A Tiny Patch of Heart Muscle Sets Every Beat Without Orders From the Brain
Learn the outside and inside of the human heart and the vessels that enter and leave it, follow systole and diastole through one heartbeat and the SA node that starts it, trace blood through the pulmonary and systemic circuits, and understand pulse, blood pressure and the hepatic portal system.
How does the heart keep blood moving through the whole body?
Place two fingers on the inside of your wrist and you can feel a steady beat — about once a second while you sit still. Each beat is your heart squeezing blood out into the arteries. It has been doing that since before you were born, and it will keep doing it without rest, day and night, whether you are awake or asleep.
The heart is really two pumps side by side.
- The right side receives blood that has given up its oxygen in the body and pumps it to the lungs
- The left side receives oxygen-rich blood from the lungs and pumps it to the rest of the body
So blood passes through the heart twice on every complete journey round the body — the double circulation that keeps oxygen-rich and oxygen-poor blood apart.
What makes it beat? Not the brain. A tiny patch of special muscle in the wall of the heart, the SA node, sends out an electrical signal with every beat, and the rest of the heart follows. Nerves and hormones can speed it up or slow it down, but they do not start it.
This part covers:
- The external and internal structure of the heart, and the main blood vessels connected to it
- How the heart works — systole, diastole and the role of the SA node
- Double circulation, tracing blood through the pulmonary and systemic circuits
- Pulse, blood pressure and the hepatic portal system
Why this matters in everyday life. Doctors feel the pulse, listen to the heart with a stethoscope and measure blood pressure at almost every check-up. Each of those three tests measures something described in this lesson, and high blood pressure is one of the most common health problems among adults in India.
The link to Part 1. The red blood cells that carry oxygen, and the plasma that carries food and wastes, are all moved by this pump. Part 3 then examines the arteries, veins and capillaries through which the heart sends them.
This page covers the second part of the ICSE Class 10 Biology chapter on the circulatory system: structure of the heart, its working, double circulation, pulse, blood pressure and the hepatic portal system.
The heart is really two pumps side by side.
- The right side receives blood that has given up its oxygen in the body and pumps it to the lungs
- The left side receives oxygen-rich blood from the lungs and pumps it to the rest of the body
So blood passes through the heart twice on every complete journey round the body — the double circulation that keeps oxygen-rich and oxygen-poor blood apart.
What makes it beat? Not the brain. A tiny patch of special muscle in the wall of the heart, the SA node, sends out an electrical signal with every beat, and the rest of the heart follows. Nerves and hormones can speed it up or slow it down, but they do not start it.
This part covers:
- The external and internal structure of the heart, and the main blood vessels connected to it
- How the heart works — systole, diastole and the role of the SA node
- Double circulation, tracing blood through the pulmonary and systemic circuits
- Pulse, blood pressure and the hepatic portal system
Why this matters in everyday life. Doctors feel the pulse, listen to the heart with a stethoscope and measure blood pressure at almost every check-up. Each of those three tests measures something described in this lesson, and high blood pressure is one of the most common health problems among adults in India.
The link to Part 1. The red blood cells that carry oxygen, and the plasma that carries food and wastes, are all moved by this pump. Part 3 then examines the arteries, veins and capillaries through which the heart sends them.
This page covers the second part of the ICSE Class 10 Biology chapter on the circulatory system: structure of the heart, its working, double circulation, pulse, blood pressure and the hepatic portal system.
What is the external and internal structure of the human heart, and which vessels enter and leave it?
The heart is a four-chambered muscular organ enclosed in a double membrane; its two upper atria receive blood and its two thicker lower ventricles pump it out, with valves keeping blood flowing one way, and it is connected to the vena cavae, pulmonary artery, pulmonary veins and aorta.
External structure.
- A hollow, muscular organ about the size of a closed fist
- Lies in the chest cavity between the lungs, tilted slightly towards the left
- Enclosed in the pericardium, a double membrane with pericardial fluid between its layers that reduces friction as the heart beats
- Supplied with its own blood by the coronary arteries on its surface
Internal structure — four chambers.
