TOPIC 6.4 AND H6 GAS EXCHANGE

Gas Exchange

Overview: Gas exchange

Gas exchange makes it possible animals to put to work the food molecules the digestive system provides.

3 phases of gas exchange: 1. Breathing 2. Transport of gases 3. Exchange of O2 and CO2

Overview: 3 phases of gas exchange1. Breathing

1. When an animal breathes, a large, moist internal surface is exposed to air.

2. O2 diffuses across the cells lining the lungs and into surrounding blood vessels.

3. Simultaneously, CO2 diffuses out of the blood and into the lungs, released from the body when exhaling occurs.

2. Transport of gases1. The O2 that has diffused into the blood attaches to

hemoglobin in red blood cells and is carried from the lungs to the body’s tissues.

2. The CO2 is also transported in blood from the tissues back to the lungs.

3. Exchange of O2 and CO2

1. Body cells take up O2 from the blood and release CO2 to the blood.

2. O2 is required for cells to obtain energy from the food molecules the body has digested and absorbed (cell respiration!)

1. Our body needs O2 to function as the final electron acceptor in ETC to produce ATP

Respiratory Surface

• Respiratory surface is the part of an animal where gases are exchanged with the environment

• Made up of living cells, whose plasma membranes must be wet to function properly

• Surface area must be extensive enough to take up sufficient O2 for every cell in the body and to dispose of all waste CO2

• Must also be thin and moist enough to allow CO2 to diffuse rapidly into the circulatory system or body tissues and allow CO2 to diffuse out.

Respiratory Surface

Humans have lungs, which are internal sacs lined with epithelium

Inner surfaces of the lungs branch extensively, forming a large respiratory surface.

Gases are carried between the lungs and the body cells by the circulatory system.

Respiratory Structures

• Lungs are located in the chest cavity, which is bounded at the bottom by a sheet of muscle called the diaphragm.

1. Air usually enters our respiratory system through the nostrils.– Air is filtered by hairs and warmed, humidified, and

sampled for odors

We can also draw in air through the mouth– Does not allow the air to be processed by the nasal

cavity.

Respiratory Structures2. From the nasal cavity or mouth, air passes to the

pharynx, where the paths for air and food cross.*the air passage in the pharynx is always

open for breathing except when we swallow3. From the pharynx, air is inhaled into the larynx

(voice box)*when we exhale, the outgoing air rushes by a pair of vocal cords in the larynx, and

we can produce sounds by voluntarily tensing muscles in the voice box, stretching the cords and making them vibrate.

*we produce high-pitched sounds when our vocal cords are tense and therefore vibrating very fast.

*when the cords are less tense, they vibrate slowly and produce low-pitched sounds

Respiratory Structures

4. From the larynx, inhaled air passes toward the lungs through the trachea, or windpipe.*rings of cartilage maintain the shape of the trachea, much as metal rings keep the hose of a vacuum cleaner from collapsing.

5. The trachea forks into two bronchi (singular, bronchus), one leading to each lung. *within the lung, the bronchus branches repeatedly into finer and finer tubes called bronchioles.

Respiratory Structures

6.Bronchioles dead-end in grape-like clusters of air sacs called alveoli*each of our lungs contains millions of these tiny sacs*the inner surface of each alveolus is lined with a thin layer of epithelial cells that form the respiratory surface.*The O2 in inhaled air dissolves in a film of moisture on the epithelial cells*It then diffuses across the epithelium and into a web of blood capillaries that surrounds each alveolus.

*The CO2 diffuses the opposite way—from the capillaries, across the epithelium of the alveolus, into the air space of the alveolus, and finally out in the exhaled air.

Structure of alveoli

Respiratory Structures

The trachea and major branches are lined by a moist epithelium covered by cilia and a thin film of mucus.

Cilia and mucus serve as the system’s cleaning elements: Mucus traps dust, pollen, and other contaminants Beating cilia move the mucus upward to the pharynx,

where it is usually swallowed.

Respiratory Structures

Breathing

Breathing is the alternation of inhalation and exhalation.

This ventilation of our lungs maintains high O2 and low CO2 concentrations at the respiratory surface.

We breathe by pulling air into the lungs, and then pushing it back out.

Breathing: Inhalation

During inhalation: Both the rib cage and chest cavity expand, and the

lungs follow suit. Ribs move upward and the rib cage expands as

muscles between the ribs contract. At the same time, the diaphragm contracts, moving

downward and expanding the chest cavity as it goes Increase in volume of the lungs lowers the air pressure

in the alveoli to less than atmospheric pressure Negative pressure breathing

Air rushes through the nostrils into the alveoli from a region of higher pressure to one of lower pressure.

Breathing: Exhalation

During exhalation: Rib muscles and diaphragm both relax, decreasing the

volume of the rib cage and the chest cavity and forcing air out of the lungs.

The diaphragm curves upward into the chest cavity Decrease in volume of the lungs increases the air

pressure in the alveoli to be greater than atmospheric pressure

Breathing

Each year, a human adult may take between 4 million and 10 million breaths.

