Bio 6.2: The Circulatory System PDF

# Unit 6: The Circulatory System ## The Pulmonary and Systemic Circuits The circulatory system is composed of two circuits that work closely together to deliver oxygen and transport waste around the body. These are called the **Pulmonary Circuit** and the **Systemic Circuit**. The difference betwe...

# Unit 6: The Circulatory System ## The Pulmonary and Systemic Circuits The circulatory system is composed of two circuits that work closely together to deliver oxygen and transport waste around the body. These are called the **Pulmonary Circuit** and the **Systemic Circuit**. The difference between them lies in where blood is travelling and what it is carrying: pay close attention to whether the blood is **oxygenated** or **deoxygenated**. ### Pulmonary Circuit The Pulmonary Circuit is comprised of the right atrium + ventricle and pulmonary trunk, as well as arteries that deal strictly with the heart and the lungs. The Pulmonary arteries are the only arteries in the body that carry deoxygenated blood, while the pulmonary veins are the only veins in the body that carry oxygenated blood. Remember, the general function of the arteries is to carry blood away from the heart _(efferent)_ and the function of veins is to carry blood to the heart _(afferent)_. The function of the pulmonary arteries is to bring deoxygenated blood from the right side of the heart (right ventricle) to the lungs to pick up oxygen for eventual transport to the whole body, while the pulmonary veins return oxygenated blood from the lungs to the left atrium of the heart, before pumping it all over the body. #### Key Summary: - Pulmonary Arteries bring deoxygenated blood from the right side of the heart to lungs to pick up oxygen; pulmonary veins bring oxygenated blood from lungs to the left side of the heart. - right ventricle -> pulmonary trunk -> pulmonary arteries -> lung capillaries -> pulmonary veins -> left atrium ## Systemic Circuit The Systemic Circuit comprises the left atrium + ventricle of the heart, as well as the aorta. It receives oxygenated blood from the lungs and pumps it to the entire body. The left atrium receives the blood from the pulmonary veins, pushes it down to the left ventricle, and up through the aorta, which has several branches that feed the rest of the body. Once the blood is in the tissues and becomes deoxygenated again, it returns through the veins back into heart via the vena cava. Therefore, the heart is divided in half: - the right half makes up the pulmonary circuit and contains deoxygenated blood, while the - left half makes up the Systemic circuit and contains oxygenated blood. ### Blood Flow: - left ventricle -> aorta -> arteries -> capillaries -> veins -> vena cava -> right atrium ## Cross Sectional Area There are several relationships governing the efficiency of blood vessels. These include: 1. Blood vessel diameter 2. Blood velocity 3. Blood pressure 4. Total cross-sectional Area You will need to understand the differences between each concept, as well as how each affect blood flow in the systemic circuit. The following graphs are a great representation of these concepts: (Image of graphs depicting vessel diameter, total cross-sectional area of vessels, blood pressure, and velocity of blood flow in different types of blood vessels) ### 1. Vessel Diameter Diameter refers to the size of the space inside a blood vessel. Generally, diameter is largest in the Veins, similarly large in the arteries, and very small in the capillaries. Diameter has a great effect on blood pressure. Arteries are surrounded by muscular walls that can dilate _(vasodilate)_ or constrict _(vasoconstrict)_ to adjust vessel diameter. Because diameter closely affects blood pressure, dilating blood vessels decreases blood pressure by allowing blood to flow through with less restriction. Constricting blood vessels causes more resistance, thus increasing blood pressure. ### 2. Blood Velocity Blood velocity refers to how quickly blood travels through a vessel. Velocity is highest in the arteries and slows down drastically in the capillaries. This allows for more efficient gas exchange in the capillary networks. Blood velocity remains low in the veins, but it is faster than capillaries. Blood velocity is inversely proportional to total cross-sectional area. As cross-sectional area increases, velocity of the blood decreases. Velocity decreases from arteries to arterioles to capillaries and increases slightly in venules and veins. ### 3. Blood Pressure Blood Pressure is highest in the arteries and lowest in the capillaries. Once blood pressure is "lost" in the capillaries, it does not increase again when blood moves into the veins. The veins are too far from the heart, and veins do not contain muscles for vasoconstriction or vasodilation. ### 4. Cross-Sectional Area Total cross-sectional area refers to the sum of the area of all blood vessels of a specific type in the circulatory system. Essentially, the more numerous the vessel type, the more total cross-sectional area there is. Capillaries have the highest cross-sectional area, while veins are second, and arteries are third. Despite arteries being large, they are less numerous and have a smaller overall diameter. (Image of a graph depicting the relationships between blood pressure, blood velocity, and total cross-sectional area) As the total cross-sectional area increases, vessel diameter and blood velocity decrease. ### Can you answer these questions without checking your notes? 1. Which type of blood vessel has the highest blood pressure? Lowest? - **Highest: Arteries** - **Lowest: veins + capillaries** 2. Which type of blood vessel has the highest velocity? Lowest? - **Highest: Arteries** - **Lowest: Capillaries** 3. Which type of blood vessel has the highest CSA? Lowest? - **Highest: Capillaries** - **Lowest: Arteries** 4. Which type of blood vessel has the highest blood pressure? Lowest? - **Highest: Veins** - **Lowest: Capillaries**