Human Physiology: The Skin and Its Parts PDF

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EthicalPegasus

Uploaded by EthicalPegasus

University of Northampton

Rodrigo Diaz-Martinez

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human physiology skin anatomy biology

Summary

This document provides a detailed overview of human skin physiology, covering its various functions and structure. It explores the different layers of the skin, including the epidermis, dermis, and hypodermis. The document also details important processes such as sensation, excretion, absorption, and thermoregulation.

Full Transcript

**The Skin and Its Appendages** The skin is the largest organ of the human body, serving as a vital barrier between the internal environment and the external world. It performs multiple essential functions, ranging from protection and sensation to temperature regulation and vitamin D synthesis. In...

**The Skin and Its Appendages** The skin is the largest organ of the human body, serving as a vital barrier between the internal environment and the external world. It performs multiple essential functions, ranging from protection and sensation to temperature regulation and vitamin D synthesis. In this unit, we will explore the structure, function, and appendages of the skin in detail. **1. Functions of the Skin** **Sensation:**\ The skin is richly innervated with sensory receptors that detect a wide range of stimuli, including touch, pressure, temperature, and pain. These receptors allow the body to respond to environmental changes, providing critical information about the surroundings. **Excretion and Secretion:**\ The skin plays a role in the excretion of waste products through sweat, which contains water, salts, and other metabolites. Sebaceous glands secrete sebum, an oily substance that lubricates and protects the skin and hair. **Absorption:**\ While the skin is primarily a protective barrier, it can also absorb certain substances, such as medications delivered through transdermal patches. The absorption capacity of the skin varies depending on factors such as thickness, hydration, and the presence of hair follicles. **Protection Against UVA and UVB Radiation:**\ The skin protects the body from harmful ultraviolet (UV) radiation. Melanin, a pigment produced by melanocytes in the epidermis, absorbs and dissipates UV radiation, reducing the risk of DNA damage and skin cancer. **Microbiome:**\ The skin hosts a diverse community of microorganisms, including bacteria, fungi, and viruses, collectively known as the skin microbiome. This microbiome plays a crucial role in maintaining skin health, protecting against pathogenic microbes, and modulating the immune response. **Protection Against Trauma:**\ The skin acts as a physical barrier, protecting underlying tissues from mechanical injury. The dermis, with its dense collagen fibers, provides tensile strength, while the epidermis serves as a resilient outer layer that resists abrasion. **Vitamin D Synthesis:**\ The skin is involved in the synthesis of vitamin D, a crucial hormone for calcium metabolism and bone health. When exposed to UVB radiation, a precursor molecule in the skin is converted to vitamin D3, which is then activated in the liver and kidneys. **Thermoregulation:**\ The skin regulates body temperature through mechanisms such as sweating, vasodilation, and vasoconstriction. Sweat glands produce sweat, which evaporates and cools the body, while blood vessels in the dermis adjust their diameter to control heat loss or retention. **2. Structure of the Skin** The skin is composed of three primary layers: the epidermis, dermis, and hypodermis (subcutaneous tissue). Each layer has distinct functions and cellular compositions. **Epidermis:**\ The epidermis is the outermost layer of the skin, providing a waterproof barrier and creating our skin tone. It consists of several sub-layers: - **Stratum Basale:** The deepest layer of the epidermis, where keratinocytes are generated through mitosis. Melanocytes, responsible for pigment production, and Merkel cells, which function in touch sensation, are also found here. - **Stratum Spinosum:** The layer above the stratum basale, characterized by spiny-shaped cells that are connected by desmosomes. Langerhans cells, which are part of the immune system, are present here. - **Stratum Granulosum:** In this layer, keratinocytes begin to undergo keratinization, accumulating