Volume of air is about 500 mL when we breathe quietly Vital capacity

Maximum volume of air that we can inhale and exhale during forced breathing.

Averages about 3.4 L and 4.8 L for college-age females and males (women tend to have smaller rib cages and lungs)

Lungs actually hold more air than the vital capacity Because the alveoli do not completely collapse, a residual

volume of “dead” air remains in the lungs even after we exhale

As lungs lose resilience with age or as result of disease (emphysema), the residual volume increases at the expense of vital capicity.

Negative Pressure Breathing

Breathing Control Centers

Although we can voluntarily hold our breath, most of the time, it is controlled by automatic control centers in our brain to regulate our breathing movements.

Automatic control is essential; it ensures coordination between the respiratory and circulatory systems and the body’s metabolic needs for gas exchange.

Breathing Control Centers

Breathing control centers Located in parts of the brain called the pons and

medulla oblongata Nerves from the medulla’s control center signal the

diaphragm and rib muscles to contract, making us inhale.

Nerves send out signals that result in about 10 to 14 inhalations per minute when we are at rest

Between inhalations, the muscles relax and we exhale. Control center in pons smooths out the basic rhythm of

breathing set by the medulla.

Breathing control centers

Adjusting our breathing rate in response to the body’s needs: Medulla’s control center monitors the CO2 level of the blood

and regulates breathing rate in response. Its main cues about CO2 concentration come form slight

changes in the pH of the blood and in the fluid bathing the brain (cerebrospinal fluid) pH decreases when CO2 in blood increases

When we exercise vigorously: our metabolism speeds up and our body cells generate more CO2 as

a waster product. Co2 goes into the blood, where it reacts with water to form carbonic

acid, lowering the pH of the blood and cerebrospinal fluid slightly The medulla senses this drop and its b.c.c. increases the breathing

rate and depth. More CO2 is eliminated in the exhaled air, and the pH returns to

normal.

Breathing Control Centers

Hyperventilating Excessively taking rapid, deep breaths Action of your b.c.c. that is hard on your body Deep, rapid breathing purges blood of so much CO2

that the control center temporarily ceases to send signals to the rib muscles and diaphragm

Breathing stops until the CO2 levels increases enough to switch the breathing center back on.

Breathing Control Centers

B.C.C. responds directly to CO2 levels, but it usually does not respond directly to oxygen levels.

Since the same process that consumes O2, cell respiration, also produces CO2, a rise in CO2 (drop in pH) is generally a good indication of a drop in blood oxygen.

Thus, by responding to lowered pH, the b.c.c. also controls blood O2 level.

Breathing Control Centers

Secondary control over breathing is exerted by sensors in the aorta and carotid arteries that monitor concentrations of O2 and CO2.

When O2 level in the blood is severely depressed, these sensors signal the control center via nerves to increase the rate and depth of breathing. May occur at high altitudes, where the air is thin that

we cannot get enough O2 by breathing normally. B.C.C. respond to a variety of nervous and chemical

signals to keep the breathing rate and depth in tune with the changing demands of the body.

Breathing rate must be coordinated with the circulatory system.

Transport of Gases

Heart pumps oxygen-poor blood to the alveolar capillaries in the lungs .

Gases are exchanged between air in the alveolar spaces and blood in the alveolar capillaries.

Blood leaves the alveolar capillaries, having lost CO2 and gained O2

Oxygen-rich blood returns to the heart and is pumped out to body tissues.

Transport of gases

Exchange of gases between capillaries and cells around them occurs by the diffusion of gases down gradients of pressure.

A mixture of gases, such as air, exerts prsessure.

Each kind of gas in a mixture accounts for a portion, called the partial pressure, of the mixture’s total pressure.

Molecules of each kind of gas will diffuse down a gradient of its own partial pressure independent of other gases.

Transport of gases

For example, O2 moves from oxygen-rich blood through the

interstitial fluid and into tissue cells because it diffuses from a region of higher partial pressure to a region of lower partial pressure.

The tissue cells maintain this gradient as they consume O2 in cell respiration.

CO2 produced as a waste product of cellular respiration diffuses down its own partial-pressure gradient out of the cells and into the capillaries.

Diffusion also accounts for gas exchange in the alveoli

Transport of gases

Oxygen is not very soluble in water, and most of the O2 in blood is carried by hemoglobin in the red blood cells.

A hemoglobin molecule consists of four polypeptide chains.

Attached to each polypeptide chain is a chemical group called a heme, at the center of which is an iron atom.

Each iron atom can carry one O2 molecule; every hemoglobin can carry up to four oxygen molecules.

Transport of gases

Hemoglobin loads up with O2 in the lungs and transports it to the body tissues.

