Each organ system performs specific functions for the body, and each organ system is typically studied independently. However, the organ systems also work together to help the body maintain homeostasis.

For example, the cardiovascular, urinary, and lymphatic systems all help the body control water balance. The cardiovascular and lymphatic systems transport fluids throughout the body and help sense both solute and water levels and regulate pressure. If the water level gets too high, the urinary system produces more dilute urine (urine with a higher water content) to help eliminate the excess water. If the water level gets too low, more concentrated urine is produced so that water is conserved. The digestive system also plays a role with variable water absorption. Water can be lost through the integumentary and respiratory systems, but that loss is not directly involved in maintaining body fluids and is usually associated with other homeostatic mechanisms.

Similarly, the cardiovascular, integumentary, respiratory, and muscular systems work together to help the body maintain a stable internal temperature. If body temperature rises, blood vessels in the skin dilate, allowing more blood to flow near the skin’s surface. This allows heat to dissipate through the skin and into the surrounding air. The skin may also produce sweat if the body gets too hot; when the sweat evaporates, it helps to cool the body. Rapid breathing can also help the body eliminate excess heat. Together, these responses to increased body temperature explain why you sweat, pant, and become red in the face when you exercise hard. (Heavy breathing during exercise is also one way the body gets more oxygen to your muscles, and gets rid of the extra carbon dioxide produced by the muscles.)

Conversely, if your body is too cold, blood vessels in the skin contract, and blood flow to the extremities (arms and legs) slows. Muscles contract and relax rapidly, which generates heat to keep you warm. The hair on your skin rises, trapping more air, which is a good insulator, near your skin. These responses to decreased body temperature explain why you shiver, get “goose bumps,” and have cold, pale extremities when you are cold.

As you have learned, blood glucose homeostasis is regulated by two hormones from the pancreas. This glucose provides the fuel for ATP production by all body cells. But the endocrine system is not the only system involved.

Many body cells respond to insulin and glucagon, but the liver of the digestive system plays in important role in ensuring the availability of fuel in-between meals. Under the influence of insulin, the anabolic process of glycogenesis (-genesis means “origin” or “birth”) in the liver converts excess glucose entering liver cells to polymerize into glycogen for storage. Under the influence of glucagon, the reverse catabolic reaction of glycogenolysis (-lysis means “break up”) will convert the glycogen back into glucose for release into the blood stream. The liver cells can also perform gluconeogenesis (-neo means “new”), which creates glucose from non-carbohydrate sources, mainly from specific amino acids.

The nervous system also plays a role in maintaining blood glucose levels. When the stomach is empty and blood glucose levels are low, the digestive system receptors and the brain respond by making you feel hungry—your stomach may “growl,” and you may feel pain or discomfort in your midsection. These sensations prompt you to eat, which provides new nutrient sources to raise blood glucose levels. The exocrine part of the pancreas is also part of the digestive system. It produces enzymes that help digest the nutrients you have eaten so they can be absorbed by the small intestine into the blood. The circulatory system is important in transporting the glucose and pancreatic hormones in blood to all body cells.

Blood Calcium Levels

As you have learned, proper calcium levels are important for normal function of several systems. Calcium ions are used for blood clotting, the contraction of muscles, the activation of enzymes, and cellular communication. The parathyroid gland of the endocrine system is the main receptor and control center for blood calcium levels. When the parathyroid glands detect low blood calcium levels, they communicate with several organ systems and alter their function to restore blood calcium levels back to normal. The skeletal, urinary, and digestive systems all act as effectors to achieve this goal through negative feedback.

The release of parathyroid hormone from the endocrine system triggers osteoclasts of the skeletal system to breakdown (resorb) bone and release calcium into the blood. Similarly, this hormone causes the kidneys of the urinary system to reabsorb calcium and return it to the blood instead of excreting calcium into the urine. Through altered function of the kidneys to form active vitamin D, the small intestine of the digestive system increases the absorption of calcium.

When blood calcium levels are elevated, the parathyroid gland senses that as well. But in this case, instead of increasing its secretion of parathyroid hormone, it decreases secretion of the hormone. This decreases bone reabsorbtion, increases calcium levels in the urine and decreases calcium absorption in the intestines.

Blood Glucose Levels

The endocrine functions of the pancreas and liver coordinate efforts to maintain normal blood glucose levels. When pancreatic cells detect low blood glucose levels, the pancreas synthesizes and secretes the hormone glucagon. Glucagon causes the liver to convert the polymerized sugar glycogen into glucose through a process known as glycogenolysis. Glucose then travels through the blood to allow all cells of the body to use it.

If pancreatic cells detect high blood glucose levels, the pancreas synthesizes and releases the hormone insulin. Insulin causes polymerization of glucose into glycogen, which is then stored in the liver through a process known as glycogenesis.

The nervous and digestive systems also play a role in maintaining blood glucose levels. When the stomach is empty and blood glucose levels are low, the digestive system and the brain respond by making you feel hungry—your stomach may “growl,” and you may feel pain or discomfort in your midsection. These sensations prompt you to eat, which raises blood glucose levels.

Cell Count

All organ systems require a balance of cell division and apoptosis during development, growth, and repair to maintain tissue structure and function. The endocrine and immune systems are important regulators for cell populations. The endocrine system delivers steroids and growth hormones that send survival signals to specific tissues so that apoptosis is prevented. Additionally, the endocrine system delivers some hormones that work to induce apoptosis under some physiological conditions.

The cells of the immune system screen the blood for cells that divide at inappropriate times. Immune cells produce antibodies to mark these out-of-control cells for destruction. A breakdown in these processes can lead to the formation of tumors.