Circulatory & Respiratory Systems

Why This Chapter Matters

Blood and breath together are worth 3-5 questions in almost every SSC CGL, CHSL, MTS and RRB NTPC paper, and the questions repeat with almost boring predictability: who discovered blood groups, how many chambers does the heart have, what is the normal blood pressure reading, which gas do we exhale in larger quantity. You are not being asked to think here. You are being asked to remember, and remember correctly under time pressure.

This chapter covers the heart and how blood moves through it, the blood itself and what each component does, blood groups and the science of transfusion, blood pressure numbers you must know cold, the respiratory organs and how gas exchange actually happens, breathing rate facts, and the disorders exams love to test. The single biggest mistake aspirants make here is mixing up pulmonary and systemic circulation, they assume all arteries carry oxygenated blood and all veins carry deoxygenated blood, and lose a guaranteed mark on the pulmonary artery question. Fix that mistake today and it never trips you up again.

1. Heart Structure and Blood Flow

Your heart sits slightly left of centre in your chest, roughly the size of your closed fist, and works non-stop from before you are born until the day you die. It is a four-chambered organ: two upper chambers called atria (singular atrium) and two lower chambers called ventricles. The right side handles deoxygenated blood, the left side handles oxygenated blood, and a muscular wall called the septum keeps the two sides from mixing.

Think of the heart as a two-lane flyover with a strict no-mixing rule. One lane carries "used" blood to the lungs for a refill of oxygen, the other lane carries "refilled" blood out to the whole body. If the two lanes ever merge, the body suffocates from the inside even while you keep breathing normally, which is exactly what happens in some congenital heart defects.

Blood flow path, step by step:

1. Deoxygenated blood from the body enters the right atrium through two large veins, the superior and inferior vena cava.
2. It passes into the right ventricle, which pumps it through the pulmonary artery to the lungs.
3. In the lungs, blood picks up oxygen and releases carbon dioxide.
4. Oxygenated blood returns via the pulmonary vein to the left atrium.
5. It moves into the left ventricle, the heart's strongest chamber, which pumps it out through the aorta to the entire body.

Exam trap: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood, the exact reverse of every other artery and vein in the body. Examiners love this because it looks like a "gotcha" but it is simple once you remember the rule is about direction, not oxygen content: arteries carry blood away from the heart, veins carry blood toward the heart, regardless of what is in them.

Four valves keep blood flowing in one direction only, like one-way gates on a busy staircase: the tricuspid valve between right atrium and right ventricle, the bicuspid or mitral valve between left atrium and left ventricle, and the pulmonary and aortic semilunar valves at the exit points of the ventricles. The "lub-dub" sound your doctor hears through a stethoscope is these valves snapping shut.

The heart has its own electrical pacemaker, the sinoatrial (SA) node, sitting in the right atrium. It fires an electrical signal roughly 70-72 times a minute in a resting adult, and that signal is what makes the heart beat in rhythm without you having to think about it. A separate network called the atrioventricular (AV) node relays the signal down to the ventricles with a slight delay, so the atria finish contracting before the ventricles start.

2. Blood: Components and Functions

Blood is a connective tissue in liquid form, and an average adult carries about 5 to 5.5 litres of it. Spin a blood sample in a centrifuge and it separates into two clear layers: a pale yellow liquid called plasma (about 55% of blood volume) sitting on top of a dense layer of cells (about 45%).

Plasma is roughly 90% water and carries dissolved proteins, nutrients, hormones, and waste products from place to place, like a delivery van running errands across the whole body at once.

The cellular part has three main players:

• Red blood cells (RBCs / erythrocytes): made in the bone marrow, they carry oxygen using a red pigment called haemoglobin. RBCs are unusual because mature human RBCs have no nucleus, which frees up space for more haemoglobin. Their lifespan is about 120 days, after which the spleen and liver break them down.
• White blood cells (WBCs / leucocytes): the body's defence force, they fight infection. Unlike RBCs, they do have a nucleus. WBCs are far fewer in number than RBCs but are larger in size.
• Platelets (thrombocytes): small cell fragments that clump together at a wound site and trigger clotting, stopping bleeding before it becomes dangerous.

Memory hook: think "RWP" for the three cellular components in order of how much blood volume they take up, Red cells (most), White cells, Platelets (least) — Red Wins the most, Platelets come last.

Haemoglobin deserves its own mention because it is one of the most-repeated single facts in this chapter. It is an iron-containing protein, and a shortage of it, whether from low iron, blood loss, or poor RBC production, causes anaemia, the most commonly tested blood disorder in these exams.

