Today we're going to discuss the respiratory and excretory systems --
We have used the term "respiration" at the cellular level to describe the oxygen-requiring chemical reactions in the mitochondria that convert glucose to carbon dioxide and ATP (energy).
In single-celled or simple multicellular organisms, oxygen from the environment enters and carbon dioxide leaves the organism by diffusion.
In higher animals, cells may be far-removed from the environment -- a respiratory system has evolved to deliver oxygen to cells and to transport carbon dioxide back into the environment.
Note: the respiratory system is closely integrated with the circulatory system -- difficult to see where one stops and the other begins -- circulatory system provides means of transport of oxygen and carbon dioxide.
Various respiratory structures and systems have evolved -- important factors are the size of the organism and its energy needs -- e.g. small organisms such as single-celled organisms may move oxygen and carbon dioxide between themselves and the environment directly by diffusion -- large, active organisms that maintain constant body temperatures need elaborate respiratory systems to efficiently deliver greater quantities of oxygen.
Respiratory surface -- site of gas interchange between organism and its environment -- usually moist and with a large surface area.
1. integument (skin) -- some simple organisms respire through their skin -- e.g. earthworms have a capillary network one-cell layer beneath skin -- oxygen from environment diffuses through this layer into blood, while carbon dioxide in blood diffuses back into the environment -- oxygen delivered to cells by blood and carbon dioxide is picked up (again: note concentration gradients and diffusion).
Earthworms must keep their skin moist for respiration to occur -- being small, earthworms have a relatively high surface area/volume ratio -- also have low energy demands.
2. tracheal system -- simple respiratory system found in insects -- oxygen enters valve-like openings called spiracles -- tubes called tracheae lead away from spiracle and branch into smaller tubes called tracheoles -- tracheoles in contact with cells of body -- air pumped in and out by body movements -- Note: circulatory system not involved in respiration and blood is colorless (no respiratory pigment).
Figure 35.5. Tracheal System of Insects.
3. gills -- highly vascularized (lots of capillaries) structures specialized for gas exchange in water -- may be external or internal -- gills composed of very fine filaments containing capillaries -- protected by an outer covering called the operculum -- you have probably observed fish opening and closing operculi as they pump water over gills -- as water moves over the filaments, dissolved oxygen in water diffuses into capillary and carbon dioxide in blood diffuses out into water -- oxygenated blood then pumped to cells where diffusion of oxygen into cells and of carbon dioxide into blood occurs.
Figure 35.6. Anatomy of Fish Gills.
4. lungs -- specialized respiratory structure of land animals -- not well-developed in all animals -- e.g. frogs use integument and lungs for respiration -- lungs are internal cavities into which air is brought and where gas exchange between the organism and the environment occurs -- making respiratory surface internal makes it easier to keep them moist.
Human Respiratory System:
1. mouth and nose -- site of intake/exhaust.
2. trachea -- "windpipe" leading away from mouth/nose.
3. bronchi -- trachea divides into two large tubes each leading to a lung.
4. bronchioles -- smaller divisions of bronchus.
5. alveolus -- each bronchiole ends in a series of highly vascularized air pockets called alveoli -- gas exchange occurs between blood in alveolus capillary and air that has been inhaled (again by diffusion) -- there are about 300 million alveoli -- surface area of lungs = 70 m2 or about 40X that of body surface area -- more surface area means more gas exchange.
Figure 35.10. Human Respiratory Tract.
Breathing -- lungs are contained in the thoracic cavity -- separated from abdominal cavity by a muscular structure called the diaphragm -- breathing occurs by changing the volume of the thoracic cavity.
1. inhalation -- diaphragm drops, ribs raised by muscle contraction -- this enlarges the volume of thoracic cavity creating negative pressure between thoracic cavity and environment -- air rushes in and fills lungs.
2. exhalation -- ribs lowered as muscles relax and diaphragm raises -- cavity volume reduced and air forced out.
Breathing Rate -- controlled by the respiratory center located in the medulla oblongata portion of the brain -- gas content of blood monitored by chemoreceptors in aorta and carotid arteries -- monitor concentrations of CO2 and H+ -- rate of respiration increases/or decreases as necessary.
