Which one of the following is considered normal blood pressure in man?

1. The Human Circulatory System: An Overview (basic)

In our body, the human circulatory system functions as a sophisticated internal transport network, ensuring that every cell receives the 'supplies' it needs and disposes of its 'trash.' As you'll find in Science-Class VII, Life Processes in Animals, p.133, this system is comprised of three primary components: the heart (the pump), blood (the carrier medium), and blood vessels (the delivery pipes). While it transports nutrients and oxygen to tissues, it also plays the vital role of carrying waste products, like carbon dioxide and nitrogenous compounds, away to the excretory organs. One of the most remarkable features of our system is called Double Circulation. Unlike fish, which have a two-chambered heart where blood passes through the heart only once per cycle, humans and other mammals have a four-chambered heart. This ensures that blood passes through the heart twice during one complete trip through the body: once to be pumped to the lungs for oxygen, and a second time to be pumped out to the rest of the body Science, Class X, Life Processes, p.92. This separation of oxygen-rich and oxygen-poor blood is what makes our metabolism so efficient. The mechanics of this pump involve four chambers: the atria (upper chambers) and the ventricles (lower chambers). The ventricles have much thicker muscular walls than the atria because they must exert enough force to push blood to distant organs Science, Class X, Life Processes, p.92. This force exerted against the arterial walls is measured as Blood Pressure. In a healthy adult, the standard reading is 120/80 mm Hg. The '120' (systolic) reflects the pressure during heart contraction, while the '80' (diastolic) reflects the pressure when the heart relaxes Science, Class X, Life Processes, p.93.

Feature	Single Circulation (e.g., Fish)	Double Circulation (e.g., Humans)
Heart Chambers	Two	Four
Passage through heart	Once per cycle	Twice per cycle
Efficiency	Lower (Oxygenated/Deoxygenated blood mixes more)	Higher (Strict separation of blood types)

Sources: Science-Class VII, Life Processes in Animals, p.133; Science, Class X, Life Processes, p.92; Science, Class X, Life Processes, p.93

2. Composition and Functions of Blood (basic)

Blood is much more than just a red liquid; it is a specialized fluid connective tissue that acts as the body's vital transport and repair system. To understand blood from first principles, imagine it as a complex delivery network where a liquid "highway" carries specialized "vehicles" to every corner of the body Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91.

The foundation of blood is plasma, a fluid medium in which various cells are suspended. Plasma is responsible for transporting substances that dissolve easily in water, such as nutrients (food), carbon dioxide, and nitrogenous wastes. While the plasma handles these dissolved materials, the Red Blood Corpuscles (RBCs) serve a specialized role: they carry life-sustaining oxygen to all tissues. This division of labor ensures that every cell receives energy and gets rid of waste efficiently Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91.

Beyond transportation, blood has an ingenious "self-repair" mechanism. If our system of blood vessels develops a leak due to an injury, it can lead to a dangerous drop in pressure, making the heart work less efficiently. To prevent this, blood contains platelets. These cells circulate throughout the body and act as a mobile repair crew, plugging leaks by helping the blood clot at the site of injury Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94.

Component	Primary Function
Plasma	Transports food, CO₂, salts, and nitrogenous wastes in dissolved form.
Red Blood Corpuscles (RBCs)	Transports Oxygen throughout the body.
Platelets	Clots blood to plug leaks and prevent blood loss.

Finally, for this system to work, the blood must stay under a certain amount of force, known as blood pressure. In a healthy adult, this is typically 120/80 mm Hg. The higher number (systolic) represents the pressure when the heart contracts, while the lower number (diastolic) is the pressure when the heart rests between beats Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.93.

Sources: Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91; Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.93; Science, class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.94

3. Anatomy of the Heart and Cardiac Cycle (intermediate)

The human heart is a sophisticated muscular organ designed to act as a double pump, ensuring that oxygen-rich blood and oxygen-poor blood remain strictly separated. It is divided into four chambers: the upper atria (singular: atrium) and the lower ventricles. To ensure blood moves in only one direction and does not leak backward when these chambers contract, the heart is equipped with valves Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 92.

