Circulatory System: Overview

Introduction to Circulation

The circulatory system is essential for transporting nutrients, gases, hormones, and waste products throughout the body. It plays a critical role in maintaining homeostasis and supporting cellular function in multicellular organisms.

• Key Functions: Transport of oxygen and carbon dioxide, distribution of nutrients, removal of metabolic wastes, immune responses, and regulation of body temperature.

• Types of Circulatory Systems: Open and closed systems, with evolutionary variations among animal groups.

Types of Circulatory Systems

Open vs. Closed Circulatory Systems

• Open Circulatory System: Found in arthropods and most mollusks. Hemolymph (a fluid equivalent to blood) is pumped by a heart into the body cavities, where tissues are directly bathed in the fluid.

• Closed Circulatory System: Found in annelids, cephalopods, and all vertebrates. Blood is confined to vessels, allowing for more efficient transport and regulation.

Chambered Hearts and Evolutionary Trends

• Two-Chambered Heart: Typical of fish; consists of one atrium and one ventricle. Blood passes through the heart once per circuit.

• Three-Chambered Heart: Found in amphibians and most reptiles; consists of two atria and one ventricle, allowing some mixing of oxygenated and deoxygenated blood.

• Four-Chambered Heart: Found in birds and mammals; consists of two atria and two ventricles, completely separating oxygenated and deoxygenated blood for efficient double circulation.

Anatomy of the Vertebrate / Human Circulatory System

Structure of the Human Cardiovascular System

The human circulatory system is a closed, double-circuit system composed of the heart, blood vessels, and blood.

• Heart: Muscular organ divided into four chambers: right atrium, right ventricle, left atrium, and left ventricle.

• Blood Vessels: Arteries (carry blood away from the heart), veins (carry blood toward the heart), and capillaries (sites of exchange between blood and tissues).

• Double Circulation: Pulmonary circuit (heart to lungs and back) and systemic circuit (heart to body and back).

Pathway of Blood Flow in the Human Heart

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava.

2. Blood flows through the tricuspid valve into the right ventricle.

3. Right ventricle pumps blood through the pulmonary valve into the pulmonary arteries to the lungs.

4. Oxygenated blood returns from the lungs via pulmonary veins to the left atrium.

5. Blood passes through the mitral (bicuspid) valve into the left ventricle.

6. Left ventricle pumps blood through the aortic valve into the aorta and systemic circulation.

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including contraction and relaxation of the heart chambers.

• Diastole: Heart muscle is relaxed, chambers fill with blood. AV (atrioventricular) valves are open; semilunar valves are closed.

• Atrial Systole: Atria contract, pushing blood into the ventricles.

• Ventricular Systole: Ventricles contract, AV valves close ("lub" sound), semilunar valves open, blood is ejected into arteries.

• Heart Sounds: "Lub-dub" sounds correspond to closure of AV and semilunar valves, respectively.

Control of the Cardiac Cycle

• Sinoatrial (SA) Node: The heart's natural pacemaker, located in the right atrium, initiates electrical impulses that set the heart rate.

• Atrioventricular (AV) Node: Receives impulses from the SA node and relays them to the ventricles.

• Autonomic Nervous System: Sympathetic stimulation increases heart rate; parasympathetic stimulation decreases it. The medulla oblongata in the brainstem regulates these responses.

Electrocardiogram (ECG/EKG)

• P Wave: Atrial depolarization (atria about to contract).

• QRS Complex: Ventricular depolarization (ventricles contract).

• T Wave: Ventricular repolarization (ventricles recover).

• Abnormal Patterns: Tachycardia (fast), bradycardia (slow), fibrillation (irregular), and asystole (flatline).

Blood Vessels: Structure and Function

Types of Blood Vessels

• Arteries: Thick-walled, muscular vessels that carry blood away from the heart under high pressure.

• Arterioles: Small branches of arteries that regulate blood flow into capillaries.

• Capillaries: Microscopic vessels with thin walls for exchange of gases, nutrients, and wastes between blood and tissues.

• Venules: Small vessels that collect blood from capillaries and transport it to veins.

• Veins: Vessels with thinner walls and valves that carry blood back to the heart under lower pressure.

Blood Pressure and Flow

• Blood Pressure: The force exerted by circulating blood on vessel walls. Measured in mmHg (millimeters of mercury).

• Systolic Pressure: Pressure during ventricular contraction.

• Diastolic Pressure: Pressure during ventricular relaxation.

• Typical Values: 120/70 mmHg (systolic/diastolic) in healthy adults.

• Measurement: Sphygmomanometer is used to measure blood pressure.

• Hypertension: High blood pressure; Hypotension: Low blood pressure.

Mechanisms of Blood Flow in Veins

• Valves prevent backflow of blood.

• Muscle contractions and breathing movements help propel blood toward the heart.

• Gravity can affect venous return, especially in the legs.

Blood: Composition and Function

Components of Blood

• Plasma: Liquid component (~55% of blood) containing water, proteins, ions, nutrients, hormones, and waste products.

• Red Blood Cells (Erythrocytes): Transport oxygen using hemoglobin; biconcave shape increases surface area for gas exchange.

• White Blood Cells (Leukocytes): Defend against infection and disease; several types including lymphocytes, monocytes, and neutrophils.

• Platelets (Thrombocytes): Cell fragments involved in blood clotting.

Blood Cell Formation

• All blood cells originate from stem cells in the bone marrow.

• Erythropoietin: Hormone that stimulates red blood cell production.

Blood Clotting and Disorders

Blood Clot Formation

• Platelets adhere to damaged vessel walls and form a temporary plug.

• Clotting factors trigger the conversion of fibrinogen to fibrin, forming a stable clot.

Common Blood Disorders

• Anemia: Deficiency of red blood cells or hemoglobin.

• Leukemia: Cancer of white blood cells.

• Thrombosis: Formation of a blood clot inside a vessel.

• Embolism: Obstruction of a vessel by a clot or other material.

Cardiovascular Diseases and Treatments

Heart Disease and Atherosclerosis

• Atherosclerosis: Buildup of fatty deposits (plaques) in arteries, leading to reduced blood flow and risk of heart attack or stroke.

• Myocardial Infarction (Heart Attack): Blockage of blood flow to heart muscle, causing tissue death.

• Stroke: Blockage or rupture of blood vessels in the brain.

Treatments for Cardiovascular Disease

• Angiogram: Imaging technique to visualize blood vessels.

• Angioplasty: Procedure to open narrowed arteries, often with a stent.

• Coronary Bypass Surgery: Surgical creation of a new pathway for blood flow around blocked arteries.

• Defibrillation: Delivery of electrical energy to restore normal heart rhythm in cases of arrhythmia or cardiac arrest.

Summary Table: Types of Circulatory Systems

Key Equations

• Blood Pressure:

• Cardiac Output:

• Flow Rate (Q):

where is flow rate, is pressure difference, and is resistance.