Circulatory System

The circulatory system moves blood around the body; in humans it is made of a muscular four-chambered heart, a closed network of blood vessels and blood. This page covers open and closed circulation, vertebrate hearts, the human heart and its nodal tissue, the cardiac cycle with the heart sounds, and the ECG, following the NCERT Class 11 chapter Body Fluids and Circulation. For the circulatory system, NEET often asks the order of cardiac cycle events, the role of the SAN and the meaning of each ECG wave.

1. Circulatory Pathways

1.1 Open and closed systems

• Circulatory patterns are of two types: open or closed.
• ★ Exam imp Open circulatory system: blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses. It is present in arthropods and molluscs.
• ★ Exam imp Closed circulatory system: blood pumped by the heart always circulates through a closed network of blood vessels. It is present in annelids and chordates.
• The closed pattern is considered more advantageous, because the flow of fluid can be regulated more precisely.
• In short, all vertebrates and a few invertebrates have a closed circulatory system.

1.2 Hearts and circulation in vertebrates

• All vertebrates have a muscular chambered heart.
• Fishes: the heart pumps out deoxygenated blood. The gills oxygenate it and it is supplied to the body parts; deoxygenated blood then returns to the heart. This is single circulation.
• Amphibians and reptiles: the left atrium receives oxygenated blood from the gills, lungs or skin; the right atrium receives deoxygenated blood from other body parts.
• In them, both kinds of blood mix in the single ventricle, which pumps out mixed blood. This is incomplete double circulation.
• ★ Exam imp Birds and mammals: oxygenated and deoxygenated blood, received by the left and right atria, pass to the ventricles of the same sides and are pumped out without mixing.
• So birds and mammals have two separate circulatory pathways: double circulation.

Examples to Remember

2. The Human Heart

• The human circulatory system, also called the blood vascular system, has three parts: a muscular chambered heart, a network of closed branching blood vessels, and blood, the fluid that circulates.

2.1 Position and covering

• The heart is a mesodermally derived organ.
• It lies in the thoracic cavity, between the two lungs, slightly tilted to the left.
• It is about the size of a clenched fist.
• It is protected by the pericardium, a double-walled membranous bag that encloses the pericardial fluid.

2.2 Chambers and septa

• The heart has four chambers: two relatively small upper chambers, the atria, and two larger lower chambers, the ventricles.

• Each atrio-ventricular septum has an opening through which the atrium and ventricle of the same side are connected.

2.3 Valves

• The valves allow blood to flow in one direction only: from the atria to the ventricles, and from the ventricles to the pulmonary artery or aorta.
• They prevent any backward flow of blood.
• ★ Exam imp The entire heart is made of cardiac muscles; the walls of the ventricles are much thicker than those of the atria.

3. Nodal Tissue and the Pacemaker

• Nodal tissue: a specialised cardiac musculature distributed in the heart.
• ★ Exam imp Sino-atrial node (SAN): a patch of nodal tissue in the right upper corner of the right atrium.
• Atrio-ventricular node (AVN): a mass of nodal tissue in the lower left corner of the right atrium, close to the atrio-ventricular septum.
• Atrio-ventricular bundle (AV bundle), or bundle of His: a bundle of nodal fibres that continues from the AVN.
• The AV bundle passes through the atrio-ventricular septa, emerges on top of the inter-ventricular septum and immediately divides into a right and a left bundle.
• These branches give rise to minute fibres throughout the ventricular musculature of their own sides, called Purkinje fibres.

1. SAN generates the action potential.
2. AVN receives it from the atrial side.
3. AV bundle (bundle of His) carries it through the atrio-ventricular septa to the top of the inter-ventricular septum.
4. Right and left bundles carry it down the two sides.
5. Purkinje fibres spread it throughout the ventricular musculature.

• ★ Exam imp The nodal musculature is autoexcitable: it can generate action potentials without any external stimuli.
• The number of action potentials generated in a minute varies in different parts of the nodal system.
• The SAN generates the maximum number: 70-75 ${\text{min}}^{-1}$.
• Our heart normally beats 70-75 times per minute (average 72 beats ${\text{min}}^{-1}$).

4. Cardiac Cycle

• The cardiac cycle is described starting from joint diastole, when all four chambers are relaxed.

