Chapter 6.1: Pulmonary and Systemic Circulation
Overview of the Circulatory System
Blood serves as the primary transportation system within the human body, facilitating the movement of nutrients, gases, and waste products through complex networks of vessels.
Types of Blood Vessels
Arteries * Function: Transports blood away from the heart. The largest arteries, such as the aorta and pulmonary artery, are characterized as elastic. * Mechanics: These vessels stretch when the ventricles eject blood from the heart and subsequently recoil, which helps move the blood along the systemic and pulmonary circuits. * Branching: Large arteries branch into smaller vessels known as arterioles. * Structure: They possess thick, muscular, and elastic walls to withstand high pressure. * Location: Typically found deep within the body, situated along bones for protection.
Veins * Function: Transports blood back toward the heart. * Structure: Characterized by thin walls and the presence of internal valves to prevent the backflow of blood. * Location: Often located near the surface of the body and surrounded by skeletal muscle, which aids in blood return via contraction. * Venules: Defined as very small veins, specifically those responsible for collecting blood directly from the capillary networks.
Capillaries * Function: Act as the interconnecting link between arteries and veins; they are the site of nutrient and gas exchange. * Structure: Extremely thin walls, consisting of a single layer of cells (1 cell thick). * Comparison: Capillaries are so small that it requires ten of them to equal the thickness of a single human hair. * Location: Distributed throughout the entire body; most body cells are within a few cells' distance of a capillary. * Regulation: Equipped with sphincter muscles that can dilate or constrict. * If all capillary beds were open simultaneously, systemic blood pressure would decrease. * If all capillary beds were closed, systemic blood pressure would increase.
Arterioles and Venules * These vessels share the same structural and functional features as arteries and veins but exist on a much smaller scale. * Arterioles leading into specific organs or regions frequently contain sphincter muscles. * Regulation of Blood Flow: When triggered, these muscles dilate or constrict to regulate blood pressure and increase or decrease blood flow to specific capillary beds. * Terminology: * Afferent Arteriole: Refers to the incoming arteriole. * Efferent Arteriole: Refers to the outgoing arteriole.
Major Blood Vessels of the Human Body
Aorta * This is the largest artery in the body, with a diameter approximately equal to that of a garden hose. * Path: It exits the left ventricle, loops over the top of the heart (creating the aortic arch), and descends along the inside of the backbone. * Function: Carries oxygenated blood away from the heart to the rest of the systemic body.
Coronary Arteries and Veins * The coronary arteries are the very first branches of the aorta and are visible on the heart's surface. * Function of Arteries: Carry nutrients and oxygen to the heart cells. * Physiological Rationale: The heart muscle is too dense and thick to receive nutrients from the blood passing through its chambers, and the internal blood flow is too aggressive and fast for diffusion. * Function of Veins: The coronary veins return "spent blood" to the heart. * Definition of Spent Blood: A term describing blood that has delivered its oxygen to cells and picked up carbon dioxide; it is low in oxygen and high in carbon dioxide.
Carotid Arteries * These branch off the aortic arch to deliver blood to the head and brain. * Specialized Sensors: They contain nerve endings for homeostasis: * Chemoreceptors: Detect oxygen content in the blood. * Pressure receptors: Detect changes in blood pressure.
Jugular Veins * The venous counterpart to the carotid arteries. * Unlike most veins, they do not contain valves; blood flow is facilitated primarily by gravity. * Function: Conduct deoxygenated blood out of the head toward the superior vena cava and then to the heart.
Subclavian Arteries and Veins * Branch from the aorta and travel underneath the clavicle. * Function: Supply blood to the arms (via the brachial artery) and return blood from the arms (via the brachial vein).
Mesenteric Arteries and Veins * Branch off the aorta as it travels posteriorly. * Function: These vessels deliver blood to the organs of the digestive system and branch into capillaries identified as villi to pick up newly digested nutrients from the digestive tract. * Return: Large veins return the nutrient-rich blood from the intestines.
Hepatic Portal Vein * Formed from one of the mesenteric veins. * Function: Transports nutrient-rich blood from the digestive tract directly to the liver.
Hepatic Vein and Hepatic Artery * Hepatic Vein: Carries filtered blood from the liver to the posterior vena cava. * Hepatic Artery: Carries oxygenated blood to the liver; it branches from the celiac artery, which originates from the aorta.
