Chapter 34: Circulation and Gas Exchange

Essential vocabulary covering circulatory systems, heart anatomy and cycle, hemodynamics, blood components, respiratory surfaces, ventilation mechanics, and respiratory pigments from Campbell Biology Chapter 34.

Gastrovascular Cavity

A central internal cavity found in cnidarians (such as hydras) and flatworms that functions in both the distribution of substances throughout the body and digestion, placing body cells in direct contact with the exchange medium.

Open Circulatory System

A circulatory arrangement found in arthropods and some mollusks in which circulatory fluid is pumped through open-ended vessels into sinuses, bathing the organs and body tissues directly.

Hemolymph

The circulatory fluid characteristic of open circulatory systems that is identical to and continuous with the interstitial fluid bathing body cells.

Closed Circulatory System

A circulatory system found in annelids, cephalopods, and all vertebrates in which blood is completely confined within vessels and remains distinct from the interstitial fluid.

Single Circulation

A circulatory pattern found in sharks, rays, and bony fishes where blood leaves a two-chambered heart and passes through two capillary beds (gill capillaries, then body capillaries) in a single loop before returning to the heart.

Double Circulation

A circulatory system found in amphibians, reptiles, and mammals where blood is pumped separately by the right side (to respiratory gas exchange tissues) and the left side (to systemic capillary beds) of the heart in two distinct circuits.

Pulmocutaneous Circuit

The respiratory circuit in amphibians that delivers oxygen-poor blood from the right atrium to gas exchange capillary beds situated in both the lungs and the skin.

Foramen of Panizza

A specialized vascular shunt located where the pulmonary artery and left aorta exit the heart in crocodilians, allowing blood to be redirected away from the lungs and toward the systemic circulation when submerged.

Myocardium

The thick, muscular tissue wall of the heart (labeled $H$ on the anatomical heart diagram) that performs coordinated contractions during systole.

Tricuspid Valve

The atrioventricular (AV) valve situated between the right atrium and right ventricle (labeled $E$ on the heart diagram key) that prevents the retrograde flow of blood into the right atrium during ventricular systole.

Bicuspid (Mitral) Valve

The atrioventricular (AV) valve positioned between the left atrium and left ventricle (labeled $J$ on the heart diagram key) that prevents backflow of blood into the left atrium during ventricular contraction.

Semilunar Valves

Valves positioned at the exits of the ventricles into the pulmonary artery and the aorta that prevent blood from leaking back into the ventricles during ventricular diastole.

Cardiac Cycle

One complete rhythmic sequence of pumping and filling of the heart, composed of a contraction phase (systole) and a relaxation phase (diastole).

Systole

The contraction phase of the cardiac cycle during which heart chambers actively pump blood out (lasting approximately $0.1\,\text{sec}$ for atrial systole and $0.3\,\text{sec}$ for ventricular systole).

Diastole

The relaxation phase of the cardiac cycle during which heart chambers relax and fill with blood, including a combined atrial and ventricular diastole period of about $0.4\,\text{sec}$.

Heart Murmur

A hissing or abnormal sound generated when blood squirts backward through a defective or malfunctioning valve in the heart.

Sinoatrial (SA) Node

A group of autorhythmic cardiac muscle cells in the wall of the right atrium that serves as the heart's pacemaker, setting the rate and timing of all electrical impulses driving heart contractions.

Atrioventricular (AV) Node

A region of specialized heart tissue that receives impulses from the SA node and delays them for approximately $0.1\,\text{sec}$ before transmitting them to the heart apex via bundle branches, allowing the atria to empty completely before ventricular contraction.

Endothelium

The simple squamous epithelial layer lining the central lumen of all blood vessels, presenting a smooth surface that minimizes friction and resistance to blood flow.

Vasoconstriction

The narrowing of arteriole lumen diameter driven by the contraction of smooth muscle in arteriole walls, which increases resistance and elevates upstream arterial blood pressure.

Vasodilation

The widening of arteriole lumen diameter resulting from the relaxation of smooth muscles in arteriole walls, which reduces vascular resistance and lowers blood pressure.

Precapillary Sphincters

Rings of smooth muscle positioned at the entrance of capillary beds that contract and relax to regulate the distribution and volume of blood entering specific capillary networks.

Lymphatic System

A specialized drainage and defense network that recovers lost interstitial fluid and proteins (lymph) from capillary beds, filters the fluid through lymph nodes, and returns it to venous blood via veins in the neck.

Plasma

The liquid extracellular matrix of vertebrate blood, accounting for approximately $55\%$ of total blood volume, consisting of water, electrolytes, nutrients, waste products, metabolic gases, and dissolved proteins.

Erythrocytes

Biconcave cellular elements present at concentrations of $5{,}000{,}000$ to $6{,}000{,}000\,\mu\text{L}^{-1}$ of blood that lack nuclei at maturity in mammals and function specifically to transport $O_2$ bound to hemoglobin.

Leukocytes

Nucleated white blood cells numbering between $5{,}000$ and $10{,}000\,\mu\text{L}^{-1}$ of blood that defend the body against pathogens; includes basophils, lymphocytes, eosinophils, neutrophils, and monocytes.

Platelets

Cell fragments measuring $250{,}000$ to $400{,}000\,\mu\text{L}^{-1}$ of blood derived from specialized bone marrow stem cells that play an indispensable role in blood clotting.

Countercurrent Exchange

The anatomical arrangement in fish gills where capillary blood flows in the opposite direction to water passing over the lamellae, maintaining a favorable partial pressure gradient for continuous $O_2$ diffusion along the entire length of the capillary.

Tracheal System

A respiratory network of air tubes (tracheae and tracheoles) branching directly to internal body cells in insects, facilitating rapid gas exchange independent of an open circulatory system.

Alveoli

Terminal, microscopic, pocketed air sacs clustered at the tips of bronchioles in mammalian lungs across which oxygen diffuses into surrounding capillaries and carbon dioxide diffuses into the alveolar lumen.

Surfactant

A surface-active secretion coating the moist inner epithelial surface of alveoli that reduces surface tension and prevents alveolar collapse during exhalation.

Negative Pressure Breathing

The mammalian ventilation mechanism whereby contraction of the diaphragm (moving down) and rib muscles increases lung volume, lowering intrathoracic pressure relative to the atmosphere to draw air into the lungs.

Tidal Volume

The precise volume of air inhaled and exhaled during a normal, unforced breath at rest.

Medulla Oblongata

The primary control center in the brain stem that regulates the rate and depth of ventilation in response to $pH$ drops in cerebrospinal fluid caused by increased blood $CO_2$ levels.

Hemoglobin

An iron-containing respiratory pigment composed of four polypeptide subunits, each carrying a heme cofactor capable of reversibly binding a molecule of $O_2$, transporting up to four $O_2$ molecules simultaneously.

Bicarbonate Ion ($HCO_3^-$)

The primary chemical form in which carbon dioxide is transported in blood plasma, formed when cellular $CO_2$ reacts with water in red blood cells to yield $H_2CO_3$, which readily dissociates into $H^+$ and $HCO_3^-$.

Myoglobin

An oxygen-binding hemoprotein found in high concentrations within the skeletal muscles of diving mammals that acts as an oxygen store during prolonged submersion.