- Right atrium and left atrium — the upper, thin-walled chambers that receive blood
- Right ventricle and left ventricle — the lower, thick-walled chambers that pump blood out
- The left ventricle has the thickest wall, because it pumps blood to the whole body
- A wall called the septum separates the left and right sides, so their blood never mixes
Valves — keeping blood moving one way.
- Tricuspid valve — between the right atrium and right ventricle
- Bicuspid or mitral valve — between the left atrium and left ventricle
- Semilunar valves — at the openings of the pulmonary artery and the aorta
- Tendon-like cords attached to the flaps of the tricuspid and bicuspid valves stop them turning inside out when the ventricles contract
Main blood vessels:
- Superior and inferior vena cava — bring deoxygenated blood from the body into the right atrium
- Pulmonary artery — carries deoxygenated blood from the right ventricle to the lungs
- Pulmonary veins — bring oxygenated blood from the lungs into the left atrium
- Aorta — carries oxygenated blood from the left ventricle to the body
Worked example — follow the valves. A drop of blood enters the right atrium. Name, in order, the valves it passes before it leaves the heart for the lungs.
- Tricuspid valve — from right atrium to right ventricle
- Semilunar valve of the pulmonary artery — from right ventricle into the pulmonary artery
An everyday example. Through a doctor's stethoscope, a healthy heart makes two sounds with each beat, often described as lub and dub. The first is made by the tricuspid and bicuspid valves closing; the second by the semilunar valves closing — the valves in this section, heard directly.
The boundary case — two vessels that break the usual rule. Arteries usually carry oxygenated blood and veins deoxygenated blood, but the pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood. An artery is defined by carrying blood away from the heart, not by the oxygen in it.
External structure.
- A hollow, muscular organ about the size of a closed fist
- Lies in the chest cavity between the lungs, tilted slightly towards the left
- Enclosed in the pericardium, a double membrane with pericardial fluid between its layers that reduces friction as the heart beats
- Supplied with its own blood by the coronary arteries on its surface
Internal structure — four chambers.
- Right atrium and left atrium — the upper, thin-walled chambers that receive blood
- Right ventricle and left ventricle — the lower, thick-walled chambers that pump blood out
- The left ventricle has the thickest wall, because it pumps blood to the whole body
- A wall called the septum separates the left and right sides, so their blood never mixes
Valves — keeping blood moving one way.
- Tricuspid valve — between the right atrium and right ventricle
- Bicuspid or mitral valve — between the left atrium and left ventricle
- Semilunar valves — at the openings of the pulmonary artery and the aorta
- Tendon-like cords attached to the flaps of the tricuspid and bicuspid valves stop them turning inside out when the ventricles contract
Main blood vessels:
- Superior and inferior vena cava — bring deoxygenated blood from the body into the right atrium
- Pulmonary artery — carries deoxygenated blood from the right ventricle to the lungs
- Pulmonary veins — bring oxygenated blood from the lungs into the left atrium
- Aorta — carries oxygenated blood from the left ventricle to the body
Worked example — follow the valves. A drop of blood enters the right atrium. Name, in order, the valves it passes before it leaves the heart for the lungs.
- Tricuspid valve — from right atrium to right ventricle
- Semilunar valve of the pulmonary artery — from right ventricle into the pulmonary artery
An everyday example. Through a doctor's stethoscope, a healthy heart makes two sounds with each beat, often described as lub and dub. The first is made by the tricuspid and bicuspid valves closing; the second by the semilunar valves closing — the valves in this section, heard directly.
The boundary case — two vessels that break the usual rule. Arteries usually carry oxygenated blood and veins deoxygenated blood, but the pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood. An artery is defined by carrying blood away from the heart, not by the oxygen in it.
How does the heart work through systole and diastole, and what does the SA node do?
Each heartbeat is a cardiac cycle in which the atria contract, then the ventricles contract, then the whole heart relaxes and refills; contraction is systole and relaxation is diastole, and every cycle is started by an electrical impulse from the SA node, the heart's natural pacemaker.
Systole and diastole.
