Just like the atrial chambers (the left atrium and right atrium), the ventricular chambers also have a wall dividing them. The left ventricle and the right ventricle are divided by the ventricular septum and it is this septum that prevents arterial blood from mixing with venous blood.
When arterial blood mixes with venous blood, there are a multitude of potential side effects that could occur to a person as a result of this mixing. First and foremost, the main effect of the mixing the blood together is that the oxygenated and non-oxygenated blood groups will intermingle with each other, creating an imbalance between the blood coming in and leaving the heart. Some of the blood coming into the heart will already be oxygenated and some blood will leave the heart with poor oxygenation. This poorly oxygenated blood will circulate around the body and organs that are very sensitive to changes in blood oxygen levels (such as the Brain) may become damaged if a prolonged period of poor blood oxygenation takes place.
Additionally, there are also blood pressure concerns that can arise from the mixing of arterial and venous blood, not to mention the possibility of chronic pulmonary hypertension problems. These are the kinds of problems that can result from a hole in heart condition, especially if that condition is a ventricular septal defect (a hole in the ventricular septum). In general it is a very undesirable problem to have because of the large number of complications that can result from it.
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