1.1: HUMAN RESPIRATORY SYSTEM

The human respiratory system consists of the nose, trachea, bronchi & a pair of lungs. The air passage during inhalation:Nose trachea Bronchus Bronchiole Alveolus

Air inhaled through the nose or the mouth cavity. The nostrils open into a nasal cavity with many olfactory hairs. The olfactory hairs filter & prevent dust particles from entering the lungs. The wall of the nasal cavity is lined with mucus membranes that keeps it moist & helps to trap the dust. The nasal cavity is connected to the trachea. The trachea connects the larynx with two bronchi that lead into the lungs. The lung is a soft spongy organ with a network of air sacs. In each of the lungs, the bronchus divides into tiny ducts called bronchioles. Each bronchioles ends in an air sac known as an alveolus. Gas exchange takes place in the alveolus

O2 from the inhaled air diffuses through the thin wall of the alveolus into the blood capillary blood plasma. In the plasma, O2 diffuse into the red blood cells & combines with Hb to form oxyhaemoglobin. At the same time, CO2 & water molecules from the blood capillary diffuse into the alveolus. The CO2 & water (in the form of vapour) is expelled from the body during exhalation.

Mechanism of Ventilation

Breathing occurs in 2 stages: Inspiration/inhalation is the process in which air is actively inhaled into the lungs. Expiration/exhalation is the process in which air is exhaled from the lungs.

Adaptations for gaseous exchange in the lungs.

The alveoli provide a large surface area for gaseous exchange. The respiratory surface of a human is made-up of over 700 million alveoli, inside a pair of lungs. The surfaces of the alveoli are moist for gases to dissolve before diffusion can occur. The alveoli have thin walls (one cell thick) which minimize the distance for gas diffusion. The walls are permeable to diffusion of O2 & CO2. The alveoli are surrounded by numerous blood capillaries, which bring CO2 for diffusion into the alveoli & carry away O2 to the circulatory blood system.

Structure of Hb & the transport of O2

Hb is a respiratory pigment, which is found in the red blood cells to assist in the transport of O2. Hb is a conjugated protein with a quaternary structure. It consists of 4 p/peptide chains, 2- & 2- chains, that coil closely together to form a nearly spherical structure. Each p/peptide chain contains a haem group. One Hb molecule can bind loosely & reversibly with 4 molecules of O2 to form oxyhaemoglobin molecule.Hb + 4O2 Hb (O2)4 Each red blood cell contain about 250 x 106 molecules of Hb. Because each Hb molecule consists of 4 haem groups, each red blood cell of mammal can transport a total of about 1000 x 106 O2 molecules.

The high capacity of red blood cells in transporting O2 molecules is due to the fact that the red blood cells do not contain nuclei & the surface area to volume ratio of the biconcave disc shaped cells is extremely high.The high efficiency of the Hb in absorbing O2 can be explained as follows:1. Hb & O2 can be easily bonded even at low O2 concentrations. Therefore, blood can be saturated with O2 immediately once in contact with water & air.2. The O2 content in red blood cells is much higher than the content in the plasma.3. Despite having a high affinity for O2, Hb can also release O2 quite easily when there is a drop in partial O2 pressure. Low partial O2 pressure occurs frequently in inner tissues of the body.

Oxygen Dissociation Curves

O2 dissociation curves show the relationship between the degree of Hb saturation with O2 at different values of partial pressure of O2. When the PO2 is high as in the lung capillaries, Hb has a high affinity for O2 to form Hb(O2)4.

When the PO2 is low as in the respiring tissues, the Hb(O2)4 dissociates & O2 is liberated. The sigmoid shape of the O2 dissociation curve shows that blood can be saturated with O2 even at a very low PO2.This is what is meant by the high affinity pf Hb for O2. The sigmoid shape of the O2 dissociation curve also shows that the percentage of O2 saturation in Hb declines steeply as the O2 pressure falls. The Hb responds by releasing more O2 as shown by the steep curve at the lower level of PO2 as in respiring tissues. The O2 dissociation curve for Hb is different under different condition of CO2 pressure as in respiring tissues. Increase in CO2 pressure will shift the O2 dissociation curve to the right. This effect is known as Bohrs effect.