- Systole — contraction of a heart chamber, pushing blood out
- Diastole — relaxation of a heart chamber, letting it fill with blood
The cardiac cycle — one heartbeat:
- Atrial systole — the atria contract, pushing the last of their blood into the ventricles through the open tricuspid and bicuspid valves
- Ventricular systole — the ventricles contract. The tricuspid and bicuspid valves close, making the first heart sound, and blood is forced through the semilunar valves into the pulmonary artery and aorta
- Joint diastole — the whole heart relaxes. The semilunar valves close, making the second heart sound, and blood flows into the atria from the vena cavae and pulmonary veins, ready for the next beat
Timing of one cycle. At about beats per minute, one cycle lasts:
Commonly used textbook values divide this as atrial systole , ventricular systole and joint diastole :
Half of every cycle is rest, which is how the heart can beat continuously without tiring.
Worked example — cardiac output. Each contraction of the left ventricle pushes out about of blood. At beats per minute, how much blood does it pump each minute?
Roughly the whole blood volume of an adult passes through the heart every minute.
The SA node — the natural pacemaker.
- The sinoatrial node is a small patch of specialised muscle in the wall of the right atrium, near where the superior vena cava enters
- It produces electrical impulses on its own, at a regular rate
- Each impulse spreads over the atria, making them contract
- It then reaches the atrioventricular node between the atria and ventricles, and passes along special conducting fibres through the ventricle walls, making the ventricles contract a moment later
Because the beat starts in the heart muscle itself, the heart is called myogenic. Nerves and hormones such as adrenaline only change the rate at which the SA node fires.
An everyday example. Some people, often elderly, have a small electronic device called an artificial pacemaker placed under the skin of the chest. It sends regular electrical pulses to the heart when the natural SA node no longer fires reliably — doing, with a battery, the job the SA node normally does on its own.
The boundary case. The atria and ventricles do not contract at the same time. The short delay at the atrioventricular node lets the atria finish emptying into the ventricles before the ventricles contract — without it, the heart would pump far less blood.
Systole and diastole.
- Systole — contraction of a heart chamber, pushing blood out
- Diastole — relaxation of a heart chamber, letting it fill with blood
The cardiac cycle — one heartbeat:
- Atrial systole — the atria contract, pushing the last of their blood into the ventricles through the open tricuspid and bicuspid valves
- Ventricular systole — the ventricles contract. The tricuspid and bicuspid valves close, making the first heart sound, and blood is forced through the semilunar valves into the pulmonary artery and aorta
- Joint diastole — the whole heart relaxes. The semilunar valves close, making the second heart sound, and blood flows into the atria from the vena cavae and pulmonary veins, ready for the next beat
Timing of one cycle. At about beats per minute, one cycle lasts:
Commonly used textbook values divide this as atrial systole , ventricular systole and joint diastole :
Half of every cycle is rest, which is how the heart can beat continuously without tiring.
Worked example — cardiac output. Each contraction of the left ventricle pushes out about of blood. At beats per minute, how much blood does it pump each minute?
Roughly the whole blood volume of an adult passes through the heart every minute.
The SA node — the natural pacemaker.
- The sinoatrial node is a small patch of specialised muscle in the wall of the right atrium, near where the superior vena cava enters
- It produces electrical impulses on its own, at a regular rate
- Each impulse spreads over the atria, making them contract
- It then reaches the atrioventricular node between the atria and ventricles, and passes along special conducting fibres through the ventricle walls, making the ventricles contract a moment later
Because the beat starts in the heart muscle itself, the heart is called myogenic. Nerves and hormones such as adrenaline only change the rate at which the SA node fires.
An everyday example. Some people, often elderly, have a small electronic device called an artificial pacemaker placed under the skin of the chest. It sends regular electrical pulses to the heart when the natural SA node no longer fires reliably — doing, with a battery, the job the SA node normally does on its own.
The boundary case. The atria and ventricles do not contract at the same time. The short delay at the atrioventricular node lets the atria finish emptying into the ventricles before the ventricles contract — without it, the heart would pump far less blood.
What is double circulation, and what path does blood take through the pulmonary and systemic circuits?
Double circulation means blood passes through the heart twice in each complete round of the body — once in the pulmonary circuit between the heart and lungs, and once in the systemic circuit between the heart and the rest of the body.
