WEEKS 12 AND 13 HANDOUT, ZOOLOGY 03051: INTERNAL FLUIDS
4/13/00 Text (7th ed.): Ch. 8. Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://sln.fi.edu/biosci/heart.html (exploration of the heart)
CIRCULATION
All animals try to maintain an internal physiological balance (homeostasis). Since vertebrates are large and multicellular, they must have a means of shuttling materials from one part of the animal to another. If this system involves the movement of fluid through tubes, then it can be considered a circulatory system.
Vertebrates have a closed circulatory system, where blood is always surrounded by blood vessels. The general path is:
heart à artery à arteriole à capillary à venule à vein à heart
The vessels of the circulatory system vary. As you saw in the chicken wing, arteries have thicker, more muscular walls than veins. Veins also have valves to prevent the backflow of blood. Capillary walls are one cell thick for maximal diffusion of nutrients/waste products to/from the blood and tissues. The biggest pressure drop, and hence the greatest point of control of blood pressure or blood flow to different organs, is across the arterioles.
We have discussed the differences in circulation among the different vertebrate groups, with the single-pass circulation of the fishes, the common ventricle of the amphibians, and the separate pulmonary and systemic circuits of the tetrapods (non-fish vertebrates). Generally speaking, the evolutionary selective pressure is for higher efficiency in circulation (and gas exchange) in increasingly metabolically active organisms.
GAS EXCHANGE
In addition to these functions, the blood is also the site of gas exchange. Individual cells use oxygen in aerobic respiration (which occurs in the mitochondrion--remember from previous courses?). The circulatory system brings oxygen to cells and takes back carbon dioxide to the site(s) of gas exchange, whether gills, skin, or lungs. Regardless of the site, a thin epithelial tissue and many capillaries are involved. Gas exchange occurs by diffusion through moist surfaces.
Gills can either be external (e.g., tadpole) or internal (e.g., bony fish, covered with the operculum).
The skin of an amphibian is moist and has many capillaries underneath. As a result, some terrestial salamanders are lungless.
Lungs in vertebrates vary greatly, from the blind sac of an amphibian to the highly extractive bird lung. The lung originated in a homologous fashion to the swim bladder seen in today's bony fishes, but whereas the swim bladder affected bouyancy, the lungs became modified to take in oxygen. Amphibians ventilate by pushing air into their lungs with positive pressure, but reptiles, birds, and mammals pull air in with negative pressure. Mammals are the only ones with a diaphragm to aid in chest expansion and hence, generate negative pressure that draws in the air. Relaxation of the diaphragm and rib muscles allows for expiration.
Generally speaking, the carbon dioxide (CO2) levels in the blood drives involuntary respiration (when blood CO2 levels are too high, inspiratory signals are sent from the respiratory center in the medulla or brainstem). Oxygen (O2) is mostly found bound to hemoglobin in the red blood cells, and so does not reflect the ventilatory status as quickly as the carbon dioxide level does.
Air enters the: pharynx à larynx à trachea à bronchi à bronchioles à alveolar ducts à alveoli. An alveolus (singular) is the site of lung gas exchange, with its thin epithelial lining and rich capillarization.
Spirometry, or pulmonary function tests, to follow the mechanics of breathing.
These values can be of clinical significance by assisting in diagnosis of lung diseases:1
restrictive lung diseases, e.g., pulmonary fibrosis. Vital capacity reduced, but rate of forcible exhalation normal.
obstructive lung diseases, e.g., asthma. Vital capacity normal, but increased airway resistance. FEV (forced expiratory volume). % of vital capacity exhaled in 1 sec, or FEV1.0. If < 75%, then obstructive disease.
EXERCISE PHYSIOLOGY
During exercise, the goal is to deliver more oxygen to the muscles. If the muscles do not have enough oxygen, they will undergo (anaerobic) glycolysis (in the cytoplasm of the cells, right?) and produce lactic acid.
How to enhance O2 delivery to the muscles?1
cardiac output (CO) = heart rate (HR) X stroke volume (SV)
Muscles are initially under anaerobic conditions when cardiovascular system is still adjusting, and we can get a"second wind" after adjustment. The anaerobic threshold is the maximum rate of oxygen consumption that can be obtained before blood lactic acid levels rise.1
As exercise progresses, vasodilation and increased skeletal muscle blood flow occur because of intrinsic metabolic control. With exercise, more local metabolic factors including more CO2, lowered pH from carbonic acid and lactic acid, lowered O2, etc., combine so that skeletal muscle can receive much more blood flow during exercise. Note that rhythmic, pumping contractions allow more blood flow, but when you're doing an isometric contraction, blood flow slows and stops (think about being tired when holding one position for a while).
Also in exercise, the cardiac output can go way up (e.g., up to 25 L/min), due to an increase in heart rate. But there is a maximum value of heart rate increase, depending on age. Stroke volume (SV) can also be increased by training. How can SV go up, if there is less time to fill between the quickened beats? There is improved venous return from skeletal muscle pumps and increased respiratory efforts. SV is improved by increased contractility from sympathetic stimulation. Also, what the heart is pumping against is decreased with peripheral resistance decreased from skeletal muscle vasodilation. Training: lowering of resting heart rate and increase in resting stroke volume (increase in blood volume is part of this). So, it's the large cardiac output that is the ultimate factor in improved oxygen delivery to skeletal muscles as a result of endurance training.1
1 SI Fox, Human Physology, 5th ed. (WC Brown, Dubuque, IA, 1996), pp. 404-405, 472-473, 496.
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This web page last updated on April 24, 2000