keratohyalin granules that contribute to the formation of the skin's protective barrier. - **Stratum Lucidum:** A thin, clear layer found only in thick, hairless skin (e.g., palms of the hands and soles of the feet). It provides an additional barrier to water and other substances. - **Stratum Corneum:** The outermost layer, consisting of dead, flattened keratinocytes that are continuously shed and replaced. This layer provides the primary barrier to environmental threats. **Dermis:**\ The dermis lies beneath the epidermis and is composed of dense connective tissue. It is divided into two regions: - **Papillary Dermis:** The upper portion of the dermis, which forms finger-like projections (dermal papillae) that extend into the epidermis. This layer contains capillaries, nerve endings, and Meissner's corpuscles (touch receptors). - **Reticular Dermis:** The deeper and thicker portion of the dermis, containing dense collagen and elastin fibers, which provide structural support and elasticity to the skin. This layer houses hair follicles, sweat glands, sebaceous glands, and sensory receptors. **Dermo-Epidermal Junction:**\ This is the interface between the epidermis and dermis, where the epidermal ridges interlock with the dermal papillae. This junction provides mechanical stability and helps in the exchange of nutrients and waste products between the two layers. **Hypodermis (Subcutaneous Tissue):**\ Though not technically part of the skin, the hypodermis is the layer of fat and connective tissue beneath the dermis that insulates the body, absorbs shock, and anchors the skin to underlying structures. **Cells of the Epidermis:** - **Keratinocytes:** The most abundant cells in the epidermis, responsible for producing keratin, a protein that provides strength and waterproofing to the skin. - **Melanocytes:** Located in the stratum basale, these cells produce melanin, the pigment that gives skin its colour and protects against UV radiation. - **Langerhans Cells:** Immune cells found in the stratum spinosum that help protect against pathogens by presenting antigens to T-cells. - **Merkel Cells:** Sensory cells located in the stratum basale that are involved in detecting light touch. **3. The Process of Maturation, Keratinization, and Desquamation** **Maturation and Keratinization:**\ Keratinocytes, produced in the stratum basale, gradually move upwards through the epidermal layers as they mature. During this process, they synthesize keratin and other proteins, undergo a flattening process, and form a tough, protective layer. By the time they reach the stratum corneum, they are fully keratinized, dead cells that form a strong barrier against environmental damage. **Desquamation:**\ Desquamation is the process by which the outermost layer of dead keratinocytes in the stratum corneum is shed from the skin\'s surface. This process is continuous and helps maintain the skin\'s barrier function by removing old cells and making way for new ones. **4. Skin Color and Langer Lines** **Skin Colour:**\ Skin colour is primarily determined by the amount and type of melanin produced by melanocytes. Other factors influencing skin colour include carotene (a yellow-orange pigment) and the oxygenation level of haemoglobin in the blood. Genetic factors, exposure to sunlight, and hormonal influences can all affect melanin production. **Langer Lines (Relaxed Skin Tension Lines - RSTL):**\ Langer lines are natural lines of cleavage in the skin, corresponding to the orientation of collagen fibres in the dermis. Incisions made parallel to these lines tend to heal better with less scarring, which is important in surgical practices. **5. Accessory Structures of the Skin** The skin is equipped with various accessory structures, including glands, hair, and nails, each serving specific functions. **Skin Glands:** - **Sebaceous Glands:** These glands are associated with hair follicles and secrete sebum, an oily substance that lubricates and waterproofs the skin and hair. - **Sweat Glands:** - **Merocrine (Eccrine) Glands:** Found throughout the body, these glands produce a watery sweat that helps regulate body temperature. - **Apocrine Glands:** Located in areas such as the armpits and groin, these glands produce a thicker sweat, often associated with body odour due to bacterial breakdown. - **Ceruminous Glands:** Specialized apocrine glands found in the ear canal, producing earwax (cerumen), which protects