1. Pulmonary circulation — heart to lungs and back.
- Deoxygenated blood leaves the right ventricle
- In the lung capillaries, it picks up oxygen and gives up carbon dioxide
- Oxygenated blood returns to the left atrium
2. Systemic circulation — heart to body and back.
- Oxygenated blood leaves the left ventricle
- In the body's capillaries, it gives up oxygen and food and picks up carbon dioxide and wastes
- Deoxygenated blood returns to the right atrium
The complete path of one red blood cell, starting in the right atrium:
- Right atrium tricuspid valve right ventricle semilunar valve pulmonary artery lungs
- Pulmonary veins left atrium bicuspid valve left ventricle semilunar valve aorta
- Arteries capillaries of an organ veins vena cava right atrium
Advantages of double circulation:
- Oxygenated and deoxygenated blood never mix, so the body receives blood fully loaded with oxygen
- Blood returning from the lungs is pumped again, so it reaches the body at high pressure, flowing quickly to distant organs
- This supports the high energy needs of warm-blooded animals such as mammals and birds
Worked example — how many times through the heart? A red blood cell travels from the left ventricle to the big toe and back to the left ventricle. How many times does it pass through the heart, and through which chambers?
- Out through the left ventricle, round the body, back into the right atrium and right ventricle — once through the right side
- Out to the lungs, back into the left atrium and left ventricle — once through the left side
It passes through the heart twice — once through each side — completing one double circulation.
Comparison with a fish. A fish has a two-chambered heart and a single circulation: blood is pumped to the gills, and then flows on to the body without returning to the heart. Blood reaches the body at lower pressure, which suits the lower energy demands of a cold-blooded animal.
An everyday example. Climbing stairs quickly makes your heart pound. Your muscles need more oxygen, so both circuits speed up — the pulmonary circuit to load oxygen faster in the lungs, and the systemic circuit to deliver it faster to the legs.
The boundary case. The two circuits are not separate loops of blood. The same blood flows through both, one after the other; the word double refers to the two passes through the heart, not to two different supplies of blood.
1. Pulmonary circulation — heart to lungs and back.
- Deoxygenated blood leaves the right ventricle
- In the lung capillaries, it picks up oxygen and gives up carbon dioxide
- Oxygenated blood returns to the left atrium
2. Systemic circulation — heart to body and back.
- Oxygenated blood leaves the left ventricle
- In the body's capillaries, it gives up oxygen and food and picks up carbon dioxide and wastes
- Deoxygenated blood returns to the right atrium
The complete path of one red blood cell, starting in the right atrium:
- Right atrium tricuspid valve right ventricle semilunar valve pulmonary artery lungs
- Pulmonary veins left atrium bicuspid valve left ventricle semilunar valve aorta
- Arteries capillaries of an organ veins vena cava right atrium
Advantages of double circulation:
- Oxygenated and deoxygenated blood never mix, so the body receives blood fully loaded with oxygen
- Blood returning from the lungs is pumped again, so it reaches the body at high pressure, flowing quickly to distant organs
- This supports the high energy needs of warm-blooded animals such as mammals and birds
Worked example — how many times through the heart? A red blood cell travels from the left ventricle to the big toe and back to the left ventricle. How many times does it pass through the heart, and through which chambers?
- Out through the left ventricle, round the body, back into the right atrium and right ventricle — once through the right side
- Out to the lungs, back into the left atrium and left ventricle — once through the left side
It passes through the heart twice — once through each side — completing one double circulation.
Comparison with a fish. A fish has a two-chambered heart and a single circulation: blood is pumped to the gills, and then flows on to the body without returning to the heart. Blood reaches the body at lower pressure, which suits the lower energy demands of a cold-blooded animal.
An everyday example. Climbing stairs quickly makes your heart pound. Your muscles need more oxygen, so both circuits speed up — the pulmonary circuit to load oxygen faster in the lungs, and the systemic circuit to deliver it faster to the legs.
The boundary case. The two circuits are not separate loops of blood. The same blood flows through both, one after the other; the word double refers to the two passes through the heart, not to two different supplies of blood.
What are pulse and blood pressure, and why is the hepatic portal system important?
The pulse is the rhythmic stretching of an artery wall with each heartbeat, blood pressure is the force blood exerts on the artery walls, and the hepatic portal system carries blood from the digestive organs to the liver before it returns to the heart, so the liver can process absorbed nutrients and toxins.
