Biology, Textbook for Class XI, chapter 15.