Because the blood leaving the lungs gained oxygen from the lungs, and has a full amount of oxygen. However the blood going to the heart has just traveled around the body. Therefore, it lost oxygen while traveling around the body.
The blood leaving the lungs is loaded with oxygen, while blood entering the lungs is about to get oxygen from the respiratory system.
The blood leaving the kidneys have most of the waste products removed. The kidneys do use some of the oxygen in the blood, but only a tiny amount, so the outflow is essentially clean arterial blood.
if blood entering the heart gets mixed with blood leaving the heart the the blood leaving the heart will get poluted. the blood entering the heart is poluted when it enters the heart, the heart cleans it up; so when the blood leaves the heart it is clean so if it gets polluted the person may get sick and this leads to his/her death.
carbon dioxide
Blood entering the kidneys has more water; the water is extracted by the kidneys and sent through the ureter to the bladder.
Blood entering the pulmonary circuit is deoxygenated. Blood leaving the pulmonary circuit is oxygenated.
pumping
if blood entering the heart gets mixed with blood leaving the heart the the blood leaving the heart will get poluted. the blood entering the heart is poluted when it enters the heart, the heart cleans it up; so when the blood leaves the heart it is clean so if it gets polluted the person may get sick and this leads to his/her death.
All blood leaving the GI tract first goes to the liver through the hepatic portal vein for processing and detoxification before entering the general circulation.
You just broke my brain cells
The blood entering from the lungs has a high oxygen content and the blood entering from the body is high in carbon dioxide.
Blood coming from the lungs has lower pressure while that from the heart has higher pressure. Blood coming from the lungs also has higher oxygen content and lower carbon dioxide content compared to that coming from the heart.