It is important to understand how the iguana body works, this will help you detect when there is a problem and why certain foods or habitat considerations are necessary.

Respiratory System

Iguanas can keep their breath for an extended period.
When Forest went for an operation it took the vet over an hour to put him under anesthesia. Every time that he had to breath in the gas he held his breath leaving my vet waiting for up to 20 minutes before Forest will take a quick breath again. Of course by that time my vet got distracted and missed it and had to wait again.

The reason for this remarkable talent is because in the wild they jump into the water when they detect a predator and stay under water until the thread is gone.

An iguana that is frightened by something usually start to breath very fast and if left in the situation the breathing can become laboured and the iguana becomes more frightened. When I see that Forest is scared I always pick him up and hold him against me, he then press his head against my face and slowly turn back to normal.

Humans have a diaphragm and ribcage that assist our breathing. Iguanas do not have a diaphragm, they rely on the muscles of their ribcage to help move air in and out of their lungs.

The windpipe is an air passageway reinforced with cartilage and leads from the mouth and branches into two bronchi, each of which connects to a passageway to each lung.

The glottis is a pair of skin flaps that protects the opening of the trachea (windpipe). In humans our glottis is mostly open and we therefore can sometimes 'breathe' in our food. In iguanas these flaps remain closed except when the iguana takes a breath or exhale. The glottis forms a seal that keeps inhaled air in the lungs and that prevents an iguana from breathing in its food while he eats.

Take a look inside your iguana's mouth when you hand feed him again, you will see that he does not swallow the food after each bite like we do, instead he keeps it in his throat / mouth.

Circulatory System

Iguanas have double circulation systems in which their blood passes through the heart twice. Blood flows from the heart to the lungs, back to the heart and then out to the body tissues before returning again to the heart.

Humans have 4 chambers, 2 atria and 2 ventricles. Iguanas have 3 chambers, 2 atria and only one ventricle that keeps oxygenated blood from mixing with de-oxygenated blood by timing its contractions to control blood pressures and by a muscular ridge that functionally divides the ventricle. Iguanas are able to direct a proportion of their blood flow away from their lungs when they are not using them, for example when they are under water. This explains how iguanas can easily stay under water for 20 minutes or more.

You cant hear an iguana's heartbeat even with a stethoscope, even a trained veterinarian find it difficult to locate an iguana's pulse points.

The blood and blood vessels deliver oxygen, carries hormones, immune cells, nutrients and several other substances to where they should be, it also removes the waste created by the organs. The blood is also responsible for distributing heat evenly through the body of a cold blooded reptile. While the iguana basks the body surface and blood vessels just below the skin warms up and flows back through the body, warming up the organs.

The spleen removes old and ineffective blood cells from the bloodstream and produces new blood cells. It also produce lymphocytes, cells that patrol the body through the blood, looking for harmful bacteria.

Hormonal System:

Pituitary Gland:
The pituitary gland produces hormones that regulates an iguana's growth rate, stimulate the muscular contractions of the digestive tract, controls the reabsorption of water by the kidney and help to control the function of the reproductive organs and adrenal gland.

It also produce a hormone that acts on melanin, a dark-coloured substance in skin cells that controls their degree of pigmentation. When the melanin clumps together in the center of the cell it appears light coloured. When the melanin spreads out over the entire cell area then the skin looks darker.

Adrenal Gland:

The adrenal gland produces adrenaline, a hormone that spurs every body cell and system to work faster. Whenever a reptile faces a threat they go into fight or flight mode, adrenalin is released to prime all the body's muscles and other parts required for fighting or fleeing. Adrenaline also slows or shuts down functions not crucial to handling immediate crisis, such as the digestion or the immune response. For this reason it is important that your reptile's life is not stressful, which causes adrenaline to flow unnecessarily. It affects proper digestion and weaken the immune system, making the reptile more vulnerable to infection and illness.

The adrenal gland also produce hormones that regulate the movement of sodium through the iguana's body cells. It helps the salt glands decide how much sodium to remove from the body through snorting.

The adrenal gland also produce hormones that affect reproductive organs and the body's use of carbohydrates.

Thyroid Gland:
The thyroid gland produces a hormone that increases metabolism. If the hormone is at low levels the iguana will be lethargic or slow and grows slower than normal. High levels of the hormone can cause the iguana's body to put too much effort into growing, causing the iguana to form thick bones and become hyperactive and nervous. The hormones produces by the thyroid gland are also involved in the actual process of skin shedding in iguanas, including the growth of new skin layers.

The thyroid gland needs iodine to create the hormones it releases and some plants contains goitrogens which reduce the amount of iodine an iguana can extract from the food that are eaten. Kale, cabbage, broccoli, cauliflower, brussels sprouts and bok choy are the cruciferous foods that cause Hypothyroidism (low levels of the hormone). Sweet natured iguanas that did not need much work in taming are usually sufferers of hypothyroidism. Fortunately this can be corrected by changing the diet and a difference will be seen within a couple of weeks.

Parathyroid Gland:

This gland regulates the levels of calcium and phosphorus in the iguana's body. Feeding the correct ration of calcium to phosphorus Ca:P is one of the most crucial balances needed for a healthy iguana. The body constantly shuffles small amounts of calcium between the blood and bones and if the diet is incorrect the iguana can easily suffer from metabolic bone disease.

There are 2 hormones, calcitonin and parathormone that help maintain the proper levels of calcium and phosphorus in the iguana's blood. Calcitonin remove the calcium from the blood and deposit it into the bone. Parathormone removes calcium form the bone and returns it to the blood and signals the kidney to retain calcium. If the diet is low on calcium the parathormone will extract calcium from the bones making it brittle and metabolic bone disease sets in.

This gland is an important part of the immune system. It protects the body from infection by producing antibodies against viruses and by making lymphocytes, which patrol the body for bacteria and other things that shouldn't be there. The hormones produced by the thymus also seem to be involved in the frequency of skin shedding.

Skeletal System
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Iguana Skeleton - lateral view of the pectoral girdle & hyobranchial apparatus

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Iguana Skeleton - dorsal view pectoral limbs

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Iguana - pelvic girdle & hind limbs

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Iguana - sternum with a bit of the interclavicle exposed medially.