The movement of blood throughout the body depends on the type of vessel it travels through. Arteries carry blood away from the heart. Because the heart pumps blood into them with significant force, arteries have thick, elastic walls to withstand high pressure. In contrast, veins collect blood from the body and bring it back to the heart. Since the pressure is much lower in veins, they have thinner walls but possess internal valves to prevent the backflow of blood against gravity Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 93.

Feature	Arteries	Veins
Direction	Away from the heart	Towards the heart
Wall Thickness	Thick and elastic	Thin
Valves	Absent (except at heart exit)	Present to prevent backflow
Pressure	High	Low

The rhythmic sequence of contraction and relaxation is known as the Cardiac Cycle. This cycle is measured via blood pressure, typically expressed as 120/80 mm Hg. The higher number, systolic pressure, occurs during the contraction phase when the heart forcefully pushes blood into the arteries. The lower number, diastolic pressure, occurs when the heart relaxes between beats and refills with blood Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 93.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.92; Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.93

4. Blood Groups and Rh Factor (intermediate)

While blood looks identical under a microscope, it is chemically distinct based on specific proteins called antigens found on the surface of Red Blood Cells (RBCs). Blood is a complex fluid connective tissue that transports oxygen and nutrients Science, Class X (NCERT 2025 ed.), Life Processes, p.91, but its "identity" is determined by the ABO system. In this system, your blood group (A, B, AB, or O) depends on whether you have Antigen A, Antigen B, both, or neither. Crucially, your body also produces antibodies in the plasma that act as a defense mechanism against any antigen you do not possess. For instance, a person with Blood Group A has A-antigens but carries anti-B antibodies.

The Rh Factor (Rhesus factor) adds another layer of classification. It is a specific protein (the D-antigen) that is either present (Rh positive) or absent (Rh negative). This creates the eight common blood types we know today (A+, A-, B+, etc.). Understanding these types is vital for safe blood transfusions. If incompatible blood is mixed, the recipient’s antibodies will attack the donor cells, causing dangerous clumping or agglutination. Because Group O negative lacks A, B, and Rh antigens, it is the "Universal Donor," while Group AB positive, which lacks antibodies against these markers, is the "Universal Recipient."

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

From a genetic perspective, blood groups are inherited traits where some alleles are dominant over others. For example, the alleles for A and B are co-dominant, while the allele for O is recessive Science, Class X (NCERT 2025 ed.), Heredity, p.133. This is why two parents with Blood Group A could potentially have a child with Blood Group O, provided both parents carry the recessive O allele. Clinical awareness of the Rh factor is also essential during pregnancy; if an Rh-negative mother carries an Rh-positive fetus, it can lead to Rh incompatibility, where the mother’s immune system may attack the baby's red blood cells in subsequent pregnancies.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.91; Science, Class X (NCERT 2025 ed.), Heredity, p.133

5. The Lymphatic System: The Other Fluid (intermediate)

While we often focus on the heart and blood vessels, our bodies possess a secondary, silent circulatory network known as the lymphatic system. Think of blood as the main delivery highway and the lymphatic system as the critical drainage and recycling network. As blood flows through the narrow capillaries under pressure, some amount of plasma, proteins, and even some blood cells leak out through tiny pores in the capillary walls into the spaces between cells. This fluid, once it leaves the blood vessels, is known as tissue fluid or lymph Science, Class X (NCERT 2025 ed.), Life Processes, p.94.

Lymph is remarkably similar to the plasma found in blood, but with two key differences: it is colorless (as it lacks red blood cells) and contains significantly less protein. This fluid doesn't just sit there; it is collected by a specialized network. It drains from the intercellular spaces into lymphatic capillaries, which merge into larger lymph vessels. These vessels eventually dump their contents back into the major veins, effectively returning the "leaked" fluid to the main bloodstream Science, Class X (NCERT 2025 ed.), Life Processes, p.94.

