The Circulatory System

The circulatory system, also known as the cardiovascular system, consists of the heart and blood vessels. It is responsible for pumping blood throughout the body, delivering oxygen and nutrients to organs and tissues, and removing waste products.

Anatomy of the Heart

• The heart is about the size of a person's fist and is located in the middle of the chest cavity.

• It sits on top of the diaphragm, behind the sternum, and in front of the vertebral column.

• The heart is protected by ribs and surrounded by a fluid-filled sac called the pericardium.

• The pericardium has two layers: parietal (outer layer) and visceral (inner layer).

• These layers secrete a protein-rich fluid that lubricates the heart.

Layers of the Heart

• Epicardium: Outer layer of the heart formed by the visceral layer of pericardium.

• Myocardium: Middle muscular layer responsible for contracting and pumping blood.

• Contains cardiac muscle cells and collagen fibers that form a fibrous cardiac skeleton.

• Coronary vessels supply blood to this layer for energy production.

• Endocardium: Innermost thin layer made up of endothelium that lines heart chambers and valves.

Right Side of the Heart

• Deoxygenated blood enters through superior vena cava, inferior vena cava, or coronary sinus into right atrium.

• Blood flows through tricuspid valve into right ventricle.

• Tricuspid valve prevents backflow with cordae tendineae attached to papillary muscles.

• Blood is pumped out through pulmonary valve into pulmonary arteries towards lungs.

Pulmonary Circulation

• In lungs, carbon dioxide diffuses from capillaries to alveoli while oxygen moves from alveoli to capillaries.

• Oxygen-rich blood returns to the heart through pulmonary veins, entering left atrium.

• Blood flows through mitral valve into left ventricle.

• Mitral valve prevents backflow with cordae tendineae attached to papillary muscles.

• Blood is pumped out through aortic valve into the aorta for systemic circulation.

Aortic Circulation

• Aorta branches into smaller arteries called arterioles and eventually capillaries.

• Exchange of oxygen, nutrients, and waste products occurs in capillaries within tissues.

• Deoxygenated blood returns to the heart via venules and veins.

The Heart's Chambers and Valves

This section focuses on the chambers and valves of the heart, explaining their structure and function.

Right Atrium

• Receives deoxygenated blood from superior vena cava, inferior vena cava, and coronary sinus.

• Tricuspid valve separates right atrium from right ventricle.

Right Ventricle

• Receives deoxygenated blood from right atrium through tricuspid valve.

• Contracts to pump blood through pulmonary valve into pulmonary arteries towards lungs.

Left Atrium

• Receives oxygenated blood from pulmonary veins returning from lungs.

• Mitral (bicuspid) valve separates left atrium from left ventricle.

Left Ventricle

• Receives oxygenated blood from left atrium through mitral valve.

• Contracts to pump blood through aortic valve into the aorta for systemic circulation.

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This section discusses the journey of blood from the body back to the heart through the systemic circulation.

Blood Flow in Systemic Circulation

• Blood returns to the heart through small venules and larger veins.

• The lower half of the body drains into the inferior vena cava, while the upper half drains into the superior vena cava.

• Both veins dump blood back into the right atrium.

• The trip from the left ventricle of the heart to the body and back to the right atrium is called systemic circulation.

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This section explains how systemic circulation differs from pulmonary circulation and highlights its greater resistance to blood flow.

Systemic Circulation vs. Pulmonary Circulation

• Systemic circulation has more blood vessels compared to pulmonary circulation.

• There is about five times greater resistance to blood flow in systemic circulation.

• The left ventricle needs to be stronger due to this difference, resulting in a thicker myocardium compared to the right ventricle.

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This section describes each heartbeat and its sounds, known as S1 and S2.

Heartbeat Sounds

• Each heartbeat produces a sound like "ldub ldub ldb."

• The first heart sound (S1) occurs when tricuspid and mitral valves snap shut during contraction of both ventricles.

• After S1, aortic and pulmonic valves open, allowing blood to be pushed out to the body. This period is called systole.

• The second heart sound (S2) occurs when aortic and pulmonic valves snap shut after blood leaves the ventricles, preventing backward flow. This marks diastole.

• After S2, tricuspid and mitral valves open again, allowing blood to fill the ventricles. This period is called diastole.

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This section explains systolic and diastolic blood pressure and introduces the concepts of cardiac output and venous return.