3. Blood Groups

In 1900-01, Austrian scientist Karl Landsteiner discovered that not all blood is the same, and mixing incompatible types can be fatal. This one discovery is asked about almost every year in some form, so lock it in: Landsteiner discovered the ABO blood group system, and he later received the Nobel Prize for this work. He also co-discovered the Rh factor later in his career.

Blood is grouped by which antigens sit on the surface of red blood cells:

Blood Group	Antigen on RBC	Antibody in Plasma	Can Donate To	Can Receive From
A	A	Anti-B	A, AB	A, O
B	B	Anti-A	B, AB	B, O
AB	A and B	None	AB only	A, B, AB, O
O	None	Anti-A and Anti-B	A, B, AB, O	O only

O group is called the universal donor because its RBCs carry no antigens for the immune system to attack, so almost anyone can safely receive it in an emergency. AB group is the universal recipient because its plasma carries no antibodies, so it will not reject incoming A or B antigens.

Think of antigens as name tags and antibodies as security guards checking those tags. If your plasma's guards recognise an incoming blood cell's tag as "not one of us," they attack, and that mismatched transfusion can be fatal. O group RBCs simply have no name tag to object to, that is the entire trick behind why they work as universal donors.

The Rh factor is a separate marker, present (Rh+) or absent (Rh-) on the RBC surface. About 80% or more of the Indian population is Rh+. Rh incompatibility between an Rh- mother and an Rh+ baby matters in pregnancy and is why doctors check it during antenatal visits.

Exam trap: students often say "O group can receive from anyone" — wrong, it is the opposite. O is the universal donor, not the universal recipient. AB is the universal recipient. Mixing these two up is the single most common error on this topic.

4. Blood Pressure

Blood pressure is the force blood exerts against the walls of arteries as the heart pumps. It is recorded as two numbers: systolic over diastolic, measured in mm Hg (millimetres of mercury).

• Systolic pressure is the higher number, recorded when the heart contracts and pushes blood out.
• Diastolic pressure is the lower number, recorded when the heart relaxes between beats.

Normal blood pressure for a healthy adult at rest is taken as 120/80 mm Hg. Consistently elevated readings, generally 140/90 mm Hg or above, are classified as hypertension (high blood pressure). Readings persistently below roughly 90/60 mm Hg indicate hypotension (low blood pressure).

The instrument used to measure blood pressure is called a sphygmomanometer. Pulse rate, the number of times an artery expands per minute as blood surges through it, is normally 72 beats per minute for a resting adult, closely tied to the SA node's firing rate mentioned earlier.

5. Respiratory System and Gas Exchange

Breathing looks simple from the outside, air goes in, air comes out, but the exam wants you to know the exact route and the exact chemistry.

Path of air: nostrils → pharynx → larynx (voice box) → trachea (windpipe) → bronchi (one tube per lung) → bronchioles (finer branches) → alveoli (tiny air sacs, the actual site of gas exchange).

Picture the respiratory tract as an upside-down tree: the trachea is the trunk, the bronchi are the main branches, the bronchioles are twigs, and the alveoli are millions of leaves clustered at the very ends. Your two lungs together hold an estimated 300 million alveoli, and it is this sheer surface area, roughly the size of a badminton court when spread flat, that makes efficient gas exchange possible in such a compact chest cavity.

Alveoli are thin-walled, single-cell-thick sacs surrounded by capillaries. Oxygen from inhaled air diffuses across this thin wall into the blood, while carbon dioxide diffuses out of the blood into the alveoli to be exhaled. This exchange happens purely by diffusion, gases move from an area of higher concentration to lower concentration, no pump or muscle is needed at this last step.

The diaphragm, a dome-shaped sheet of muscle below the lungs, drives the mechanics of breathing. It contracts and flattens during inhalation, increasing chest cavity volume and pulling air in. It relaxes and domes upward during exhalation, pushing air out. This is why a hard blow to the diaphragm region ("solar plexus") can briefly knock the breath out of you, the muscle spasms and cannot do its job for a few seconds.

Inhaled air is roughly 21% oxygen and 0.04% carbon dioxide. Exhaled air is roughly 16% oxygen and about 4% carbon dioxide, a large jump in carbon dioxide content because the body has just used the oxygen and released this waste gas from cellular respiration.

Exam trap: students confuse breathing (a purely mechanical, physical act of moving air in and out) with respiration (the biochemical process inside cells that actually releases energy from food using oxygen). Breathing is how you get oxygen to your cells. Respiration is what your cells do with it.