Feature	Blood	Lymph
Color	Red (due to Hemoglobin)	Colorless
Protein Content	High	Low
Direction of Flow	Circular (Heart → Body → Heart)	One-way (Tissues → Heart)

Beyond fluid balance, the lymphatic system plays a vital role in nutrition. While the blood carries most nutrients, digested and absorbed fats from the intestine are too large to enter the blood capillaries directly. Instead, they are picked up by the lymphatic system and transported into the circulation. This ensures that every bit of energy we consume is effectively utilized Science, Class X (NCERT 2025 ed.), Life Processes, p.94.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.94

6. Blood Pressure: Systole, Diastole, and Measurement (exam-level)

To understand blood pressure, we must first look at the heart as a rhythmic pump. As blood is forced out of the heart and into the network of tubes (arteries) that reach all our tissues, it exerts a force against the walls of these vessels. This force is what we call blood pressure. It is important to note that this pressure is significantly greater in the arteries than in the veins because arteries receive blood directly from the heart's high-pressure pumping chambers Science, Class X, Life Processes, p. 93.

Blood pressure is recorded using two distinct values, representing the two phases of the cardiac cycle:

• Systolic Pressure: This is the maximum pressure in the arteries during ventricular systole (when the heart muscles contract to push blood out). In a healthy adult, this is typically around 120 mm Hg.
• Diastolic Pressure: This is the minimum pressure in the arteries during ventricular diastole (when the heart muscle relaxes and the chambers refill with blood). A normal reading is about 80 mm Hg.

We measure this vital sign using an instrument called a sphygmomanometer. While modern digital monitors are popular, the universal standard unit remains mm Hg (millimeters of mercury). When blood pressure remains consistently high—a condition known as hypertension—it is often due to the constriction of arterioles (tiny branches of arteries). This constriction increases the resistance to blood flow, forcing the heart to work harder and potentially damaging the vessel walls over time Science, Class X, Life Processes, p. 93.

Feature	Systolic Pressure	Diastolic Pressure
Heart Status	Ventricular Contraction	Ventricular Relaxation
Normal Value	~120 mm Hg	~80 mm Hg
Vessel Impact	Peak force on arterial walls	Resting force between beats

Sources: Science, Class X, Life Processes, p.93; Science, Class X, Life Processes, p.91

7. Solving the Original PYQ (exam-level)

Now that you have mastered the mechanics of the circulatory system and the cardiac cycle, this question brings those concepts into a clinical context. You have learned that the heart functions as a dual-action pump, creating pressure during contraction (systole) and maintaining a baseline pressure during relaxation (diastole). As noted in Science, class X (NCERT 2025 ed.), these phases are represented by the values 120 and 80 respectively. This question tests whether you can distinguish the standardized unit of measurement from the physical components of the circulatory system itself.

To arrive at the correct answer, you must recall that blood pressure is traditionally measured using a sphygmomanometer, which utilizes a column of mercury (Hg) because of its high density and stable properties. Thinking like a coach: when you see the numbers 120/80, your first instinct should be to look for the unit "mm Hg." Therefore, (C) 120/80 mm mercury is the only scientifically accurate option. The mercury scale allows for a precise reading of the force exerted by the blood against the arterial walls, which is essential for diagnosing hypertension or hypotension.

UPSC frequently uses "familiarity traps" by pairing the correct numerical values with plausible-sounding but incorrect units. Options like "mm water" or "mm blood" are decoys; while blood is the fluid being measured and air is often used to inflate the pressure cuff, neither serves as the universal manometric standard for blood pressure. Always remember to verify the unit of measurement as carefully as the numbers themselves, as this is a common strategy used to catch students who rely on rote memorization rather than conceptual precision.