Circulation and Gas Exchange

Overview: Trading with the Environment

All organisms must exchange materials with their environment, a process that occurs at the cellular level. In unicellular organisms, exchanges happen directly with the environment, while multicellular organisms require specialized systems for internal transport and exchange.

• Direct exchange: Unicellular organisms exchange materials directly with their surroundings.

• Specialized exchange systems: Multicellular organisms, such as salmon, use structures like gills for efficient exchange.

• Diffusion limitations: Diffusion alone is inadequate for large organisms; circulatory systems ensure substances do not need to diffuse far.

Concept 42.1: Circulatory Systems Link Exchange Surfaces with Cells

Transport systems connect organs of exchange with body cells. Most complex animals have internal circulatory systems that circulate fluid, overcoming the limitations of diffusion.

• Closed circulatory system: Vertebrates have a cardiovascular system with blood vessels and a heart.

• Heart chambers: Vertebrate hearts have two to four chambers (atria and ventricles).

• Blood vessels: Arteries carry blood to capillaries; veins return blood to the heart.

• Metabolic correlation: The number of blood vessels in an organ correlates with its metabolic needs.

Survey of Vertebrate Circulation

Different vertebrate groups have distinct heart and circulatory system structures:

• Fish: Two-chambered heart (one atrium, one ventricle); blood flows through gills for gas exchange.

• Amphibians: Three-chambered heart (two atria, one ventricle); ventricle pumps blood to pulmocutaneous (lungs and skin) and systemic circuits.

• Reptiles (except birds): Three-chambered heart with partial septum; double circulation (pulmonary and systemic circuits).

• Mammals and birds: Four-chambered heart; complete separation of oxygen-rich and oxygen-poor blood; essential for endothermy.

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

The mammalian circulatory system is a model for double circulation, ensuring efficient oxygen delivery and waste removal. Heart valves maintain one-way blood flow.

• Pathway of blood: Right ventricle pumps blood to lungs; oxygen-rich blood returns to left atrium and is pumped to body tissues by left ventricle.

• Heart valves: Atrioventricular (AV) and semilunar valves prevent backflow.

• Cardiac cycle: Rhythmic contraction (systole) and relaxation (diastole) of the heart.

• Heart rate and cardiac output: Heart rate (beats/min); cardiac output (volume/min) = heart rate × stroke volume.

Maintaining the Heart’s Rhythmic Beat

• Self-excitable cells: Some cardiac muscle cells contract without nervous input.

• Sinoatrial (SA) node: Pacemaker sets contraction rate; impulses travel to atrioventricular (AV) node, then to Purkinje fibers.

• Electrocardiogram (ECG): Records electrical impulses during the cardiac cycle.

• Regulation: Pacemaker influenced by nerves, hormones, temperature, and exercise.

Concept 42.3: Physical Principles Govern Blood Circulation

Blood vessel structure and function are governed by physical principles similar to plumbing systems. All vessels have three layers: connective tissue, smooth muscle, and endothelium.

• Arteries: Thick walls to withstand high pressure.

• Veins: Thinner walls; blood flow aided by skeletal muscle contraction and valves.

• Capillaries: Thin walls for efficient exchange.

Blood Flow Velocity and Pressure

• Velocity: Slowest in capillaries due to high resistance and large cross-sectional area.

• Blood pressure: Hydrostatic pressure exerted by blood; systolic (during ventricular contraction) and diastolic (during relaxation).

• Regulation: Cardiac output and peripheral resistance determine blood pressure.

Capillary Function and Fluid Exchange

• Blood distribution: Regulated by smooth muscle contraction and precapillary sphincters.

• Exchange: Driven by differences in blood pressure and osmotic pressure.

• Lymphatic system: Returns fluid to circulation and aids in defense.

Concept 42.4: Blood is a Connective Tissue with Cells Suspended in Plasma

Blood is a specialized connective tissue composed of plasma and cellular elements. Plasma is mostly water, with dissolved ions and proteins.

