Body Fluids and Circulation — NCERT Solutions

Evolutionary Changes in the Heart Among Vertebrates:

The heart has undergone progressive evolutionary changes from lower to higher vertebrates, increasing in complexity and efficiency:

1. Fishes (2-chambered heart):

• Heart consists of one atrium and one ventricle.
• Only single circulation — blood flows: Heart → Gills (oxygenation) → Body → Heart.
• Deoxygenated blood is pumped to gills; oxygenated blood goes to body.
• No separation of oxygenated and deoxygenated blood in the heart.

2. Amphibians (3-chambered heart):

• Heart has two atria and one ventricle.
• Left atrium receives oxygenated blood from lungs; right atrium receives deoxygenated blood from body.
• Both types of blood mix in the single ventricle — incomplete double circulation.
• Some mixing of oxygenated and deoxygenated blood occurs.

3. Reptiles (incompletely 4-chambered heart):

• Heart has two atria and two ventricles, but the inter-ventricular septum is incomplete (except in crocodiles).
• Some mixing of blood still occurs between ventricles.
• Crocodiles have a completely 4-chambered heart — most advanced among reptiles.

4. Birds and Mammals (completely 4-chambered heart):

• Heart has two atria and two completely separated ventricles.
• Complete double circulation — pulmonary and systemic circuits are fully separated.
• No mixing of oxygenated and deoxygenated blood.
• This is the most efficient type, supporting high metabolic rates of warm-blooded animals.

Conclusion: The evolutionary trend shows a progression from a 2-chambered heart (fishes) → 3-chambered (amphibians) → incompletely 4-chambered (reptiles) → completely 4-chambered (birds and mammals), ensuring increasingly efficient separation of oxygenated and deoxygenated blood.

Definition: A heart is called myogenic when the heartbeat is initiated by a specialised muscle tissue within the heart itself, without requiring any external nervous stimulation.

Explanation:

• In humans (and all mammals), the heartbeat originates from the Sino-Atrial Node (SAN), which is a specialised patch of modified cardiac muscle tissue located in the right atrium.

• The SAN is capable of self-excitation — it spontaneously generates electrical impulses (action potentials) at a rate of 70–75 per minute without any nerve signal.

• Since the impulse for contraction arises from the myocardium (cardiac muscle) itself, the human heart is called myogenic.

• This is in contrast to neurogenic hearts (found in some invertebrates like crustaceans), where the heartbeat is initiated by nerve impulses from ganglia.

Conclusion: Our heart is called myogenic because its rhythmic contraction is initiated by the cardiac muscle (SAN) itself, not by external nerve stimulation.

Given: The Sino-Atrial Node (SAN) is a specialised mass of modified cardiac muscle tissue located in the upper right corner of the right atrium.

Reason for being called the Pacemaker:

1. The SAN has the ability to spontaneously generate action potentials (electrical impulses) without any external stimulus.

2. It generates action potentials at the highest frequency among all auto-excitable tissues of the heart — approximately 70–75 action potentials per minute.

3. The impulse generated by SAN spreads first to the atria, causing atrial contraction (atrial systole), then reaches the Atrio-Ventricular Node (AVN), and subsequently spreads to the ventricles via the Bundle of His and Purkinje fibres, causing ventricular contraction.

4. Since the SAN sets the rhythm and rate of the heartbeat for the entire heart, it is called the Pacemaker.

5. If the SAN is damaged, other nodes (like AVN) can take over but at a lower rate, confirming that SAN normally dominates and controls the pace.

Conclusion: SAN is called the pacemaker because it generates the maximum number of action potentials per minute and thereby determines the rate and rhythm of the heartbeat.

1. Atrio-Ventricular Node (AVN):

• The AVN is located at the base of the right atrium, near the inter-atrial septum.
• It receives the electrical impulse from the SAN after atrial depolarisation.
• Significance:
• It acts as a relay station — it receives the impulse from the atria and transmits it to the ventricles.
• It introduces a brief delay (approx. 0.1 second) before passing the impulse to the ventricles. This delay ensures that the atria complete their contraction (and blood is pushed into ventricles) before the ventricles contract. This is crucial for efficient filling of ventricles.
• It is the only electrical connection between the atria and ventricles (since the fibrous skeleton of the heart otherwise electrically insulates them).

2. Atrio-Ventricular Bundle (Bundle of His):

• The AV bundle arises from the AVN and passes through the inter-ventricular septum, dividing into right and left bundle branches (Purkinje fibres).
• Significance:
• It conducts the impulse from AVN to the ventricular myocardium rapidly via Purkinje fibres.
• It ensures simultaneous and coordinated contraction of both ventricles from the apex upward, so blood is efficiently pumped into the pulmonary artery and aorta.
• It maintains the proper sequence of contraction: atria first, then ventricles.

