How a Frog Breathes Through Its Skin as Well as Its Lungs
Study the digestive, circulatory and respiratory systems of the frog, its excretory, nervous and reproductive systems, and how the frog's organ systems reflect the basic vertebrate body plan.
Why do biologists study the frog to understand vertebrates?
The common Indian frog, Rana tigrina, is easy to find near ponds in the monsoon and has every major organ system a vertebrate needs, laid out in a simple form. Studying it shows how digestion, circulation, breathing, excretion, control and reproduction fit together in one body.
This lesson covers the frog's digestive, circulatory and respiratory systems, its excretory, nervous and reproductive systems, and how it reflects the vertebrate body plan.
This lesson covers the frog's digestive, circulatory and respiratory systems, its excretory, nervous and reproductive systems, and how it reflects the vertebrate body plan.
How do the digestive, circulatory and respiratory systems of a frog work?
The frog digests food in a short alimentary canal suited to a carnivorous diet, pumps blood through a closed system with a three-chambered heart, and breathes through its skin in water and through its skin, mouth lining and lungs on land.
Digestive system:
- Mouth, buccal cavity, pharynx, oesophagus, stomach, intestine, rectum and cloaca; the canal is short because frogs are carnivores
- A tongue bilobed at the tip catches insects
- The liver secretes bile, stored in the gall bladder, and the pancreas secretes pancreatic juice
- Digested food is absorbed by finger-like villi and microvilli in the intestine
Circulatory system:
- A closed system with a three-chambered heart — two atria and one ventricle — covered by a pericardium
- A triangular sinus venosus joins the right atrium, and a conus arteriosus leaves the ventricle
- Hepatic portal and renal portal systems carry blood to the liver and kidneys
Respiratory system:
- In water — gas exchange through the moist skin, called cutaneous respiration
- On land — through the skin, the lining of the buccal cavity and a pair of lungs
- During aestivation in summer and hibernation in winter, breathing happens only through the skin
An everyday example. A frog that stays buried in dry mud through the hot months and reappears with the monsoon rains survives by breathing only through its skin while dormant.
The substance. A three-chambered heart lets some oxygen-rich and oxygen-poor blood mix in the single ventricle — enough for a cold-blooded animal with modest energy needs.
Digestive system:
- Mouth, buccal cavity, pharynx, oesophagus, stomach, intestine, rectum and cloaca; the canal is short because frogs are carnivores
- A tongue bilobed at the tip catches insects
- The liver secretes bile, stored in the gall bladder, and the pancreas secretes pancreatic juice
- Digested food is absorbed by finger-like villi and microvilli in the intestine
Circulatory system:
- A closed system with a three-chambered heart — two atria and one ventricle — covered by a pericardium
- A triangular sinus venosus joins the right atrium, and a conus arteriosus leaves the ventricle
- Hepatic portal and renal portal systems carry blood to the liver and kidneys
Respiratory system:
- In water — gas exchange through the moist skin, called cutaneous respiration
- On land — through the skin, the lining of the buccal cavity and a pair of lungs
- During aestivation in summer and hibernation in winter, breathing happens only through the skin
An everyday example. A frog that stays buried in dry mud through the hot months and reappears with the monsoon rains survives by breathing only through its skin while dormant.
The substance. A three-chambered heart lets some oxygen-rich and oxygen-poor blood mix in the single ventricle — enough for a cold-blooded animal with modest energy needs.
How do the excretory, nervous and reproductive systems of a frog work?
The frog removes nitrogenous waste as urea through a pair of kidneys, is controlled by a brain, spinal cord, nerves and endocrine glands, and reproduces sexually with separate sexes and external fertilisation in water.
Excretory system:
- A pair of dark red, bean-shaped kidneys near the backbone, each made of many uriniferous tubules
- In males the ureter carries both urine and sperm as a urinogenital duct; in females the ureters and oviducts open separately into the cloaca
- A thin-walled urinary bladder lies below the rectum and opens into the cloaca
- Frogs are ureotelic, excreting urea
Nervous system:
- Central — the brain, protected by a cranium, and the spinal cord in the vertebral column
- Peripheral — ten pairs of cranial nerves from the brain, and spinal nerves
- Brain in three parts: forebrain with olfactory lobes, cerebral hemispheres and diencephalon; midbrain with optic lobes; hindbrain with cerebellum and medulla oblongata
- Endocrine glands include the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads
Reproductive system:
- Male — a pair of yellowish, oval testes attached to the kidneys; vasa efferentia carry sperm through the kidneys to the urinogenital duct
- Female — a pair of ovaries near the kidneys, with no functional connection to them; the oviducts open into the cloaca
- External fertilisation in water, and development through a tadpole larva that undergoes metamorphosis
An everyday example. Clumps of jelly-coated frog eggs floating in a paddy field after heavy rain soon hatch into tadpoles.