6. Breathing Rate

Normal breathing rate for a healthy adult at rest is about 12 to 16 breaths per minute, considerably slower than the heart's roughly 72 beats a minute, since one breath supplies enough oxygen for several heartbeats worth of circulation. Breathing rate rises sharply during exercise or exertion because muscles demand more oxygen and produce more carbon dioxide that needs clearing.

Memory hook: remember "12-16-72" as a single climbing ladder for the three big resting numbers this chapter tests, breathing rate (12-16 per minute), then a jump to pulse rate (72 per minute), and you already know blood pressure sits at 120/80.

7. Common Disorders

• Anaemia: low haemoglobin or RBC count, leading to fatigue and paleness. The most common type, iron-deficiency anaemia, is widespread in India and frequently tested alongside the vitamin/mineral deficiency table from the nutrition chapter.
• Hypertension: chronically high blood pressure, a major risk factor for heart attack and stroke, closely linked to excess salt intake, obesity, and stress.
• Hypotension: abnormally low blood pressure, can cause dizziness and fainting.
• Asthma: a chronic condition where airways narrow and swell, producing wheezing and breathlessness, often triggered by allergens, cold air, or pollution.
• Bronchitis: inflammation of the bronchial tubes, often following a respiratory infection.
• Arteriosclerosis: hardening and narrowing of arteries, restricting blood flow and raising heart attack risk.
• Leukaemia: cancer of blood-forming tissue, marked by an abnormal rise in white blood cells.

You will have seen at least one of these words on a hospital prescription slip or a relative's medical report, that real-world familiarity is exactly why these disorder names stick better than abstract terms, use it.

Quick Revision — One-Line Facts

• The heart has four chambers: two atria (upper) and two ventricles (lower).
• The right side of the heart pumps deoxygenated blood, the left side pumps oxygenated blood.
• The pulmonary artery carries deoxygenated blood; the pulmonary vein carries oxygenated blood.
• The aorta is the largest artery in the body, carrying oxygenated blood out from the left ventricle.
• The vena cava brings deoxygenated blood into the right atrium.
• The heart's natural pacemaker is the SA node, located in the right atrium.
• Normal resting pulse rate is about 72 beats per minute.
• Average adult blood volume is roughly 5 to 5.5 litres.
• Blood has two main parts: plasma (about 55%) and blood cells (about 45%).
• RBCs carry oxygen using haemoglobin and live about 120 days.
• Mature human RBCs have no nucleus.
• WBCs fight infection and are the body's main defence cells.
• Platelets help blood clot at wound sites.
• Karl Landsteiner discovered the ABO blood group system in 1900-01 and won a Nobel Prize for it.
• O group is the universal donor; AB group is the universal recipient.
• The Rh factor marks blood as Rh-positive or Rh-negative.
• Normal blood pressure is 120/80 mm Hg.
• Systolic pressure is the higher reading (heart contracting); diastolic is the lower (heart relaxing).
• Blood pressure is measured using a sphygmomanometer.
• Hypertension is chronic high blood pressure, generally 140/90 mm Hg or above.
• The path of air is: nostrils, pharynx, larynx, trachea, bronchi, bronchioles, alveoli.
• Alveoli are the actual site of gas exchange in the lungs.
• Gas exchange in alveoli happens by diffusion.
• The diaphragm is the main muscle that drives breathing.
• Normal resting breathing rate is 12 to 16 breaths per minute.
• Exhaled air has far more carbon dioxide (about 4%) than inhaled air (about 0.04%).
• Anaemia is caused by low haemoglobin or RBC count.
• Asthma narrows the airways, causing wheezing and breathlessness.
• Leukaemia is cancer of the blood-forming tissue, marked by excess WBCs.
• Breathing is a mechanical act; respiration is the biochemical energy-release process inside cells.

Memory Tables

Table 1: Heart Chambers and Blood Vessels at a Glance

Chamber/Vessel	Blood Type Carried	Direction
Right atrium	Deoxygenated	Receives from body via vena cava
Right ventricle	Deoxygenated	Pumps to lungs via pulmonary artery
Left atrium	Oxygenated	Receives from lungs via pulmonary vein
Left ventricle	Oxygenated	Pumps to body via aorta
Vena cava	Deoxygenated	Body → right atrium
Pulmonary artery	Deoxygenated	Right ventricle → lungs
Pulmonary vein	Oxygenated	Lungs → left atrium
Aorta	Oxygenated	Left ventricle → body

Table 2: Blood Groups, Donors and Recipients

Blood Group	Can Donate To	Can Receive From	Special Status
A	A, AB	A, O	—
B	B, AB	B, O	—
AB	AB only	A, B, AB, O	Universal recipient
O	A, B, AB, O	O only	Universal donor