1. Pulse.
- Each ventricular systole pushes a surge of blood into the arteries, which stretch and then recoil
- That rhythmic expansion can be felt where an artery lies near the skin over a bone — at the wrist or the side of the neck
- The pulse rate equals the heart rate, about ** beats per minute at rest in an adult
Worked example — measuring pulse rate.** A student counts pulse beats in seconds. Find the pulse rate per minute.
2. Blood pressure.
- Blood pressure is the pressure exerted by blood on the walls of the arteries
- Systolic pressure — the higher value, during ventricular contraction, about in a healthy adult
- Diastolic pressure — the lower value, during relaxation, about
- It is written as systolic over diastolic — for example,
- Measured with a sphygmomanometer, an inflatable cuff wrapped round the upper arm
- Persistently high blood pressure is called hypertension, which strains the heart and blood vessels
Worked example — pulse pressure. For a reading of , find the difference between the systolic and diastolic pressures.
3. The hepatic portal system.
- A portal system is one in which a vein starts in the capillaries of one organ and ends in the capillaries of another, instead of taking blood straight back to the heart
- The hepatic portal vein collects blood from the stomach, intestines, spleen and pancreas
- It carries that blood to the liver, where it passes through capillaries before draining into the hepatic vein and on to the heart
Significance of the hepatic portal system:
- The liver receives digested food first — glucose, amino acids and other nutrients absorbed from the intestine
- Excess glucose is stored as glycogen, keeping blood sugar steady
- Excess amino acids are broken down, forming urea
- Harmful substances absorbed with food, including many drugs and toxins, are removed or made harmless
- So the blood reaching the rest of the body has already been processed
An everyday example. Blood pressure machines are now common in pharmacies and homes across India, and doctors advise adults to check their readings regularly. **A reading repeatedly well above is a signal to reduce salt, stay active and seek medical advice.
The boundary case. Blood pressure is highest in the arteries near the heart and falls steadily as blood passes through smaller arteries and capillaries into the veins. That is why pulse and blood pressure are measured in arteries**, and why veins show no pulse.
1. Pulse.
- Each ventricular systole pushes a surge of blood into the arteries, which stretch and then recoil
- That rhythmic expansion can be felt where an artery lies near the skin over a bone — at the wrist or the side of the neck
- The pulse rate equals the heart rate, about ** beats per minute at rest in an adult
Worked example — measuring pulse rate.** A student counts pulse beats in seconds. Find the pulse rate per minute.
2. Blood pressure.
- Blood pressure is the pressure exerted by blood on the walls of the arteries
- Systolic pressure — the higher value, during ventricular contraction, about in a healthy adult
- Diastolic pressure — the lower value, during relaxation, about
- It is written as systolic over diastolic — for example,
- Measured with a sphygmomanometer, an inflatable cuff wrapped round the upper arm
- Persistently high blood pressure is called hypertension, which strains the heart and blood vessels
Worked example — pulse pressure. For a reading of , find the difference between the systolic and diastolic pressures.
3. The hepatic portal system.
- A portal system is one in which a vein starts in the capillaries of one organ and ends in the capillaries of another, instead of taking blood straight back to the heart
- The hepatic portal vein collects blood from the stomach, intestines, spleen and pancreas
- It carries that blood to the liver, where it passes through capillaries before draining into the hepatic vein and on to the heart
Significance of the hepatic portal system:
- The liver receives digested food first — glucose, amino acids and other nutrients absorbed from the intestine
- Excess glucose is stored as glycogen, keeping blood sugar steady
- Excess amino acids are broken down, forming urea
- Harmful substances absorbed with food, including many drugs and toxins, are removed or made harmless
- So the blood reaching the rest of the body has already been processed
An everyday example. Blood pressure machines are now common in pharmacies and homes across India, and doctors advise adults to check their readings regularly. **A reading repeatedly well above is a signal to reduce salt, stay active and seek medical advice.
The boundary case. Blood pressure is highest in the arteries near the heart and falls steadily as blood passes through smaller arteries and capillaries into the veins. That is why pulse and blood pressure are measured in arteries**, and why veins show no pulse.
Exam tip
What earns full marks on the heart and circulation?
