Human Circulatory System Overview

Human Circulatory System

• The circulatory system is a closed system of vessels (intravascular compartment)
• Blood volume in an average human is approximately 5 liters
• Blood cells have short lifespans and are continually replaced through hematopoiesis
• Blood carries oxygen and nutrients to all body cells and carries carbon dioxide and waste products to the organs for elimination
• Blood carries ions, hormones, proteins, and other signaling molecules to their final destinations
• Blood helps regulate body temperature and maintains the acid/base and osmotic balance of body fluids
• Blood provides optimum hydration for extravascular compartments
• Blood forms in the bone marrow, circulates in blood vessels
• Red blood cells (RBCs) carry oxygen
• Platelets stop bleeding
• White blood cells (WBCs) protect the body
• The heart pumps circulation
• Blood vessels control what stays in circulation and what leaks out to different tissues
• Blood components are continuously recycled

Blood Components

• Blood is a mixture of cellular elements (erythrocytes, leukocytes, and platelets), fluid (plasma), ions, proteins, nutrients, wastes, and gases
• Red blood cells (RBCs) = (4.8-5.4 million/µL of blood)
• White blood cells (WBCs) = (4,000 - 11,000/μL of blood)
• Platelets = (150,000-400,000/ µL of blood)
• RBC life span: 90-120 days
• WBC life span: 13-20 days (varies by type)
• Platelet life span: 8-12 days

The Red Blood Cell

• Biconcave disc (20-30% more surface area than a sphere)
• No nucleus or organelles
• Filled with hemoglobin (iron-containing protein; capable of carrying oxygen)
• Average diameter: 7 µm (smallest capillary is 3-4 µm)
• 2-4 million new RBCs are produced per second in the bone marrow
• Recycled by macrophages in the reticuloendothelial system (RES) (liver, spleen, and lymph nodes)

White Blood Cells (Leukocytes)

• Colorless cells involved in fighting foreign substances and diseases
• Two main types: Granulocytes and Agranulocytes
• Types of granulocytes include: basophils, neutrophils, and eosinophils
• Types of agranulocytes include: monocytes and lymphocytes

Platelets (Thrombocytes)

• Small, colorless, disc-shaped cell fragments
• No nucleus
• Found in large numbers in blood
• Involved in clotting
• Primary function is to rapidly release the content of their granules upon contact with collagen or other materials outside of the endothelium to begin clot formation

Mechanism of Blood Clotting

• Vascular spasm: Smooth muscle in blood vessel walls contracts immediately blood vessel is broken
• Platelet plug formation: Platelets form a platelet plug to close the gap in the broken blood vessel
• Coagulation: Blood normally remains liquid in vessels. When it leaves, it thickens and forms a gel (coagulation). This is a complex process involving many clotting factors (calcium ions, enzymes, platelets, damaged tissues) activating each other.

Hematopoiesis

• Earliest hematopoiesis in the fetus occurs in the yolk sac
• During the second trimester, it occurs mainly in the liver and spleen
• By the end of the second trimester, it also involves the bone marrow in the medullary cavity of long bones
• In adults, it occurs mainly in bones of the axial skeleton

The Heart

• A bag of strong muscle sheets enforced with fibrous tissue
• Receives blood from different regions of the body through the venous system and pushes it into the arterial system
• Located in the mediastinum between the two lungs
• Powered by its own internal pulse generator
• Four chambers (two atria for receiving blood and two ventricles for sending blood)
• Each chamber fills with blood when the muscle relaxes (diastole), then pushes blood when the muscle contracts (systole).
• Ventricles need more force than atria (thicker muscles)
• Atria must push blood to ventricles first
• Blood MUST move in one direction

Heart Valves

• Heart valve disease can affect any of the four heart valves
• These doors separate different parts of the heart.
• A diseased valve strains the heart, complicating to heart failure or sudden cardiac death.
• Common valve diseases are regurgitation (leakage) or stenosis (narrowing)

Blood Vessels

• Walls have three layers: tunica intima, tunica media, and tunica externa
• Arteries are thicker than veins

Blood Pressure

• Blood pressure is the pressure of blood as it flows through blood vessels
• It exists in all blood vessels
• Highest in arteries, lowest in veins
• Factors controlling blood pressure include blood volume, arterioles diameter, and blood viscosity

Heart Sounds

• Two distinct sounds are heard, often described as "lub-dub"
• The first sound (lub) is caused by vibrations and abrupt closure of atrioventricular (AV) valves during ventricular contraction (prevents backflow)
• The second sound (dub) is caused by closure of aortic and pulmonary valves when ventricles relax

Blood Pressure Conditions

• Hypertension - high blood pressure throughout the arteries in the body
• Hypotension- low blood pressure throughout the body

Exercises

• Diuretics reduce mean blood pressure
• Vasoconstrictors increase mean blood pressure

Pulse

• Pulse is the rhythmic expansion and recoiling of arteries
• Pulse points are named after the arteries and can be used to assess heart rate, strength, and rhythm

Circulatory System Disorders

• Atherosclerosis: Plaque buildup in arteries leading to reduced blood flow and risk of blood clots.
• Carotid Artery Stenosis: Plaque buildup in neck arteries, blood supply to the brain
• Coronary Artery Disease (CAD): Plaque buildup in arteries supplying the heart
• Peripheral Artery Disease: Plaque buildup in arteries supplying arms and legs
• CHF / Congestive Heart Failure: Caused by the heart's inability to pump effectively.
• Arrhythmias: Irregular or abnormal heartbeats, can have various causes and types.
• Aneurysms: Weak spots in artery walls that can expand, potentially rupturing or causing blood clots

Other Conditions

• Vasculitis: Inflammation of blood vessels caused by an overactive immune system
• Venous Diseases: Conditions affecting the veins (e.g., Chronic venous insufficiency, Deep vein thrombosis)
• Angina Pectoris: Chest pain or discomfort due to insufficient oxygen-rich blood supply to the heart
• Stroke: Life-threatening emergency; blood flow to the brain is interrupted (ischemic or hemorrhagic)
• High Cholesterol: Elevated levels of fats in the blood, increasing the risk of other heart conditions
• Cardiomyopathy: Weakening or hardening of the heart muscles
• Congenital Heart Disease: Heart problems present at birth

Explore the intricacies of the human circulatory system with this quiz. Discover how blood circulates, the roles of various blood components, and the regulatory functions it performs in the body. Test your knowledge on blood volume, cell types, and the mechanics of the heart.