Gas Exchange & Circulation

External Respiration Across Animals

5 types of respiratory systems

1. Simple diffusion across plasma membranes

2. Cutaneous (integument or body surface)

3. Tracheae

4. Gills

5. Lungs

Tracheal System of Insects

• O2 and CO2 enter/exit through openings through spiracles and travel through air-filled tubes called tracheae

• Tracheae branch into tracheoles which end in or near tissues where gases diffuse

• Air sacs extend from tracheoles to aid in ventilation

Countercurrent Exchange in Fish Gills:

• Primary organs of respiration = gills

• Blood flows through the gills in the opposite direction as the water flowing over the gills maintaining a concentration gradient to promote diffusion = counter-current gas-exchange

• Low-oxygen blood (from body) enters gills meets low-oxygen water, while high-oxygen blood, after passing through gills, meets high-oxygen water— ensures diffusion of O2 from water to blood all along gills

Avian Respiration

• Birds have the greatest oxygen demand of any vertebrate, so their respiratory system has evolved to be the most efficient of all vertebrates

• Unique system of air sacs and parabronchi allow for a nearly-continuous, unidirectional movement of oxygen-rich air over respiratory surfaces during both inspiration and expiration

• Quantity of “dead air” in the (non-inflatable) lungs is sharply reduced compared with other vertebrates

• Air flows through parabronchi during both inspiration and expiration

• It takes two ventilatory cycles to move a particular volume of air through the bird’s respiratory system

Air Conduction

• Air enters/leaves this system through either nasal or oral cavities

• From these cavities, air moves into the pharynx, common area for respiratory/digestive tracts

• Pharynx connects with larynx (voice box) and w/esophagus that leads to stomach

• Epiglottis, flap of cartilage allows air to enter trachea during breathing

• Covers trachea during swallowing to prevent food/water from entering

• During inhalation, air from larynx moves into the trachea (windpipe) which branches into right and left bronchus

Gas Exchange

• Each bronchus enters the lungs where it branches into smaller respiratory bronchioles, then terminal bronchioles

• Small tubes called alveolar ducts connect respiratory bronchioles to alveoli—the functional units of the lungs (surface area)

• The alveoli cluster to form an alveolar sac

• Surrounding the alveoli are many capillaries; passive diffusion moves oxygen from alveoli to the blood, and carbon dioxide from the blood into the alveoli = respiration (external)

Ventilation

Inhalation:

• Intercostal muscles and diaphragm contract causing rib cage to expand → volume of thoracic cavity increases

• Increased volume of thoracic cavity causes decrease in pressure (drops below the atmospheric pressure) → air rushes into the lungs and lungs inflate

Exhalation:

• Intercostal muscles and the diaphragm relax causing thoracic cavity to return to its original, smaller size and increasing pressure in the thoracic cavity

• Lungs contract, compressing air in the alveoli → alveolar pressure increases (becomes greater than the atmospheric pressure) causing air to be exhaled from the lungs

Respiratory Pigments

• Respiratory pigments are organic compounds that have either metallic copper or iron that binds oxygen

• Respiratory pigments raise the oxygen carrying capacity of body fluids

• In general, the pigments respond to a high oxygen concentration by combining with oxygen and respond to low oxygen concentrations by releasing oxygen

• The four most common respiratory pigments: hemoglobin, hemocyanin, hemerythrin and chlorocruorin

Types of Circulatory Systems

Open Circulatory System

• Blood is pumped into a cavity called a hemocoel where it bathes the organs then returns to the heart through openings called ostia

• Blood & interstitial fluid is combined = hemolymph

• Found in most invertebrates (insects, crustaceans, some mollusks)

Closed Circulatory System

• Blood is contained within blood vessels and circulates unidirectionally from the heart, through the systemic route, then back to the heart

• Blood is separate from interstitial fluid

• Found in all vertebrates and some invertebrates (ex. cephalopods)

Human Heart Anatomy

•The heart sits within a thin-walled, double-layered structure called a pericardium •Pericardial cavity contains pericardial fluid which reduces friction between membranes

•The outer protective covering of the heart is fibrous connective tissue called visceral pericardium (or epicardium)

•Most of the heart is composed of cardiac muscle called myocardium

•Connective tissue and endothelium form the inside of the heart, the endocardium •The left and right halves of the heart are two separate pumps, each containing two chambers

• Four chambers: right atrium, right ventricle, left atrium and left ventricle • Valves— prevent backflow of blood

• Tricuspid valve: RA → RV

• Mitral valve: LA → LV Atrioventricular (A-V) valves

• Pulmonary valve: RV → pulm. artery

• Aortic valve: LV → aorta Semilunar valves

Diastole

• The relaxation phase of the cardiac cycle is called diastole, during which blood flows into all four chambers

• Blood enters the right atrium and the left atrium from the superior/inferior vena cava and pulmonary veins (respectively)

• The valves between the atria and the ventricles (AV- valves) are open and blood passively fills both ventricles

• The semilunar valves are closed to prevent backflow from pulmonary artery/aorta

• Diastole lasts only about 0.4 seconds, during which the ventricles nearly fill with blood

Atrial Systole

• The contraction phase of the cardiac cycle is called systole

• Systole begins with a very brief (0.1 second) contraction of the atria that completely fills the ventricles with blood (atrial systole)

Ventricular Systole

• Ventricles contract for about 0.3 seconds = ventricular systole

• The force of their contraction closes the A-V valves & opens the semilunar valves— pumps blood into the large arteries (pulmonary & aorta)

