Digestive System

The digestive system is composed of the esophagus, stomach, small intestine, colon, and glands. The esophagus runs adjacent to the air sac from the pharynx, or throat, to the stomach. In mammals, the esophagus is very muscular and moves food to the stomach. In the snake, however, the esophagus has very little muscle and food is moved to the stomach more by movement of the entire body. The junction between the esophagus and the stomach is not well defined, and the stomach itself is not very advanced. It is a short, filiform shape with interior longitudinal folds to increase the surface area for digestion and absorption. The small intestine is likewise relatively simple. There may be a few loops or folds, but for the most part it is a long tube that receives food from the stomach, absorbs nutrients from it, and transports it to the colon, or large intestine. The colon then carries the fecal matter to the cloacal opening where it is disposed. The cloaca is a common chamber, receiving products from the digestive, urinary, and reproductive systems.

The liver, gall bladder, and pancreas are all associated with the digestive system. The liver is the largest internal organ in a snake, filling the space between the heart and stomach. One of the many functions of the liver is to produce bile, a digestive enzyme. The gall bladder and spleen are found near the posterior tip of the liver. The gall bladder stores bile produced by the liver and releases it into the small intestine when needed. The pancreas also secretes digestive enzymes into the small intestine, as well as producing hormones that regulate blood sugar.

Cardiovascular System

Two atria and one ventricle make up the three-chambered heart of a snake. The right and left atria receive blood from the lungs and body, respectively, and pass it to the ventricle to be circulated again. Encased in a sac, called the "pericardium," the heart is located at the bifurcation of the bronchi. The heart is able to move around, however, due to the lack of a diaphragm. This adjustment protects the heart from potential damage when large ingested prey is passed through the esophagus. The spleen is attached to the gall bladder and pancreas and functions to filter the blood and recycle old red blood cells. The thymus gland is located in fatty tissue above the heart and is responsible for the maturation of special immune cells in the blood.

Endocrine System

The endocrine system is made up of glands that secrete hormones essential to normal body function. Snakes have the same endocrine glands as mammals. A few examples are the thyroid, parathyroid, and adrenal glands. The thyroid gland located in the throat area is responsible for proper growth and development, such as normal shedding of the skin. The parathyroid is a paired structure located near the thyroid and helps in the metabolism of calcium. The two adrenal glands are located in the tail region, suspended in a mesentery (membrane sheet attaching organs to the body wall) near the reproductive organs. They secrete the hormone epinephrine (adrenaline) that increases heart and respiratory rates when the animal is in a dangerous situation.

Genitourinary System

The kidneys are the organs responsible for urinary output. In the snake, the kidneys are elongated, and the right kidney is situated closer to the head than the left. These organs filter the blood and remove waste products, which are then concentrated and transported, via the ureters, to the cloaca. The ureters are hollow tubes for transporting urine. In mammals, the ureters empty into the urinary bladder where the urine is stored and later expelled through another tube called the "urethra." Because snakes do not have a urinary bladder, the urine is not stored, and the ureters empty directly in the cloaca.

The paired gonads, testes in the male and ovaries in the female, are situated in a similar fashion, with the right being closer to the head than the left. They are also located closer to the head than the kidneys. In the female, the ovaries are near the oviducts, which carry eggs to the uterus before they enter the cloaca. Some snakes are oviparous (egg-laying) and some are viviparous (having live birth). In mammals, males have two ducts associated with each teste — the epididymis and ductus deferens. Snakes lack epidiymides and the sperm are simply transported from the teste through the ductus deferens to the cloaca. The male also has organs called "hemipenes" that are located posterior to the cloacal opening. The hemipenes are paired copulatory organs, and they are both fully functional, though only one at a time is used to transfer sperm to the female. The hemipenes are closely associated with the scent glands, or musk glands, which are also present in the female.