Concepts: Double circulation, regulation and disorders

4.1 Why double circulation

The blood passes through the heart twice in one complete circuit. In the pulmonary circulation the right ventricle pumps deoxygenated blood to the lungs, and oxygenated blood returns to the left atrium. In the systemic circulation the left ventricle pumps oxygenated blood to the whole body, and deoxygenated blood returns to the right atrium. The two circuits are joined in series through the heart, and the arrangement is called double circulation.

The advantage of double circulation is that oxygenated and deoxygenated blood do not mix. A complete separation of the two sides allows the blood going to the body to carry the maximum amount of oxygen at a high pressure, while the lungs receive blood at a lower pressure. Birds and mammals have four completely separated chambers and are therefore the animals with the most efficient double circulation. Amphibians have three chambers with mixing in the single ventricle, and reptiles have a partial septum; only crocodiles have four chambers.

4.1.1 Regulation of heart rate

The heart beats on its own because the sino-atrial node is myogenic, but the rate is adjusted by the autonomic nervous system through the special neural centre in the medulla oblongata. Sympathetic nerves increase both the rate and the force of contraction, which is the response during exercise or excitement. Parasympathetic nerves, mainly the vagus, slow the heart down and reduce the force. Hormones such as adrenaline and noradrenaline also raise the heart rate. The result is that the heart can increase its output several times over when the body needs more oxygen, and settle back when the demand falls.

4.1.2 Disorders of the circulatory system

Hypertension is persistently high blood pressure. It puts extra load on the heart and the arteries and raises the risk of stroke and heart attack. Atherosclerosis is the deposition of fatty material and cholesterol in the walls of arteries, which narrows the lumen and makes the wall less elastic. Coronary artery disease happens when the coronary arteries that supply the heart muscle itself are narrowed, and the reduced blood flow causes angina, a chest pain on exertion; complete blockage causes a heart attack. Heart failure is a condition in which the heart cannot pump enough blood to meet the needs of the body, and the tissues become short of oxygen while fluid collects in the lungs and legs.

The routine checks that follow from this are simple: blood pressure measurement catches hypertension, lipid levels and lifestyle explain much of the atherosclerosis risk, and an ECG and a stress test pick up coronary artery disease. Smoking, a high fat diet, obesity and a sedentary life are the risk factors students are expected to name.

4.1.3 Exercise practice

The chapter has one exercise section of fourteen questions and all are represented across the pages: blood and its components on page 1, lymph and vessels on page 2, the heart and cardiac cycle on page 3, and the remaining comparisons and disorders here.

Exercise 6. Double circulation means that the blood passes through the heart twice in one complete circuit. In the pulmonary circuit the right ventricle sends deoxygenated blood to the lungs and oxygenated blood returns to the left atrium, and in the systemic circuit the left ventricle sends oxygenated blood to the body and deoxygenated blood returns to the right atrium. Its significance is that the two circuits are kept separate, so the tissues receive fully oxygenated blood at a high pressure while the lungs receive blood at a lower pressure, and the output of the two sides can be regulated independently.

Exercise 7. Blood is a fluid connective tissue with red cells, white cells and platelets in plasma, while lymph is tissue fluid with lymphocytes and fewer proteins. An open system lets blood flow through sinuses, while a closed system keeps blood in vessels. Systole is contraction and diastole is relaxation. The P wave is atrial depolarisation and the T wave is ventricular repolarisation.

Exercise 8. Fish have two chambers, amphibians three, reptiles three with a partial septum, crocodiles four, and birds and mammals four with complete separation. The trend is towards complete separation of oxygenated and deoxygenated blood and towards double circulation.

Exercise 9. The heart is myogenic because the impulse for each beat is generated inside the heart itself, by the sino-atrial node, and does not require a nerve impulse from outside.

Exercise 10. The sino-atrial node is called the pacemaker because it generates the impulse that sets the rhythm of the whole heart; the rest of the conducting system follows its lead.

Exercise 11. The atrio-ventricular node delays the impulse so that the atria finish emptying before the ventricles contract, and the atrio-ventricular bundle carries the impulse from the atria into the ventricular muscle so that both ventricles contract together.

Exercise 12. A cardiac cycle is one complete heartbeat, from the start of one contraction to the start of the next. Cardiac output is the volume of blood pumped by each ventricle per minute, equal to stroke volume multiplied by heart rate.

Exercise 13. The first heart sound is produced when the two atrioventricular valves shut at the beginning of ventricular systole, and the second sound is produced when the semilunar valves shut at its end.

Exercise 14. A standard ECG shows the P wave, the QRS complex and the T wave; the P wave is atrial depolarisation, the QRS complex is ventricular depolarisation and the T wave is ventricular repolarisation.