Chapter 44GAS EXCHANGE - RESPIRATORY SYSTEM

The exchange of gases between an organism and its environment is called respiration.

  • Organismic respiration brings oxygen from the environment to the cells.
  • Aerobic respiration occurs within the cell in the mitochondria.

Ventilation is the movement of air or water over the respiratory surfaces.

In order for oxygen and carbon dioxide to diffuse across a cell membrane, they must dissolve in water.

Water is more viscous and dense than air and the aquatic animal must spend a lot of its energy moving water over the gills.

Aquatic animals spend 20% of its energy while terrestrial animals spend 1 -2 % of its total energy.

Air contains much larger concentration of oxygen than water.

Terrestrial animals have to compensate for water loss during breathing.

Respiratory surfaces must be maintained moist and air has to pass through a long series of tubes to reach these surfaces.

TYPES OF RESPIRATORY SURFACES

  1. Body surface. Used by small animals with low metabolic rate.
  1. Tracheal tubes that deliver oxygen to all parts of the body.
  • It consists of a network of tracheal tubes that open on the body surface through up to 20 tiny openings called spiracles.
  1. Gills containing capillaries.
  • Echinoderms have dermal gills.
  • Chordates usually have internal gills.
  • In bony fish the gills are protected by a bony plate, the operculum
  • Counter current system is an efficient method of obtaining oxygen.
  1. Lungs formed by in-growth of the body surface or from the wall of the body cavity.

Spiders have "book lungs".

  • Located in an inpocketing of the abdominal wall.
  • Open to the outside by a spiracle.
  • A series of plates rich in hemolymph separated by air spaces.

Osteichthyes have a swim bladder.

  • It is used to control buoyancy.
  • Lungfishes use it breath air at certain times in their life cycle.

Amphibians and reptiles have simple lungs.

  • The lungs of toads and frogs are simple sacs with ridges that increase the respiratory surface.
  • Some amphibians do not have lungs and exchange gases through the skin.
  • Reptiles have sacs with folding of the wall to increase the respiratory surface.

Birds have the most efficient respiratory system of any living vertebrate.

Their lungs have air sac extensions that reach into many parts of the bird's body.

Hemoglobin increases the capacity to transport oxygen by about 75 times.

There are several types of hemoglobin.

  • All contain iron as part of a heme group.
  • Heme group is bound to a protein called globin.
  • Protein portion varies in size and AA in different species.
  • It is present in some invertebrates like annelids, nematodes, mollusks and arthropods.

Hemocyanins are copper containing proteins found in arthropods and some mollusks.

  • Lack heme group.
  • Copper-containing proteins.
  • Dissolved in the blood rather than contained in cell.
  • Blue when combined with oxygen; without oxygen is colorless.
  • It is found many species of mollusks and arthropods.

HUMAN RESPIRATORY SYSTEM

The human respiratory system is typical of air-breathing vertebrates.

Nostrils are the opening of the nose.

Nasal cavities moisten, warm and filter the air.

Pharynx or throat is used also by the digestive system.

Larynx also called "voice box" contains the vocal cords and is supported by a cartilage.

Epiglottis is a small flap of tissue that closes the larynx during swallowing.

Trachea or windpipe is supported by rings of cartilage.

Bronchi are branches of the trachea that lead to each lung.

  • Both trachea and bronchi are lined with a mucous membrane containing ciliated cells.

Bronchioles and alveoli make most the lungs.

Each lung is covered with a pleural membrane, which also lines the thoracic cavity.

Pleural cavity is the space in between the pleural membranes and it's filled with a fluid.

  • The pleural fluid provides lubrication between the lungs and the body wall.

The alveoli are tiny air sacs at the end of the bronchioles and are lined with a very thin epithelium.

Capillaries surround the alveoli.

Gas exchange occurs in the alveoli of the lungs.

The lungs as such consist mostly of air tubes and elastic tissue with a very large internal surface.

Passage of air:

Nostrils → nasal cavities → pharynx → larynx → trachea → bronchi→ bronchioles → alveoli.

BREATHING

Ventilation is accomplished by breathing.

Breathing is the mechanical processes of moving air from he environment into the lungs (inspiration) and expelling the air from the lungs (expiration).

During inspiration, the volume of the thoracic cavity is increased by contraction of the diaphragm.

  • Contraction moves the diaphragm downward increasing the volume of the thoracic cavity.
  • The pressure of the air in the lungs decreases by 2 or 3 mm Hg below the atmospheric pressure.

With the increase in volume in the thoracic cavity, the pressure drops and air is forced in by the atmospheric pressure.

Expiration occurs when the diaphragm relaxes.

The normal amount of air inhaled at rest is called tidal volume: ~ 500 ml.

