Understanding the Circulatory and Respiratory Systems

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Blood Flow and the Circulatory System

Blood flow refers to the circulation of blood pumped by the heart through the circulatory system. In humans, blood circulation is:

  • Double: Blood passes through the heart twice in every complete cycle.
  • Complete: Oxygen-rich blood and carbon dioxide-rich blood do not mix because the two sides of the heart are separated by the septum.

Pulmonary Circuit

The pulmonary circuit, or shorter circuit, is the path blood takes between the heart and the lungs. The right side of the heart pumps blood through this circuit.

  1. Blood carrying carbon dioxide, waste, and nutrients picked up throughout the body enters the right atrium through the inferior vena cava and then passes into the right ventricle.
  2. The blood leaves the right ventricle through the pulmonary arteries and moves into the vessels of the lungs, where it releases carbon dioxide and picks up oxygen.
  3. Oxygenated, nutrient-rich blood travels through the pulmonary veins from the lungs to the left atrium.

Systemic Circuit

The systemic circuit, or longer circuit, is the path blood takes between the heart and the different organs of the body. The left side of the heart pumps blood through this circuit.

  1. Oxygenated, nutrient-rich blood passes from the left atrium into the left ventricle and then into the aorta. This artery branches off into smaller arteries, which spread throughout the body.
  2. These arteries, in turn, branch into capillaries, which distribute oxygen and nutrients to all the body's cells and pick up the waste produced by them. They also pick up nutrients obtained from digestion in the intestinal villi.
  3. The capillaries merge into veins, which flow into the venae cavae.
  4. The venae cavae lead to the right atrium, where the pulmonary circuit begins.

Components of Blood

  • Red Blood Cells: Transport oxygen and carbon dioxide around the body. They are shaped like biconcave disks.
  • White Blood Cells: Larger and generally spherical. They defend our bodies against pathogens and tumor cells. There are different types of white blood cells.
  • Platelets: Help our bodies to clot. They are not true cells but pieces of cytoplasm.

Types of Blood Vessels

  • Arteries: Branch off into smaller arteries. Their walls are thick and elastic. They carry blood from the heart to the organs.
  • Veins: Their walls are thinner and less elastic. They have valves to prevent the backflow of blood. They carry blood from the organs to the heart.
  • Capillaries: Microscopic blood vessels that reach all the cells. They join the arterioles and the venules. Their walls are very thin and allow the exchange of substances.

The Respiratory System


The respiratory system exchanges gases with the external environment. It takes oxygen from the air and transports it to the blood system while taking carbon dioxide from the blood and expelling it from the body.

Pleura and Pleural Liquid

The pleural liquid protects the lungs from rubbing against the rib cage and facilitates the movements involved in breathing.


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