Digestive System and Blood Circulation in Vertebrates

Digestive System

Initial Stage: Stomach

The first stage of digestion occurs in the stomach. Here, digestive substances break down food. The stomach cavity is wider than the intestinal walls and secretes gastric juices. This section also includes glands like the liver, which secrete substances that convert food molecules into smaller ones.

Second Stage: Small Intestine

In the second stage, nutrients are absorbed in the small intestine. This organ's extensive length and folded inner walls maximize absorption surface area.

Final Stage: Large Intestine

The last stage takes place in the large intestine (colon), where water is absorbed. This section is wider than the small intestine and lacks the folds of the previous section.

Transport of Nutrients

Blood Vessels: Arteries, Capillaries, and Veins

Arteries, with their larger diameter and thicker walls, branch into a network of smaller ducts called capillaries. Capillaries have thin walls, allowing blood to flow close to body cells. This proximity facilitates nutrient absorption and waste removal (e.g., CO₂). Veins, similar in size to arteries but with thinner walls, collect blood from capillaries and return it to the heart, completing the circulatory system.

Circulatory Routes

Vertebrates have two main circulatory routes: systemic circulation and pulmonary circulation. Systemic circulation carries oxygenated blood throughout the body. Once the blood is deoxygenated, it enters pulmonary circulation, which carries it to the respiratory system to be re-oxygenated.

The Heart

Systole and Diastole

During systole, the ventricles (larger heart cavities) contract, pushing blood into the arteries. Simultaneously, the atria (smaller cavities) receive blood returning to the heart. During diastole, the atria contract, opening valves between the atria and ventricles, allowing blood to flow between them.