Renal Arteries and Veins * Branch off the dorsal aorta as it passes through the lumbar region. * Function: Renal arteries deliver blood to the kidneys, while renal veins carry filtered blood away from the kidneys back to the posterior vena cava.
Iliac and Femoral Arteries and Veins * The dorsal aorta branches in the pelvic area into two iliac arteries (one for each leg), which lead to the femoral arteries. * The femoral veins drain into the left and right common iliac veins, which merge in the abdomen at the level of the fifth lumbar vertebra to form the inferior vena cava. * Function: Supply oxygenated blood to the legs and pelvis and return deoxygenated blood.
Vena Cava * The largest vein in the body, which collects all "spent" blood from smaller veins to return it to the right atrium. * Anterior (Superior) Vena Cava: Collects blood from the upper body. * Posterior (Inferior) Vena Cava: Collects blood from the lower body.
The Pulmonary and Systemic Circuits
Pulmonary Circuit * Scope: Comprised of the pulmonary trunk and arteries; deals strictly with the heart and lungs. * Exceptions: This circuit contains the only arteries in the body that carry deoxygenated blood and the only veins that carry oxygenated blood. * Function: Arteries bring deoxygenated blood from the right side of the heart to the lungs for oxygenation. Veins return oxygenated blood from the lungs to the left atrium. * Path: Right ventricle $\rightarrow$ pulmonary trunk $\rightarrow$ pulmonary arteries $\rightarrow$ lung capillaries $\rightarrow$ pulmonary veins $\rightarrow$ left atrium. * Chemical states: Carries carbaminohemoglobin (carbon dioxide filled) to the lungs and returns oxyhemoglobin (oxygen rich) to the heart.
Systemic Circuit * Scope: The path from the left ventricle to the body tissues and back to the right atrium. * Function: Delivers highly oxygenated arterial blood to body tissues and returns carbon dioxide-filled blood. * Internal Respiration: The site of the chemical reaction: .
Hemodynamics: Cross-Sectional Area and Velocity
Total Cross-Sectional Area (CSA): The sum of the cross-sectional area of all blood vessels of a specific type. This has a major effect on the mechanics of blood flow.
Velocity and Area Relationship: As the cross-sectional area increases, the velocity of the blood decreases. * Velocity decreases from arteries to arterioles to capillaries. * Velocity increases as blood moves into venules and veins.
Pressure and Area Relationship: As CSA increases, blood pressure decreases. * Capillaries have the highest CSA, leading to the lowest blood pressure and lowest velocity. * Critical Note: Once blood pressure is lost in the capillaries, it cannot be regained, even though velocity increases in the venules and veins as vessel diameter increases.
Physiological Data Summary
Blood Pressure Peaks: Highest in the arteries; lowest in the veins.
Blood Velocity: Highest in the arteries (with an increase seen returning in the veins); lowest in the capillaries (necessary to provide time for nutrient and waste exchange).
Cross-Sectional Area (CSA): Highest in the capillaries; lowest in the arteries and veins.
Tracing the Path of Blood
From the Head to the Toe: * Jugular vein $\rightarrow$ Vena cava $\rightarrow$ Right atrium $\rightarrow$ Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary vein $\rightarrow$ Left atrium $\rightarrow$ Left ventricle $\rightarrow$ Aorta $\rightarrow$ Iliac artery $\rightarrow$ Femoral artery.
From the Arm to the Liver: * Brachial vein $\rightarrow$ Subclavian vein $\rightarrow$ Vena cava $\rightarrow$ Right atrium $\rightarrow$ Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary vein $\rightarrow$ Left atrium $\rightarrow$ Left ventricle $\rightarrow$ Aorta $\rightarrow$ Mesenteric artery $\rightarrow$ Small intestine $\rightarrow$ Hepatic portal vein $\rightarrow$ Liver. * Alternative route from aorta: Aorta $\rightarrow$ Celiac artery $\rightarrow$ Hepatic artery $\rightarrow$ Liver.
From the Heart Muscle to the Kidney: * Coronary vein $\rightarrow$ Vena cava $\rightarrow$ Right atrium $\rightarrow$ Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary vein $\rightarrow$ Left atrium $\rightarrow$ Left ventricle $\rightarrow$ Aorta $\rightarrow$ Renal artery $\rightarrow$ Kidney.