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PNG Grade 10 Science study materials
PNG Grade 10 Science study resources

PNG Grade 10 Science practice questions

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Question 1Biology / Life Science

Which part of a plant cell controls the movement of substances into and out of the cell?

  1. Cell wall
  2. Cell membrane Correct answer.
  3. Large vacuole
  4. Chloroplast

Explanation

The cell membrane is a thin, partially permeable layer of lipid and protein that regulates which substances enter or leave the cell. The cell wall lies outside it and, being fully permeable, does not control entry.

Question 2Biology / Life Science

During photosynthesis a green plant releases which gas into the surrounding air?

  1. Nitrogen
  2. Oxygen Correct answer.
  3. Hydrogen
  4. Carbon dioxide

Explanation

Chlorophyll traps light energy, which is used to split water molecules and to combine hydrogen with carbon dioxide to form glucose. Oxygen is released as a by-product through the stomata.

Question 3Biology / Life Science

A blood vessel has thick elastic muscular walls and carries blood away from the heart under high pressure. Which vessel is described?

  1. Capillary
  2. Lymph vessel
  3. Artery Correct answer.
  4. Vein

Explanation

Arteries have thick walls with elastic and muscular layers that stretch and recoil as the heart pumps, so they can withstand high pressure. Veins carry blood back to the heart at low pressure and have thinner walls with valves.

Question 4Biology / Life Science

Salivary amylase begins the chemical digestion of starch in the mouth. What is the product of this reaction?

  1. Maltose Correct answer.
  2. Glucose
  3. Amino acids
  4. Fatty acids and glycerol

Explanation

Amylase breaks the long starch chain into maltose, a disaccharide; a second enzyme, maltase, later splits maltose into glucose in the small intestine. Amino acids and fatty acids come from other food types and other enzymes.

Question 5Biology / Life Science

In guinea pigs, black coat colour (B) is dominant to white (b). Two heterozygous black guinea pigs are crossed. What fraction of the offspring is expected to be white?

  1. 1/4 Correct answer.
  2. 1/2
  3. 3/4
  4. All of them

Explanation

A cross of Bb with Bb gives the genotypes BB, Bb, Bb and bb in the ratio 1:2:1. Only the double-recessive bb offspring show the white coat, so one quarter of the young are expected to be white.

Question 6Biology / Life Science

Which statement correctly describes the normal transport of water in a flowering plant?

  1. Sugars move up the xylem from the roots to the leaves
  2. Water moves up the phloem from the roots to the leaves
  3. Water moves up the xylem from the roots to the leaves Correct answer.
  4. Oxygen moves up the xylem from the roots to the leaves

Explanation

Xylem tissue carries water and dissolved mineral ions upward from the roots to the leaves, driven mainly by the transpiration pull created as water evaporates from the leaves. Phloem, by contrast, carries dissolved sugars made in the leaves to the rest of the plant.

Question 7Biology / Life Science

Decomposers such as bacteria and fungi are important in an ecosystem because they

  1. prevent energy loss from the food chain
  2. convert light energy into chemical energy
  3. release mineral ions into the soil Correct answer.
  4. supply energy directly to primary consumers

Explanation

Decomposers break down dead organisms and waste, releasing mineral ions and other nutrients back into the soil so that producers can absorb them and reuse them. They do not create energy or feed energy directly to consumers.

Question 8Biology / Life Science

During vigorous exercise, human muscle cells may respire anaerobically. Which product is formed in this process?

  1. Carbon dioxide and water
  2. Lactic acid and carbon dioxide
  3. Ethanol and carbon dioxide
  4. Lactic acid Correct answer.

Explanation

In human muscle, anaerobic respiration breaks glucose down into lactic acid without using oxygen and releases only a small amount of energy. Carbon dioxide and water are produced in aerobic respiration, and ethanol is formed in yeast.

Question 9Biology / Life Science

A man of blood group AB has children with a woman of blood group O. Which blood groups are possible among their children?

  1. AB and O only
  2. O only
  3. A and B only Correct answer.
  4. A, B, AB and O

Explanation

The father's genotype is IAIB, so each sperm carries either IA or IB, while the mother's genotype ii means every egg carries i. The children are therefore IAi (group A) or IBi (group B) in roughly equal numbers, so groups AB and O cannot occur.