This means that as the CO2 pressure increases, the rate of O2 combination with Hb decreases, or the dissociation of O2 from oxyhemoglobin is more efficient. Therefore, the rate of O2 dissociate from Hb(O2)4 is higher in the tissues that contain higher CO2 pressure due to respiration. Conversely, the rate of O2 combination with Hb is higher in the lungs that contain lower CO2 pressure due to release of CO2 to the atmosphere.O2 dissociation curve of Hb & myoglobin in comparison

Myoglobin acts as O2 store in the muscles. Myoglobin is a protein conjugate having chemical similarities to Hb. Compared to Hb, myoglobin shows a higher affinity towards O2 & becomes easily saturated even at very low O2 concentration. Its O2 dissociation curve is displaced well to the left of Hb. It only begins to release O2 when the partial pressure of O2 is below 20mmHg. In this way, it acts as a store of O2 in resting muscle, only releasing it when supplies of HbO2 have been exhausted. (e.g; strenuous exercise)

O2 Dissociation Curves of Hb in Fetus & Mother.

The fetus has a very high O2 demand. The fetal Hb is one of a type which has a higher affinity for O2 than the mothers Hb. O2 is therefore readily unloaded from the mothers blood to the fetal blood. A graph for fetal Hb shows a shift to the left from curve for adult Hb.

CO2 TRANSPORT IN BLOOD

CO2 is transported in the blood in 3 ways;1. In aqueous solution (5%) About 5% of the CO2 does not diffuse into the red blood cells, but dissolves into the plasma & becomes carbonic acid, H2CO3, and carried as such in the blood.

2. Combined with protein (10-20%) CO2 can combine with the amino group at one end of the haemoglobin p/peptide to form a neutral carbamino-haemoglobin compound. The amount of CO2 that is able to combine with Hb depends on the amount of O2 already being carried bythe Hb. The less the amount of O2 being carried by the Hb molecule, the more CO2 that can be carried by the Hb. 3. As Hydrogen carbonate (85%) CO2 produced by the tissue diffuses passively into the bloodstream & passes into the erythrocytes where it combines with water to form carbonic acid. The reaction is catalysed by carbonic anhydrase.

The carbonic acid quickly dissociates into a hydrogen ion (H+) & a hydrogencarbonate ion (HCO3-).H2CO3 H+ + HCO3- The hydrogen carbonate ions (HCO3-) diffuse out of the erythrocyte into the plasma .

The erythrocyte membrane is relatively impermeable to cations such as Na+ & K+. To maintain electrical neutrality, the chloride ions (Cl-) diffuse into the erythrocyte to balance the hydrogen carbonate ions diffusing out. The process is called a chloride shift. The concentration of H+ in the cell increases, pH decreases. The Hb(O2)4 dissociates & O2 released to the cell for cellular respiration. The Hb acts as a buffer combines with H+ to form haemoglobinic acid (HHb).

Exchange of gases between air in the alveolar space & blood in the pulmonary capillaries.

1.2: Control of Breathing/Respiration

The control is involuntary & involves negative feedback mechanisms. The breathing rate is controlled involuntary by respiratory center, which is located in the medulla of the brain. The respiratory center is divided into;i) Inspiratory center control inspiration & found at the ventral part of the respiratory center.ii) Expiratory center controls expiration & found at the dorsal & lateral parts of the respiratory center. Chemoreceptors are receptor cells that can be stimulated by chemicals including H+ concentration caused by CO2. These chemoreceptors are stimulated by increase in [H+] (low pH) & they can produce impulse that is sent to the respiratory center in the brain by sensory nerve.

