Frog: digestion, respiration and circulation

Frogs eat insects and other small animals (they are carnivores), so their gut is short. Food passes through the mouth, buccal cavity, pharynx, oesophagus, stomach, intestine and rectum, and leftovers leave through the cloaca. Prey is caught with a bilobed (forked) sticky tongue that flips out.

In the stomach, hydrochloric acid and gastric juices start digestion, turning food into a semi-liquid chyme. This passes into the first part of the intestine, the duodenum. There it receives bile (made by the liver, stored in the gall bladder) and pancreatic juice, through a common bile duct. Bile breaks fat into tiny droplets (emulsifies it). Digested food is absorbed by tiny finger-like villi and microvilli in the intestine wall.

A frog breathes in two ways. In water, it breathes through its moist skin (cutaneous respiration). On land, it uses the buccal cavity, the skin and the lungs (pulmonary respiration). During aestivation and hibernation, when it hardly moves, it breathes only through the skin.

Blood flows in closed vessels, and there is also a lymphatic system. The heart has three chambers: two atria and one ventricle, inside a protective sac, the pericardium. A chamber called the sinus venosus collects blood and joins the right atrium; the ventricle pumps blood into the conus arteriosus.

Two special routes: the hepatic portal system carries blood from the intestine to the liver, and the renal portal system from the lower body to the kidneys. Blood is plasma with red blood cells that have a nucleus (nucleated, with haemoglobin), white blood cells and platelets. Lymph is like blood without the red cells and some proteins.