Systolic Blood Pressure, Diastolic Blood Pressure, Cardiac Output, and Venous Return

• Systolic blood pressure is the pressure in the arteries when the ventricles contract and squeeze out blood.

• Diastolic blood pressure is the pressure in the arteries when the ventricles are filling up with more blood.

• Cardiac output refers to the amount of blood pumped out by either ventricle over a period of time.

• Venous return is the rate at which veins return blood back to the atria.

• In a closed-loop circulatory system, cardiac output and venous return are equal.

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This section provides numerical examples to illustrate cardiac output and discusses the distribution of systemic arterial blood.

Numerical Examples: Cardiac Output and Systemic Arterial Blood Distribution

• Assuming 70 mlit is ejected per squeeze with a heart rate of 70 beats per minute (bpm), each minute pumps about 4.9 L (cardiac output).

• An average adult has approximately 5 liters (lers) of total blood volume in their body.

• About 10% (.5 L) is in pulmonary circulation, while another 5% (.25 L) resides in one of the four chambers of the heart itself.

• Approximately 15% (75 lers) is found in systemic arteries traveling away from the heart.

• Systemic capillaries contain about 5% (.25 L), while systemic veins hold around 65% (3.25 L).

• The distribution of systemic arterial blood is as follows: 15% to the brain, 5% to nourish the heart, 25% to the kidneys, another 25% to gastrointestinal organs, 25% to skeletal muscles, and 5% to the skin.

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This section highlights the differences between arteries and veins in terms of volume and pressure.

Arteries vs. Veins

• Arteries generally have lower volume but higher pressure compared to veins.

• Veins are high-volume, low-pressure vessels.

• Veins often have valves that help prevent backward flow and assist blood flow back to the heart against gravity.

• Arteries do not require valves due to their higher pressure.

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This section explains the three layers of blood vessels and their functions.

Layers of Blood Vessels

• Blood vessels have three layers or tunics: tunica intima (innermost), tunica media (middle), and tunica externa (outermost).

• The tunica intima includes endothelial cells that create a smooth surface for blood flow.

• The tunica media consists mostly of smooth muscle cells and elastin protein sheets.

• The tunica externa is made up of collagen fibers that protect and reinforce the vessel.

• Some large vessels have a thick tunica externa with its own blood supply through tiny vessels called vasa vasorum.

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This section focuses on elastic arteries and their ability to stretch and maintain shape.

Elastic Arteries

• The largest arteries closest to the heart, such as the aorta, main branches, and pulmonary arteries, are known as elastic arteries.

• Elastic arteries contain a significant amount of elastin in their tunica externa and tunica media.

• Elasticity allows these arteries to stretch, maintain their shape, and absorb and even out systolic and diastolic pressures.

New Section

This section concludes the discussion on blood vessels by highlighting the differences between arteries and veins.

Arteries vs. Veins (Continued)

• Arteries have a bulky tunica media that can contract in response to hormones and the autonomic nervous system (vasoconstriction).

• Veins are usually under lower pressure compared to arteries.

• Arteries do not require valves due to higher pressure, while veins often have valves to prevent backward flow and aid blood return to the heart.

• The structure of blood vessels reflects their different functions in the circulatory system.

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This section explains the role of blood vessels in regulating body temperature and the functions of capillaries.

Blood Vessels and Body Temperature Regulation

• Blood carries heat, and when it gets close to the skin surface, more heat is lost.

• Vasodilation of arterials allows more heat to be lost, helping lower body temperature.

• Vasoconstriction reduces blood flow, resulting in less heat loss through the skin.

Functions of Capillaries

• Capillary walls are thin and allow for exchange of oxygen, carbon dioxide, nutrients, and fluid.

• Water-soluble substances cross capillary walls through water-filled spaces or large pores.

• Lipid-soluble molecules like oxygen and carbon dioxide can dissolve and diffuse across endothelial cell membranes.

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This section discusses the connection between capillary beds, venules, and arteries in circulation.

Capillary Beds and Circulation

• Capillary beds connect arterioles (arterial) with venules (venous).

• The pulmonary circulation starts with the right ventricle delivering blood to the lungs.

• Fresh oxygenated blood from the lungs enters systemic circulation via the left atrium.

• Deoxygenated blood returns to the right atrium, completing the cycle.

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The video concludes with acknowledgments and suggestions for further learning on osmosis.org.

Conclusion and Further Learning

• Acknowledgments given to those involved in creating the video.

• Viewers are encouraged to visit osmosis.org for a deeper dive into the content.

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