BiologyCBSE Class 11 · Structural Organisation in Animals

Frog: Circulatory, Excretory and Reproductive Systems

Updated 17 October 2026 · Fact-checked

The frog has a closed circulation with a three-chambered heart (two atria, one ventricle), a sinus venosus and conus arteriosus, and portal systems. Paired kidneys remove urea (frog is ureotelic). Males have a pair of testes; females have ovaries and oviducts; fertilisation is external in water and development is indirect through a tadpole by metamorphosis.

Understand Frog: Circulation, Excretion and Reproduction

Circulatory system. Closed blood vascular system with a lymphatic system.

Heart: muscular, three-chambered (two atria and one ventricle), covered by a double-layered pericardium. A triangular sinus venosus joins the right atrium; a conus arteriosus emerges from the ventricle.
Blood flows: veins carry blood to the heart (sinus venosus to right atrium); arteries carry blood away. Atrial contraction pushes mixed blood into the ventricle. The ventricle pumps blood into the conus arteriosus.
Portal systems: 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 returning to the heart.
Red blood cells are nucleated and contain haemoglobin; white cells and platelets (thrombocytes) also occur. The lymph has fewer proteins than blood plasma.

Excretory system. Elimination of nitrogenous wastes. Frogs are ureotelic: they excrete urea. Organs: a pair of compact, dark red, bean-shaped kidneys on either side of the vertebral column in the posterior part of the body cavity. Each kidney has many structural and functional units, nephrons. Ureters emerge from the kidneys and in males act as urinogenital ducts carrying sperm and urine, opening into the cloaca. A thin-walled urinary bladder on the ventral side of the cloaca stores urine, which is passed out through the cloacal aperture. In females the ureters and oviducts open separately into the cloaca.

Nervous and sense organs (brief). Brain enclosed in the cranium, 10 pairs of cranial nerves; sense organs of touch, taste, smell, vision and hearing; eyes have a transparent nictitating membrane; the ear is an internal ear with a tympanum on the outer surface.

Male reproductive system. A pair of yellowish, ovoid testes attached to the upper side of the kidneys by a double fold of peritoneum, the mesorchium. About 10 to 12 vasa efferentia arise from the testes, enter the kidneys, and open into Bidder's canal, which finally communicates with the urinogenital duct, opening into the cloaca. The cloaca is the common chamber for faeces, urine and sperm. Male frogs have vocal sacs and a copulatory pad on the first digit of the forelimbs (sexual dimorphism).

Female reproductive system. A pair of ovaries near the kidneys, with no functional connection to them. A pair of coiled oviducts opens into the cloaca. A mature female can lay about 2500 to 3000 ova at a time (textbook figure).

Fertilisation and development. External fertilisation in water, typically during the rainy season. The zygote develops into a tadpole, which undergoes metamorphosis to the adult frog (indirect development).

Economic importance. Frogs eat insects, protecting crops, and maintain ecological balance as a link in the food chain; in some countries frog legs are consumed as food.

Draw and label: the heart of a frog (right atrium, left atrium, ventricle, sinus venosus, conus arteriosus).

Frog: Circulation, Excretion and Reproduction: key facts

Heart: 2 atria + 1 ventricle
sinus venosus
conus arteriosus; Three-chambered
Excretion: $U r eo t e l i c ∣ u r e a ∣ k i d n ey s ∣ b l a dd er ∣ c l o a c a$; Nitrogenous waste
Male path: Testis
vasa efferentia
Bidder's canal
urinogenital duct
cloaca; Sperm route
Development: External fertilisation
tadpole
metamorphosis; Indirect

How to answer Frog: Circulation, Excretion and Reproduction questions

Questions ask for flow paths or structure.

1Trace the route stepwise using arrows.
2For the heart, name chambers and the vessels entering and leaving.
3Give the nitrogenous waste type for the animal.
4Use sexual dimorphism features to distinguish sexes.
5Mention external fertilisation and metamorphosis for the life cycle.

Frog: Circulation, Excretion and Reproduction 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.

Pond in monsoon

Suppose a pond fills with croaking males and masses of jelly-covered eggs after the first monsoon rains.

Explain the sound, the fertilisation and what hatches.

Try it first, then show the solutionSolution

Males have vocal sacs which amplify the call.
Eggs and sperm meet in water: external fertilisation.
Eggs develop into tadpoles which metamorphose into frogs.

Answer: Vocal sacs call; external fertilisation; tadpole by metamorphosis.

Memory hook: Rain calls, water fertilises.

Tricks to remember Frog: Circulation, Excretion and Reproduction

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

2-1 heart
Two atria, One ventricle.
Use it when: Heart
VBU
Vasa efferentia, Bidder's canal, Urinogenital duct.
Use it when: Male route
Ureo-frog
Frogs excrete urea.
Use it when: Excretion

Frog: Circulation, Excretion and Reproduction: solved examples

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

Example 1: Frog heart · 3 marks · Short answerBoard level

State the roles of the sinus venosus, the conus arteriosus and the pericardium in the frog heart.

Try it first, then show the solutionSolution

Sinus venosus collects blood from the veins and opens into the right atrium (1 mark).
Conus arteriosus carries blood from the ventricle to the arteries (1 mark).
Pericardium is the double membrane covering and protecting the heart (1 mark).