Label the heart with every chamber, valve and vessel, describe the cardiac cycle in order with valve actions, trace double circulation as a complete sequence, and define pulse, blood pressure and portal system precisely.
- Label the four chambers, septum, tricuspid, bicuspid and semilunar valves, and the four main vessels
- State which vessels carry oxygenated and deoxygenated blood, noting the pulmonary exception
- Explain why the left ventricle wall is thickest
- Define systole and diastole
- Describe the cardiac cycle in three stages, saying which valves open and close
- Name the SA node, its location in the right atrium and its role as pacemaker; say the heart is myogenic
- Trace the pulmonary and systemic circuits with arrows, starting and ending in the correct chambers
- Give two advantages of double circulation
- Define pulse and give its normal resting rate
- Define blood pressure with systolic and diastolic values and the instrument used
- Explain the hepatic portal system and what the liver does with the blood
The misconception to name. Arteries do not always carry oxygenated blood. The pulmonary artery carries deoxygenated blood, and the pulmonary veins carry oxygenated blood. Arteries carry blood away from the heart; veins carry it towards the heart — that is the true definition.
A second trap. Saying the brain starts each heartbeat. The SA node starts it; nerves and hormones only alter the rate.
- Label the four chambers, septum, tricuspid, bicuspid and semilunar valves, and the four main vessels
- State which vessels carry oxygenated and deoxygenated blood, noting the pulmonary exception
- Explain why the left ventricle wall is thickest
- Define systole and diastole
- Describe the cardiac cycle in three stages, saying which valves open and close
- Name the SA node, its location in the right atrium and its role as pacemaker; say the heart is myogenic
- Trace the pulmonary and systemic circuits with arrows, starting and ending in the correct chambers
- Give two advantages of double circulation
- Define pulse and give its normal resting rate
- Define blood pressure with systolic and diastolic values and the instrument used
- Explain the hepatic portal system and what the liver does with the blood
The misconception to name. Arteries do not always carry oxygenated blood. The pulmonary artery carries deoxygenated blood, and the pulmonary veins carry oxygenated blood. Arteries carry blood away from the heart; veins carry it towards the heart — that is the true definition.
A second trap. Saying the brain starts each heartbeat. The SA node starts it; nerves and hormones only alter the rate.
Did you know
Why does your heart pound when you get a sudden fright?
A loud bang at night, a near-miss on a busy road or the moment before your name is called in an exam hall — within seconds your heart is pounding and your breathing speeds up. The SA node is still in charge of every beat, but it has just received new instructions.
Two messages arrive at the heart almost together.
- Nerves from the brain carry signals directly to the SA node, making it fire faster
- The adrenal glands, just above the kidneys, release the hormone adrenaline into the blood, which reaches the heart within moments and speeds up the SA node further while making each contraction stronger
The result is more blood pumped every minute. Using the figures from this lesson, **a resting output of about per beat at beats per minute** is about litres a minute. **If fright raised the rate to beats and the stroke to **:
More than double the resting flow.
Why the body does this. A sudden fright prepares the body to run or fight. More blood carries more oxygen and glucose to the muscles, while faster breathing loads extra oxygen in the lungs — the respiratory and circulatory systems working together for an emergency.
Other changes happen at the same time.
- Blood vessels to the muscles widen, sending them more blood
- Blood vessels to the skin and digestive system narrow, which is why a frightened person may look pale and lose their appetite
- Blood pressure rises
Once the danger passes, the extra signals stop, adrenaline is broken down, and the SA node gradually returns to its resting rate.
This is the clearest proof that the heart is myogenic. The nerves and adrenaline never create the beat; they only change the speed of a rhythm the SA node was already producing — which is why a heart can keep its own beat even when its nerve connections are cut.
Two messages arrive at the heart almost together.
- Nerves from the brain carry signals directly to the SA node, making it fire faster
- The adrenal glands, just above the kidneys, release the hormone adrenaline into the blood, which reaches the heart within moments and speeds up the SA node further while making each contraction stronger
The result is more blood pumped every minute. Using the figures from this lesson, **a resting output of about per beat at beats per minute** is about litres a minute. **If fright raised the rate to beats and the stroke to **:
More than double the resting flow.
