Red blood cells | Human anatomy and physiology | Health & Medicine | Khan Academy

• Red blood cells (RBCs), or erythrocytes, are responsible for transporting oxygen from the lungs to the body's tissues and returning carbon dioxide from the tissues back to the lungs for exhalation.

• They have a unique biconcave shape, which increases their surface area for gas exchange and allows them to deform as they pass through narrow capillaries.

• RBCs are produced in the bone marrow through a process called erythropoiesis and have a lifespan of about 120 days before being recycled by the spleen and liver.

1. Alveoli: The Gas Exchange Units

Structure of Alveoli:

• Small, sac-like structures found at the terminal ends of bronchioles.

• Arranged in clusters resembling grape-like formations to maximize surface area.

• Walls are extremely thin, one cell thick, allowing efficient gas diffusion.

Alveolar Function:

• Gas Exchange:

  • Oxygen enters the alveoli from inhaled air.

  • Carbon dioxide from the bloodstream diffuses into the alveoli to be exhaled.

• Close Proximity to Capillaries:

  • Surrounded by pulmonary capillaries for efficient exchange of gases.

Diffusion Process:

• Oxygen from alveoli (high concentration) moves into capillary blood (low concentration).

• Carbon dioxide from blood (high concentration) moves into alveoli (low concentration).

Key Adaptations:

• Large surface area and thin membrane enhance the speed and efficiency of gas exchange.

• Elastic walls enable alveoli to expand and contract with each breath.

2. Bronchioles: Pathways to the Alveoli

Structure of Bronchioles:

• Small, tube-like structures branching from larger bronchi.

• Gradually narrow as they extend deeper into the lungs.

• Lack cartilage, unlike bronchi, but have smooth muscle that controls their diameter.

Function of Bronchioles:

• Air Distribution:

  • Deliver air from bronchi to alveolar sacs.

  • Act as conduits, ensuring air reaches all alveoli.

• Regulation of Airflow:

  • Smooth muscle in the bronchiole walls adjusts diameter:

    • Bronchodilation: Expands the airways to increase airflow.

    • Bronchoconstriction: Narrows the airways during irritant exposure or asthma.

Importance of Bronchioles in Gas Exchange:

• Control the volume of air reaching alveoli.

• Ensure oxygen delivery is matched to metabolic demands.

3. Oxygen: The Life-Sustaining Gas

Role of Oxygen:

• Essential for cellular respiration, where cells convert glucose into energy (ATP).

Journey of Oxygen:

1. Inhalation:

   • Oxygen-rich air enters the lungs and reaches alveoli.

2. Diffusion into Blood:

   • Oxygen diffuses across the thin alveolar membrane into capillaries.

   • Driven by the concentration gradient (higher oxygen in alveoli vs. blood).

Oxygen Transport:

• Binding to Hemoglobin:

  • Oxygen binds to hemoglobin in red blood cells.

  • Each hemoglobin molecule can carry four oxygen molecules.

• Efficiency:

  • 98.5% of oxygen is carried by hemoglobin, while only a small fraction dissolves in plasma.

Oxygen Delivery to Tissues:

• Oxygenated blood is pumped by the heart to tissues where oxygen is released.

• Enables ATP production necessary for metabolic functions.

4. Heart and Arteries: Circulatory Powerhouses

Heart’s Role in Gas Exchange:

1. Deoxygenated Blood:

   • Blood low in oxygen and high in carbon dioxide returns to the heart from tissues.

   • Pumped from the heart to the lungs via pulmonary arteries.

2. Oxygenated Blood:

   • Blood absorbs oxygen in the lungs and releases carbon dioxide.

   • Pumped back to the heart through pulmonary veins, then distributed to the body.

Pulmonary Arteries and Veins:

• Pulmonary Arteries:

  • Transport deoxygenated blood from the heart to the lungs.

  • Unique as arteries typically carry oxygenated blood.

• Pulmonary Veins:

  • Carry oxygenated blood from the lungs back to the heart.

  • Unique as veins typically carry deoxygenated blood.

Artery and Vein Transition:

• Occurs when blood absorbs oxygen and releases carbon dioxide in alveoli.

• Oxygenated blood appears bright red, while deoxygenated blood looks purplish-blue

5. Red Blood Cell Structure: Specialized Oxygen Carriers

Shape and Adaptations:

• Biconcave Shape:

  • Increases surface area for oxygen exchange.

  • Enhances flexibility for navigating narrow capillaries.

• No Nucleus or DNA:

  • Mature RBCs expel their nucleus, maximizing space for hemoglobin.

  • Lack of DNA limits their lifespan to about 80–120 days.

Hemoglobin: Oxygen Transport Molecule:

• Structure:

  • Contains four heme groups, each centered around an iron atom.

  • Each heme group binds one oxygen molecule, enabling hemoglobin to carry four oxygen molecules.

• Color Change:

  • Oxygenated hemoglobin appears bright red.

  • Deoxygenated hemoglobin looks bluish due to the absence of oxygen.

Transport Efficiency:

• Hemoglobin absorbs the majority of oxygen (98.5%).

• Remaining oxygen dissolves in plasma for immediate use.

Philosophical Note:

• Without DNA or the ability to replicate, RBCs are highly specialized and efficient but not “alive” traditionally.