At the tissues, hemoglobin unloads O2, depending in the needs of the cells (partial pressure of O2 in the tissue reflects how much

O2 the cells are using)

Hemoglobin also helps blood transport CO2 and assists in buffering the blood (preventing harmful changes in pH)

Transport of gases

Hemoglobin also helps blood transport CO2 and assists in buffering the blood Helps to prevent harmful changes in pH When CO2 leaves a tissue cell, it diffuses through the

interstitial fluid, across the wall of a capillary, and into the blood fluid (plasma)

Most of the CO2 enters the rbc’s where some of it combines with hemoglobin

Rest reacts with water molecules, forming carbonic acid (H2CO3) with the help of an enzyme (carbonic anhydrase) on the rbc’s

H2CO3 then breaks apart into H+ and HCO3- Hemoglobin binds most of the H+, minimizing change in

blood pH HCO3- diffuses into the plasma, where they are carried to

the lungs

Transport of Gases

Hemoglobin also helps blood transport CO2 and assists in buffering the blood (continued) Process is reversed as blood flows through capillaries

in the lungs. Carbonic acid forms when bicarbonate combines with

H+ Carbonic acid is then converted back to CO2 and

water Finally, the CO2 diffuses from the blood into the

alveoli and out of the body in exhaled air.

Transport of Gases

Oxygen dissociation curves important tool for understanding how our blood

carries and releases oxygen relates oxygen saturation (SO2) and partial pressure

of oxygen in the blood (PO2) is determined by what is called "hemoglobin's affinity

for oxygen"; that is, how readily hemoglobin acquires and releases oxygen molecules into the fluid that surrounds it

Transport of gases

Bohr shift a property of hemoglobin which states that at lower

pH (more acidic environment), hemoglobin will bind to oxygen with less affinity.

Since carbon dioxide is in direct equilibrium with the concentration of protons in the blood, increasing blood carbon dioxide levels leads to a decrease in pH, which ultimately leads to a decrease in affinity for oxygen by hemoglobin.

promotes the dissociation of oxygen from hemoglobin to the tissue, allowing the tissue to obtain enough oxygen to meet its demands

Transport of gases

Bohr Shift: Oxygen Dissociation curve shifts to the right when carbon dioxide or hydrogen ion

concentration is increased. This facilitates increased oxygen dumping. This mechanism allows for the body to adapt the problem

of supplying more oxygen to tissues that need it the most. When muscles are undergoing strenuous activity, they

generate CO2 and lactic acid as products of cellular respiration and lactic acid fermentation.

In fact, muscles generate lactic acid so quickly that pH of the blood passing through the muscles will drop to around 7.2. As lactic acid releases its protons, pH decreases, which causes hemoglobin to release ~10% more oxygen

This shifting of the curve to the RIGHT from these 4 factors is referred to as the Bohr

effect.

Breathing Challenges

Asthma an inflammatory disorder of the airways, which causes

attacks of wheezing, shortness of breath, chest tightness, and coughing

When an asthma attack occurs, the muscles surrounding the airways become tight and the lining of the air passages swell. This reduces the amount of air that can pass by, and can lead to wheezing sounds.

Excess mucus is also produced, further contributing to airflow restriction, making it difficult for oxygen to get through to the alveoli and into the bloodstream.

Asthma attacks can last minutes to days and can become dangerous if the airflow becomes severely restricted.

Breathing Challenges

High altitudes The percentage of oxygen in the air at two miles (3.2 km.) is the same as

at sea level (21%).  However, the air pressure is 30% lower at the higher altitude because the atmosphere is less dense--that is, the air molecules are farther apart

At high altitudes, the lower air pressure makes it more difficult for oxygen to enter our vascular systems.  The result is hypoxia, or oxygen deprivation. 

Hypoxia usually begins with the inability to do normal physical activities, such as climbing a short flight of stairs without fatigue. 

Other early symptoms of "high altitude sickness" include a lack of appetite, distorted vision, and difficulty with memorizing and thinking clearly.  

In serious cases, pneumonia-like symptoms (pulmonary edema) and an abnormal accumulation of fluid around the brain (cerebral edema) develop, leading to death within a few days if there is not a return to normal air pressure levels.  There is also an increased risk of heart failure due to the added stress placed on the lungs, heart, and arteries at high altitudes.

Breathing challenges

High Altitudes: Body response When we travel to high altitudes, our bodies initially

develop inefficient physiological responses.  There is an increase in breathing and heart rate to as

much as double even while resting.   Pulse rate and blood pressure go up sharply as our hearts

pump harder to get more oxygen to the cells.  These are stressful changes, especially for people with

weak hearts.

Breathing challenges

High Altitudes: Body response Later, a more efficient response normally develops as

acclimatization takes place. More red blood cells and capillaries are produced to

carry more oxygen.  T the lungs increase in size to facilitate the osmosis of

oxygen and carbon dioxide.  There is also an increase in the vascular network of

muscles which enhances the transfer of gases.

Breathing Challenges

On returning to sea level after successful acclimatization to high altitude, the body usually has more red blood cells and greater lung expansion capability than needed. 

Since this provides athletes in endurance sports with a competitive advantage, the U.S. maintains an Olympic training center in the mountains of Colorado. 

Several other nations also train their athletes at high altitude for this reason.  However, the physiological changes that result in increased fitness are short term at low altitude.  In a matter of weeks, the body returns to a normal fitness level.