Why the body does this. A sudden fright prepares the body to run or fight. More blood carries more oxygen and glucose to the muscles, while faster breathing loads extra oxygen in the lungs — the respiratory and circulatory systems working together for an emergency.
Other changes happen at the same time.
- Blood vessels to the muscles widen, sending them more blood
- Blood vessels to the skin and digestive system narrow, which is why a frightened person may look pale and lose their appetite
- Blood pressure rises
Once the danger passes, the extra signals stop, adrenaline is broken down, and the SA node gradually returns to its resting rate.
This is the clearest proof that the heart is myogenic. The nerves and adrenaline never create the beat; they only change the speed of a rhythm the SA node was already producing — which is why a heart can keep its own beat even when its nerve connections are cut.
Exam relevance
How is the human heart tested in NEET Biology?
This is foundation work for Class 11 Body Fluids and Circulation in NEET Biology, a core chapter of human physiology.
Where heart structure leads. Class 11 describes the chambers, valves, septa and blood vessels of the heart in detail. Identifying a valve from its position, and naming which chambers and vessels carry oxygenated or deoxygenated blood, are standard NEET questions, often diagram-based.
Where the cardiac cycle leads. The chapter describes the cardiac cycle with its timing — **a cycle of about seconds at about beats per minute — and the opening and closing of valves at each stage. Stroke volume and cardiac output are calculated exactly as in this lesson, and the heart sounds are linked to valve closure.
Where the SA node leads. Class 11 explains the nodal tissue — the sinoatrial node, atrioventricular node and the conducting bundle and fibres — and describes the heart as myogenic. The electrocardiogram, with its waves corresponding to the spread of impulses through the atria and ventricles, is examined directly, often as a labelled graph.
Where double circulation leads. The chapter covers double circulation, the hepatic portal system and the coronary circulation, and compares circulation in different animal groups. Tracing the path of blood and identifying portal systems are recurring question types.
Where blood pressure leads. Disorders of the circulatory system — hypertension, coronary artery disease, angina and heart failure — are described, with normal blood pressure given as .
Question types to expect. At this level: labelled heart diagrams, the cardiac cycle, the SA node, double circulation, pulse, blood pressure and the portal system. In NEET: valve positions, cardiac output calculations, ECG waves, nodal tissue sequence, comparative circulation and disorders, often as diagram-based or match-the-column questions.
The single trap that costs marks. Linking heart sounds to the wrong valves. The first sound comes from the closing of the atrioventricular valves; the second from the closing of the semilunar valves.
A second trap. Treating all arteries as carrying oxygenated blood. The pulmonary artery carries deoxygenated blood — a fact NEET tests repeatedly.
Board versus competitive emphasis. The ICSE paper marks labelled diagrams, ordered descriptions and definitions; NEET marks timings, calculations, nodal sequence and ECG interpretation. The transferable habit is following the electrical signal and the blood through the heart in the same order** — impulse first, contraction second, valves third.
Where heart structure leads. Class 11 describes the chambers, valves, septa and blood vessels of the heart in detail. Identifying a valve from its position, and naming which chambers and vessels carry oxygenated or deoxygenated blood, are standard NEET questions, often diagram-based.
Where the cardiac cycle leads. The chapter describes the cardiac cycle with its timing — **a cycle of about seconds at about beats per minute — and the opening and closing of valves at each stage. Stroke volume and cardiac output are calculated exactly as in this lesson, and the heart sounds are linked to valve closure.
Where the SA node leads. Class 11 explains the nodal tissue — the sinoatrial node, atrioventricular node and the conducting bundle and fibres — and describes the heart as myogenic. The electrocardiogram, with its waves corresponding to the spread of impulses through the atria and ventricles, is examined directly, often as a labelled graph.
Where double circulation leads. The chapter covers double circulation, the hepatic portal system and the coronary circulation, and compares circulation in different animal groups. Tracing the path of blood and identifying portal systems are recurring question types.
Where blood pressure leads. Disorders of the circulatory system — hypertension, coronary artery disease, angina and heart failure — are described, with normal blood pressure given as .