• Blood flows into the atria during the second part of systole

Cardiac Electrical Conduction Pathway

1. The sinoatrial (SA) node (aka “the pacemaker”)— situated in upper wall of RA— generates electrical signals, which spread quickly through both atria, causing them to contract in unison (atrial systole)

2. The signals then pass through a relay point called the atrioventricular (AV) node located in the wall between the right and left ventricles

3. Specialized cardiac muscle fibers (AV bundle/bundle of His) then relay the signals through the septum to the apex of the heart then up through the walls of the ventricles (Purkinje fibers) triggering the strong ventricular contractions (ventricular systole) that drive blood out of the heart

The Vertebrate Circulatory System — Blood Vessels

• Arteries are elastic blood vessels that carry blood away from heart to tissues of body

• Walls of arteries are 3 layers and thicker than arterial walls

• Veins are large vessels that carry blood from body tissues to heart

• Walls contain same three layers as arteries, but middle layer is much thinner

• Relatively inelastic with one or more valves

• Capillaries are composed of single layer of cells, most numerous blood vessels in an animal’s body

• Provide enormous surface area for exchange of gases, fluids, nutrients, wastes between nearby cells

• Aorta → Arteries → arterioles → capillaries → venules → veins → vena cava

Blood Pressure

• Blood pressure rises and falls in a pattern corresponding to the phases of the cardiac cycle

• Ventricular contraction generates fluid pressure called blood pressure

• BP = force that blood exerts against inner walls of blood vessels (measured in mm Hg)

• Max pressure achieved during ventricular contraction is called systolic pressure • When ventricles relax, (ventricular diastole), arterial pressure drops; the lowest pressure that remains in arteries before next ventricular contraction is called diastolic pressure

Formed Elements of Vertebrate Blood

• Vertebrate blood is classified as special type of connective tissue

• Contains a fluid matrix called plasma and formed elements— erythrocytes (RBC’s), leukocytes (WBC’s) & thrombocytes (platelets)

• Functions of vertebrate blood:

• Transports oxygen, carbon dioxide and nutrients

• Defends against harmful microorganisms, cells, viruses

• Homeostasis— helps regulate body temperature/pH

Blood Cells

• Platelets (thrombocytes), red blood cells (erythrocytes) & white blood cells (leukocytes)

• Hematopoiesis— development of mature blood and immune cells from stem cells in bone marrow (spongy bone)

Erythrocytes & Hemoglobin

• Red blood cells

• Nearly the entire mass of RBC is made up of hemoglobin

• Major function of RBC’s is to pick up oxygen from the environment, bind it to hemoglobin to form oxyhemoglobin, and transport if to body tissues

• Blood rich in oxyhemoglobin is bright red

• As oxygen diffuses into the tissues, blood becomes darker and appears blue when observed through the blood vessel walls

• Hemoglobin also carries waste CO2 from tissues to lungs (or gills) for removal from body

Thrombocytes

• Platelets

• Involved in hemostasis (thickening of blood), leading to the formation of blood clots and blood coagulation

• Coagulation cascade:

• Clotting factors trigger conversion of prothrombin (inactive protein) into thrombin (active enzyme)

•Also cause platelets to become sticky and adhere to damaged region, forming a “plug”

• Thrombin catalyzes conversion of soluble plasma protein fibrinogen into insoluble fibers called fibrin

• Fibrin strands form mesh around platelet “plug” and trap blood cells (causing temporary clot)

• When damaged region is completely repaired, an enzyme (plasmin) is activated to dissolve the clot

Leukocytes

• Leukocytes = white blood cells

• Scavengers that destroy microorganisms at infection sites, remove foreign chemicals and remove debris that result from dead or injured cells

• Agranular— basophils, neutrophils, eosinophils & mast cells

• Granular— lymphocytes, monocytes, macrophages & dendritic cells

Lymphatic System

• The lymphatic system is a network of vessels and secondary lymphoid organs (structures dedicated to the circulation and production of lymphocytes)

• Major functions:

• Collect and drain excess interstitial fluid that has accumulated in tissues (returned to blood circulation)

• Transport fatty acids from digestive system (in form of chylomicrons) into blood circulation to the liver to be broken down

• Transport cellular debris & foreign particles (including pathogens, microbes) to disposal centers called lymph nodes

• Transport of immune cells (lymphocytes and antigen presenting cells) to lymph nodes— location of many macrophages— to trigger immune response

Lymphatic Vessels

• As blood moves through the capillaries, the high pressure causes small amounts of proteins and fluids to seep out and become part of the interstitial fluid • Most gets reabsorbed by the capillaries, but some gets stuck in the tissues, and needs a way back into the blood

• Lymphatic capillaries pick up this excess fluid, which is now called lymph

• The small lymphatic capillaries merge to form larger lymphatic vessels (called lymphatics), which merge into trunks and then ducts • The lymphatic ducts empty lymph back into the venous circulation near the heart

Lymph nodes & other lymphoid organs

• On their way back towards the heart, the lymphatic vessels pass through lymph nodes, which play an important role in the body’s defense against pathogens

• Lymph nodes are concentrated in several areas of the body and contain macrophages, dendritic cells, which monitor lymph and present antigens to immune cells (B and T)

• Lymphoid organs:

• Primary— bone marrow & thymus

• Secondary— lymph nodes, spleen, tonsils, adenoids, Peyer's patches, mucous membranes (respiratory, urinary tract, bowels)