With reference to a normal human being consider the following statements : I. Sugar appears in the urine if blood sugar exceeds 180 mg per 100 ml. II. Liver synthesises cholesterol and secretes it into blood. III. Blood plasma contains 92 per cent water and 8 per cent solids. IV. The total number of erythrocytes per microlitre of blood in adult male human beings averages 5-million. Which of these statements are correct?

1. Composition of Human Blood: Plasma vs. Formed Elements (basic)

Welcome to your first step in mastering human physiology! To understand how our body functions, we must start with the "river of life" — blood. At its most fundamental level, blood is a fluid connective tissue that acts as the body's primary transport system, carrying oxygen, nutrients, and waste products to and from every cell Science, Class X (NCERT 2025 ed.), Chapter 5, p.91. If you were to spin a tube of blood in a centrifuge, it would separate into two distinct layers: a straw-colored liquid called plasma and a dense collection of cells known as formed elements.

Plasma makes up about 55% of your total blood volume. Think of it as the watery matrix that keeps everything flowing. It is remarkably consistent, composed of approximately 90-92% water. The remaining 8-10% consists of vital solutes, including proteins (like albumin and fibrinogen), electrolytes (salts), and nutrients such as glucose. Plasma is the vehicle for transporting dissolved substances like carbon dioxide and nitrogenous wastes Science, Class X (NCERT 2025 ed.), Chapter 5, p.91.

The remaining 45% of blood consists of formed elements, which are the specialized cells suspended in the plasma. These include:

• Erythrocytes (Red Blood Cells/RBCs): These are the most numerous, with a healthy adult male averaging about 5 to 5.5 million RBCs per microlitre (mm³) of blood. Their primary job is to carry oxygen using a pigment called haemoglobin Science, Class X (NCERT 2025 ed.), Chapter 5, p.91.
• Leukocytes (White Blood Cells/WBCs): The "soldiers" of the immune system.
• Thrombocytes (Platelets): Small cell fragments essential for blood clotting and repair.

Understanding this balance is crucial because any deviation — such as a drop in RBC count or a change in plasma glucose levels — can significantly impact your health.

Component	Percentage of Blood	Primary Composition/Role
Plasma	~55%	90-92% Water; transports nutrients, CO₂, and urea.
Formed Elements	~45%	RBCs (Oxygen), WBCs (Immunity), Platelets (Clotting).

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91

2. Erythrocytes (RBCs): Structure and Average Counts (basic)

Erythrocytes, commonly known as Red Blood Cells (RBCs), are the most abundant cellular components of human blood. Their primary role is the transportation of oxygen from the lungs to the body's tissues and the return of carbon dioxide to the lungs. To perform this high-stakes logistics job, the RBC has a highly specialized structure. Unlike most cells in your body, a mature human erythrocyte is enucleated—meaning it lacks a nucleus. This absence, along with the lack of other organelles like mitochondria, provides maximum internal space for hemoglobin, the iron-rich protein that chemically binds to oxygen molecules.

The physical shape of an RBC is described as a biconcave disc (think of a donut where the hole didn't go all the way through). This specific geometry is brilliant for two reasons: it provides a high surface-area-to-volume ratio for efficient gas exchange, and it grants the cell incredible flexibility. This allows RBCs to twist and fold as they squeeze through tiny capillaries that are sometimes narrower than the cell itself. As noted in Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 91, the amount of hemoglobin within these cells is a critical indicator of health and can vary based on age and sex.

In terms of quantity, the blood is densely packed with these cells. The average RBC count in a healthy adult male is approximately 5 to 5.5 million per microliter (mm³) of blood. Women generally have a slightly lower count, typically ranging from 4.5 to 5.0 million per mm³. This difference is often attributed to hormonal influences (like testosterone stimulating RBC production) and physiological factors such as menstruation.

Feature	Description
Shape	Biconcave disc for flexibility and surface area.
Organelles	Lacks nucleus and mitochondria (in mature state) to maximize hemoglobin space.
Avg. Count (Male)	5.0 – 5.5 million cells per mm³.
Lifespan	Approximately 120 days.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.91

3. The Liver: Metabolism and Cholesterol Synthesis (intermediate)

Think of the liver as the body’s premier chemical refinery and distribution hub. While we often associate it with detoxification, its role in metabolism and cholesterol synthesis is what truly keeps our internal machinery running. One of its most vital contributions to digestion is the secretion of bile juice. Since the food arriving from the stomach is highly acidic, bile—which is mildly basic—neutralizes it so that pancreatic enzymes can function effectively Science, Class X (NCERT 2025 ed.), Chapter 5, p.86. Furthermore, bile contains salts that perform emulsification: they break down large, stubborn fat globules into tiny droplets, vastly increasing the surface area for enzymes to act upon Science-Class VII, NCERT (Revised ed 2025), Chapter 9, p.125.