The substance. The adult frog is ureotelic, while its tadpole is ammonotelic — urea is less toxic and needs less water to remove, which suits life on land.
Excretory system:
- A pair of dark red, bean-shaped kidneys near the backbone, each made of many uriniferous tubules
- In males the ureter carries both urine and sperm as a urinogenital duct; in females the ureters and oviducts open separately into the cloaca
- A thin-walled urinary bladder lies below the rectum and opens into the cloaca
- Frogs are ureotelic, excreting urea
Nervous system:
- Central — the brain, protected by a cranium, and the spinal cord in the vertebral column
- Peripheral — ten pairs of cranial nerves from the brain, and spinal nerves
- Brain in three parts: forebrain with olfactory lobes, cerebral hemispheres and diencephalon; midbrain with optic lobes; hindbrain with cerebellum and medulla oblongata
- Endocrine glands include the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads
Reproductive system:
- Male — a pair of yellowish, oval testes attached to the kidneys; vasa efferentia carry sperm through the kidneys to the urinogenital duct
- Female — a pair of ovaries near the kidneys, with no functional connection to them; the oviducts open into the cloaca
- External fertilisation in water, and development through a tadpole larva that undergoes metamorphosis
An everyday example. Clumps of jelly-coated frog eggs floating in a paddy field after heavy rain soon hatch into tadpoles.
The substance. The adult frog is ureotelic, while its tadpole is ammonotelic — urea is less toxic and needs less water to remove, which suits life on land.
How does the frog's body show the basic vertebrate body plan?
The frog shows the core vertebrate plan — a vertebral column and cranium, a dorsal nerve cord, a ventral heart, closed circulation, paired limbs and a coelom holding the organs — while its three-chambered heart, moist skin and external fertilisation mark it as an amphibian.
Vertebrate features seen in the frog:
- A notochord in the embryo, replaced by a vertebral column
- A dorsal, hollow nerve cord forming the spinal cord, with the brain protected by a cranium
- A ventral heart and closed circulation with portal systems
- Two pairs of limbs, with webbed toes on the hind limbs
- A coelom lined by mesoderm and an organ-system level of organisation
Amphibian features:
- Moist, scaleless skin that also breathes
- A three-chambered heart
- A cloaca shared by the digestive, excretory and reproductive tracts
- A tympanum receiving sound, with no external ear
- Body temperature that varies with the surroundings, making frogs poikilotherms
Linking to other classes. Fish have a two-chambered heart and gills, while reptiles add dry, scaly skin and birds and mammals add a four-chambered heart and a constant body temperature — in these features the frog sits between fish and reptiles.
An everyday example. A frog hopping from a sunny path into a cool, shaded drain is managing its body temperature by choosing where to sit, since it cannot produce enough heat of its own.
The substance. Frogs do not drink water through the mouth — they absorb it through the skin, a reminder that amphibian skin serves breathing, water uptake and protection at once.
Vertebrate features seen in the frog:
- A notochord in the embryo, replaced by a vertebral column
- A dorsal, hollow nerve cord forming the spinal cord, with the brain protected by a cranium
- A ventral heart and closed circulation with portal systems
- Two pairs of limbs, with webbed toes on the hind limbs
- A coelom lined by mesoderm and an organ-system level of organisation
Amphibian features:
- Moist, scaleless skin that also breathes
- A three-chambered heart
- A cloaca shared by the digestive, excretory and reproductive tracts
- A tympanum receiving sound, with no external ear
- Body temperature that varies with the surroundings, making frogs poikilotherms
Linking to other classes. Fish have a two-chambered heart and gills, while reptiles add dry, scaly skin and birds and mammals add a four-chambered heart and a constant body temperature — in these features the frog sits between fish and reptiles.