1. Joint diastole: the tricuspid and bicuspid valves are open. Blood from the vena cava and the pulmonary veins flows through the right and left atria into the right and left ventricles. The semilunar valves are closed.
2. Atrial systole: the SAN generates an action potential, and both atria contract at the same time. This increases the flow of blood into the ventricles by about 30 per cent.
3. The action potential is conducted to the ventricular side by the AVN and the AV bundle; the bundle of His then transmits it through the entire ventricular musculature.
4. Ventricular systole: the ventricles contract. At the same time the atria relax (atrial diastole).
5. Ventricular pressure rises, and the tricuspid and bicuspid valves close because blood tries to flow back into the atria.
6. Ventricular pressure rises further, and the semilunar valves of the pulmonary artery (right side) and the aorta (left side) are forced open. Blood flows from the ventricles into the circulatory pathways.
7. Ventricular diastole: the ventricles relax, ventricular pressure falls, and the semilunar valves close, preventing backflow into the ventricles.
8. Ventricular pressure falls further, and the tricuspid and bicuspid valves are pushed open by the pressure of the blood that the veins have emptied into the atria. Blood again moves freely into the ventricles.
9. All chambers are relaxed again (joint diastole). The SAN soon generates a new action potential, and the events repeat in the same sequence.

4.1 Duration, stroke volume and cardiac output

• The heart beats 72 times per minute, so 72 cardiac cycles take place every minute.
• So one cardiac cycle lasts about 0.8 seconds: $\frac{60\text{s}}{72}\approx 0.8\text{s}$.
• Stroke volume (also called beat volume): the volume of blood, about 70 mL, pumped out by each ventricle during one cardiac cycle.
• Cardiac output: the volume of blood pumped out by each ventricle per minute.

• Cardiac output averages 5000 mL (5 litres) in a healthy individual.
• The body can change the stroke volume and the heart rate, and so the cardiac output.
• ★ Exam imp For example, the cardiac output of an athlete is much higher than that of an ordinary man.

4.2 Heart sounds

• Two prominent sounds are produced in each cardiac cycle; they are easily heard through a stethoscope.

• These sounds are of clinical diagnostic significance.

5. Electrocardiogram (ECG)

• In a hospital, a patient can be connected to a monitoring machine that shows voltage traces of the heart on a screen. It makes a short repeated sound with each beat, which becomes one long continuous tone when the patient goes into cardiac arrest.
• Such a machine is an electro-cardiograph; it records the electrical activity of the heart from the body surface to give an electrocardiogram (ECG).
• For a standard ECG, the patient is connected to the machine with three electrical leads: one to each wrist and one to the left ankle. They continuously monitor the activity of the heart.
• For a detailed evaluation of heart function, multiple leads are attached to the chest region.
• Each peak in the ECG is identified with a letter from P to T, which corresponds to a specific electrical activity of the heart.

• ★ Exam imp The heart beat rate is found by counting the number of QRS complexes in a given time.
• For a given lead configuration, ECGs of different individuals have roughly the same shape.
• Any deviation from this shape indicates a possible abnormality or disease, so the ECG is of great clinical significance.

6. Exam Essentials

Pairs to Match

Numbers to Remember

• Heart chambers: fishes 2; amphibians and most reptiles 3; crocodiles, birds and mammals 4.
• SAN: 70-75 action potentials ${\text{min}}^{-1}$; heart rate 70-75, average 72 beats ${\text{min}}^{-1}$.
• Atrial systole: increases blood flow into the ventricles by about 30%.
• Duration of one cardiac cycle: about 0.8 s.
• Stroke volume: about 70 mL; cardiac output: about 5000 mL (5 L) per minute.
• Standard ECG: 3 leads (two wrists, left ankle).

7. Quick Revision

• Open system (sinuses): arthropods, molluscs. Closed system: annelids, chordates; flow regulated more precisely.
• Fishes: 2 chambers, single circulation. Amphibians and reptiles: 3 chambers, mixed blood, incomplete double circulation.
• Crocodiles, birds and mammals: 4 chambers; birds and mammals have double circulation without mixing.
• Human heart: mesodermal, in the thoracic cavity, tilted left, fist-sized, inside a pericardium with pericardial fluid.
• Inter-atrial septum thin and muscular; inter-ventricular septum thick; atrio-ventricular septum thick and fibrous.
• Tricuspid valve on the right, bicuspid (mitral) on the left, semilunar valves at the pulmonary artery and aorta.
• The whole heart is cardiac muscle; ventricle walls are much thicker than atrial walls.
• SAN (right upper corner of right atrium) and AVN (lower left corner of right atrium); AV bundle, bundles, Purkinje fibres.
• Nodal tissue is autoexcitable; the SAN fires 70-75 ${\text{min}}^{-1}$ and is the pacemaker; average heart rate 72.
• Cardiac cycle: joint diastole, atrial systole (+30% filling), ventricular systole, ventricular diastole, joint diastole.
• One cycle about 0.8 s; stroke volume about 70 mL; cardiac output about 5 L per minute, higher in athletes.
• Lub: closure of tricuspid and bicuspid valves; dub: closure of semilunar valves.
• ECG: graphical record of the heart's electrical activity; standard ECG uses three leads.
• P: atrial depolarisation; QRS: ventricular depolarisation (systole starts after Q); T: ventricular repolarisation (its end ends systole).

8. Solved Examples

9. Practice Questions

Common Mistakes to Avoid