The vital capacity is the maximum amount of air a person can exhale after filling the lungs to the maximum extent.

O2 and CO2 are exchanged between alveoli and blood by diffusion.

The difference in partial pressure of oxygen between the inhaled air and the blood allows the oxygen to diffuse.

  • Pox= 160 mm in air and Pox= 40 mm in venous blood.
  • Therefore oxygen diffuses into the blood.
  • Pox= 100 mm in arterial blood; Pox= 0- 40 mm in tissues.
  • Therefore oxygen diffuses into the tissues.
  • Fick's law: the greater the partial pressure difference and the larger the surface area, the faster the gas will diffuse.

About 97% of the oxygen is transported as oxyhemoglobin and 3% dissolves in the plasma.

The maximum amount of oxygen that can be transported by hemoglobin is called the oxygen carrying capacity.

The actual amount of oxygen bound to hemoglobin is the oxygen content.

The ration of oxygen content to oxygen capacity is the percent oxygen saturation.

Oxyhemoglobin dissociates faster in an acid medium.

Lactic acid released during muscular activity lower the pH and therefore decreases the affinity of hemoglobin to oxygen and oxygen is released more easily in the muscles.

Bohr effect: Changing the blood pH affects the % of O2 saturation of blood.

CARBON DIOXIDE TRANSPORT.

Carbon dioxide is transported mainly as bicarbonate ions.

  • About 70% of the CO2 dissolves in the plasma and forms HCO3- and H+ lowering the pH.
  • About 7% - 10% dissolves in the plasma.
  • About 20% - 23% enter the red blood cells and combines with hemoglobin forming carbaminohemoglobin.

This reaction occurs in the RBC catalyzed by carbonic anhydrase.

Carbonic

anhydrase

CO2 + H2O ------→ H2CO3 → H+ → HCO3-

Most of the H+ released from carbonic acid combine with hemoglobin and do not change the pH of the blood.

Many of the bicarbonate ions leave the RBCs and diffuse into the plasma.

Chloride ions diffuse into the RBC to replace the bicarbonate ions. This is known as the chloride shift.

As CO2 diffuses out of the alveolar capillaries, the resulting lower CO2 concentration reverses the previous reaction.

BREATHING CONTROL CENTERS

Breathing is regulated by respiratory centers in the pons, medulla and in the walls of the carotid arteries and aorta.

Chemoreceptors sensitive to increases in CO2 and H+ and to low O2 concentrations regulate the respiratory centers.

Neurons originating in the medulla send messages to the diaphragm and external intercostal muscles causing them to contract and inspiration occurs.

Negative feedback mechanism prevents our lungs from overexpanding; stretch sensors in the lung tissue send nerve impulses back to the medulla, inhibiting its breathing control center.

After several seconds the neurons become inactive, the muscles relax, and expiration occurs.

The medulla control center maintains homeostasis by monitoring the amount of CO2 in the blood.

Slight drop in the pH of the blood and cerebrospinal fluid means an increase in CO2 in the tissues and blood.

The medulla registers these changes, and increases the depth and rates of breathing so the excess of CO2 is eliminated in exhaled air.

Oxygen concentration generally does not play an important role in breathing regulation.

Only if the partial pressure of oxygen drops markedly, the aortic and carotid centers become stimulated to send messages to the respiratory centers in the brain.

Hyperventilation reduces the concentration of CO2 in the blood.

Certain amount of CO2 is needed to maintain normal blood pressure.

CO2 stimulates vasoconstrictors in the brain in order to maintain muscle tone in the walls of blood vessels.

As altitude increases, barometric pressure decreases and less oxygen enters the blood and leads to hypoxia.

A rapid drop of barometric pressure that produces gas bubbles in the blood causes decompression sickness or bends.

Dissolved gases and some liquids bubble out of solution when the barometric pressure drops below the total pressure of the gases in solution.

Diving mammals have high concentration of myoglobin, an oxygen binding pigment found in muscles.

Myoglobin stores oxygen in diving mammals up to ten times more than in terrestrial mammals.

  • Weddell seal store about 25% of its oxygen in muscle, compared to only 13% in humans.

Diving reflex reduces the heart rate, blood is redistributed and other physiological changes occur that allow the diving mammal to conserve oxygen.

Polluted air results in bronchial constriction, increase mucus secretion and damage to ciliated cells.

  • Chronic bronchitis causes the production of too much mucus and damaged ciliated cells cannot expel it. The body resorts to coughing as the way of clearing respiratory passages.
  • Emphysema is caused the loss of elasticity in the alveoli and air cannot be expelled effectively. Also, gas exchanged is impaired. The right ventricle compensates by pumping harder and becoming enlarged. Heart failure is common among emphysema patients.
  • Lung cancer.