Answer: Inlet chamber, outlet vessel and protective covering.

Example 2: Ureotelism · 2 marks · Very short answerBoard level

What does ureotelic mean? Where is urea formed and eliminated in the frog?

Try it first, then show the solutionSolution

An animal whose main nitrogenous waste is urea (1 mark).
Urea is formed in the liver, carried by blood to the kidneys, where it is filtered out; it is stored in the urinary bladder and passed out through the cloaca (1 mark).

Answer: Urea is the main nitrogenous waste; it is formed in the liver and eliminated by the kidneys, bladder and cloaca.

Example 3: Male reproductive route · 3 marks · Short answerHarder board level

In a male frog the vasa efferentia open into a canal inside the kidney. Name it, name the duct beyond, and explain why that duct has a double name.

Try it first, then show the solutionSolution

The canal is Bidder's canal (1 mark).
Beyond it is the urinogenital duct (1 mark).
It carries both urine and sperm to the cloaca, hence urogenital in name (1 mark).

Answer: Testis - vasa efferentia - Bidder's canal - urinogenital duct - cloaca.

Example 4: Portal systems · 3 marks · Short answerHarder board level

Explain the two portal systems of the frog and why they are called portal.

Try it first, then show the solutionSolution

A portal vein carries blood from one capillary network to another before it returns to the heart (1 mark).
Hepatic portal: intestine to liver (1 mark).
Renal portal: hind limbs to kidneys (1 mark).

Answer: Blood passes through a second capillary bed in liver or kidney.

Example 5: Metamorphosis · 5 marks · Long answerHOTS / competency

Frog development is indirect. Describe the stages and explain why external fertilisation needs water and why so many eggs are laid.

Try it first, then show the solutionSolution

Eggs are fertilised outside the body in water (1 mark).
Zygote develops to a tadpole that swims and breathes with gills (1 mark).
Tadpole undergoes metamorphosis into the adult frog (1 mark).
Water keeps gametes moist and lets sperm swim to eggs (1 mark).
Many eggs (2500 to 3000) compensate for losses to predators and unfertilised eggs (1 mark).

Answer: Egg - tadpole - frog; water needed; many eggs balance loss.

Where students lose marks in Frog: Circulation, Excretion and Reproduction

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

Mistake: Four chambers
Mammal heart assumed.
Wrong working
Frog heart has four chambers.
Right working
Frog heart has three.
Do this: 2 atria + 1 ventricle.
Usually costs: 1 mark.
Mistake: Ureters as only urinary duct in male
Urinogenital duct forgotten.
Wrong working
Male frog has separate sperm duct.
Right working
The urinogenital duct carries both.
Do this: Shared duct.
Usually costs: 1 mark.
Mistake: Internal fertilisation
Reptile fact mixed.
Wrong working
Fertilisation internal in frogs.
Right working
Fertilisation external in water.
Do this: Frogs: external.
Usually costs: 1 mark.

Exam tips

Use arrows for sperm route.
Name the portal systems.
Mention vocal sacs in sexual dimorphism.
Describe metamorphosis in order.

Practice questions on Frog: Circulation, Excretion and Reproduction

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
A frog has a heart with
(a) two chambers (b) three chambers (c) four chambers (d) one chamber
Hint
Two atria and one ventricle.
Answer
(b).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The frog is ureotelic.
Reason (R): Its main nitrogenous waste is urea.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Definition.
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
What are vocal sacs and the copulatory pad? Which sex has them?
Hint
Sexual dimorphism.
Answer
Vocal sacs help amplify the croak; the copulatory pad on the first digit of the forelimb helps the male grip the female. Both occur in males.
Q4 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a dissected frog shows P: bean-shaped red-brown organs by the backbone; Q: a thin-walled sac at the end of the cloaca; R: coiled tubes opening into the cloaca of a female.
(a) Name P, Q, R. (1)
(b) What is stored in Q? (1)
(c) Name the nitrogenous waste. (1)
(d) Name the kidney unit. (1)
Hint
Kidney, bladder, oviduct.
Answer
(a) Kidneys, urinary bladder, oviducts. (b) Urine. (c) Urea. (d) Nephron.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a frog's heart mixes oxygenated and deoxygenated blood in a single ventricle. Explain why this may be sufficient for a frog but not for a bird.
Hint
Cold-blooded and skin respiration.
Answer
Frog is cold-blooded, with a low metabolic rate, and obtains additional oxygen through skin so mixed blood supplies enough. Birds are warm-blooded with high demand; a four-chambered heart keeps oxygenated blood separate for efficient delivery.

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.How many chambers has a frog heart?Show answer
Three.
2.What is Bidder's canal?Show answer
A canal receiving vasa efferentia in the kidney.
3.What is the frog's waste?Show answer
Urea.
4.Where are ova fertilised?Show answer
Outside the body, in water.
5.What is a tadpole?Show answer
The larva of the frog.
6.Where do the oviducts open?Show answer
Into the cloaca.

Frog: Circulation, Excretion and Reproduction: frequently asked questions

What is ureotelic?

Excreting urea as main nitrogenous waste.

How many ova can a frog lay?

About 2500 to 3000 at a time.

What is metamorphosis?

The change from larva to adult.

Is frog blood closed?

Yes, closed circulation.

What is the pericardium?

The double membrane around the heart.