BiologyISC Class 11 · Structural Organisation in Animals: Frog

The Frog: Circulatory, Excretory, Nervous and Reproductive Systems

Updated 17 October 2026 · Fact-checked

The frog has a closed circulatory system with a three-chambered heart (two atria, one ventricle), a hepatic and a renal portal system and nucleated red cells. It is ureotelic, with a pair of kidneys; the brain has ten pairs of cranial nerves; fertilisation is external and the tadpole undergoes metamorphosis.

Notice that two atria empty into a single ventricle, so blood from the body and lungs mixes before leaving through the truncus arteriosus.

Illustration drawn for QuestionBuddy.

Understand Frog: Circulation, Excretion, Nervous and Reproductive Systems

Circulatory system. Closed, with blood vessels, lymph and a muscular heart in the pericardium. The heart has two atria and one ventricle, plus sinus venosus (receiving blood from the vena cavae) and truncus arteriosus (a conus leading out of the ventricle). Blood from the body enters the right atrium through the sinus venosus; oxygenated blood from the lungs enters the left atrium; both are emptied into the single ventricle, where mixing occurs, and the mixed blood goes out via the truncus arteriosus. The red blood cells are nucleated and contain haemoglobin. The *hepatic portal system* carries blood from the intestine to the liver, and the *renal portal system* carries blood from the hind limbs to the kidneys, before it returns to the heart.

Excretion. A pair of compact, dark red, bean-shaped kidneys on either side of the vertebral column. Each has nephrons. Ureters carry urine to the cloaca; the urinary bladder (ventral) stores urine. In males the ureters also carry sperm (urinogenital duct). Frogs excrete urea, so they are ureotelic.

Nervous system. CNS: brain (forebrain with olfactory lobes and cerebral hemispheres, midbrain with optic lobes, hindbrain with cerebellum and medulla oblongata) enclosed in the cranium, and the spinal cord; ten pairs of cranial nerves; PNS and autonomic system. Sense organs: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), eyes (simple) and ears (internal ears only, with the tympanum visible externally).

Reproduction. Male: a pair of yellow, ovoid testes attached to the kidneys, with vasa efferentia entering the kidneys and then the Bidder's canal; the ureter carries sperm to the cloaca. Female: a pair of ovaries and oviducts opening into the cloaca; a female may lay 2500 to 3000 ova at a time. Fertilisation is external, in water. The zygote develops into a tadpole (larva) with gills and a tail; through metamorphosis (a hormone-controlled process) it becomes the adult. The frog is helpful as it eats insects and as an experimental animal.

Frog: Circulation, Excretion, Nervous and Reproductive Systems: key facts

Heart: Two atria
One ventricle
Sinus venosus
Truncus arteriosus; Mixed blood in the ventricle.
Portal systems: Hepatic portal: intestine to liver | Renal portal: hind limbs to kidneys; Veins that end in capillaries.
Excretion: $U r eo t e l i c ∣ K i d n ey s ∣ U r e t er ∣ B l a dd er ∣ C l o a c a$; Males: ureters also carry sperm.
Reproduction: Fertilisation external
Zygote
Tadpole
Metamorphosis
Adult; Takes place in water.

Frog: Circulation, Excretion, Nervous and Reproductive Systems in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

From tadpole to frog

Suppose a student keeps frog spawn in a tank and watches the tadpoles change over many weeks.

Describe the change of the respiratory organ and give the term for the process.

Try it first, then show the solutionSolution

The fertilised egg develops into a tadpole with gills and a tail, living in water.
During metamorphosis the gills are lost, lungs develop, the tail is absorbed and limbs form.
The process is metamorphosis, controlled by thyroid hormones.

Answer: Gills of the tadpole are replaced by lungs and skin; the process is metamorphosis.

Memory hook: Metamorphosis means a change in form during development.

Tricks to remember Frog: Circulation, Excretion, Nervous and Reproductive Systems

Short memory aids. Say each one aloud once, then use it on a question the same day.

2 atria, 1 ventricle
The frog's heart is three-chambered, so the blood in the ventricle is mixed.
Use it when: Describing the heart.
External fertilisation in water
Eggs and sperm are shed into water by the female and male.
Use it when: Reproduction questions.

Frog: Circulation, Excretion, Nervous and Reproductive Systems: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Chambers of the frog's heart · 1 mark · MCQBoard level

The heart of a frog has:
(a) two chambers
(b) three chambers
(c) four chambers
(d) one chamber

Try it first, then show the solutionSolution

Two atria and one ventricle.

Answer: (b) three chambers

Example 2: Blood vessels · 2 marks · Short answerBoard level

Define the hepatic portal system and renal portal system in the frog.

Try it first, then show the solutionSolution

A portal system is a vein that begins and ends in capillaries. The hepatic portal vein carries blood from the intestine to the liver. (1 mark)
The renal portal vein carries blood from the hind limbs to the kidneys. (1 mark)

Answer: Intestine to liver; hind limbs to kidneys.

Example 3: Heart and blood flow · 4 marks · Short answerHarder board level

Trace the flow of blood through the heart of a frog and explain why the blood in the ventricle is mixed.

