Digestive System
When blood is pumped from the left ventricle of the heart, much of it passes down the dorsal aorta to the organs of the abdomen. Chief among these organs are those of the digestive tract
Within the digestive tract the food is broken down to nutrient molecules small enough to be absorbed by the villi of the small intestine. Digestive enzymes are produced by the digestive tract and by the pancreas. In addition the liver produces bile, an emulsifier that plays a role in the digestion of fats. Bile, which is stored in the gallbladder, enters the small intestine along with the pancreatic enzymes. Following the absorption of nutrients, blood passes from the region of the small intestine to the liver by way of the hepatic portal vein.
The liver, which monitors the blood, is a very important organ of homeostasis. The liver breaks down toxic substances like alcohol and other drugs, and it produces urea, the end product of nitrogenous metabolism. The liver produces the plasma proteins and stores glucose as glycogen after eating. In between eating it releases glucose, thereby keeping the blood glucose concentration constant. The liver destroys old blood cells and breaks down hemoglobin--hemoglobin breakdown products are excreted in bile.
Urinary System
As blood passes through the kidneys, urine is made and excreted. Urine is composed of substances not needed by cells: end-products of metabolism (e.g., urea) and excess salts and water. In the process of making urine, blood is first filtered and all small molecules, including both nutrients and wastes, enter a nephron. Then the nutrient molecules and much of the salts and water are reabsorbed back into the blood, while unwanted substances remain within the nephron to become a part of urine. Tubular secretion is another way by which certain molecules enter a nephron just before urine enters a collecting duct.