Circulatory System

Circulatory System

  • The circulatory system can be compared to a system of interconnected, one-way roads that range from superhighways to back alleys.
  • Like a network of roads, the job of the circulatory system is to allow the transport of materials from one place to another.
  • As described in Figure below, the materials carried by the circulatory system include hormones, oxygen, cellular wastes, and nutrients from digested food.
  • Transport of all these materials is necessary to maintain homeostasis of the body. The main components of the circulatory system are the heart, blood vessels, and blood. Each of these components is described in detail belowCirculatory System

The Heart

  • The heart is a muscular organ in the chest. It consists mainly of cardiac muscle tissue and pumps blood through blood vessels by repeated, rhythmic contractions.
  • The heart has four chambers, as shown in Figurebelow: two upper atria (singular, atrium) and two lower ventricles. Valves between chambers keep blood flowing through the heart in just one direction



Blood Flow through the Heart

Blood flows through the heart in two separate loops, which are indicated by the arrows in Figure above.

  1. Blood from the body enters the right atrium of the heart. The right atrium pumps the blood to the right ventricle, which pumps it to the lungs. This loop is represented by the blue arrows in Figure
  2. Blood from the lungs enters the left atrium of the heart. The left atrium pumps the blood to the left ventricle, which pumps it to the body. This loop is represented by the red arrows in Figure



  • Unlike skeletal muscle, cardiac muscle contracts without stimulation by the nervous system. Instead, specialized cardiac muscle cells send out electrical impulses that stimulate the contractions.
  • As a result, the atria and ventricles normally contract with just the right timing to keep blood pumping efficiently through the heart


Blood Vessels

  • Blood vessels form a network throughout the body to transport blood to all the body cells.
  • There are three major types of blood vessels: arteries, veins, and capillaries. All three are shown in Figurebelow and described below


  • Arteriesare muscular blood vessels that carry blood away from the heart. They have thick walls that can withstand the pressure of blood being pumped by the heart. Arteries generally carry oxygen-rich blood. The largest artery is the aorta, which receives blood directly from the heart.
  • Veinsare blood vessels that carry blood toward the heart. This blood is no longer under much pressure, so many veins have valves that prevent backflow of blood. Veins generally carry deoxygenated blood. The largest vein is the inferior vena cava, which carries blood from the lower body to the heart.
  • Capillariesare the smallest type of blood vessels. They connect very small arteries and veins. The exchange of gases and other substances between cells and the blood takes place across the extremely thin walls of capillaries


Blood Vessels and Homeostasis

  • Blood vessels help regulate body processes by either constricting (becoming narrower) or dilating (becoming wider).
  • These actions occur in response to signals from the autonomic nervous system or the endocrine system.
  • Constriction occurs when the muscular walls of blood vessels contract. This reduces the amount of blood that can flow through the vessels. Dilation occurs when the walls relax.
  • This increases blood flows through the vessels. Constriction and dilation allow the circulatory system to change the amount of blood flowing to different organs.
  • For example, during a fight-or-flight response, dilation and constriction of blood vessels allow more blood to flow to skeletal muscles and less to flow to digestive organs.
  • Dilation of blood vessels in the skin allows more blood to flow to the body surface so the body can lose heat. Constriction of these blood vessels has the opposite effect and helps conserve body heat.


Blood Vessels and Blood Pressure

  • The force exerted by circulating blood on the walls of blood vessels is called blood pressure. Blood pressure is highest in arteries and lowest in veins.
  • When you have your blood pressure checked, it is the blood pressure in arteries that is measured. High blood pressure, or hypertension, is a serious health risk but can often be controlled with lifestyle changes or medication


Pulmonary Circulation

  • Pulmonary circulation is the part of the circulatory system that carries blood between the heart and lungs (the term pulmonary means “of the lungs”).
  • It is illustrated in Figure below. Deoxygenated blood leaves the right ventricle through pulmonary arteries, which transport it to the lungs.
  • In the lungs, the blood gives up carbon dioxide and picks up oxygen. The oxygenated blood then returns to the left atrium of the heart through pulmonary veins

Systemic Circulation

  • Systemic circulation is the part of the circulatory system that carries blood between the heart and body. It is illustrated in Figure below.
  • Oxygenated blood leaves the left ventricle through the aorta.
  • The aorta and other arteries transport the blood throughout the body, where it gives up oxygen and picks up carbon dioxide.
  • The deoxygenated blood then returns to the right atrium through veins


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