An everyday example. A frog hopping from a sunny path into a cool, shaded drain is managing its body temperature by choosing where to sit, since it cannot produce enough heat of its own.
The substance. Frogs do not drink water through the mouth — they absorb it through the skin, a reminder that amphibian skin serves breathing, water uptake and protection at once.
Exam tip
What earns full marks on frog anatomy?
Label frog diagrams with paired structures clearly — two atria, a pair of kidneys, a pair of testes or ovaries — and add the sinus venosus and conus arteriosus to the heart.
- Heart: three chambers, two atria and one ventricle
- Breathing: through the skin in water; through skin, buccal cavity and lungs on land
- Excretion: kidneys with uriniferous tubules; ureotelic
- Male ureter doubles as a urinogenital duct; female ureters and oviducts open separately
The trap. Writing that frogs have internal fertilisation. Fertilisation in frogs is external, in water.
- Heart: three chambers, two atria and one ventricle
- Breathing: through the skin in water; through skin, buccal cavity and lungs on land
- Excretion: kidneys with uriniferous tubules; ureotelic
- Male ureter doubles as a urinogenital duct; female ureters and oviducts open separately
The trap. Writing that frogs have internal fertilisation. Fertilisation in frogs is external, in water.
Did you know
How do male frogs call so loudly on monsoon nights?
On monsoon nights, the chorus from a pond can be heard from far away. Male frogs produce it by pushing air from the lungs over the vocal cords and into vocal sacs that swell like balloons.
The sacs act as resonators and amplify the call. Each species has its own call, so females can find males of their own kind even in a crowded, noisy pond.
Females lack vocal sacs, and males also have a copulatory pad on the first finger of each forelimb — two easy ways to tell male and female frogs apart.
The sacs act as resonators and amplify the call. Each species has its own call, so females can find males of their own kind even in a crowded, noisy pond.
Females lack vocal sacs, and males also have a copulatory pad on the first finger of each forelimb — two easy ways to tell male and female frogs apart.
Exam relevance
How does NEET test the anatomy of the frog?
Structural Organisation in Animals, studied through the frog, is part of the NEET syllabus, and its questions are mostly detail-based.
What gets asked. Heart structure with the sinus venosus and conus arteriosus, modes of respiration, the male urinogenital duct, the number of cranial nerves, features that identify male frogs such as vocal sacs and copulatory pads, and parts of the brain.
Question types. Mostly statement-based questions, where one wrong detail makes a statement false, and match-the-column questions pairing a structure with its function.
Why it matters later. Portal systems, kidneys and cranial nerves return in human physiology chapters such as Body Fluids and Circulation, Excretory Products and their Elimination and Neural Control and Coordination.
The trap that costs marks. Calling the adult frog ammonotelic — the adult is ureotelic, while its tadpole excretes ammonia.
What gets asked. Heart structure with the sinus venosus and conus arteriosus, modes of respiration, the male urinogenital duct, the number of cranial nerves, features that identify male frogs such as vocal sacs and copulatory pads, and parts of the brain.
Question types. Mostly statement-based questions, where one wrong detail makes a statement false, and match-the-column questions pairing a structure with its function.
Why it matters later. Portal systems, kidneys and cranial nerves return in human physiology chapters such as Body Fluids and Circulation, Excretory Products and their Elimination and Neural Control and Coordination.
The trap that costs marks. Calling the adult frog ammonotelic — the adult is ureotelic, while its tadpole excretes ammonia.
Key takeaways
What must you be able to do from this lesson?
- Digestion, circulation and breathing: a short gut for a carnivore, closed circulation with a three-chambered heart, and breathing through skin, mouth lining and lungs
- Excretion, control and reproduction: ureotelic kidneys, ten pairs of cranial nerves with endocrine glands, and separate sexes with external fertilisation
- Vertebrate plan: backbone, dorsal nerve cord, ventral heart and coelom, with amphibian features such as moist skin and a cloaca
Which structure in a male frog carries both urine and sperm — and why does a female frog not have one?
- Excretion, control and reproduction: ureotelic kidneys, ten pairs of cranial nerves with endocrine glands, and separate sexes with external fertilisation
- Vertebrate plan: backbone, dorsal nerve cord, ventral heart and coelom, with amphibian features such as moist skin and a cloaca
Which structure in a male frog carries both urine and sperm — and why does a female frog not have one?