Circulation and Gas Exchange

Circulation and Gas Exchange Notes

Overview: Trading Places

  • Every organism exchanges materials with its environment at the cellular level.
  • Unicellular organisms exchange directly with the environment.
  • Multicellular organisms cannot directly exchange with the environment, requiring specialized exchange systems like gills.
  • Internal transport and gas exchange are functionally related in most animals.

Concept 42.1: Circulatory Systems

  • Small/thin animals exchange materials directly with the surrounding medium.
  • Most animals have transport systems connecting exchange organs with body cells.
  • Complex animals have internal transport systems circulating fluid.
Gastrovascular Cavities
  • Simple animals like cnidarians have a two-cell-thick body wall enclosing a gastrovascular cavity.
  • This cavity functions in digestion and distribution.
  • Jellies have elaborate gastrovascular cavities.
  • Flatworms have a gastrovascular cavity and a large surface area-to-volume ratio.

Open and Closed Circulatory Systems

  • Complex animals have open or closed circulatory systems.
  • Both systems have three components:
    • Circulatory fluid (blood or hemolymph)
    • Tubes (blood vessels)
    • Muscular pump (the heart)
Open Circulatory System
  • Insects, arthropods, and most mollusks have an open circulatory system where blood bathes organs directly.
  • There is no distinction between blood and interstitial fluid; the fluid is called hemolymph.
Closed Circulatory System
  • Blood is confined to vessels and distinct from interstitial fluid.
  • Closed systems are more efficient at transporting fluids to tissues and cells.

Organization of Vertebrate Circulatory Systems

  • Humans and vertebrates have a closed circulatory system called the cardiovascular system.
  • Three main types of blood vessels: arteries, veins, and capillaries.
  • Arteries branch into arterioles, carrying blood to capillaries.
  • Capillary beds are sites of chemical exchange between blood and interstitial fluid.
  • Venules converge into veins, returning blood to the heart.
  • Vertebrate hearts have two or more chambers.
  • Blood enters through an atrium and is pumped out through a ventricle.

Single Circulation

  • Bony fishes, rays, and sharks have single circulation with a two-chambered heart.
  • Blood passes through two capillary beds before returning to the heart.

Double Circulation

  • Amphibians, reptiles, and mammals have double circulation.
  • Oxygen-poor and oxygen-rich blood are pumped separately from the right and left sides of the heart.
Adaptations of Double Circulatory Systems
  • Hearts vary in different vertebrate groups.
  • Amphibians: Three-chambered heart (two atria, one ventricle). The ventricle pumps blood into a forked artery that splits output into the pulmocutaneous and systemic circuits. Underwater, blood flow to the lungs is nearly shut off.
  • Reptiles: Three-chambered heart (two atria, one ventricle). Alligators, caimans, and crocodilians have a septum dividing the ventricle. They have double circulation with pulmonary and systemic circuits.
  • Mammals and Birds: Four-chambered heart (two atria, two ventricles). The left side pumps/receives oxygen-rich blood, while the right side receives/pumps oxygen-poor blood. Mammals and birds are endotherms and require more O2O_2 than ectotherms.

Concept 42.2: Mammalian Heart Contraction

  • Coordinated cycles of heart contraction drive double circulation.
  • The cardiovascular system meets the body’s continuous oxygen demand.
Mammalian Circulation
  • Blood flow begins with the right ventricle pumping blood to the lungs.
  • In the lungs, blood loads O<em>2O<em>2 and unloads CO</em>2CO</em>2.
  • Oxygen-rich blood enters the left atrium and is pumped to the body tissues via the aorta by the left ventricle.
  • The aorta provides blood to the heart through the coronary arteries.
  • Blood returns to the heart through the superior vena cava (from head, neck, forelimbs) and inferior vena cava (from trunk, hind limbs).
  • The vena cavas flow into the right atrium.
The Mammalian Heart: A Closer Look
  • The heart contracts and relaxes in a rhythmic cycle called the cardiac cycle.
  • Systole: Contraction/pumping phase.
  • Diastole: Relaxation/filling phase.
  • Four valves prevent backflow of blood:
    • Atrioventricular (AV) valves separate each atrium and ventricle.
    • Semilunar valves control blood flow to the aorta and pulmonary artery.
  • The "lub-dup" sound is caused by the recoil of blood against the AV valves (lub) then against the semilunar valves (dup).
  • Backflow through a defective valve causes a heart murmur.

