Reptile Anatomy

Reptiles differ from mammals in several ways. They have a single bone where the skull attaches to the first vertebra, a single auditory bone (the stapes) that transmits sound vibrations from the eardrum to the inner ear and a very strong jaw.


The skin of lizards and snakes contains blood vessels, nerves, and special cells called chromatophores that contain guanine crystals. These crystals allow these animals to change color.

Respiratory System

Like all reptiles, a snake is an air-breathing animal. The lungs are simple saclike structures with small pockets, or alveoli, in their walls that facilitate gas exchange. A snake’s gills, like those of amphibians, respond to oxygen partial pressure changes by increasing or decreasing ventilation and may be controlled by peripheral chemoreceptors. The respiratory system also includes central chemoreceptors that detect the concentration of blood carbon dioxide.

As in mammals, the heart of a reptile is a four-chambered organ. The atria are separated by the sinoatrial valve, and the ventricle is joined to the cloaca (the site of excretion). Most snakes have paired aortae that exit each side of the single ventricle and fuse into the abdominal aorta. The pulmonary artery that leads to the lung(s) arises from this common aorta.

A reptile’s lungs are irrigated by two pulmonary arteries that come off the left ventricle. These arteries are joined by the renal portal system, which also supplies blood to the gills and bladder.

The kidneys of a reptile provide a constant extracellular environment by maintaining normal concentrations of water and salt, removing waste products, regulating acid-base balance, producing hormones, and producing vitamins. They also produce a hypertonic urine by a process called osmosis, which involves the cloaca (if present) and the urinary bladder (if present). Like in birds, reptiles have a Loop of Henle that is absent.

Digestive System

The main digestive system of a reptile is a long, J-shaped organ that begins with the mouth and salivary glands. The food is swallowed and peristalstic movement within the esophagus pushes it downward toward the stomach where cells secrete gastric juices that break down proteins. The stomach empties into the intestines, and waste from the large intestines passes through the cloaca.

Carnivorous snakes chew their food to a manageable state, but herbivorous ones do not, and a whole animal can take a lot of energy to digest. As a result, it is very important that they have a fast and efficient digestive system.

In addition to the esophagus and stomach, there is also an enlarged pharynx and an operculum. The trachea bifurcates in the cranial thorax to supply two lungs, which are characterized by large bulla-like divisions and alveoli. In a lizard, the lungs can be inflated by contraction of abdominal and intercostal muscles to aid respiration.

The paired kidneys are situated in the posterior half of the body and attached to the dorsal body wall. The right kidney is cranial to the left and both have a single ureter that travels across their ventral surface and empty into the cloaca. As with birds and snakes, the cloaca receives waste from the digestive, urinary and reproductive systems and empty into the proctodeum portion.

Reproductive System

Unlike mammals, reptiles do not have a uterus or vagina. Instead, they have a cloaca, which is a shared exit-hole for gametes, urine and feces. Reptiles are all oviparous, meaning they lay eggs that must hatch in order to reproduce. The eggs are surrounded by a membrane that protects the embryo and contains amniotic fluid to keep it moist.

General reptile anatomy includes a body divided into head, neck, trunk and tail, as well as a three-chamber heart with two atria and one partly split ventricle. The skin is covered in rough, horny scales or bony plates composed of a tough material called keratin. The keratin in the scales helps to prevent water loss from the body.

Many lizards have a small opening in the lower jaw called the glottis. When a lizard eats, it can extend this slit open to allow it to swallow its prey. Snakes have a more complex gill slit, which is actually a flexible ligament that is capable of expanding to the size of its meal.

Male reptiles have a copulatory organ, which can be a single penis (turtles and tortoises, crocodiles) or a pair of hemipenes (snakes and lizards). They also have paired internal gonads that connect through ducts to the cloaca. Reptiles have a kidney called the metanephric kidney, which is found in the posterior part of the body and has twenty-five to thirty lobes.


The skin of reptiles, particularly in snakes, is covered by scales that form armor that helps them defend themselves from injury. These scales are also keratinized to make them water-proof. This is an important feature that allows them to live on land in contrast to amphibians.

The scales are formed from a tough protein called keratin, which is similar to the protein in hair and fingernails. The keratin is reinforced in the hinge area by a different protein called a-keratin. The a-keratin is deposited from the outer layer of the epidermis to the inner layers of the scales. Both the a-keratin and the b-keratin have a waxy coating that makes the skin very waterproof.

Snakes shed their skin regularly. The shedding process is called ecdysis. During sloughing, the old epidermis pushes its way through the scales to be removed, thus doubled in thickness. The new skin is up to 20% larger than the old skin. This is one of the reasons why a sloughed snake looks shrunken, as the scales are shed in their entirety rather than individually.

In crocodiles and many turtles the skin is coated by a tough layer of overlapping dermal scutes, which protects them from both external damage and dehydration. The scutes are formed from the upper part of the epidermis and the subcutaneous tissue below.