the ear from debris and pathogens. **Hair:** - **Anatomy of the Hair Follicle:** The hair follicle consists of the hair shaft (the visible part of the hair), the hair bulb (the base of the follicle where hair growth begins), and the arrector pili muscle (a small muscle attached to the follicle that causes hair to stand erect when contracted). - **Growth of Hair:** Hair growth occurs in cycles, with phases of active growth (anagen), transition (catagen), and rest (telogen). Hair growth is influenced by factors such as genetics, hormones, and overall health. **Sensory Receptors in the Skin:** - **Meissner's Corpuscles:** Located in the papillary dermis, these receptors detect light touch and texture. - **Pacinian Corpuscles:** Found in the deeper dermis and hypodermis, they respond to deep pressure and vibration. - **Ruffini Endings:** These receptors detect skin stretch and sustained pressure, contributing to the sensation of object manipulation and hand positioning. - **Merkel Discs:** Located in the stratum basale, these receptors are sensitive to light touch and texture. - **Nociceptors:** Free nerve endings that detect pain and noxious stimuli, helping protect the body from injury. **Nails:** - **Anatomy of the Nail:** The nail is composed of the nail plate (the hard, visible part), the nail bed (the skin beneath the nail plate), the proximal nail fold (the skin covering the base of the nail), the matrix (the tissue under the nail fold where nail growth occurs), the lunula (the white, crescent-shaped area at the nail base), and the cuticle (a protective layer of skin at the base of the nail). - **Nail Formation:** Nails are formed in the matrix, where keratinocytes produce hard keratin. As cells are pushed forward, they harden and form the nail plate. - **Embryology of Nails:** Nails develop from the ectoderm during fetal development. The nail matrix begins to form around the 10th week of gestation, and the nails are fully formed by the 32nd week. **Multiple Choice Questions (MCQs)** 1. **Which layer of the epidermis is responsible for the production of new keratinocytes?** - a\) Stratum corneum - b\) Stratum basale - c\) Stratum granulosum - d\) Stratum lucidum 2. **Which of the following is not found in the epidermis?** - a\) Melanocytes - b\) Keratinocytes - c\) Langerhans cells - d\) Blood vessels 3. **What is the function of the arrector pili muscle?** - a\) Produce sweat - b\) Cause hair to stand erect - c\) Detect light touch - d\) Lubricate the skin 4. **Which type of sweat gland is most involved in thermoregulation?** - a\) Sebaceous gland - b\) Merocrine (eccrine) gland - c\) Apocrine gland - d\) Ceruminous gland 5. **Which structure is responsible for detecting deep pressure and vibration?** - a\) Meissner's corpuscle - b\) Pacinian corpuscle - c\) Ruffini ending - d\) Merkel disc **Clinical Cases** **Case 1: Skin Trauma and the Healing Process** **Presentation:**\ A 30-year-old male sustains a deep laceration on his forearm after an accident. The wound extends through the epidermis and dermis, requiring sutures. Over the following weeks, the patient notices that the wound has healed with minimal scarring. **Discussion:** - **Question:** Explain the role of the dermo-epidermal junction in wound healing and why the wound healed with minimal scarring. - **Answer:** The dermo-epidermal junction provides mechanical stability and facilitates the exchange of nutrients between the epidermis and dermis, promoting effective healing. The alignment of collagen fibres along Langer lines may also contribute to reduced scarring. **Case 2: Hyperpigmentation and UV Exposure** **Presentation:**\ A 45-year-old woman presents with dark patches of skin on her face and hands, which she notices have become more pronounced after spending time in the sun. She is concerned about the potential risks of these changes. **Discussion:** - **Question:** Discuss the role of melanocytes in skin pigmentation and the effects of UV exposure on skin colour. What advice would you give this patient regarding sun protection? - **Answer:** Melanocytes produce melanin, which absorbs UV radiation and protects the skin from damage. Increased UV exposure stimulates melanin production, leading to hyperpigmentation. The patient should be advised to use sunscreen and protective clothing to prevent further UV damage and reduce the risk of skin cancer.

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