From days 23 through 28, the heart tube folds and twists, with the future ventricles moving left of center (the ultimate location of the heart) and the atria moving towards the head.On day 28, areas of tissue in the heart tube begin to expand inwards; after about two weeks, these expansions, the membranous "septum primum" and the muscular "endocardial cushions", fuse to form the four chambers of the heart. A failure to fuse properly will result in a defect that may allow blood to leak between chambers. After this happens, cells that have migrated from the neural crest begin to divide the bulbus cordis, the main outflow tract is divided in two by the growth a spiraling septum, becoming the great vessels—the ascending segment of the aorta and the pulmonary trunk. If the separation is incomplete, the result is a "persistent truncus arteriosus". The vessels may be reversed ("transposition of the great vessels"). The two halves of the split tract must migrate into the correct positions over the appropriate ventricles. A failure may result in some blood flowing into the wrong vessel (e.g.overriding aorta). The four-chambered heart and the great vessels have features required for fetal growth. The lungs are unexpanded and cannot accommodate the full circulatory volume. Two structures exist to shunt blood flow away from the lungs.
Low humidity
Low humidity from air conditioning was found to be the most common cause of physical irritant contact dermatitis. To the lay person a definition of low humidity being a physical irritant can be confusing because low humidity is a deficit (or absence) of an elemental substance, whereas all other irritants implicated in contact dermatitis are in concentrations of relative abundance. So the irritant is actually a lack of water vapour. This confusion is further compounded with the use of the term contact implying touching (as is the case with all other forms of physical irritant contact dermatitis) whereas in the case of low humidity physical irritant contact dermatitis there is an absence of contact with water vapour. Plants
Many plants cause irritant contact dermatitis through their spines or irritant hairs. Some plants such as the buttercup, spurge, and daisy act by chemical means. The sap of these plants contains a number of alkaloids, glycosides, saponins, anthraquinones, and (in the case of plant bulbs) irritant calcium oxalate crystals - all of which can cause crystal irritant contact dermatitis.Butternut squash and Acorn squash have been known to cause an allergic reaction in many individuals, especially in food preparation where the squash skin is cut and exposed to the epidermis.