• Plasma: 90% water; contains electrolytes and proteins (albumin, fibrinogen, immunoglobulins).

• Cellular elements: Erythrocytes (red blood cells), leukocytes (white blood cells), and platelets.

Cellular Elements

• Erythrocytes: Transport oxygen; lack nuclei and mitochondria; contain hemoglobin.

• Leukocytes: Five major types; defense and immunity.

• Platelets: Cell fragments involved in clotting.

Stem Cells and Replacement

• Pluripotent stem cells: Source of all blood cells; located in bone marrow.

• Erythropoietin (EPO): Hormone stimulates erythrocyte production.

• Leukemia: Overproduction of leukocytes impairs red blood cell formation.

Blood Clotting

• Clotting mechanism: Cascade converts fibrinogen to fibrin, forming a clot.

• Hemophilia: Defects in clotting cause excessive bleeding.

• Thrombus: Blood clot within a vessel.

Cardiovascular Disease

Cardiovascular diseases are disorders of the heart and blood vessels, often caused by lifestyle factors and cholesterol buildup.

• Atherosclerosis: Buildup of cholesterol (LDL) in arteries; HDL reduces deposition.

• Hypertension: High blood pressure increases risk of heart attack and stroke.

• Heart attack: Death of cardiac muscle due to blocked coronary arteries.

• Stroke: Death of nervous tissue in the brain from blocked or ruptured arteries.

Concept 42.5: Gas Exchange Occurs Across Specialized Respiratory Surfaces

Gas exchange supplies oxygen for cellular respiration and removes carbon dioxide. Animals require large, moist respiratory surfaces for efficient diffusion.

• Respiratory surface size: Correlates with metabolic needs; endotherms have larger surfaces.

• Lungs: Internal organs for gas exchange in terrestrial vertebrates.

Mammalian Respiratory System

• Air pathway: Nostrils → pharynx → trachea → bronchi → bronchioles → alveoli.

• Alveoli: Site of gas exchange; covered with capillaries.

Concept 42.6: Breathing Ventilates the Lungs

Breathing is the process of ventilating the lungs through inhalation and exhalation. Mammals use negative pressure breathing, while amphibians use positive pressure.

• Negative pressure breathing: Rib muscles and diaphragm contract to increase lung volume.

• Tidal volume: Amount of air inhaled/exhaled per breath (500 mL).

• Vital capacity: Maximum tidal volume during forced breathing.

• Residual volume: Air remaining after forceful exhalation.

Bird Respiration

• Air sacs: Keep air flowing in one direction through lungs.

• Parabronchi: Site of gas exchange.

• Efficient ventilation: Air is completely renewed with each exhalation.

Control of Breathing in Humans

• Medulla oblongata and pons: Control centers in the brain regulate breathing rate and depth.

• pH regulation: Medulla responds to pH changes in cerebrospinal fluid.

• O2 and CO2 sensors: Located in aorta and carotid arteries; exert secondary control.

Concept 42.7: Respiratory Pigments Bind and Transport Gases

Respiratory pigments, such as hemoglobin, increase the oxygen-carrying capacity of blood. Gases diffuse down partial pressure gradients.

• Partial pressure: Determines direction of gas diffusion; oxygen diffuses from high to low partial pressure.

• Hemoglobin: Binds oxygen reversibly; cooperative binding enhances loading and unloading.

• Bohr shift: Drop in pH lowers hemoglobin's affinity for oxygen, facilitating release in tissues.

• CO2 transport: Hemoglobin also transports CO2 and buffers blood.

Equation for partial pressure of oxygen:

Equation for hemoglobin binding:

Equation for CO2 transport:

Additional info: Academic context was added to clarify the function of each system, the importance of partial pressure gradients, and the physiological relevance of circulatory and respiratory adaptations in vertebrates.