Circulation: Structure and Function of the Cardiovascular System

Circulation: Chapter 23

Introduction to Circulatory Systems

The circulatory system is essential for sustaining life in animals by facilitating the exchange of nutrients, gases, and waste products with all body tissues. Internal transport systems bring resources close enough to cells for diffusion to be effective.

• Key Functions: Acquire nutrients, exchange gases, dispose of waste products.

• Circulatory systems facilitate these exchanges in most animals.

Types of Circulatory Systems

Gastrovascular Cavities

Found in simple animals such as cnidarians and flatworms, gastrovascular cavities function in both digestion and transport.

• Example: Aurelia (moon jelly), Dugesia (planarian)

Open Circulatory Systems

Open circulatory systems are found in all arthropods and most molluscs. They consist of:

• A tubular heart

• Open-ended vessels

• Blood (called haemolymph) that directly bathes the cells and functions as the interstitial fluid

There is no distinction between blood and interstitial fluid in open systems.

Closed Circulatory Systems

Closed circulatory systems are found in vertebrates, earthworms, squids, and octopuses. They consist of:

• A circulatory fluid, blood, confined to vessels

• Blood is kept distinct from the interstitial fluid

Cardiovascular System in Vertebrates

The vertebrate circulatory system is called the cardiovascular system and includes three main types of vessels:

1. Arteries/Arterioles: Carry blood away from the heart to organs and tissues

2. Veins/Venules: Return blood to the heart

3. Capillaries: Convey blood between arteries and veins within each tissue

Evolution of Vertebrate Cardiovascular Systems

Vertebrate cardiovascular systems have evolved to reflect different circulatory needs:

• Fish: Two-chambered heart, single circulation

• Amphibians and many reptiles: Three-chambered hearts, double circulation (pulmonary and systemic circuits)

• Birds and mammals: Four-chambered hearts, double circulation

Single vs. Double Circulation

• Single circulation: Blood passes through the heart once per circuit (e.g., fish)

• Double circulation: Blood passes through the heart twice per circuit (e.g., mammals, birds, amphibians)

The Human Cardiovascular System and Heart

Heart Structure

The human heart consists of four chambers: two atria and two ventricles. It is divided into right and left sides, each serving different circuits:

• Pulmonary circuit: Heart and lungs

• Systemic circuit: Heart and the rest of the body

Heart Interior

• Valves: Ensure one-way flow of blood (e.g., tricuspid, mitral, semilunar valves)

• Major vessels: Aorta, pulmonary arteries and veins, vena cava

The Cardiac Cycle

The heart contracts and relaxes rhythmically in a cycle:

• Diastole: Heart chambers are relaxed; blood flows in from veins

• Systole: Atria contract to push blood into ventricles; ventricles contract to propel blood into arteries

Cardiac Output and Heart Rate

• Cardiac output: Volume of blood each ventricle pumps per minute

• Heart rate: Number of heart beats per minute

Equation:

• $ext{Cardiac Output} = ext{Heart Rate} imes ext{Stroke Volume}$

Electrical Control of the Heart

SA and AV Nodes

• SA (sinoatrial) node: Generates electrical impulses in atria; sets heart contraction rate

• AV (atrioventricular) node: Relays signals to ventricles; triggers ventricular contractions

Electrocardiogram (ECG/EKG)

• Records electrical changes in the heart

• Used to assess heart health and diagnose arrhythmias

Heart Rate Regulation

• Heart rates adjust to body needs

• Abnormal rhythms may occur in heart attacks

• Automatic external defibrillators (AEDs) can reset the SA node

Structure and Function of Blood Vessels

Arteries

• Elastic fibers allow recoil after stretching

• Thick smooth muscle layer can constrict and reduce blood flow

Veins

• Thinner walls, lower blood pressure and velocity

• One-way valves permit blood flow only toward the heart

Capillaries

• Thin walls allow exchange of gases and fluids with interstitial fluid

Blood Pressure and Velocity

• Blood pressure: Force exerted by blood on vessel walls

• Depends on cardiac output and resistance of vessels to expansion

• Decreases as blood moves away from the heart

Blood Pressure Equation

• $ext{Blood Pressure} = ext{Cardiac Output} imes ext{Total Peripheral Resistance}$

Capillary Exchange

• Blood pressure forces fluid and small solutes out at the arterial end

• Osmotic pressure draws fluid in at the venous end

• Excess fluid is returned via lymph vessels

Types of Capillaries

Type	Structure	Function
Continuous	Few/no pores	Limited permeability
Fenestrated	Many pores	High permeability
Discontinuous (sinusoids)	Large gaps	Allows passage of large molecules

The Lymphatic System

• Maintains fluid balance by returning excess fluid to the circulatory system

• Requires a solute concentration gradient for fluid movement via osmosis

• Albumin and other plasma proteins maintain osmotic pressure

Blood Vessel Disorders

• Varicose veins: Gnarled, enlarged veins due to valve failure

Measuring Blood Pressure and Cardiovascular Health

• Blood pressure measured as systolic/diastolic

• Hypertension: Persistent high blood pressure; major risk for heart attack, stroke, kidney failure

Category	Systolic (mmHg)	Diastolic (mmHg)
Normal	<120	<80
Prehypertension	120–139	80–89
Stage 1 Hypertension	140–159	90–99
Stage 2 Hypertension	≥160	≥100

Cardiovascular Disease

• Heart attack: Damage or death of cardiac muscle tissue, usually from blocked coronary artery

• Stroke: Death of brain tissue due to lack of oxygen from blocked/ruptured arteries

• Atherosclerosis: Chronic disease where fatty plaques narrow arteries, impeding blood flow

Structure and Function of Blood

• Plasma: Contains ions, proteins, nutrients, wastes, gases, hormones

• Red blood cells (erythrocytes): Transport oxygen via hemoglobin

• White blood cells (leukocytes): Fight infections; some are phagocytes

• Platelets: Cell fragments involved in blood clotting

Plasma Proteins

• Albumins: Maintain osmotic balance

• Globulins: Immune defense

• Fibrinogen: Clotting

Red Blood Cell Regulation

• Erythropoietin (EPO): Hormone that stimulates red blood cell production

• Artificially increasing red blood cells (e.g., blood doping) can be dangerous

Blood Clotting

• Platelets adhere to damaged tissue and help convert fibrinogen to fibrin, forming a clot

• Clotting prevents blood loss and seals vessel injuries

Summary Table: Circulatory System Components

Component	Function
Heart	Pumps blood
Arteries	Carry blood away from heart
Veins	Return blood to heart
Capillaries	Exchange substances with tissues
Lymphatic vessels	Return excess fluid to blood
Red blood cells	Transport oxygen
White blood cells	Immune defense
Platelets	Blood clotting

Additional info: The notes include expanded definitions, examples, and tables for clarity and completeness, suitable for college-level General Biology study.