BIOLOGY Volume 2 - A Guide to General Biology - 2004

19. HOMEOSTASIS

19.5. Endothermic Animals

Endothermic animals include birds and mammals, whose activity level is relatively independent of ambient environmental temperatures. Maintaining a constant body Temperature—typically higher than that of the surrounding air—requires an intense METABOLISM and efficient mechanisms to regulate heat loss from the body surface. The Skin serves as the primary organ in contact with the external environment, and consequently, it detects changes in ambient temperature. Signals from cutaneous thermoreceptors are processed in the Hypothalamus (a region of the Brain), which orchestrates thermoregulation through various metabolic processes.

19.5.1. Skin Structure

The term "skin" refers to the external covering of vertebrates. As the body's largest organ, it performs A wide variety of Functions. While skin Structure varies depending on the species, it is outlined here using the human skin as a representative example (Fig. 19.11).

The skin consists of two main layers: the epidermis and the dermis (or true skin). Beneath the dermis lies the subcutaneous layer (hypodermis), which contains specialized fat Cells that form adipose tissue. The thickness of this subcutaneous layer varies across different body regions and among individuals.

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Fig. 19.11. A. Transverse section of human skin. B. Detailed diagram of the skin section shown in A.

Epidermis

Epidermal cells are separated from the dermis by a basement membrane. The epidermis is composed of multiple layers of cells forming a Stratified Epithelium (section 6.3.2). Cuboidal epithelial cells (section 6.3.1) rest directly on the basement membrane, forming the germinative layer (also known as the stratum germinativum or Malpighian layer). Through repeated mitotic divisions of these cells, the epidermis is continuously renewed. Above the germinative layer lies the stratum granulosum (granular layer), whose living cells no longer divide; as they are pushed outward, they flatten and transition into the outermost layer of the epidermis, the stratum corneum (cornified layer). As the Cells of the cornified layer approach the skin surface, they flatten even further (becoming squamous epithelium) and begin to synthesize and accumulate keratin, which makes them waterproof—a process known as keratinization or cornification. Eventually, the nuclei in these cells disintegrate, and the cells die, essentially transforming into cornified scales. The stratum corneum is thickest in areas subject to constant, heavy friction, such as the palms of the hands and the soles of the feet. Overall, the epidermis forms a thin, translucent, durable, elastic, and waterproof covering perforated by numerous pores (the openings of sweat gland ducts) and Hair follicle openings. The outermost layer of cornified epithelial cells is continuously shed As a result of friction.

In many vertebrates, the stratum corneum can be modified to form structures such as Nails, claws, hooves, horns, scales, feathers, and hair. The primary component of all these structures is the protein keratin.

Dermis

The dermis consists of a dense mass of Connective Tissue containing elastic fibers, Blood capillaries, Lymphatic vessels, Muscle and nerve fibers, sensory cells, chromatophores (pigment cells), sweat and Sebaceous Glands, and hair follicles.

Hair follicles are epidermal invaginations. At the base of each follicle lies a connective tissue ROOT known as the hair papilla, which is rich in blood capillaries and from which the hair shaft develops. A hair consists of cuboidal epithelial cells that become keratinized through the accumulation of keratin. The cortex of the hair contains varying amounts of the pigment melanin, which determines hair color, while the medulla contains air spaces. With advancing age, as these air spaces increase and melanin synthesis declines, hair turns gray. Blood capillaries supply the growing hair with nutrients and remove Metabolic waste products. The upper part of the hair extends beyond the epidermal surface; it remains flexible and Water-repellent thanks to the oily secretion of sebaceous glands that empty into the hair follicle. This secretion contains Fatty acids, Waxes, and Steroids, and it spreads over the hair and skin surface to protect the follicles from dust and microorganisms while forming a thin waterproof barrier. This barrier prevents both excessive water loss from the skin and The entry of water from the outside into the body.

A smooth muscle, extending from the basement membrane, is attached to the base of each hair follicle. When these arrector pili Muscles contract, they reduce the angle of the hair relative to the skin, increasing the thickness of the trapped air layer above the surface. This serves as a thermoregulatory mechanism and, in some vertebrates, as a behavioral response to danger. When hair "stands on end," the apparent body size increases, which may be enough to deter a potential predator.

Sweat Glands are coiled tubular glands located in the dermis, connected by a duct to a pore on the skin surface. In humans, they are distributed across the entire body surface, but in some mammals, they are restricted to specific areas, such as the paw pads. They are absent in birds. The secretory activity of these glands is regulated by sympathetic nerve fibers and involves the Excretion of Water, salts, and urea brought by numerous capillaries. There are two MAIN TYPES OF sweat glands: eccrine (merocrine) and apocrine. Eccrine glands are far more numerous (approximately 2.5 million in humans) and are found over most of the body. Apocrine glands are concentrated in the armpits, around the nipples, in the groin, palms, soles, and around the anus. These glands secrete an odorless fluid that can later develop an unpleasant odor due to bacterial breakdown. Certain zinc and aluminum compounds inhibit sweat gland activity and kill Bacteria, which is why they are included as active ingredients in antiperspirants and deodorants.

Blood capillaries in the dermis are extremely abundant, supplying blood to all structures within it. Many capillaries form loops associated with arteriovenous anastomoses (Fig. 19.12), making it possible to alter blood flow to the skin surface. This is one of the key thermoregulatory mechanisms described in section 19.5.3.

Fig. 19.12. Mechanism of blood flow regulation in the skin. A. Blood flow distribution that minimizes heat loss. Arteriolar constriction reduces capillary blood flow. Only enough blood reaches the skin to maintain its viability. Most of the warm blood arriving from the body core bypasses the skin via arteriovenous anastomoses, thereby reducing heat dissipation. B. Blood flow distribution that maximizes heat loss. Arteriolar dilation increases Blood flow through the capillaries. Driven by the incoming blood pressure, the capillaries dilate. Heat is lost from the blood through radiation, convection, and conduction. In addition, Blood supply to the sweat glands increases, resulting in elevated perspiration.

Motor Neurons innervate the muscles and Glands of the dermis, while sensory neurons transmit nerve impulses from numerous cutaneous receptors to the Central Nervous system. These receptors detect warmth and cold (thermoreceptors), Touch, pressure, and vibration (mechanoreceptors), pain (nociceptors), and likely related sensations such as itching and tickling. Some receptors have a very simple structure consisting of free nerve endings, whereas others, such as Pacinian corpuscles (Fig. 17.32), are quite complex.