Question 10Biology / Life Science

On a beach most mice are light coloured, but a few are dark. A dark lava flow covers part of the beach, and hawks hunt the mice they can see most easily against the background. Which statement best predicts what will happen over many generations?

  1. Hawks stop hunting mice once the lava has cooled
  2. Light mice darken during their lifetime to match the lava
  3. Dark mice on the lava survive and leave more offspring Correct answer.
  4. All mice on the lava become dark within one generation

Explanation

Dark mice are better camouflaged on the lava and are less likely to be caught, so they survive longer and produce more offspring, passing on the dark allele. The frequency of the dark allele therefore rises over many generations through natural selection.

Question 11Biology / Life Science

Which organelle in a cell is the main site of aerobic respiration and releases energy from glucose?

  1. Chloroplast
  2. Ribosome
  3. Golgi body
  4. Mitochondrion Correct answer.

Explanation

Mitochondria contain the enzymes needed for aerobic respiration, which releases usable energy from glucose. Chloroplasts carry out photosynthesis, while ribosomes make proteins and Golgi bodies modify and package materials.

Question 12Biology / Life Science

After digestion, most of the water from food is absorbed back into the blood in which part of the human alimentary canal?

  1. Small intestine
  2. Oesophagus
  3. Stomach
  4. Large intestine Correct answer.

Explanation

The large intestine absorbs most of the remaining water from undigested food, helping to form faeces. The small intestine absorbs digested nutrients, but most water reabsorption occurs later in the colon.

Question 13Biology / Life Science

An organism that uses sunlight to make its own food is best described as a

  1. primary consumer
  2. producer Correct answer.
  3. carnivore
  4. decomposer

Explanation

Producers, such as green plants and algae, trap light energy by photosynthesis and build organic molecules. Consumers must eat other organisms, and decomposers break down dead material rather than making food from sunlight.

Question 14Biology / Life Science

Within a DNA molecule, which base always pairs with adenine on the opposite strand?

  1. Thymine Correct answer.
  2. Guanine
  3. Uracil
  4. Cytosine

Explanation

In DNA, adenine forms hydrogen bonds with thymine, while cytosine pairs with guanine. Uracil is found in RNA and replaces thymine there, so it does not pair with adenine in DNA.

Question 15Biology / Life Science

Which component of human blood is mainly responsible for carrying oxygen from the lungs to body tissues?

  1. Platelets
  2. Plasma
  3. White blood cells
  4. Red blood cells Correct answer.

Explanation

Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in respiring tissues. White blood cells defend against pathogens, platelets help clotting, and plasma transports dissolved substances.

Question 16Biology / Life Science

Pollination in flowering plants is best defined as the transfer of

  1. male gametes from the pollen tube to the ovule
  2. pollen from the anther to the stigma Correct answer.
  3. seeds from the ovary to the soil
  4. water through the xylem to the leaves

Explanation

Pollination is the transfer of pollen grains from a flower's anther to a stigma, either by wind, insects, or other agents. Fertilisation happens later, when a male gamete from the pollen tube fuses with an ovule.

Question 17Biology / Life Science

On a hot day, a person's body temperature is kept near 37 °C. Which response helps to lower body temperature?

  1. Sweating and evaporation from the skin Correct answer.
  2. Shivering by skeletal muscles
  3. Erection of body hair
  4. Narrowing of skin arterioles

Explanation

Sweat glands release water onto the skin, and as this water evaporates it takes heat from the body, cooling it. Shivering and narrowing skin arterioles conserve heat instead, which is useful in cold conditions.

Question 18Biology / Life Science

In a reflex action, which part of the nervous system detects the stimulus that starts the response?

  1. Effector
  2. Synapse
  3. Receptor Correct answer.
  4. Motor neurone

Explanation

A receptor is a specialised cell or organ, such as a pain receptor in the skin, that detects a stimulus and starts a nerve impulse. The effector is the muscle or gland that responds, while a synapse is a junction between neurones.

Question 19Biology / Life Science

In humans, which statement correctly explains why the sex of a baby is determined by the father?

  1. The mother produces only X-bearing eggs, while the father produces X- and Y-bearing sperm. Correct answer.
  2. Both parents produce only Y-bearing gametes, so the mother's egg decides the sex.
  3. The father's diet during pregnancy controls whether an X or Y sperm fertilises the egg.
  4. The mother produces both X- and Y-bearing eggs, while the father produces only X-bearing sperm.

