The physiology of cartilage centers around its unique structure and avascular nature, enabling it to provide support, reduce friction, and distribute load within the body.
Components of Cartilage
Cartilage is a specialized connective tissue composed primarily of:
- Chondrocytes: These are the only cells found in cartilage. They produce and maintain the extracellular matrix.
- Extracellular Matrix (ECM): This matrix comprises most of the cartilage's volume and provides its unique properties. It consists of:
- Collagen fibers: Primarily type II collagen, providing tensile strength.
- Proteoglycans: These molecules, like aggrecan, attract and retain water, providing resilience and compressive stiffness.
- Water: Cartilage has a high water content (60-80%), which is crucial for nutrient transport and resisting compression.
Unique Physiological Aspects
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Avascularity: Cartilage lacks blood vessels. This impacts nutrient supply and waste removal, relying on diffusion from surrounding tissues (perichondrium or synovial fluid). This contributes to slow healing.
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Load Distribution: The interaction between collagen and proteoglycans allows cartilage to distribute compressive forces evenly, protecting underlying bone.
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Low Metabolic Rate: Chondrocytes have a relatively low metabolic rate due to the limited oxygen availability, which is another consequence of being avascular.
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Types of Cartilage and Their Specific Physiology: There are three main types of cartilage, each with slight physiological variations:
- Hyaline Cartilage: Found in articular surfaces of joints, the nose, and trachea. It provides smooth, low-friction surfaces for joint movement and structural support in other locations. Its high water content and specific proteoglycan composition are critical for joint lubrication and shock absorption.
- Elastic Cartilage: Found in the ear and epiglottis. Contains abundant elastic fibers, providing flexibility and the ability to return to its original shape after deformation.
- Fibrocartilage: Found in intervertebral discs and menisci. Contains a high proportion of type I collagen (compared to the type II in hyaline and elastic), providing resistance to tension and compression. This is crucial in areas bearing high loads.
Nutrient Supply and Waste Removal
Because cartilage is avascular, nutrient supply and waste removal occur via:
- Diffusion from the perichondrium: Nutrients diffuse from blood vessels in the perichondrium (a layer of connective tissue surrounding cartilage) to the chondrocytes.
- Synovial Fluid: Articular cartilage relies on the synovial fluid in joints for nutrient supply and waste removal. The movement of the joint helps to "pump" fluid in and out of the cartilage matrix, facilitating exchange.
Response to Injury
- Cartilage has a limited capacity for self-repair due to its avascular nature and the limited proliferative capacity of chondrocytes.
- Damage to cartilage can lead to osteoarthritis, a degenerative joint disease characterized by cartilage breakdown.
Physiological Functions Summarized
| Function | Description |
|---|---|
| Support | Provides structural support to various tissues and organs. |
| Friction Reduction | Creates smooth, low-friction surfaces in joints, facilitating movement. |
| Load Distribution | Distributes compressive forces evenly, protecting underlying bone from damage. |
| Flexibility | Allows for flexibility and elastic recoil in structures like the ear and epiglottis (elastic cartilage). |
| Shock Absorption | Absorbs shock and cushions joints from impact. |
In summary, the physiology of cartilage is dictated by its unique composition and avascularity. This combination facilitates load distribution, reduces friction in joints, provides flexible support, and makes nutrient supply and repair challenging.