Conclusion: The AVN and AV bundle together ensure that the electrical impulse is properly delayed and then rapidly distributed to the ventricles, resulting in coordinated and efficient pumping of blood.

1. Cardiac Cycle:

Definition: The cardiac cycle refers to the sequential series of events that occur in the heart during one complete heartbeat — from the beginning of one heartbeat to the beginning of the next.

• It includes:
• Atrial systole (contraction of atria) — 0.1 sec
• Ventricular systole (contraction of ventricles) — 0.3 sec
• Joint diastole (relaxation of both atria and ventricles) — 0.4 sec
• Total duration of one cardiac cycle = 0.8 seconds
• A healthy person completes approximately 72 cardiac cycles per minute.

2. Cardiac Output:

Definition: Cardiac output is defined as the volume of blood pumped out by each ventricle per minute.

$$\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$$

Where:

• Stroke Volume = volume of blood pumped by each ventricle per beat ≈ 70 mL
• Heart Rate = number of beats per minute ≈ 72 beats/min

$$\text{Cardiac Output} = 70 \text{ mL} \times 72 = 5040 \text{ mL} \approx 5 \text{ litres/min}$$

Conclusion: A healthy adult has a cardiac output of approximately 5 litres per minute, which means the entire blood volume is circulated once every minute.

Heart Sounds:

During each cardiac cycle, the heart produces two main sounds that can be heard through a stethoscope:

1. First Heart Sound — 'LUBB':

• Produced by the closure of the tricuspid and bicuspid (mitral) valves (Atrio-Ventricular valves / AV valves).
• Occurs at the beginning of ventricular systole (when ventricles start contracting).
• It is louder and longer in duration.
• Indicates the start of ventricular contraction.

2. Second Heart Sound — 'DUPP':

• Produced by the closure of the semilunar valves (pulmonary and aortic valves).
• Occurs at the beginning of ventricular diastole (when ventricles start relaxing).
• It is shorter and sharper in duration.
• Indicates the end of ventricular systole and beginning of diastole.

Summary:

Clinical Significance: Any abnormal heart sounds (murmurs) may indicate defective valves or other cardiac abnormalities.

Conclusion: Heart sounds are produced by the closure of valves and help in assessing the proper functioning of the heart.

Electrocardiogram (ECG):

Definition: An ECG is a graphical recording of the electrical activity of the heart over time, recorded from the body surface using an electrocardiograph.

Standard ECG — Description of Waves:

A normal ECG shows the following distinct waves:

$$\boxed{P \rightarrow Q \rightarrow R \rightarrow S \rightarrow T}$$

(Note: Draw a standard ECG trace showing P wave, QRS complex, and T wave on the answer sheet. The baseline is the isoelectric line. P is a small upward deflection; QRS is a sharp tall complex; T is a broad upward dome.)

Explanation of Each Segment/Wave:

1. P-wave:

• Represents depolarisation of the atria (spread of electrical impulse from SAN over both atria).
• Results in atrial contraction (atrial systole).
• Small, rounded, upward deflection.

2. QRS Complex:

• Represents depolarisation of the ventricles (spread of impulse through Bundle of His and Purkinje fibres to ventricular muscle).
• Results in ventricular contraction (ventricular systole).
• Q: small downward deflection; R: tall upward peak; S: small downward deflection.
• The atrial repolarisation is hidden within this complex.

3. T-wave:

• Represents repolarisation of the ventricles (recovery of ventricular muscle).
• Results in ventricular relaxation (ventricular diastole).
• Broad, dome-shaped, upward deflection.

4. P-Q (P-R) Interval:

• Time from start of P-wave to start of QRS complex.
• Represents the time taken for impulse to travel from SAN to ventricles (includes AVN delay).
• Normal duration: 0.12–0.20 seconds.

5. S-T Segment:

• Represents the period when ventricles are in a sustained contracted state (plateau phase).
• Normally isoelectric (flat).
• Elevation or depression of S-T segment indicates myocardial infarction or ischaemia.

6. Q-T Interval:

• Represents the total time of ventricular activity (depolarisation + repolarisation).

Clinical Significance of ECG:

• Helps diagnose arrhythmias, myocardial infarction (heart attack), hypertrophy, electrolyte imbalances, and other cardiac conditions.
• Each cardiac cycle produces one complete PQRST pattern; a healthy person shows 72 such cycles per minute.

Conclusion: ECG is a vital diagnostic tool that records the electrical events of the heart, with each wave corresponding to a specific event in the cardiac cycle.