Circulation and Gas Exchange: Structure and Function of the Cardiovascular System

Circulatory Systems and Exchange

Concept 42.1: Circulatory Systems Link Exchange Surfaces with Cells Throughout the Body

The circulatory system is essential for transporting nutrients, gases, and wastes between exchange surfaces and cells throughout the body. Small molecules move between cells and their surroundings primarily by diffusion, which is only efficient over short distances. The time required for diffusion increases with the square of the distance, making circulatory systems necessary for larger organisms.

• Diffusion: The passive movement of molecules from an area of higher concentration to an area of lower concentration.

• Limitation: Diffusion is effective only over microscopic distances.

• Circulatory System: Links exchange surfaces (e.g., lungs, intestines) with body cells.

Open and Closed Circulatory Systems

Circulatory systems consist of a circulatory fluid, a set of interconnecting vessels, and a muscular pump (the heart). They connect the fluid surrounding cells with organs that exchange gases, absorb nutrients, and dispose of wastes. Circulatory systems can be open or closed.

• Open Circulatory System: Circulatory fluid (hemolymph) bathes organs directly. Found in insects, other arthropods, and some molluscs.

• Closed Circulatory System: Circulatory fluid (blood) is confined to vessels and is distinct from interstitial fluid. Found in annelids, cephalopods, and vertebrates.

Organization of Vertebrate Circulatory Systems

Structure of the Cardiovascular System

Humans and other vertebrates have a closed circulatory system called the cardiovascular system, which includes the heart and blood vessels. The three main types of blood vessels are arteries, veins, and capillaries. Blood flows only within these vessels.

• Arteries: Carry blood away from the heart and branch into arterioles.

• Capillaries: Networks called capillary beds are the sites of chemical exchange between blood and interstitial fluid.

• Veins: Converge from venules and return blood from capillaries to the heart.

• Direction of Blood Flow: Arteries and veins are distinguished by the direction of blood flow, not by oxygen content.

• Heart Chambers: Vertebrate hearts contain two or more chambers. Blood enters through atria and is pumped out through ventricles.

Single and Double Circulation

• Single Circulation: Found in sharks, rays, and bony fishes. Blood passes through two capillary beds before returning to the heart, which has two chambers.

• Double Circulation: Found in amphibians, reptiles, and mammals. Blood is pumped from the right side of the heart to the lungs (pulmonary circuit) and from the left side to the body (systemic circuit).

Heart Function and Blood Flow

Concept 42.2: Coordinated Cycles of Heart Contraction Drive Double Circulation in Mammals

The mammalian cardiovascular system meets the body's continuous demand for oxygen through coordinated heart contractions.

Mammalian Circulation

• Contraction of the right ventricle pumps blood to the lungs via pulmonary arteries.

• Blood flows through capillary beds in the lungs, loading oxygen and unloading carbon dioxide.

• Oxygen-rich blood returns to the left atrium via pulmonary veins, then flows into the left ventricle and is pumped out to body tissues via the systemic circuit.

• Blood leaves the left ventricle via the aorta, which branches to supply the heart muscle and other body regions.

• Oxygen diffuses from blood to tissues, and carbon dioxide diffuses from tissues to blood.

• Veins return oxygen-poor blood to the heart via the superior and inferior vena cava.

The Mammalian Heart: Structure and Cycle

• The human heart is about the size of a fist and consists mainly of cardiac muscle.

• The two atria have thin walls and serve as collection chambers for blood returning to the heart.

• The ventricles have thicker walls and contract more forcefully to pump blood.

• The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle.

• Systole: Contraction (pumping) phase.

• Diastole: Relaxation (filling) phase.

Cardiac Output and Heart Rate

• Cardiac Output: Volume of blood pumped into systemic circulation per minute. Depends on heart rate and stroke volume.

• Heart Rate: Number of beats per minute.

• Stroke Volume: Amount of blood pumped in a single contraction.

• Formula: $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

Heart Valves and Sounds

• Four valves prevent backflow of blood in the heart.

• Atrioventricular (AV) valves: Separate each atrium and ventricle.

• Semilunar valves: Control blood flow to the aorta and pulmonary artery.

• The "lub-dup" sound of a heartbeat is caused by the recoil of blood against the AV valves (lub) and then against the semilunar valves (dup).

• Backflow through a defective valve causes a heart murmur.

Maintaining the Heart's Rhythmic Beat

• Some cardiac muscle cells are autorhythmic, contracting without nervous system signals.

• The Sinoatrial (SA) node, or pacemaker, sets the rate and timing of cardiac muscle contractions.

• Impulses during the cardiac cycle can be recorded as an electrocardiogram (ECG).

• Impulses from the SA node travel to the Atrioventricular (AV) node, are delayed, then travel to bundle branches and Purkinje fibers that make the ventricles contract.

• The pacemaker is regulated by the sympathetic (increases rate) and parasympathetic (decreases rate) divisions of the nervous system, as well as by hormones and temperature.

Blood Vessels and Circulation

Concept 42.3: Patterns of Blood Pressure and Flow Reflect Structure and Arrangement of Blood Vessels

• All blood vessels contain a lumen lined with an epithelial layer called endothelium.

