The Circulatory System

• The human circulatory system is a closed system, meaning blood is confined to vessels and the heart.
• Its primary function is to transport oxygen, nutrients, hormones, and waste products throughout the body.
• It plays important roles in immune response and the regulation of body temperature and pH.

Components

• The circulatory system consists of the heart, blood vessels (arteries, veins, capillaries), and blood.

Heart

• The human heart is a muscular organ that pumps blood throughout the body.
• It is located in the thoracic cavity between the lungs.
• The heart has four chambers: two atria (right and left) and two ventricles (right and left).
• The right atrium receives deoxygenated blood from the body via the superior and inferior vena cavae and the coronary sinus.
• The left atrium receives oxygenated blood from the lungs via the pulmonary veins.
• The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery.
• The left ventricle pumps oxygenated blood to the body via the aorta.
• Valves within the heart ensure unidirectional blood flow.
• The atrioventricular valves (tricuspid on the right, mitral/bicuspid on the left) prevent backflow from the ventricles into the atria during ventricular contraction (systole).
• The semilunar valves (pulmonary and aortic) prevent backflow from the arteries into the ventricles during ventricular relaxation (diastole).
• The heart's pumping action is driven by rhythmic contractions and relaxations of the cardiac muscle (myocardium).
• The sinoatrial (SA) node, located in the right atrium, initiates the electrical impulse that triggers each heartbeat.
• The SA node is the heart's natural pacemaker.
• The impulse spreads through the atria, causing them to contract.
• The impulse then reaches the atrioventricular (AV) node, which delays the signal briefly before passing it to the ventricles.
• The impulse travels down the bundle of His and through the Purkinje fibers, causing the ventricles to contract.

Blood Vessels

• Arteries carry blood away from the heart.
• They typically carry oxygenated blood, except for the pulmonary artery, which carries deoxygenated blood to the lungs.
• Arteries have thick, elastic walls that can withstand the high pressure of blood pumped from the heart.
• Arterioles are smaller branches of arteries that regulate blood flow to capillaries.
• Capillaries are tiny, thin-walled vessels that allow for the exchange of gases, nutrients, and waste products between blood and tissues.
• They form a vast network throughout the body, ensuring that no cell is far from a blood supply.
• Veins carry blood back to the heart.
• They typically carry deoxygenated blood, except for the pulmonary veins, which carry oxygenated blood from the lungs.
• Veins have thinner walls than arteries and contain valves to prevent backflow, especially in the limbs where blood has to flow against gravity.
• Venules are small veins that collect blood from capillaries and merge into larger veins.

Blood

• Blood is a connective tissue composed of plasma, red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
• Plasma is the liquid component of blood, consisting mostly of water, along with dissolved proteins, electrolytes, nutrients, and waste products.
• Red blood cells contain hemoglobin, a protein that binds to oxygen and transports it from the lungs to the tissues.
• They are anucleate in mammals to maximize space for hemoglobin.
• White blood cells are involved in the immune response, defending the body against infection and disease.
• There are different types of white blood cells, each with a specific function (e.g., neutrophils, lymphocytes, monocytes, eosinophils, basophils).
• Platelets are small, cell fragments involved in blood clotting.
• They aggregate at the site of injury and release factors that initiate the clotting cascade.

Circulation Pathways

• The circulatory system consists of two main circuits: pulmonary circulation and systemic circulation.
• Pulmonary circulation carries blood between the heart and the lungs for gas exchange.
• Deoxygenated blood is pumped from the right ventricle to the lungs via the pulmonary artery.
• In the lungs, carbon dioxide is removed from the blood, and oxygen is absorbed.
• Oxygenated blood returns to the left atrium via the pulmonary veins.
• Systemic circulation carries blood between the heart and the rest of the body.
• Oxygenated blood is pumped from the left ventricle to the body via the aorta.
• Blood delivers oxygen and nutrients to the tissues and picks up carbon dioxide and waste products.
• Deoxygenated blood returns to the right atrium via the superior and inferior vena cavae.
• Coronary circulation supplies blood to the heart muscle itself:
  • The heart has its own network of blood vessels.
  • The left and right coronary arteries originate from the aorta near its origin.
  • These arteries and their branches provide the heart muscle (myocardium) with oxygen and nutrients.
  • Blockage of these arteries can lead to a heart attack (myocardial infarction).
• Hepatic portal circulation is a specialized pathway that directs blood from the digestive organs to the liver before it returns to the heart:
  • Blood from the stomach, intestines, spleen, and pancreas flows into the hepatic portal vein.
  • The hepatic portal vein carries this blood to the liver.
  • The liver processes the nutrients, filters toxins, and regulates blood glucose levels before the blood returns to the systemic circulation.

Blood Pressure

• Blood pressure is the force exerted by blood against the walls of blood vessels.
• It is typically measured using a sphygmomanometer and expressed as systolic pressure over diastolic pressure (e.g., 120/80 mmHg).
• Systolic pressure is the pressure when the heart contracts (systole).
• Diastolic pressure is the pressure when the heart relaxes (diastole).
• Blood pressure is influenced by factors such as cardiac output, blood volume, and peripheral resistance.
• The nervous system and endocrine system regulate blood pressure through various mechanisms, including vasoconstriction, vasodilation, and hormone release.
• High blood pressure (hypertension) can damage blood vessels and increase the risk of heart disease, stroke, and kidney disease.
• Low blood pressure (hypotension) can cause dizziness, fainting, and other symptoms.

Regulation of Circulation

• The circulatory system is regulated by both nervous and endocrine mechanisms.
• The autonomic nervous system controls heart rate, blood vessel diameter, and blood pressure.
• The sympathetic nervous system increases heart rate and blood pressure, while the parasympathetic nervous system decreases heart rate and blood pressure.
• Hormones such as epinephrine (adrenaline), norepinephrine, and antidiuretic hormone (ADH) can also affect circulation.
• Epinephrine and norepinephrine increase heart rate and blood pressure.
• ADH increases water reabsorption in the kidneys, which increases blood volume and blood pressure.

Common Disorders

• Atherosclerosis is a condition in which plaque builds up inside the arteries, narrowing them and reducing blood flow.
• Hypertension (high blood pressure) is a major risk factor for heart disease, stroke, and kidney disease.
• Coronary artery disease is a condition in which the coronary arteries become narrowed or blocked, reducing blood flow to the heart muscle.
• Heart failure is a condition in which the heart is unable to pump enough blood to meet the body's needs.
• Stroke occurs when blood flow to the brain is interrupted, either by a blood clot or a burst blood vessel.
• Arrhythmias are irregular heartbeats, which can be too fast, too slow, or erratic.
• Anemia is a condition in which the blood has a lower than normal number of red blood cells or hemoglobin.
• Deep vein thrombosis (DVT) is a blood clot that forms in a deep vein, usually in the leg.