Question types to expect. At this level: labelled heart diagrams, the cardiac cycle, the SA node, double circulation, pulse, blood pressure and the portal system. In NEET: valve positions, cardiac output calculations, ECG waves, nodal tissue sequence, comparative circulation and disorders, often as diagram-based or match-the-column questions.
The single trap that costs marks. Linking heart sounds to the wrong valves. The first sound comes from the closing of the atrioventricular valves; the second from the closing of the semilunar valves.
A second trap. Treating all arteries as carrying oxygenated blood. The pulmonary artery carries deoxygenated blood — a fact NEET tests repeatedly.
Board versus competitive emphasis. The ICSE paper marks labelled diagrams, ordered descriptions and definitions; NEET marks timings, calculations, nodal sequence and ECG interpretation. The transferable habit is following the electrical signal and the blood through the heart in the same order** — impulse first, contraction second, valves third.
Key takeaways
What must you be able to do from this part?
One organ, one cycle, one pacemaker, two circuits and three measurements.
- Heart: fist-sized, between the lungs, in the pericardium, supplied by coronary arteries
- Chambers: two thin-walled atria receive blood; two thick-walled ventricles pump it; left ventricle thickest
- Valves: tricuspid on the right, bicuspid on the left, semilunar at the pulmonary artery and aorta
- Vessels: vena cavae into right atrium; pulmonary artery from right ventricle; pulmonary veins into left atrium; aorta from left ventricle
- Pulmonary artery carries deoxygenated blood; pulmonary veins carry oxygenated blood
- Systole is contraction; diastole is relaxation
- Cardiac cycle: atrial systole, ventricular systole with AV valves closing, joint diastole with semilunar valves closing
- **At beats per minute**, one cycle is about —
- Cardiac output: per minute
- SA node in the right atrium is the pacemaker; impulses pass to the AV node and conducting fibres; the heart is myogenic
- Pulmonary circuit: right ventricle, pulmonary artery, lungs, pulmonary veins, left atrium
- Systemic circuit: left ventricle, aorta, arteries, capillaries, veins, vena cavae, right atrium
- Double circulation keeps oxygenated and deoxygenated blood apart and sends blood to the body at high pressure
- Pulse: rhythmic stretching of arteries, about per minute; ** beats in s** is per minute
- Blood pressure: about , measured with a sphygmomanometer
- Hepatic portal system: blood from the gut goes to the liver first, which stores glucose and removes toxins
The sharpest self-test is one red blood cell and a stopwatch. Trace it from the vena cava back to the vena cava, naming every chamber, valve and vessel in order — then say where in that journey the SA node's impulse, the pulse and the portal system each come in.
- Heart: fist-sized, between the lungs, in the pericardium, supplied by coronary arteries
- Chambers: two thin-walled atria receive blood; two thick-walled ventricles pump it; left ventricle thickest
- Valves: tricuspid on the right, bicuspid on the left, semilunar at the pulmonary artery and aorta
- Vessels: vena cavae into right atrium; pulmonary artery from right ventricle; pulmonary veins into left atrium; aorta from left ventricle
- Pulmonary artery carries deoxygenated blood; pulmonary veins carry oxygenated blood
- Systole is contraction; diastole is relaxation
- Cardiac cycle: atrial systole, ventricular systole with AV valves closing, joint diastole with semilunar valves closing
- **At beats per minute**, one cycle is about —
- Cardiac output: per minute
- SA node in the right atrium is the pacemaker; impulses pass to the AV node and conducting fibres; the heart is myogenic
- Pulmonary circuit: right ventricle, pulmonary artery, lungs, pulmonary veins, left atrium
- Systemic circuit: left ventricle, aorta, arteries, capillaries, veins, vena cavae, right atrium
- Double circulation keeps oxygenated and deoxygenated blood apart and sends blood to the body at high pressure
- Pulse: rhythmic stretching of arteries, about per minute; ** beats in s** is per minute
- Blood pressure: about , measured with a sphygmomanometer
- Hepatic portal system: blood from the gut goes to the liver first, which stores glucose and removes toxins
The sharpest self-test is one red blood cell and a stopwatch. Trace it from the vena cava back to the vena cava, naming every chamber, valve and vessel in order — then say where in that journey the SA node's impulse, the pulse and the portal system each come in.