Membranes and Skin ORGAN – Two or more tissues working together performing a special function. Membranes are the simplest organs in the body. 4 types of membranes in the body in 2 major groups.

EPITHELIAL MEMBRANES 1)Serous Membranes. 2 layered membranes that line body cavities that do NOT open to the outside. Layer closest to the organ is the Visceral layer Layer closest to the wall of the body cavity is the Parietal layer Open space between the layers is filled with a watery serous fluid to lubricate the membranes while the organs move. Composed of Simple squamous epithelium over areolar tissue

Serous Membranes Examples: Pleural Membrane covers the lungs Pericardium covers the heart Peritoneum covers the organs in the abdomen

2)Mucous Membranes – Line body cavities that DO open to the outside. (oral, nasal, digestive, urinary, reproductive) Nasal and digestive membranes secrete mucous. Others are kept moist by body secretions. Composed of epithelium over a loose connective tissue.

3) Cutaneous Membrane – your skin Composed of Stratified Squamous epithelium (epidermis) over dense connective tissue (dermis). The upper layers are keratinized (filled with keratin) that makes it a dry membrane.

CONNECTIVE TISSUE MEMBRANES 4)Synovial Membranes – Line joint cavities, bursae and tendon sheaths. Secrete a thick Synovial Fluid that lubricates the joints. Composed of dense connective over areolar tissue (no epithelium present)

Skin Functions Protects deeper tissues from: –Mechanical damage and chemical damage –Bacterial and Viral invasion –Thermal damage –Ultraviolet radiation damage –Desiccation (water loss) Aids in body temperature regulation Aids in excretion of urea and uric acid Synthesizes vitamin D

Figure 4.4

Skin Structure Epidermis – upper layer –Stratified squamous epithelium –Often keratinized (hardened by keratin)

Layers of Epidermis Stratum basale - Deepest Layer –Cells undergoing mitosis –Lies next to dermis –Lacks blood vessels. Dermal cells supply nourishment. Stratum spinosum Stratum granulosum Stratum lucidum – (occurs only in thick skin) Sole of the foot Stratum corneum – Upper layer composed of shingle-like dead cells. It is waterproof and easily rubbed away.

Skin Pigment Pigment (melanin) produced by cells called melanocytes Color varies from yellow to brown to black Melanocytes are mostly in the stratum basale All humans have the same number of melanocytes. Amount of melanin produced depends upon genetics and exposure to sunlight Localized concentrations of melanin form freckles and moles

Dermis Underneath the epidermis Composed of dense connective tissue with both collagen and elastin fibers Blood vessels supply the epidermis Location of sensory & motor nerve fibers, hair follicles, sweat glands, sebaceous glands

Subcutaneous Layer (Hypodermis) Deep to dermis is the hypodermis –Not part of the skin –Anchors skin to underlying organs –Composed mostly of adipose tissue

Appendages of the Skin 1. Sebaceous glands –Produce oil (Sebum) Lubricant for skin Kills bacteria –Most with ducts that empty into hair follicles –Glands are activated at puberty

2. Sweat glands Widely distributed in skin –Two types Eccrine –Open via duct to pore on skin surface –Function throughout life –Secrete a watery secretion that contains salt and urea –Respond to elevated body temperature Apocrine –Ducts empty into hair follicles –Most numerous in the armpit and groin –Begin to function at puberty –Respond to stress, pain and fright

3. Hair –Produced by hair bulb within the hair follicle –Located on all body surfaces except: soles, palms, nipples, lips, and external reproductive organs –Consists of hard keratinized epithelial cells –Melanocytes provide pigment for hair color –Arrector Pili muscle (smooth muscle) attaches to the hair follicle –When it contracts it causes the hair to stand up.

4. Nails –Scale-like modifications of the epidermis Heavily keratinized –Stratum basale extends beneath the nail bed Responsible for growth –Lack of pigment makes them colorless

Skin’s role in body temperature regulation Slight shifts in temperature disrupts normal rates of metabolic reactions When body heat is excessive (excercise) –Blood vessels in the dermis dilate and fill with blood. Heat escapes –Eccrine Glands become active. The evaporating sweat consumes body heat When body heat loss is excessive (cold environment) Dermal vessels constrict Eccrine glands become inactive Skeletal muscles contract involuntarily