Explanation

Human females have XX sex chromosomes and produce eggs that all carry an X chromosome. Males have XY and produce sperm that carry either X or Y; a Y-bearing sperm fertilising the egg gives an XY male, while an X-bearing sperm gives an XX female.

Question 20Biology / Life Science

When the water content of the blood falls, the pituitary gland releases more ADH. What effect does ADH have on the kidney?

  1. It decreases water reabsorption, so a larger volume of dilute urine is produced.
  2. It stops the kidneys filtering blood completely during dehydration.
  3. It increases glucose reabsorption from the kidney tubules into the blood.
  4. It increases water reabsorption from the collecting ducts into the blood. Correct answer.

Explanation

ADH makes the walls of the collecting ducts more permeable to water, so more water is reabsorbed into the blood and a smaller volume of concentrated urine is produced. This negative feedback helps restore the blood water content toward normal.

Question 21Biology / Life Science

Palisade mesophyll cells in a leaf contain many chloroplasts. Which pigment inside the chloroplast absorbs the light energy used in photosynthesis?

  1. Haemoglobin
  2. Melanin
  3. Keratin
  4. Chlorophyll Correct answer.

Explanation

Chlorophyll is the green pigment in chloroplasts that absorbs mainly red and blue light. The absorbed light energy is then used to drive the reactions of photosynthesis, which produce glucose and release oxygen.

Question 22Biology / Life Science

Bile is made in the liver and stored in the gall bladder before it is released into the small intestine. What is the main function of bile in digestion?

  1. It carries absorbed glucose into the blood.
  2. It emulsifies fats into smaller droplets. Correct answer.
  3. It breaks down starch into maltose molecules.
  4. It digests proteins into amino acids.

Explanation

Bile salts break large fat droplets into many smaller ones, increasing the surface area for lipase to act on. Bile is not an enzyme, so it does not chemically digest fats, proteins or starch itself.

Question 23Biology / Life Science

Root hair cells are specialised cells in the root epidermis. Which feature is an adaptation that helps these cells absorb water and mineral ions from the soil?

  1. A long narrow extension that increases surface area Correct answer.
  2. A rigid secondary wall that strengthens the cell
  3. A large number of chloroplasts in the cell
  4. A thick waterproof cuticle covering the cell

Explanation

The long, narrow extension of a root hair cell gives it a large surface area in contact with soil water. This increases the rate at which water enters by osmosis and mineral ions are taken up, including by active transport.

Question 24Biology / Life Science

In snapdragon plants, flower colour shows incomplete dominance. Red-flowered plants (RR) crossed with white-flowered plants (WW) produce pink-flowered offspring (RW). If two pink-flowered plants are crossed, what fraction of the offspring is expected to have red flowers?

  1. One-quarter (1/4) Correct answer.
  2. Three-quarters (3/4)
  3. One-half (1/2)
  4. All of the offspring (1)

Explanation

A cross between two RW plants gives the genotypes RR, RW, RW and WW in equal proportions. Only the RR genotype produces red flowers, so one-quarter of the offspring are expected to be red.

Question 25Biology / Life Science

Sucrose made in a leaf is transported to a developing root. Which tissue carries out this transport, and in which direction does the sucrose move?

  1. Xylem, from the root to the leaf
  2. Phloem, from the leaf to the root Correct answer.
  3. Xylem, from the leaf to the root
  4. Phloem, from the root to the leaf

Explanation

Phloem tissue translocates sugars from a source, such as a photosynthesising leaf, to a sink, such as a growing root or storage organ. Xylem carries water and mineral ions upward from the roots to the shoots.

Question 26Biology / Life Science

The muscular wall of the left ventricle is much thicker than the muscular wall of the right ventricle. Which statement best explains this difference?

  1. The left ventricle holds a larger volume of blood than the right ventricle.
  2. The left ventricle pumps blood only to the lungs at low pressure.
  3. The left ventricle must pump blood at high pressure around the whole body. Correct answer.
  4. The left ventricle receives oxygenated blood directly from the lungs.

Explanation

The left ventricle pumps blood into the aorta and around the systemic circulation, which has greater resistance than the pulmonary circuit. Its thicker muscle generates the higher pressure needed, while the right ventricle pumps blood only to the lungs.