Beyond digestion, the liver is the primary biosynthetic site for cholesterol. While we often hear about cholesterol in a negative light, it is actually a fundamental building block for cell membranes, Vitamin D, and steroid hormones (like cortisol and testosterone). The liver doesn't just create cholesterol; it acts as a logistics manager, packaging cholesterol and other lipids into specialized transport vehicles called lipoproteins (such as VLDL and LDL) to be sent into the bloodstream for use by various tissues.

In the broader metabolic context, the liver acts as a glucose reservoir. When blood sugar levels are high, the liver converts excess glucose into glycogen for storage. Conversely, when levels drop, it converts that glycogen back into glucose to maintain a steady supply of energy for the body Science, Class X (NCERT 2025 ed.), Chapter 6, p.111. This metabolic flexibility ensures that even when we aren't eating, our cells—especially our brain cells—have a constant fuel source.

Process	Description	Significance
Emulsification	Bile salts breaking fat globules into tiny droplets.	Essential for efficient fat digestion by lipase.
Glycogenesis	Conversion of glucose into stored glycogen.	Regulates blood sugar levels and prevents hyperglycemia.
Cholesterol Synthesis	Endogenous production of sterol lipids.	Provides raw material for hormones and cell membranes.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.86; Science-Class VII, NCERT (Revised ed 2025), Life Processes in Animals, p.125; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.111

4. The Excretory System: Mechanism of Urine Formation (intermediate)

The primary purpose of the human excretory system is to maintain homeostasis by filtering out metabolic waste products, particularly nitrogenous wastes like urea and uric acid, which are toxic if allowed to accumulate in the blood. This vital process occurs within the kidneys, located in the abdomen on either side of the backbone Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 96. The functional unit responsible for this filtration is the nephron. Each kidney contains millions of these microscopic units, which work like sophisticated chemical processing plants to separate waste from essential nutrients.

Urine formation occurs in two distinct, high-precision stages: Ultrafiltration and Selective Reabsorption. First, blood enters a cluster of very thin-walled capillaries called the glomerulus. Under high pressure, water and small solutes (like glucose, amino acids, and salts) are forced out of the blood into a cup-shaped structure called Bowman’s capsule. This "initial filtrate" is surprisingly massive—about 180 liters per day in a healthy adult Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 97. However, we only excrete about 1 to 2 liters of urine daily. The rest is recovered through selective reabsorption as the filtrate flows through the long tubular part of the nephron, where the body reclaims water and useful substances based on its current needs.

Feature	Initial Filtrate	Final Urine
Daily Volume	Approximately 180 Liters	1 to 2 Liters
Composition	Water, Glucose, Amino acids, Salts, Urea	Water, Urea, Excess Salts, Metabolic wastes
Purpose	Raw separation of small molecules from blood	Concentrated waste ready for excretion

Once the urine is formed, it travels through the ureters to the urinary bladder. The bladder is a muscular reservoir that stores urine until it is full. Because the bladder is under nervous control, we can consciously regulate the urge to pass urine through the urethra Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p. 97. A critical aspect of this system is the renal threshold: for instance, glucose is usually 100% reabsorbed, but if blood sugar levels exceed a certain limit (approx. 180 mg/dL), the kidneys cannot reabsorb it all, and glucose begins to appear in the urine.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.96; Science, Class X (NCERT 2025 ed.), Chapter 5: Life Processes, p.97

5. Renal Threshold and Glycosuria (exam-level)

To understand how our kidneys manage sugar, we must first look at the nephron, the functional unit of the kidney. During the process of filtration, blood enters the Bowman’s capsule, where a liquid called 'initial filtrate' is formed. This filtrate contains waste products, but also highly valuable substances like glucose, amino acids, and salts Science, Class X, Chapter 5, p.97. Under normal conditions, the body cannot afford to lose these nutrients, so as the filtrate flows through the renal tubules, these substances are selectively re-absorbed back into the bloodstream. However, the kidney's ability to reabsorb glucose is not infinite. Think of the transport proteins in the tubule walls as a series of 'conveyor belts.' If the concentration of glucose in the blood (and thus the filtrate) is normal, the belts can pick up every single molecule. But if the blood sugar level rises significantly—specifically above the Renal Threshold of approximately 180 mg/dL—the conveyor belts become 'saturated' Science, Class VIII, Chapter 10, p.150. This physiological limit is known as the Transport Maximum (Tₘ). Once this limit is reached, the excess glucose cannot be reabsorbed and is instead excreted in the urine, a condition known as Glycosuria.