Table 3: Resting-Body Numbers Worth Memorising

Measurement	Normal Value
Blood volume (adult)	5 to 5.5 litres
Pulse/heart rate	72 beats per minute
Blood pressure	120/80 mm Hg
Breathing rate	12 to 16 breaths per minute
RBC lifespan	About 120 days

Practice MCQs

Q1. How many chambers does the human heart have? (a) Two (b) Three (c) Four (d) Six

Q2. Which blood vessel carries deoxygenated blood away from the heart? (a) Aorta (b) Pulmonary vein (c) Pulmonary artery (d) Vena cava

Q3. The red pigment in red blood cells that carries oxygen is called: (a) Melanin (b) Haemoglobin (c) Insulin (d) Keratin

Q4. Who discovered the ABO blood group system? (a) Edward Jenner (b) Karl Landsteiner (c) Louis Pasteur (d) Alexander Fleming

Q5. Which blood group is known as the universal donor? (a) AB (b) A (c) B (d) O

Q6. Normal blood pressure for a healthy adult is recorded as: (a) 80/120 mm Hg (b) 120/80 mm Hg (c) 100/60 mm Hg (d) 140/90 mm Hg

Q7. The site of gas exchange in human lungs is the: (a) Bronchi (b) Trachea (c) Alveoli (d) Larynx

Q8. Which muscle is primarily responsible for the mechanics of breathing? (a) Bicep (b) Diaphragm (c) Cardiac muscle (d) Trapezius

Q9. Normal resting pulse rate for an adult is approximately: (a) 40 beats per minute (b) 72 beats per minute (c) 100 beats per minute (d) 150 beats per minute

Q10. Which chamber of the heart pumps oxygenated blood to the entire body? (a) Right atrium (b) Right ventricle (c) Left atrium (d) Left ventricle

Q11. A person with which blood group can receive blood from all other groups? (a) O (b) A (c) B (d) AB

Q12. The instrument used to measure blood pressure is called a: (a) Stethoscope (b) Sphygmomanometer (c) Barometer (d) Thermometer

Q13. Anaemia is primarily caused by a deficiency related to: (a) Vitamin C (b) Haemoglobin/iron (c) Calcium (d) Vitamin D

Q14. The heart's natural pacemaker, which initiates each heartbeat, is the: (a) AV node (b) SA node (c) Septum (d) Aorta

Q15. Which of these correctly describes exhaled air compared to inhaled air? (a) Exhaled air has less carbon dioxide (b) Exhaled air has more oxygen (c) Exhaled air has more carbon dioxide (d) Exhaled air has the same composition

Answer Key

Q	Answer	Reason
Q1	(c) Four	The heart has two atria and two ventricles, forming a double-pump system that keeps oxygenated and deoxygenated blood separate.
Q2	(c) Pulmonary artery	It is the one exception where an artery carries deoxygenated blood, since it runs from the right ventricle to the lungs.
Q3	(b) Haemoglobin	This iron-containing protein in RBCs binds oxygen; its shortage causes anaemia.
Q4	(b) Karl Landsteiner	He identified the ABO system in 1900-01 and later won the Nobel Prize for this discovery.
Q5	(d) O	O-group RBCs carry no A or B antigens, so the recipient's immune system does not attack them.
Q6	(b) 120/80 mm Hg	Systolic (heart contracting) is always written first and is higher than diastolic (heart relaxing).
Q7	(c) Alveoli	These thin-walled, capillary-surrounded air sacs are where oxygen and carbon dioxide actually diffuse between air and blood.
Q8	(b) Diaphragm	Its contraction and relaxation change chest cavity volume, driving inhalation and exhalation.
Q9	(b) 72 beats per minute	This resting rate is set by the SA node and roughly matches the standard 120/80 blood pressure fact set.
Q10	(d) Left ventricle	It is the heart's strongest chamber because it must pump blood through the aorta to the whole body, not just to nearby lungs.
Q11	(d) AB	AB plasma carries no anti-A or anti-B antibodies, so it does not reject incoming blood from any ABO group.
Q12	(b) Sphygmomanometer	A stethoscope is used alongside it to listen for pulse sounds, but the pressure cuff device itself is the sphygmomanometer.
Q13	(b) Haemoglobin/iron	Low haemoglobin, most often from iron deficiency, reduces the blood's oxygen-carrying capacity, causing fatigue and paleness.
Q14	(b) SA node	Located in the right atrium, it fires the electrical signal that starts each heartbeat, distinct from the AV node which relays the signal onward.
Q15	(c) Exhaled air has more carbon dioxide	Cellular respiration produces CO2 as waste, raising exhaled CO2 from about 0.04% in inhaled air to roughly 4%.