Correct answer: (2) excretory system-remove potentially dangerous materials from the body

The primary organ of the excretory system is the kidney, which filers the blood to remove wastes, thereby maintaining homeostasis. The basic unit of the kidney is a nephron, which serves as a filtering unit. As blood flows through the nephron and then to the capillaries of the glomerulus in high pressure, water, glucose, vitamins, amino acids, protein waste products, salts, and ions from the blood pass out of the capillaries into the Bowman's capsule. Since blood cells and most proteins are too large to pass through the walls of the capillaries, these components are retained in the blood vessels.

From the Bowman's capsule, the filtered liquid passes through a u-shaped tubule where most of the ions and water, and all of the glucose and amino acids are reabsorbed into the bloodstream to maintain homeostasis. Thus, excess water, waste molecules, and excess ions become urine, which flows out of the kidneys through the ureter, then into the urinary bladder, and, after which, exits the body through the urethra.

Incorrect answers:

(1) immune system-intake and distribution of oxygen to cells of the body

The immune system protects the body against invasion of disease-producing agents such as bacteria, fungi, and viruses. The body system in charge of intake and distribution of oxygen to the cells of the body is the respiratory system.

(3) digestive system-transport energy-rich molecules to cells

The main function of the digestive system is to disassemble ingested food into its simpler form so that it can be used as energy for the body. The transport of energy-rich molecules to cells is a function of the circulatory system.

(4) circulatory system-produce building blocks of complex compounds