Question 27Biology / Life Science

The walls of the alveoli are one cell thick and are surrounded by a dense network of capillaries. How does this structure help gas exchange in the lungs?

  1. It traps dust and bacteria before they can reach the blood.
  2. It increases the total volume of air stored in the lungs.
  3. It gives a short diffusion distance and a steep concentration gradient. Correct answer.
  4. It actively pushes oxygen from the air into the blood.

Explanation

A one-cell-thick wall creates a very short distance for oxygen and carbon dioxide to diffuse across. The dense capillary network carries blood with a different oxygen concentration, maintaining a steep gradient that keeps diffusion rapid.

Question 28Biology / Life Science

A vaccine contains weakened or inactive pathogens. After vaccination, why does a person usually not become ill when exposed to the active form of the same pathogen later?

  1. The body stops making antibodies so the pathogen cannot use them.
  2. The pathogen cannot enter the body after a person has been vaccinated.
  3. Memory cells recognise the pathogen and respond quickly and strongly. Correct answer.
  4. The vaccine removes all pathogens from the body before they can multiply.

Explanation

Vaccination stimulates an immune response and produces memory B and T cells. If the active pathogen enters later, these memory cells recognise it and produce a rapid, large response that destroys it before illness develops.

Question 29Biology / Life Science

A student adds catalase from potato to hydrogen peroxide at 20 °C and records rapid oxygen production. The mixture is then heated to 80 °C and kept at that temperature. When fresh hydrogen peroxide is added, no oxygen is produced. Which explanation is most likely?

  1. The enzyme has been denatured, so its active site no longer fits the substrate. Correct answer.
  2. The hydrogen peroxide has all been used up by the first reaction.
  3. The substrate has been broken down into water and oxygen.
  4. The enzyme now works faster but produces a different gas.

Explanation

High temperature changes the shape of the enzyme's active site, so the substrate can no longer bind and the reaction stops. Fresh hydrogen peroxide is still present, but the denatured catalase cannot break it down.

Question 30Biology / Life Science

After a meal rich in starch, a person's blood glucose concentration rises. Which sequence correctly describes how the body returns blood glucose to its normal level?

  1. The pancreas releases insulin, and liver and muscle cells take up glucose. Correct answer.
  2. The pancreas releases insulin, and the liver breaks down glycogen.
  3. The pancreas releases glucagon, and the liver breaks down glycogen.
  4. The pancreas releases glucagon, and body cells take up glucose.

Explanation

A rise in blood glucose is detected by the pancreas, which releases insulin. Insulin stimulates liver and muscle cells to take up glucose and store it as glycogen, so blood glucose concentration falls back towards normal.

Question 31Biology / Life Science

Which part of a flowering plant produces the pollen grains that contain male gametes?

  1. Anther Correct answer.
  2. Stigma
  3. Sepal
  4. Ovary

Explanation

The anther is the top part of the stamen and contains pollen sacs where pollen grains, which carry male gametes, are formed. The ovary contains ovules, the stigma receives pollen, and sepals protect the flower bud.

Question 32Biology / Life Science

Which blood vessel carries oxygenated blood from the lungs back to the heart?

  1. Vena cava
  2. Pulmonary vein Correct answer.
  3. Pulmonary artery
  4. Aorta

Explanation

The pulmonary vein returns oxygenated blood from the lungs to the left atrium of the heart. The pulmonary artery carries deoxygenated blood to the lungs, while the aorta and vena cava serve the systemic circulation.

Question 33Biology / Life Science

Which gas is released into the atmosphere by a green plant during photosynthesis?

  1. Hydrogen
  2. Carbon dioxide
  3. Oxygen Correct answer.
  4. Nitrogen

Explanation

During photosynthesis, chlorophyll traps light energy and water is split, releasing oxygen as a by-product. Carbon dioxide is taken in, not released, during this process.

Question 34Biology / Life Science

In a food chain, only about 10% of the energy at one trophic level is passed to the next level. Which statement best explains why most food chains have only four or five trophic levels?

  1. Producers cannot make enough glucose
  2. Energy is destroyed when organisms are eaten
  3. Energy is stored as fat by consumers
  4. Most energy is lost as heat Correct answer.

