Aerobic Cellular Respiration and Gas Exchange

Aerobic Cellular Respiration

• Cells require oxygen for survival, using it to obtain energy from food.
• Aerobic cellular respiration is the process.
• Glucose reacts with oxygen, releasing energy, forming carbon dioxide and water.
• 64% of energy is released as thermal energy.
• Approximately 36% of the energy is stored in ATP (adenosine triphosphate) molecules.
• ADP + P + Energy → ATP (ATP formation)
• Cells use ATP for various processes like growth, movement, and building new molecules.
• ATP → ADP + P + Energy (ATP use)

Gas Exchange and Ventilation

• Gas exchange is the diffusion of oxygen into and carbon dioxide out of body cells.
• Simple organisms directly exchange gases via their cell membrane.
• Multicellular organisms use specialized systems to exchange gases.
• Gas exchange occurs in cells and lungs.
• In the lungs, oxygen diffuses from air into the bloodstream then cells.
• Oxygen diffuses from the blood into tissue fluid, then into cells.
• Carbon dioxide travels from cells to tissue fluid, then bloodstream, and lungs, finally diffusing out.
• Ventilation is the process of moving air to and from the lungs.

Respiratory Structures

• The human respiratory system has four key features: thin respiratory membrane, large surface area, good blood supply and a system to deliver oxygen-rich air.
• Lungs are the primary site of gas exchange, containing the respiratory membrane with a large surface area and rich blood supply.
• Lungs are enclosed within the thoracic cavity, protected by the rib cage.

Pathway of Air

• Air enters through the nose and mouth, warming and moistening in the nasal passages.
• Nasal passages have hairs and mucus to filter and trap particles.
• The pharynx is where the glottis stays open during breathing.
• The trachea (windpipe) is a semi-rigid tube with cartilage rings, keeping it open.
• Trachea is lined with mucus and cilia, sweeping away debris.
• Cilia are hair-like structures, propelling trapped particles upward via the trachea.
• Bronchi connect the trachea to the lungs.
• Bronchioles are smaller branches of bronchi, branching extensively.
• Alveoli are clusters of tiny sacs surrounded by capillaries.
• Each alveolus is extremely small.
• There are around 150 million alveoli in each lung.

Gas Exchange in the Alveoli

• Air in the alveoli is 37°C and saturated with moisture.
• Oxygen diffuses across the respiratory membrane when dissolved in liquid.
• Alveoli structures are ideal for gas exchange, membrane is one-cell thick.

Partial Pressures

• Air pressure is measured in kilopascals (kPa).
• Sea level air pressure is 101.3 kPa.
• Partial pressure is the pressure exerted by an individual gas.
• Oxygen makes up about 20.9% of air.
• Carbon dioxide comprises about 0.0391% of air.

Oxygen Transport and Diffusion

• PO2 in alveoli is about 13.3 kPa, which is higher than in capillaries.
• Hemoglobin is an iron-containing protein that binds to oxygen to form oxyhemoglobin.
• 98.5% of oxygen is carried by hemoglobin.
• Hemoglobin enhances oxygen carrying capacity significantly.

Cells

• Oxygen diffuses into body cells.
• In the veins, blood still contains some oxygen due to hemoglobin's capability to hold oxygen.

Carbon Dioxide Transport and Diffusion

• Carbon dioxide is a by-product of cellular respiration and needs removal.
• CO2 diffuses from tissues to blood.
• Three ways CO2 is transported: 7% as dissolved in plasma, 20% bound to hemoglobin, and 73% as bicarbonate ions.
• Hydrogen ions from carbonic acid affect blood pH.
• Increased CO2 in the body triggers increased breathing rate for removal.
• Hydrogen ions attach to hemoglobin to prevent blood acidity.
• In the lungs, bicarbonate and hydrogen ions are converted back to CO2 and water.

Altitude

• Reduced atmospheric pressure at higher altitudes lowers the partial pressure of oxygen.
• Altitude sickness results from decreased oxygen supply, causing symptoms such as shortness of breath.
• Kidneys produce EPO (erythropoietin) when oxygen supply is reduced.
• EPO stimulates red blood cell production increasing oxygen absorption.

Mechanism of Ventilation

• Inspiration: chest cavity expands, increasing lung volume, and decreasing lung pressure, drawing air in.
• Expiration: chest cavity contracts, decreasing lung volume, and increasing lung pressure, expelling air.

Lung Capacity

• Total Lung Capacity: maximum volume of air held.
• Tidal Volume: normal, involuntary breath volume.
• Inspiratory Reserve Volume: air forcibly inhaled after a normal breath.
• Expiratory Reserve Volume: air forcibly exhaled after a normal breath.
• Residual Volume: air remaining in lungs after forced exhalation (preventing collapse).
• Vital Capacity: maximum exhaled air volume.