Try it first, then show the solutionSolution

Deoxygenated blood from the body enters the right atrium through the sinus venosus. (1 mark)
Oxygenated blood from the lungs enters the left atrium. (1 mark)
Both atria empty into the single ventricle, where the two kinds of blood mix. (1 mark)
The mixed blood leaves through the truncus arteriosus to the body and lungs. (1 mark)

Answer: Right and left atrium, single ventricle (mixing), truncus arteriosus.

Example 4: Excretion and osmoregulation · 4 marks · Short answerHarder board level

Name the excretory organs of the frog and the nitrogenous waste. Explain why it is called ureotelic and how male frogs differ from females in the excretory duct.

Try it first, then show the solutionSolution

Kidneys (a pair), ureters, urinary bladder and cloaca. (1 mark)
The main nitrogenous waste is urea, which is why the frog is ureotelic. (1 mark)
The kidney filters blood; urine passes to the cloaca through the ureters and is stored in the bladder. (1 mark)
In males the ureter also carries sperm (urinogenital duct); in females urine and eggs have separate ducts. (1 mark)

Answer: Ureotelic; kidneys and ureters; male ducts carry both urine and sperm.

Example 5: Reproduction and development · 5 marks · Long answerHOTS / competency

Describe the process of reproduction in the frog from mating to the adult. Why is fertilisation said to be external and why does the female lay many eggs?

Try it first, then show the solutionSolution

In the breeding season the male calls with the vocal sacs and clasps the female with the copulatory pads. (1 mark)
The female releases eggs (many thousands, up to 2500 to 3000 at a time) into water; the male releases sperm over them: external fertilisation. (1.5 marks)
The zygote develops into a tadpole with gills and a tail, then undergoes metamorphosis to the adult. (1.5 marks)
Many eggs are laid because eggs and tadpoles in the open water are exposed to predators and other hazards, so only a few survive. (1 mark)

Answer: Mating, external fertilisation, tadpole, metamorphosis; many eggs offset heavy losses.

Where students lose marks in Frog: Circulation, Excretion, Nervous and Reproductive Systems

The slips examiners see most often, and what to write instead.

Mistake: Stating a four-chambered heart
Giving two ventricles.
Do this: The frog has two atria and one ventricle.
Usually costs: 1 to 2 marks.
Mistake: Calling the frog ammonotelic
Writing ammonia as the main waste.
Do this: The adult frog is ureotelic; the tadpole is ammonotelic.
Usually costs: 1 mark.
Mistake: Forgetting that blood mixes
Saying the blood is fully oxygenated.
Do this: Mixing occurs in the single ventricle.
Usually costs: 1 mark.

Exam tips

Draw the dorsal and ventral views of the heart with labels.
Mention both the portal systems.
State the nitrogenous waste in excretion answers.
Describe the life cycle in order.

Practice questions on Frog: Circulation, Excretion, Nervous and Reproductive Systems

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of the following is true for the frog's nervous system?
(a) 12 pairs of cranial nerves
(b) 10 pairs of cranial nerves
(c) no spinal cord
(d) no cerebellum
Hint
Recall the textbook number.
Answer
(b) 10 pairs of cranial nerves
Q2 · 2 marks · Short answerBoard level
Name the three parts of the frog's brain.
Hint
Forebrain, midbrain, hindbrain.
Answer
Forebrain (olfactory lobes and cerebral hemispheres), midbrain (optic lobes) and hindbrain (cerebellum and medulla oblongata).
Q3 · 3 marks · Short answerHarder board level
Why is the red blood cell of the frog described as nucleated? Contrast it with the human RBC.
Hint
Mammals have enucleate RBCs.
Answer
Frog RBCs are oval, biconvex and have a nucleus; mature human RBCs are biconcave and have no nucleus.
Q4 · 3 marks · Short answerHarder board level
Name the external openings of the frog's cloaca and the organs that open into it.
Hint
The cloaca is common.
Answer
The cloaca opens to the outside through the cloacal aperture. The rectum, urinary bladder, ureters and genital ducts (oviducts or urinogenital ducts) open into it.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): The frog is considered a ureotelic animal. Reason (R): Urea is the chief nitrogenous waste excreted by it. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Define ureotelism.
Answer
(a) Both are true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student studies frog spawn in a pond.
(i) Name the larva that hatches. (1 mark)
(ii) State the type of fertilisation. (1 mark)
(iii) Name the process of change from larva to adult. (1 mark)
(iv) Name the hormone-producing gland that controls it. (1 mark)
Hint
Recall the life cycle.
Answer
(i) Tadpole. (ii) External fertilisation. (iii) Metamorphosis. (iv) The thyroid.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Heart chambers of the frog?Show answer
Two atria and one ventricle.
2.Nitrogenous waste of the adult frog?Show answer
Urea.
3.Larva of the frog?Show answer
Tadpole.
4.Fertilisation in the frog?Show answer
External.

Frog: Circulation, Excretion, Nervous and Reproductive Systems: frequently asked questions

What is the function of the sinus venosus?

It is a thin-walled triangular chamber that collects deoxygenated blood from the vena cavae and pours it into the right atrium.

Why are frogs useful to farmers?

They eat insects that damage crops and are also part of food chains.

What is the Bidder's canal?

A canal in the male frog's kidney through which sperm pass from the testes to the ureter.