Connective Tissue and Epithelium - MCAT Biological and Biochemical Foundations of Living Systems

Connective tissues are defined as cells suspended in an expansive extracellular matrix. For blood, plasma serves as the matrix that suspends erythrocytes. Other types of connective tissue include cartilage, bone, and adipose.

Any kind of muscle, nervous, or epithelial cell is in its own category, separate from connective tissue. Skin is a type of epithelium, and the heart is composed of cardiac muscle tissue.

Bone is considered a connective tissue. The hydroxyapatite crystal forms a generous amount of extracellular matrix, helping to connect the cells of the bone. This large amount of extracellular material is a defining characteristic of connective tissue.

Nervous tissue carries electrical impulses through the body. Muscular tissue is involved in contraction. Epithelium lines the biological lumens and areas exposed to the external environment, including the tracts for respiration, digestion, and excretion. Endothelium lines the tracts that are fully contained in the body, namely the circulatory system and heart chambers.

Chondrin is the elastic matrix found in cartilaginous connective tissue. In order to identify where chondrin can be found in the body, we should identify where in the body we would find cartilage.

The ears and nose are some of the few structures on the adult human body that contain cartilage. Other cartilage structures include the vertebral discs, public symphysis, menisci in the knees, and most sternocostal joints.

The human skull is formed by intramembranous ossification, a process in which mesenchymal stem cells form osteoblasts and eventually bone. Note that there is no cartilage involved in this process. Long bones, such as the femur, are formed by the process of endochondral ossification, in which cartilage is converted into bone. This process occurs long before adulthood, however, and would not be a reasonable answer in this case. Diaphyses of bones are composed of compact and spongy bone. Haversian canals house blood vessels, nerves, and lymph within bone.

Tendons link skeletal muscle to bone. When skeletal muscle contracts and shortens, the tendon pulls the bone in the direction of the muscle contraction to propel movement.

Ligaments serve to link bone to bone, mainly playing a role in stability rather than force transduction. Cartilage is the main tissue of the ears and nose, and generally provides structure or biological cushioning. Adipose is fat tissue, responsible for storing water and nutrients for extended periods.

Ligaments serve to link bone to bone, mainly playing a role in stability rather than force transduction. Once a tendon has pulled on the bone, ligaments provide a mechanism for nearby bones to follow, allowing for a smooth and coordinated movement.

Tendons link skeletal muscle to bone. Cartilage is the main tissue of the ears and nose, and generally provides structure or biological cushioning. Adipose is fat tissue, responsible for storing water and nutrients for extended periods.

Cartilage does not contain blood vessels. Chondrocytes, the specialized cells of cartilage, are supplied by diffusion, which slows growth and repair. While they do produce collagen, this is not the direct reason for the relatively slow growth of cartilage.

Tissue, by definition, is made up of groups of similar cells. However, these cells may be suspended in a living, cellular matrix, or in a nonliving, noncellular matrix. For example, muscle tissue is composed of tightly packed cells in an organized matrix; however, blood is comprised of scattered cells in an aqueous plasma matrix. It is important to remember that all living components of the body are made entirely of tissue, so bone, blood, fat, etc. must fit the definiton of tissue.

Hyaline cartilage is rigid, glassy in appearance, and provides cushioning for smooth joints. Fibrocartilage is fibrous and provides support in high-stress areas, such as the pubic symphysis. Elastic cartilage is flexible and makes up fleshy appendages, such as the nose and ears.

Homeostasis, or "steady-state" physiology, is the system of feedback loops that enables an organism to create a stable, adaptive environment. For example, the pancreas and the brain together work to regulate the blood sugar levels via the hormones insulin and glucagon.

The four types of tissue are muscular, connective, nervous, and epithelial.

Muscular tissue encompasses skeletal, cardiac, and smooth muscle. Nervous tissue creates the central and peripheral nervous systems (CNS, PNS) and consists of all neurons and glial cells. Epithelial tissue makes up "borders" and lines most organs. Connective tissue generally fills spaces and provides the suspensory matrices for the body, such as bone, fat, blood, and cartilage.

Immovable joints are fused shortly after birth, so that they are no longer functional for articulation. The bones of the skull remain open, joined with strips of cartilage, until an infant has passed through the birth canal. They stay open for a short period, giving the baby's brain room to grow. After the brain has finished its major growth stages, the bones of the skull knit together, forming a protective barrier.

