Chapter 34
Circulation & Gas Exchange
Circulatory Systems Linked to the Surface
● Exchange ultimately occurs at the cellular level.
● Natural selection favors two different methods of gas and material exchange in animals.
● Gastrovascular cavities allow for exchange directly from the environment.
● A circulatory system has 3 basic parts:
circulatory fluid, interconnected vessels, and a muscular pump or heart.
● A circulatory system connects the aqueous environment of body cells to organs of gas exchange.
● Circulatory systems are either open or closed.
Organization of Vertebrate Circulatory Systems
● In a cardiovascular system blood circulates to and from the heart within a network of vessels.
● Arteries carry blood away from heart to organs branching into arterioles.
● Arterioles branch into the smallest vessels known as capillaries and capillary beds.
● Capillaries converge into venules which converge into veins that carry blood back to the heart.
● The heart of all vertebrates contain receiving chambers or atria and pumping chambers or ventricles.
● Single circulation passes blood through the heart in one circuit, whereas double circulation passes blood in two circuits.
Double Circulation in Vertebrates
The Mammalian Heart
The Cardiac Cycle
● In the human heart, blood fills ventricles from atria when the heart is relaxed.
● Ventricles are much thicker walled and contract with greater force than atria.
● The left ventricle and right ventricle pumps the same volume of blood.
● The cardiac cycle is one complete pumping and filling of the chambers.
● Cardiac output = heart rate x stroke volume
● The average adult human cardiac output is 5L/min.
● Atrioventricular or AV valves close when the ventricles contract.
● Semilunar valves open during ventricular contraction.
Diastole - relaxation Systole - contraction
The Heart’s Rhythmic Beating
● The sinoatrial or SA node or
pacemaker is a group of cells in the wall of the right atrium near the superior vena cava that sets the rate and timing of when cardiac muscle cells contract.
● An ECG or EKG monitors currents of electrical energy from these contractions.
● The atrioventricular or AV node are a group of relay cells between the left and right atria that delay impulses from the SA node via Purkinje fibers for the ventricles to fill before pumping.
The Control of Heart Rhythm
Patterns of Blood Pressure and Flow due to
Vessels ● Blood vessels are comprised of: a central cavity or lumen and a single layer of cells called endothelium that line the lumen.
● Capillaries are thin walled with endothelium and a basal lamina.
● Arteries and veins have two layers of tissue surrounding the endothelium: out connective tissue made of elastic fibers and middle layer of smooth muscle tissue.
● Arteries are thicker and stronger than veins.
● Reduced blood flow velocity or in capillaries is the result of its enormous branches.
● Blood pressure is the forces exerted in all directions from ventricular contraction.
● Systolic pressure is the pressure during ventricular contraction or systole.
● Diastolic pressure is a lower pressure during relaxation of the ventricles.
Reading Blood Pressure
The first or top number is systolic pressure and the second or bottom number is diastolic pressure
The Connection to the Lymphatic System
● Lost fluid from capillaries to interstitial fluid is returned to blood through the lymphatic system.
● Lymph travels through vessels to lymph nodes where immune system cells attack any viruses or bacteria.
● The lymphatic system drains into two large veins of the circulatory system at at the neck.
● Disorders interfere with normal
maintenance can lead to fluid retention in tissues called edema or lymphedema.
Chapter 34
Circulation & Gas Exchange
Gas Exchange Occurs Across Respiratory Surfaces
● The driving force of gas exchange is
partial pressure, or the pressure exerted by a particular gas in a mixture.
● At sea level oxygen exerts a downward force equal to 760mm Hg or a PO2 of 160mm Hg.
● Oxygen is much less soluble in water, especially warm salty water.
● Aquatic animals have evolved to be more efficient in gas exchange.
● Gas exchange is fastest when the
respiratory surface is large and diffusion has a short path.
The Variety of Respiratory Organs
● Gills generally have more surface area than the rest of the body’s exterior.
● Ventilation is the process of movement of the respiratory medium across the respiratory
surface.
● In fish gills, countercurrent exchange moves water across the gills in the opposite direction of blood flow through the capillaries of the gills.
● Terrestrial insects use a tracheal system, which is a branching tube system.
● Larger insects must ventilate their tracheal system to maximize gas exchange.
The Mammalian Respiratory System
● Lungs are localized respiratory organs that rely on the animal circulatory system.
● Important structures in the mammalian respiratory system include:
○ Larynx is the upper part of respiratory tract
○ Epiglottis protects the opening of the trachea or glottis.
○ Larynx has elastic bands of muscle called vocal folds or vocal cords (humans).
○ Trachea branches into the L and R bronchi which branch into bronchioles.
○ Gas exchange in mammals occurs at the air sacs or alveoli.
○ The alveoli have 50x the surface area of human skin and require a moist film for diffusion of gases to occur.
Breathing Is Positive
● Amphibians use positive pressure
breathing, inflating the lungs with forced air flow.
● By lowering and raising the floor of the throat frogs move air through their respiratory
system.
● Birds pass air over gas exchange surfaces in one direction and incoming fresh air does not mix with air that has carried out gas exchange.
● Birds use air sacs to keep air flowing through the lungs and use parabronchi instead of the deadend alveoli for gas exchange.
How Do Mammals Breath?
● Mammals use negative pressure breathing which is pulling air into the lungs rather than pushing.
● Muscular contractions lower air pressure in the lungs below that of the air pressure outside the body which leads to air
moving from high pressure to low pressure.
● Tidal volume is the volume of air inhaled and exhaled with each breath.
● Vital capacity is the tidal volume during maximal inhalation and exhalation.
● Residual volume is the air that remains after forced exhalation.
Control of Breathing in Humans
Adaptations for Gas Exchange
● Partial pressure of O2 and CO2 in the blood vary at different points in the circulatory system.
● Animals transport most of their O2 bound to respiratory pigments, proteins bound to metal that greatly increase the amount of O2 that can be carried in the circulatory fluid.
● CO2 production promotes unloading of O2 in active tissues.
● Low pH decreases the affinity of hemoglobin for O2.
Carbon Dioxide Transport in Blood