Explanation

At each trophic level, organisms use much of the energy they obtain for respiration, and a large amount is lost as heat. Because only a small fraction remains to be passed on, there is too little energy to support many higher trophic levels.

Question 35Biology / Life Science

A person with type 1 diabetes cannot produce enough insulin. Which treatment directly replaces the missing hormone?

  1. Antibiotic tablets
  2. Vitamin supplements
  3. Blood transfusions
  4. Insulin injections Correct answer.

Explanation

Type 1 diabetes results from the pancreas not producing enough insulin, so insulin must be injected to help cells take up glucose from the blood. Antibiotics kill bacteria, vitamins are nutrients, and blood transfusions replace blood cells or volume, not insulin.

Question 36Biology / Life Science

Damage to the nephrons in a human kidney would most directly affect which function?

  1. Digesting proteins in the stomach
  2. Pumping blood around the body
  3. Filtering waste from the blood Correct answer.
  4. Exchanging gases in the lungs

Explanation

Nephrons are the filtering units of the kidney; they remove urea and excess salts and water from the blood to form urine. Damage to them therefore reduces the kidney's ability to filter waste, while the other options describe the heart, stomach and lungs.

Question 37Biology / Life Science

A plant stem growing near a window bends toward the light. Which type of tropism is shown?

  1. Geotropism
  2. Thigmotropism
  3. Hydrotropism
  4. Phototropism Correct answer.

Explanation

Phototropism is the growth response of a plant toward or away from light. The stem bends toward the window because cells on the shaded side elongate more, causing the shoot to grow in the direction of the light.

Question 38Biology / Life Science

During vigorous exercise, a muscle cell may respire anaerobically. Which substance builds up in the muscle cell under these conditions?

  1. Oxygen molecules
  2. Ethanol (alcohol)
  3. Glucose molecules
  4. Lactic acid Correct answer.

Explanation

In human muscle cells, anaerobic respiration breaks down glucose to lactic acid, releasing a small amount of energy without using oxygen. Ethanol is produced in yeast, while oxygen and glucose are used or stored rather than being the waste product that builds up.

Question 39Biology / Life Science

Both parents have blood group A, but their child has blood group O. Which genotypes are possible for the parents?

  1. IAIB and IAi
  2. IAi and IAi Correct answer.
  3. ii and ii
  4. IAIA and IAIA

Explanation

Blood group O has the genotype ii, so the child must inherit an i allele from each parent. Each parent has blood group A and must therefore carry one IA allele and one i allele, giving the genotype IAi.

Question 40Biology / Life Science

Large areas of forest are cleared and burned. Which pair of changes most directly increases the carbon dioxide concentration in the atmosphere?

  1. More nitrogen fixation and less respiration
  2. Less photosynthesis and more combustion Correct answer.
  3. More transpiration and less decay
  4. Less osmosis and more digestion

Explanation

Trees remove carbon dioxide from the atmosphere during photosynthesis, so clearing them reduces this removal. Burning the cleared vegetation releases carbon dioxide stored in the wood, so both changes raise atmospheric carbon dioxide.

Question 41Biology / Life Science

In a cell, which structure is the main site of protein synthesis?

  1. Mitochondrion
  2. Ribosome Correct answer.
  3. Vacuole
  4. Golgi apparatus

Explanation

Ribosomes are the organelles where amino acids are joined together to form polypeptides during translation. Mitochondria release energy, the Golgi apparatus modifies and packages proteins, and vacuoles store substances.

Question 42Biology / Life Science

In an ecosystem, which term describes an organism that makes its own food using energy from sunlight?

  1. Predator
  2. Decomposer
  3. Consumer
  4. Producer Correct answer.

Explanation

A producer, such as a green plant, uses photosynthesis to make its own food from carbon dioxide and water using sunlight. Consumers feed on other organisms, decomposers break down dead material, and predators hunt other animals.

Question 43Biology / Life Science

Which blood component contains haemoglobin and carries most oxygen around the body?

  1. Red blood cells containing haemoglobin Correct answer.
  2. Platelets involved in blood clotting
  3. White blood cells defending against pathogens
  4. Plasma carrying dissolved nutrients

Explanation

Red blood cells contain haemoglobin, which binds oxygen in the lungs and releases it in tissues. White blood cells defend against pathogens, platelets help clotting, and plasma transports dissolved substances.