Now, let's conclude our discussion of the respiratory system by considering gas transport and exchange.
Diffusion of gases between the air surrounding the alveoli (O2 rich; CO2 poor) and the blood in alveolar capillaries (O2 poor; CO2 rich) results in CO2 leaving blood and being exhaled and the blood becoming oxygenated.
By themselves, fluids (e.g. water; plasma) have low capacities for containing dissolved gases (e.g. O2) -- capacity greatly enhanced by the addition of a respiratory pigment = hemoglobin (in RBC) -- increases ability of blood to carry dissolved O2 by of 70X.
Hemoglobin -- 4 polypeptide chains each containing a heme (iron) group -- each heme group can carry one O2 molecule -- diffusion of O2 into alveolar capillary results in reduced hemoglobin becoming oxyhemoglobin -- as blood is pumped to cells, concentration of O2 in cells is less than that of blood, so oxyhemoglobin gives up O2, which diffuses into cells where it is used in aerobic respiration -- hemoglobin now reduced hemoglobin and is pumped back to lungs where oxygenation can occur again.
Note: hemoglobin will bind with other gas molecules besides oxygen -- e.g. carbon monoxide (CO), a gas present in automobile exhaust, combines with hemoglobin more readily than oxygen -- death may result from carbon monoxide poisoning if hemoglobin is not available for oxygen transport.
Carbon Dioxide -- waste product of cellular respiration -- disposal of carbon dioxide is another function of respiratory system -- carbon dioxide diffuses out of cells into capillaries -- three things happen:
1. some CO2 dissolves into plasma -- recall that fluids have low capacities for dissolved gases, so only a small amount of CO2 is transported in this way.
2. some CO2 binds with hemoglobin -- not very much CO2 carried this way.
3. most CO2 transported as bicarbonate ion -- CO2 + H20 goes to H2CO3 (carbonic acid), which dissociates into H+ and HCO3- -- this reaction is facilitated by the red blood cell enzyme carbonic anhydrase -- H+ ions picked up by buffering proteins to maintain constant blood pH -- in the capillary of the alveoli, carbonic anhydrase facilitates the reverse reaction and the resulting CO2 diffuses out of blood into air surrounding the alveolus -- it is then exhaled.
Another group of waste products are those produced by the breakdown of protein, amino acids and nucleic acids -- these compounds all contain nitrogen and nitrogenous wastes are produced when they are metabolized.
Excretory System -- three functions:
1. elimination of nitrogenous wastes.
2. control of ionic concentrations in body fluids.
3. osmoregulation -- control of how much water the body contains.
Nitrogenous wastes -- different organisms produce different nitrogenous wastes when nitrogen-containing molecules are metabolized.
1. ammonia -- NH3 -- highly toxic -- produced by marine invertebrates and freshwater fishes -- highly soluble in water.
2. uric acid -- not very toxic and poorly soluble in water -- insects, birds, and reptiles -- produced in liver and transported to kidney by the blood.
3. urea -- marine fishes and mammals -- liver converts ammonia to urea -- passed out in urine.
Ionic concentrations -- Na+, K+, Mg++, etc. -- minerals required as cofactors, but too much can also be a problem.
Osmoregulation -- control of how much water is in body -- notice that this is related to controlling ionic concentrations -- one way to change ionic concentrations is to change amount of water in the body.
Water gained by drinking, eating and as a byproduct of oxidation of food in mitochondria (= metabolic water).
Water lost by breathing, sweating, defecation and, most importantly, by urination.
Let's look at the human excretory system:
1. kidneys -- paired organs lying at back of abdominal cavity -- major regulatory organ of excretory system.
2. renal arteries -- supply kidneys with blood -- come off aorta.
3. renal veins -- take blood way from kidneys -- empty into inferior vena cava.
4. ureter -- tube carrying urine away from each kidney to the urinary bladder.
5. urinary bladder -- urine reservoir.
6. urethra -- tube leading from urinary bladder to the outside.
Figure 37.3. The Human Excretory System.
Kidneys -- paired organs that are the major regulatory organs in the excretory system-- each kidney consists of an outer cortex, a middle medulla, and an inner hollow basin called the renal pelvis.
