Circulatory System

The circulatory system uses a series of organs and vessels to move blood transporting oxygen, nutrients, and chemical signals to, and waste from, the cells throughout the body. The heart serves as the system pump.

Deoxygenated blood from the body enters the upper right chamber of the heart known as the right atrium. It passes down through a one-way valve into the right ventricle, filling the chamber. When the heart beats, it squeezes this blood, pumping it through another one-way valve toward the lungs. The blood passes through the lungs where it acquires oxygen and releases carbon dioxide. Blood flows out of the lungs and into the upper left atrium. From there it passes through another one-way valve into the left ventricle. When the heart beats, it squeezes the blood in the left ventricle, pumping it into the aorta. The aorta is the large artery that branches into the aortic arch and thoracic and abdominal aortas, which transport blood to the arms, neck, and head, chest, and vital organs, and the iliac arteries, which supply blood to the lower limbs. Some of the blood passes through the digestive tract, where nutrients are absorbed, and through the liver, which converts nutrients into substances that the body can use. Some of the blood passes through the kidneys, which remove waste and excess water.

Blood is passes from large arteries to smaller vessels and into capillaries where oxygen and nutrients are delivered and carbon dioxide and waste is removed. Next, it passes into small then larger veins before returning to the heart, completing the cycle.

The system has some interesting characteristics. Essentially all the blood in the system passes through either the right or left lung. However, only a portion of the blood passes through the kidneys, which “cleans” the blood. This clean blood mixes in with the blood that was not cleaned and recirculates. One of the functions of the liver is to neutralize toxins in the blood. Similar to the kidneys, only a portion of the blood receives this treatment before being recombined with the regular flow. In this way, the human body differs from most engineered systems that pass 100% of the flow through the filter.

Blood has non-Newtonian fluid properties that can result in errors if it is modeled as a Newtonian fluid. Blood is a shear thinning fluid, which means that its viscosity decreases with shear stress. In large volume flow, it can be characterized as a homogeneous fluid. However, when passing though channels near the diameter of a red blood cell, the fluid is better characterized as a mixture of plasma (liquid) and cells.

Vessels in the human body are elastic. As a result, they swell under high blood pressure and constrict back during each heartbeat. This helps to regulate pressure spikes and maintain pressure between pumps. Engineered mechanical systems typically use rigid piping.  Accumulators that contain a flexible diaphragm or piston are often used to achieve the same purpose.

The circulatory system is the primary transport system in the body. It is used to transport hormones for the endocrine system, nutrients from the digestive system, white blood cells for the immune system, and waste from the lymphatic system. Shared responsibility is common with most body systems. This approach differs from engineered systems that typically serve one distinct role.