Question 44Biology / Life Science

A student places one piece of potato in distilled water and another piece of the same size in concentrated salt solution. After 30 minutes, which result is most likely?

  1. The distilled-water piece firms; the salt-solution piece softens. Correct answer.
  2. Both pieces become firmer because water enters each one.
  3. The piece in distilled water softens, while the piece in salt solution firms up.
  4. Both pieces become softer because salt removes water from each.

Explanation

Potato cells have a partially permeable membrane. In distilled water, water enters the cells by osmosis and the tissue becomes turgid and firm. In concentrated salt solution, water leaves the cells by osmosis, so the tissue loses turgor and becomes soft.

Question 45Biology / Life Science

An enzyme found in the human stomach has an optimum pH of about 2. If this enzyme is moved to a solution at pH 7, what is the most likely effect on its activity?

  1. Activity increases because pH 7 is closer to neutrality.
  2. Activity stops immediately because the enzyme is digested.
  3. Activity decreases because the active site changes shape. Correct answer.
  4. Activity stays the same because pH does not affect enzymes.

Explanation

Each enzyme has an optimum pH, and extreme pH changes can alter the shape of the active site so the substrate no longer fits. The stomach enzyme is adapted to pH 2, so at pH 7 its activity is likely to decrease.

Question 46Biology / Life Science

In pea plants, tall (T) is dominant to short (t). A heterozygous tall plant (Tt) is crossed with a short plant (tt). What percentage of the offspring is expected to be short?

  1. 100%
  2. 75%
  3. 0%
  4. 50% Correct answer.

Explanation

The cross Tt × tt produces gametes T and t from the tall parent and only t from the short parent. The possible offspring are Tt (tall) and tt (short) in equal numbers, so 50% are expected to be short.

Question 47Biology / Life Science

In a food chain, grass is eaten by grasshoppers, which are eaten by frogs, which are eaten by snakes. If the frog population falls sharply, which change is most likely in the short term?

  1. The grass population increases, and snakes have more food.
  2. The snake population will immediately increase.
  3. The grass population will immediately decrease.
  4. Grasshoppers increase, and snakes may have less food. Correct answer.

Explanation

Frogs are predators of grasshoppers, so fewer frogs allow more grasshoppers to survive. Snakes eat frogs, so a sharp fall in frogs can reduce the food available to snakes.

Question 48Biology / Life Science

A person's kidney nephrons are damaged so they can no longer reabsorb useful amounts of glucose, amino acids, and water. Which change in urine is most directly expected?

  1. Less glucose and fewer amino acids appear in urine.
  2. More protein and fewer salts appear in urine.
  3. More glucose and amino acids appear in dilute urine. Correct answer.
  4. Urine becomes much more concentrated and smaller in volume.

Explanation

Nephrons reabsorb glucose, amino acids, and much of the water from the filtrate back into the blood. If they are damaged, these useful substances remain in the filtrate, so urine contains more glucose and amino acids and more water, making it more dilute.

Question 49Biology / Life Science

A population of bacteria is exposed to an antibiotic. Most bacteria die, but a few survive because they already carry a gene that breaks down the antibiotic. Which statement best explains why the population becomes resistant after several generations?

  1. The antibiotic makes the bacteria mutate in a directed way to become resistant.
  2. The antibiotic causes each bacterium to develop a new resistance gene.
  3. The bacteria learn to avoid the antibiotic and then pass on this learning.
  4. The surviving bacteria reproduce, passing the resistance gene to their offspring. Correct answer.

Explanation

Antibiotic resistance is an example of natural selection. The antibiotic kills susceptible bacteria, while those with an existing resistance gene survive and reproduce, so the resistance gene becomes more common in the population.

Question 50Biology / Life Science

After a person drinks a large volume of water, blood becomes more dilute. Which response is expected?

  1. ADH secretion decreases, and urine becomes more dilute. Correct answer.
  2. ADH secretion decreases, and urine becomes more concentrated.
  3. ADH secretion increases, and urine becomes more dilute.
  4. ADH secretion increases, and urine becomes more concentrated.

Explanation

When blood becomes too dilute, the pituitary gland releases less ADH. The kidney tubules then reabsorb less water, so more water is lost in a larger volume of dilute urine. This negative feedback helps restore the normal water concentration of the blood.