Figure 37.4. Kidney structure.
Nephron -- tiny tubules that are the functional unit of kidney -- each kidney contains about 1 million nephrons -- part of nephron lies in cortex, part in medulla.
1. convoluted excretory tubule -- blind end of tubule called glomerular capsule (Bowman's capsule) -- tubule leading away bends and dips-- parts are proximal tubule, loop of Henle (dips into medulla) and distal tubule.
2. each distal convoluted tubule empties into a collecting duct, which leads and empties into the renal pelvis -- one collecting duct may service several convoluted tubules.
3. each nephron served by an afferent arteriole, which forms a bed of capillaries called a glomerulus -- glomerulus surrounded by Bowman's capsule -- efferent arteriole leads away -- becomes peritubular capillaries -- these surround the excretory tubule -- join to form venules that join the renal vein.
Figure 37.4. Nephron Anatomy
Now, let's look at how the nephron cleans the blood, regulates osmotic and ionic concentrations.
1. filtration -- blood entering glomerulus is under about 2X the pressure of blood in most capillaries -- about 1/5 of plasma entering glomerulus and the molecules dissolved in it are forced out through capillary walls into the glomerular capsule -- this liquid is called glomerular filtrate -- note that this filtrate has good stuff (e.g. water, ions, vitamins, glucose, amino acids, etc.) that should be conserved and that the nephron will "reclaim".
2. reabsorption -- removal of molecules (Na+, Cl-, glucose, water, etc.) in filtrate from convoluted excretory tubule into peritubular capillary -- may occur by diffusion or by active transport performed by cells lining convoluted tubule -- H2O reabsorption continues in the collecting duct -- about 99% of the water that entered nephron as filtrate is reaborbed and put back into blood, none of the urea is reabsorbed.
Note: every substance that is reabsorbed in nephron has a threshold level based on the normal concentration of the substance in the blood -- e.g. Na+ is reabsorbed until blood has normal concentration -- additional Na+ will remain in nephron and will be lost in the urine.
Depending on conditions, our kidneys can produce urine with ionic concentrations that are hypotonic (lower), isotonic (equal) or hypertonic (higher) to the blood.
Reabsorption of Na+ and water controlled by hormones:
1. aldosterone -- produced by adrenal glands -- regulates Na+ reabsorption in distal convoluted tubule -- increased aldosterone means more Na+ is reabsorbed and concentration of Na+ in urine will be reduced.
2. antidiuretic hormone (ADH) -- produced by pituitary gland -- acts on cells lining collecting duct and influences H2O reabsorption -- increase ADH secretion results in increased H2O absorption (increased urine concentration).
Receptors monitor concentrations of salt and adjust concentrations through hormone secretion -- system sometimes disrupted -- fear and alcohol both depress ADH secretion and, in turn, H2O reabsorption by collecting duct -- results in excessive water loss (e.g. dehydration in hangover) through urination.
Different organisms face different problems when it comes to regulating ionic and water concentrations (osmoregulation) -- let's look at some of these.
I. Freshwater fishes -- environment hypotonic to body fluids -- problem is loss of salts by diffusion and water loading by osmosis -- solutions:
A. don't drink water -- swallow food only.
B. gills remove salts from water against concentration gradient by active transport.
C. kidney reabsorbs salts by active transport.
D. large quantities of hypotonic urine are produced.
Figure 37.7.
II. Marine fishes -- reverse problem -- body fluids hypotonic to environment -- problem is water loss (dehydration) and salt loading -- solutions:
A. drink sea water to replace water loss through osmosis.
B. salt put back into water by active transport in gills.
C. produce small quantities of isotonic urine -- kidneys unable to concentrate salts in urine.
Figure 37.7.
Other marine animals also have adaptations to get rid of excess salts -- e.g. nasal salt glands in ducks, gulls -- tear glands in sea turtles secrete salt.
What about land animals such as ourselves?
Most of the time, we face problems similar to that of marine fishes -- water loss to environment and buildup of salts -- our kidneys can produce urine that is hypertonic, isotonic, or hypotonic, depending upon the situation.
Next time: Digestive System and Nutrition.