The type of muscle that lines the walls of most blood vessels in the human body is smooth muscle. We are told to make the assumption that the muscle types and cells present in Lorempis marengis are the same, structurally and functionally, as those found in humans. Therefore, the type of muscle that lines the walls of most blood vessels in Lorempis marengis is also smooth muscle. Blood vessels contain a specific subtype of smooth muscle, called vascular smooth muscle, that helps to regulate the distribution of blood volume throughout the entire body by either appropriately contracting or relaxing to restrict or increase blood flow to various parts of the body to best meet bodily needs (typically oxygen delivery).

Skeletal muscle cells, also known as striated muscles, are the type of muscle cells that make up the larger, weight-bearing muscles of the body. These are not the type of muscle cells that are found within the walls of blood vessels. The biceps muscles are the muscles responsible for flexion of the forearm at the elbow. They are not a type of muscle cell, per se, and they are made up of skeletal muscle cells.

Cardiac muscle cells are the type of muscle cells that make up the myocardium, or muscle cells of the heart. These are not the type of muscle cells that are found within the walls of blood vessels.

Thermoregulation is a homeostatic mechanism for regulating body temperature in varying conditions. This is accomplished by physiological, structural, and behavioral mechanisms. For example, many mammals will shiver as a result of small muscle contractions to generate heat. Many of the other possible answers sound similar, but are incorrect. Thermoregulation is a type of environmental adaptation, but "adaptation" is too broad of an answer in this case.

All of these mechanisms are used to either increase or decrease body temperature, allowing for proper temperature regulation. When your body becomes cold, shivering (the result of involuntary muscle contractions) helps increase body temperature. Constriction of blood vessels diverts warm blood to the most critical organs and away from extremities. In hot temperatures, blood vessels dilate to allow more blood to the surface of the skin where heat exchange can occur. Sweating allows excess heat to be released through the evaporation of water. Without all of these mechanisms, humans would have difficulty surviving in all the environments we currently inhabit around the world.

This question is asking for a thermoregulatory mechanism that is NOT active in response to an increase in temperature. Preference for a cooler environment would be an apt response to increased temperature. Activation of sweat gland excretion would secrete liquid onto the skin's surface—the evaporation of this liquid carries heat away from the skin. Dilation of arteriole sphincters would increase overall blood flow, which carries heat, to the skin; this heat can dissipate to the environment. Decrease in skeletal muscle activity would lead to less metabolic activity and subsequent heat production.

The pilorection of hairs on the skin surface would not be an appropriate response to a temperature increase. This is because when erect, hairs would reduce the flow of air over the skin and retain body heat. Goosebumps are formed in pilorection, and this is typically seen in a response to cold temperatures.

Endothelial cells are specialized to prevent clotting in their intact form. When disrupted, the endothelium retracts to expose the basement membrane. Proteins in this region will activate the clotting cascade.

Arterioles are the main type of vessel that regulates vessel size and pressure, and exchange over endothlieum can occur with or without fenestrations.

Beneath the surface of the skin, erector muscles are attached to hairs. When it is cold these muscles contract, causing hair to stand up. When hair stands it creates a collective mass of air that maintains temperature by lowering drafts. The erect hairs essentially dull the effects of convection heat transfer by trapping warm air against the skin.

The skin is part of the integumentary system, and is also partly responsible for thermoregulation.

In warm climates, the skin helps lower body temperature by sweating and promoting vasodilation. It is able to open up pores to secrete sodium, which is followed by chloride and water. When water evaporates on the skin, it lowers the temperature of the skin because the water requires an input of thermal energy to transition to the gaseous state. Vasodilation in hot climates allows blood to exchange heat with the environment by flowing close to the skin.

In cold climates, the skin helps raise body temperature by promoting vasoconstriction and goose bumps (cutis anserina). Vasoconstriction helps prevent heat loss to the environment by reducing blood flow to regions of high surface area. Goose bumps raise the hairs in the skin to prevent convection from transferring heat away from the skin.

Sweat is produced in response to increased body temperature, which triggers a response from the hypothalamus that increases sweat secretion. Sweat is composed of water, sodium, and chloride ions. The evaporation of the water requires an input of energy (it is an endothermic process). This energy comes from the heat of the body, and is dissipated when the water transitions to the gaseous state. The byproduct is sodium chloride salt.