Condition	Blood Glucose Level	Kidney Action	Urine Result
Normal Range	70–140 mg/dL	Complete Reabsorption	No Glucose
Above Threshold	>180 mg/dL	Saturation of Transporters	Glycosuria (Glucose in urine)

Glycosuria is a primary clinical sign of Diabetes Mellitus. When the pancreas fails to produce enough insulin or the body becomes resistant to it, blood sugar levels skyrocket. The kidneys try their best to reclaim the sugar, but once the 180 mg/dL threshold is breached, the sugar 'spills over' into the urine. Because glucose is osmotically active, it also pulls water with it, leading to increased frequency of urination and dehydration.

Sources: Science, Class X, Chapter 5: Life Processes, p.97; Science, Class VIII, Chapter 10: The Amazing World of Solutes, Solvents, and Solutions, p.150

6. Homeostasis and Biological Constants (exam-level)

To understand how the human body functions as a high-precision machine, we must first master the concept of Homeostasis. Derived from the Greek words for 'same' and 'steady,' homeostasis is the process by which biological systems maintain internal stability while adjusting to changing external conditions. Think of your body like a sophisticated smart home: if the temperature rises, the AC kicks in; if it gets too dark, the lights brighten. In our bodies, this 'smart' regulation is primarily managed through Feedback Mechanisms. For instance, the timing and amount of hormones released are strictly regulated; if blood sugar levels rise, they are detected by the pancreas, which responds by producing more insulin to bring the levels back to a 'set point' Science, class X (NCERT 2025 ed.), Control and Coordination, p.111.

When these feedback loops operate correctly, the body maintains specific Biological Constants—numerical ranges that signify a healthy state. For a UPSC aspirant, knowing these 'normal' values is essential for understanding pathology and health. For example, the renal threshold for glucose is approximately 180 mg/dL. This means the kidneys are designed to reabsorb all glucose back into the blood, but if sugar levels exceed this threshold, the transport capacity is overwhelmed, and sugar begins to appear in the urine—a condition known as glycosuria Science, class X (NCERT 2025 ed.), Control and Coordination, p.110.

Beyond sugar regulation, our internal environment is defined by several other critical constants. The composition of our blood is a prime example: Blood Plasma is remarkably consistent, consisting of roughly 90–92% water and 8–10% solutes like proteins and electrolytes. Similarly, the Erythrocyte (RBC) count in a healthy adult male typically stays within the narrow window of 5 to 5.5 million per microlitre (mm³). Even our cholesterol levels are tightly managed, with the liver acting as the primary factory for its biosynthesis and distribution via lipoproteins. When these constants deviate significantly, it signals a breakdown in homeostasis, often resulting in disease.

Biological Metric	Standard Constant / Value	Key Regulator
Blood Plasma Water Content	90–92%	Kidneys / ADH Hormone
Average RBC Count (Male)	5.0 – 5.5 million/mm³	Bone Marrow / Erythropoietin
Renal Threshold (Glucose)	~180 mg/dL	Kidney Tubules
Primary Cholesterol Site	Biosynthesis in Liver	HMG-CoA Reductase Enzyme

Sources: Science, class X (NCERT 2025 ed.), Control and Coordination, p.110; Science, class X (NCERT 2025 ed.), Control and Coordination, p.111

7. Solving the Original PYQ (exam-level)

This question serves as a comprehensive review of human physiology, bringing together the building blocks you have studied regarding the excretory, circulatory, and metabolic systems. Statement I tests your understanding of renal threshold and selective reabsorption in the nephrons; when blood glucose levels surpass approximately 180 mg/dL, the transport capacity of the kidneys is overwhelmed, leading to glycosuria. Meanwhile, Statement II highlights the liver's role as a central metabolic hub, responsible for synthesizing cholesterol and packaging it into the blood for cellular use. These concepts, detailed in Science, class X (NCERT 2025 ed.), demonstrate how our internal organs maintain homeostasis through precise chemical regulation.

To reach the correct answer, Option (D), you must evaluate the standard physiological constants of the blood. Statement III correctly identifies that blood plasma is roughly 92 percent water, providing the necessary fluidity for transport, while Statement IV provides the standard clinical average for erythrocyte (RBC) counts in adult males—approximately 5 million per microlitre. UPSC often uses these specific numerical benchmarks to test your confidence in core factual data. If you found yourself doubting the exactness of "92%" or "180 mg," remember that these are the standard values used to define a "normal" state in biological literature.

The common trap in this question type is second-guessing specific numbers. Options (A), (B), and (C) are designed to make you feel that one of these pillars of physiology must be an outlier or an error. However, in UPSC General Science, when statements describe fundamental, non-conflicting biological constants, they are typically all correct. By verifying that each statement aligns with the foundational principles of life processes and control systems found in your preparatory material, you can confidently conclude that all four statements are accurate representations of human biology.