Systems Biology and Tissue Types - MCAT Biological and Biochemical Foundations of Living Systems

After axons from the sinoatrial node flow through the atria to cause atrial contraction, the depolarization pauses in the atrioventricular node. Once the atrioventricular node depolarizes, the electrical signal travels though the bundle of His to the walls of the ventricles via purkinje fibers. The atrioventricular node initiates ventricular contraction, the bundle of His carries the signal, and the purkinje fibers allow for synchronized contraction of different regions of the ventricular wall.

The vagus nerve is a major nerve of the parasympathetic nervous system, responsible for mediating numerous responses in the body. In relation to the heart, the vagus nerve provides constant inhibition to the sinoatrial node, slowing the heart rate. The sinoatrial node naturally fires at about 80 to 100 beats per minute, while a healthy resting heart rate is closer to 60 due to innervation by the vagus nerve.

The radial nerve is located in the forearm; the femoral nerve is located in the thigh; the subcostal nerve is located along the lower ribs.

Hormones cortisol and aldosterone are synthesized in the adrenal glands. Adrenal glands are made up of the adrenal cortex and the adrenal medulla. Adrenal medulla is involved in the synthesize of catecholamines (epinephrine and norepinephrine) whereas adrenal cortex synthesizes mineralocorticoids (aldosterone), glucocorticoids (cortisol), and androgens (testosterone, DHT, and DHEA).

Aldosterone is released upon stimulation from the renin-angiotensin system and serves to increase reabsorption of sodium in the collecting ducts of the kidney. Cortisol is released due to stress and serves to increase the metabolic rate.

In a female, the luteal surge occurs during the menstrual cycle. Its main function is to increase the concentration of luteinizing hormone, or LH. LH is released in both males and females and its main function is to increase the concentration of testosterone in males and estrogen in females; therefore, the luteal surge in females increases the concentration of LH, which causes an increase in estrogen.

Progesterone functions to prepare the lining of the uterus for implantation of the egg, and also produces menstrual bleeding if pregnancy does not occur.

Estrogen induces the development of female sex characteristics. Follicle-stimulating hormone stimulates the synthesis and secretion of estrogen. Luteinizing hormone stimulates progesterone production. Gonadotropin-releasing hormone controls the release of follicle-stimulating hormone.

When it comes to the strength and resilience of bone, both collagen and calcium play important roles. Collagen provides bones with great tensile strength, and calcium (usually in the form of hydroxyapatite) provides bones with great compressive strength. In the event of a calcium deficiency, weight placed on bones would cause them to bend irregularly, resulting in the characteristic bending of bones found with rickets.

A deficiency in collagen would cause damage to the bone during extension or stretching forces. Bone marrow does not play an important structural role, and is more important for bone function.

Ligaments connect a bone to another bone, while tendons attach a muscle to a bone. Since two bones are being connected by the fibrous band of tissue in question, we can confirm that it is a ligament.

Muscle does not actually form a functional connection; rather, muscle will transition to a tendon, which will connect to a bone. Joints are comprised of several different structures and tissues, and are the articulations between two bones.

Bone consists of calcium-phosphate crystals, known as hydroxyapatite, in a collagen matrix. The molecular formula for hydroxyapatite is . Collagen's structure depends on hydroxylysine and hydroxyproline for stability. Potassium is not required in bone.

While these specific formulas are not generally tested knowledge, it is important to know what molecules may influence bone composition and growth.

Osteocytes are differentiated osteoblasts that have become imbedded in hydroxyapatite bone matrix. The osteocytes are found in small gaps in bone matrix called lacunae and exchange nutrients with the blood using small canals called canaliculi. Lamellae are the concentric regions of osteocytes that are arranged around the central Haversian canal. The canal houses blood vessels and nerves to nourish and stimulate the osteocytes via the canaliculi.

Bone matrices store these slightly soluble calcium salts so that they can be released into the bloodstream in times of low blood calcium levels. As a result, these salts are an example of how the skeletal system provides mineral storage for the body.

Movement and bone cell production are also primary functions of the skeletal system, but are not linked to calcium salts. Energy storage is linked to the fats stored in the yellow marrow of the bone.

Vitamin D aids in calcium absorption in the small intestine. In the absence of vitamin D, inadequate absorption causes a deficiency of calcium in the body. Without calcium, the bones will not have adequate tensile strength, which can cause a bowing effect in bones that support weight, such as the femurs. Collagen in the bones is associated with resistance to compression, rather than tensile strength, and is not linked to vitamin D.

The skeleton plays a role in musclular contraction in that it serves as an anchor and provides leverage against which muscles can exert force. It also serves as protection and support for soft tissues. Bones also maintain certain ion concentrations. The correct answer choice is the regulation of pH through hydration, which is not a function of the skeletal system.

There are organic (collagen) and inorganic (hydroxyapatite) components of the bone that contribute to its characteristic hardness. The key here is knowing the composition of hydroxypatite, which is primarily constructed of calcium and phosphorus.

Collagen is a protein that allows the bone to be somewhat flexible, so "removal of proteins" is the only correct answer. Other answer choices involve inorganic bone material, which add to hardness of the skeleton, but not to its flexibility.

Compact and spongy bone have similar chemical and structural compositions and are both hard and resistant to compression; however, due to the fact that compact bone is denser, nutrients are delievered through canals called Haversian systems in compact bone; spongy bone is less dense and lacks these specialized canals.

Endocrine organs are capable of secreting hormones into the bloodstream. Each of the given answer options performs some sort of endocrine function.

The testes secrete testosterone, and are responsible for the development of most secondary sex characteristics in males.

The thyroid gland secretes calcitonin and thyroid hormones, responsible for elevating calcium levels and increasing metabolism, respectively.

The pancreas and liver are both endocrine AND exocrine organs. As an endocrine organ, the pancreas secretes the hormones insulin and glucagon. As an exocrine organ, it secretes several enzymes that are essential for digestion in the small intestine. The liver secretes insulin-like growth factors in response to stimulation by growth hormone. As an exocrine organ, it is also responsible for producing bile to aid in digestion.

Glucagon acts in a negative feedback loop with insulin to regulate blood sugar concentration. Both hormones are secreted from the Islets of Langerhans in the pancreas. Alpha cells in the pancreas have high levels of glucagon synthase, allowing them to synthesize the hormone.

Essentially, glucagon synthase is located in alpha cells, which are found in Islets of Langerhans within the pancreas; thus, all of these answers describe the site of glucagon synthesis.

There is a key difference between exocrine and endocrine glands. Exocrine glands release their product to the external environment, while endocrine glands release their product directly into bodily fluids. The answer is the structure that accomplishes both of these actions. The pancreas does both: it releases digestive enzymes into the small intestine (external environment), and releases hormones into the bloodstream (body fluid). Remember that the entire digestive tract is considered part of the outside environment and is lined by epithelium, rather than endothelium.

The adrenal cortex and thyroid perform only endocrine functions, while the gall bladder is only exocrine.

The endocrine system involves secretions by ductless glands into the bloodstream. The pancreas, pituitary gland, thyroid, and hypothalamus are all endocrine organs that secrete hormone signals to the body. The pancreas, however, also functions as an exocrine gland. When the pancreas secretes insulin and glucagon into the blood stream, it serves as an endocrine organ. When it secretes digestive enzymes through the pancreatic duct into the small intestine, it serves an exocrine function.