There are 2 types of chemoreseptors that detect [H+];i. The Peripheral Chemoreceptors, the carotid & aortic bodies, found on the carotid arteries & aorta, are stimulated by the high concentration of CO2 & H+. Nerve impulses are sent from the peripheral chemoreceptors to the respiratory center. ii. The central chemoreceptors in the medulla are sensitive to H+ & not CO2 concentration. As arterial [CO2] increases some of the molecules diffuse into the cerebrospinal fluid surrounding the medulla. This results in a corresponding increase in [H+] & decrease in pH. CO2 + H2O H2CO3 H+ + HCO3- The central chemoreceptors in the medulla are stimulated by H+. Impulse are then sent to the respiratory center.

The sequence of events that occurs in the process of involuntary controlled breathing:

1. CO2 from respiration is detected by chemoreceptor & the impulses generated are sent to the respiratory center that controls the rate of breathing.2. From the respiratory center impulses is sent to the diaphragm & external inter-costal muscle respectively.3. Both muscles contract, resulting an expansion of thoracic cavity, which causes the lung to inflate, inspiration occurs.4. When the lung expand, stretch receptors within the walls of alveoli & bronchioles are stimulated.5. Inhibitory impulses are sent to inspiratory center, which cut off inspiratory activity.6. Such action results in the relaxation of the respiratory muscles.7. The thoracic cavity contracts deflating the lungs & resulting in expiration.8. The walls of the alveoli & bronchiole contract & no inhibitory impulse is sent to the respiratory center. The cycle can begin again.

Voluntary (conscious) control

Breathing can be altered voluntarily to a limited extent by the higher centers in the cerebral hemispheres to permit activities like singing, talking & holding the breath for a short while.

1.3:GASES EXCHANGE IN PLANTSStoma- structure & Functions Gaseous exchange in plants occurs mainly through pores called stomata. They are found on the epidermis of leaves & stems of flowering plants. Lenticels found in the bark of stem & root hairs with thin walls & large surface area also allow gaseous exchange to take place. Each stoma consist of a stomatal pore surrounded by two guard cells. Each guard cell is kidney-shapes & contains chloroplasts. It has a thinner outer wall & a thicker, less elastic, inner wall. Changes in turgor pressure of guard cells causes the opening or closing of the stomatal pore. The stomatal pore allows exchange of CO2 & O2 for photosynthesis & respiration & control water loss to the surroundings.

Mechanism of opening & closing

Stomata open & close because of changes in turgor pressure of their guard cells. Stomata opens when the guard cells become turgid. When the cells swell with water, the thin outer walls bulge out & force the inner walls into a crescent shape. As a result the guard cells buckle & the stoma opens. Stoma closes when the guard cells become flaccid. This is because when water leaves the guard cells, there is no turgor pressure, causing the leaf to wilt & the guard cells to close.

Flaccid Guard CellsTurgid Guard Cells

Hypothesis of opening & closing

1. CO2 concentration in the leaf When there is light, photosynthesis takes place & CO2 is used. The pH of the guard cell is increase. This causes the enzyme amylase to change starch to sugar, lowering of the water potential & the stoma opens. At night, the process is reversed as accumulation of CO2 by respiration lowers the pH. Sugar is changed to starch by the amylase & the starch does not affect the water potential.2. Photosynthesis in the guard cells & the accumulation of sugar When there is light, photosynthesis takes place in the chloroplasts of the guard cells resulting an accumulation of sugar in the guard cells. The sugar reduces the water potential of the guard cells & water moves in from neighbouring cells by osmosis, the guard cells become turgid & the stoma open. The water potential of the guard cells reverses & the stoma closes. At night, there is no photosynthesis, therefore no sugar production [sugar ] in guard cells.3. Potassium ion Hypothesis

When there is light, especially blue light, it stimulates the proton pump in the membrane of the guard cells. This causes a fast accumulation of H+ in the cells. This in turn causes the opening of K+ channel & a fast uptake of K+ ions. An increase in K+ concentration lowers the water potential that causes the water to diffuse in by osmosis, increasing the turgor pressure & the stoma opens. In some species, Cl- accompanies the K+ in and out of the guard cells, thus maintaining electrical neutrality. At night, the reverse process take place when there is no light. K+ diffuses out, water also leaves. The guard cells lose turgor, & the stomata close.