• The endothelium is smooth and minimizes resistance.

• Capillaries are only slightly wider than a red blood cell and have thin walls to facilitate exchange.

Blood Flow Velocity

• Blood flow slows as it moves from arteries to arterioles to capillaries due to increased total cross-sectional area.

• Flow speeds up as blood enters venules and veins, where total cross-sectional area decreases.

Blood Pressure

• Blood flows from areas of higher pressure to lower pressure.

• Blood pressure: Force exerted by blood against vessel walls.

• Elastic arterial walls help maintain blood pressure.

• Resistance in capillaries and arterioles dissipates much of the pressure.

Changes in Blood Pressure During the Cardiac Cycle

• Systolic pressure: Pressure in arteries during ventricular systole; highest pressure.

• Pulse: Rhythmic bulging of artery walls with each heartbeat.

• Diastolic pressure: Pressure in arteries during ventricular diastole; lower than systolic pressure.

Regulation of Blood Pressure

• Homeostatic mechanisms regulate arterial blood pressure by altering arteriole diameter.

• Vasoconstriction: Narrowing of arteriole walls; increases blood pressure.

• Vasodilation: Increase in arteriole diameter; decreases blood pressure.

Fluid Return by the Lymphatic System

• The lymphatic system returns fluid (lymph) that leaks out from capillary beds.

• Fluid lost by capillaries is called lymph.

• Lymphatic system drains into veins in the neck; valves prevent backflow.

• Edema: Swelling caused by disruptions in lymph flow.

• Lymph nodes: Filter lymph and play a role in immune defense; become swollen during infection.

Blood Components and Function

Concept 42.4: Blood Components Function in Exchange, Transport, and Defense

• Open circulation: Fluid is continuous with interstitial fluid.

• Closed circulation: Vertebrate blood is a specialized connective tissue.

Blood Composition and Function

• Blood consists of cells suspended in a liquid matrix called plasma.

• Cellular elements (erythrocytes, leukocytes, platelets) occupy about 45% of blood volume.

Plasma

• Contains inorganic salts as dissolved ions (electrolytes).

• Electrolytes influence blood pH and osmotic balance.

• Plasma proteins function in lipid transport, immunity, and clotting.

• Plasma is similar to interstitial fluid but has higher protein concentration.

Cellular Elements

• Red blood cells (erythrocytes): Transport oxygen using hemoglobin.

• White blood cells (leukocytes): Function in defense; five major types.

• Platelets: Cell fragments involved in clotting.

Erythrocytes

• Most numerous blood cells; contain hemoglobin, an iron-containing protein that binds up to four molecules of oxygen.

• Mature erythrocytes lack nuclei and mitochondria.

• Sickle-cell disease: Caused by abnormal hemoglobin proteins forming aggregates, deforming cells, and reducing oxygen transport.

Leukocytes

• Five major types: neutrophils, lymphocytes, monocytes, eosinophils, basophils.

• Function by phagocytizing bacteria/debris or mounting immune responses.

• Found both inside and outside the circulatory system.

Platelets

• Fragments of cells; function in blood clotting.

Blood Clotting

• Formation of a solid clot from liquid blood.

• Cascade of reactions converts inactive fibrinogen to fibrin.

• Blood clot formed within a vessel is called a thrombus and can block blood flow.

Cardiovascular Disease

Atherosclerosis, Heart Attacks, and Stroke

• Atherosclerosis: Hardening of arteries due to fatty deposits (plaque).

• Cholesterol: Steroid important for membrane fluidity; excess can contribute to plaque formation.

• Inflammation attracts leukocytes, which take up lipids and contribute to plaque growth.

• Plaque rupture can form a thrombus, triggering heart attack or stroke.

• Heart attack (myocardial infarction): Damage/death of cardiac muscle due to blocked coronary arteries.

• Stroke: Death of nervous tissue in the brain due to blocked/ruptured arteries.

• Angina pectoris: Chest pain from partial coronary artery blockage.

Risk Factors and Treatment

• LDL (Low-Density Lipoprotein): Delivers cholesterol to cells.

• HDL (High-Density Lipoprotein): Scavenges excess cholesterol for return to liver.

• Risk increases with high LDL/HDL ratio and inflammation.

• Aspirin inhibits inflammation, reducing risk.

• Hypertension (high blood pressure): Contributes to heart attack, stroke, and other problems; controlled by diet, exercise, and medication.

Table: Comparison of Open and Closed Circulatory Systems

Feature	Open Circulatory System	Closed Circulatory System
Circulatory Fluid	Hemolymph	Blood
Fluid Location	Bathes organs directly	Confined to vessels
Organisms	Insects, arthropods, some molluscs	Annelids, cephalopods, vertebrates

Table: Major Types of Blood Cells

Cell Type	Function
Erythrocytes (Red Blood Cells)	Transport oxygen
Leukocytes (White Blood Cells)	Defense (immune response, phagocytosis)
Platelets	Blood clotting

Additional info: These notes expand upon the provided questions and diagrams, filling in academic context and definitions for a comprehensive study guide on Chapter 42: Circulation and Gas Exchange.