Concept 42.3: Blood Pressure and Flow

  • Patterns of blood pressure and flow reflect blood vessel structure.
  • Physical principles governing water movement in plumbing systems also influence circulatory systems.
Blood Vessel Structure and Function
  • The epithelial layer lining blood vessels is called the endothelium.
  • Capillaries have thin walls (endothelium + basement membrane) for material exchange.
  • Arteries and veins have endothelium, smooth muscle, and connective tissue.
  • Arteries have thicker walls than veins to accommodate high pressure from the heart.
  • In thinner-walled veins, blood flows back to the heart mainly due to muscle action.
Regulation of Blood Pressure
  • Blood pressure is determined by cardiac output and peripheral resistance (arteriole constriction).
  • Vasoconstriction: Contraction of smooth muscle in arteriole walls; increases blood pressure.
  • Vasodilation: Relaxation of smooth muscles in arterioles; decreases blood pressure.
  • Vasoconstriction and vasodilation maintain adequate blood flow as demands change.
  • The peptide endothelin induces vasoconstriction.
Fluid Return by the Lymphatic System
  • The lymphatic system returns fluid that leaks out in capillary beds and aids in body defense.
  • Lymph reenters circulation directly at the venous end of the capillary bed and indirectly through the lymphatic system.
  • The lymphatic system drains into veins in the neck.
  • Lymph nodes filter lymph and play an important role in body defense.
  • Edema is swelling caused by disruptions in lymph flow.

Concept 42.4: Blood Components

  • Blood components function in exchange, transport, and defense.
  • In invertebrates with open circulation, blood (hemolymph) is not different from interstitial fluid.
  • Vertebrate blood is a specialized connective tissue.
Blood Composition and Function
  • Blood consists of cells suspended in liquid matrix (plasma).
  • Cellular elements occupy about 45% of blood volume.
Plasma
  • Blood plasma is about 90% water.
  • Solutes include inorganic salts (electrolytes).
  • Plasma proteins influence blood pH, osmotic pressure, and viscosity.
  • Plasma proteins function in lipid transport, immunity, and blood clotting.
Cellular Elements
  • Red blood cells (erythrocytes) transport oxygen.
  • White blood cells (leukocytes) function in defense.
  • Platelets are cell fragments involved in clotting.
Erythrocytes
  • Red blood cells are the most numerous blood cells.
  • They transport oxygen throughout the body.
  • Contain hemoglobin (iron-containing protein that transports oxygen).
Leukocytes
  • Five major types: monocytes, neutrophils, basophils, eosinophils, and lymphocytes.
  • Function in defense by phagocytizing bacteria/debris or producing antibodies.
  • Found both in and outside of the circulatory system.
Platelets
  • Fragments of cells that function in blood clotting.
Blood Clotting
  • When the endothelium of a blood vessel is damaged, the clotting mechanism begins.
  • Complex reactions convert fibrinogen to fibrin, forming a clot.
  • A thrombus (clot within a vessel) can block blood flow.
Stem Cells and Replacement of Cellular Elements
  • Cellular elements are constantly replaced.
  • Erythrocytes, leukocytes, and platelets develop from stem cells in the red marrow of bones.
  • Erythropoietin (EPO) stimulates erythrocyte production when oxygen delivery is low.

Cardiovascular Disease

  • Cardiovascular diseases are disorders of the heart and blood vessels.
  • Account for more than half the deaths in the United States.
Atherosclerosis
  • Caused by the buildup of plaque deposits within arteries.
Heart Attacks and Stroke
  • Heart attack: Death of cardiac muscle tissue from a blockage of coronary arteries.
  • Stroke: Death of nervous tissue in the brain, usually from rupture/blockage of arteries in the head.
Treatment and Diagnosis of Cardiovascular Disease
  • Cholesterol is a major contributor to atherosclerosis.
  • Low-density lipoproteins (LDLs) are associated with plaque formation (“bad cholesterol”).
  • High-density lipoproteins (HDLs) reduce cholesterol deposition (“good cholesterol”).
  • The proportion of LDL relative to HDL can be decreased by exercise, not smoking, and avoiding foods with trans fats.
  • Hypertension (high blood pressure) promotes atherosclerosis and increases the risk of heart attack and stroke.
  • Hypertension can be reduced by dietary changes, exercise, and/or medication.