Systems Physiology - Anatomy

The endosteum is the layer that is found deep to all layers. It is found in the inner lining of the bone. Meanwhile, the periosteum is the most superficial layer of the bone which compromises the outer covering of bones.

The heart secretes atrial natriuretic peptide, which acts on the kidneys. Oxytocin is secreted by the posterior pituitary. Thymosin is secreted by the thymus gland. Androgens are secreted by the adrenal cortex.

Peptide hormones are large polar hormones that are able to freely float in the bloodstream. Once they reach their target cells, peptide hormones attach to transmembrane receptors on the target cell. This generally initiates a second messenger signal cascade to amplify the response, eventually driving a change in genetic expression and transcription.

In contrast, steroid hormones are small and nonpolar. These characteristics allow them to freely diffuse across the plasma membrane of the cell and enter the nucleus, where they can directly act as transcription factors to regulate genetic expression.

Parathyroid hormone (PTH) is released when blood calcium levels are low, and functions to raise blood calcium levels via a negative feedback mechanism. It does this by increasing osteoclast activity and degrading bone to release calcium into the blood. It also increases absorption of calcium in the renal tubule, increases production of 25-hydroxyvitamin D3, and increases absorption of calcium in the intestines.

Hypersecretion, or oversecretion, of the growth hormone in adulthood results in a condition called acromegaly. The adult individual no longer has growth plates at this point, so the excess hormones interact with the receptors on the surface of bones. This results in coarse facial features, thickening of the dermis, and large hands and feet.

Gigantism is also a result from hypersecretion of the growth hormone, but it occurs during childhood when the individual still has his or her growth plates. Dwarfism is a result from hyposecretion, or undersecretion, of the growth hormone.

The liver secretes angiotensinogen and insulin-like growth factors. Only angiotensinogen is responsible for increasing blood pressure by acting on blood vessels. Cortisol is released by the adrenal cortex, oxytocin is released by the posterior pituitary, and vitamin D3 is found being activated in the skin. Angiotensinogen is a zymogen that is converted into angiotensin I by renin (secreted by the kidney). Then angiotensin converting enzyme (ACE) converts angiotensin I into angiotensin II, which is a potent vasoconstrictor. Angiotensin II also promotes the release of aldosterone from the adrenal cortex, which increases sodium reabsorption. Lastly, antidiuretic hormone (ADH) is released from the posterior pituitary to act on the distal convoluted tubule and the collecting duct to increase the permeability of water via upregulation of aquaporins. Together, these hormones act to increase blood pressure.

Gastrin is a hormone secreted by the stomach and small intestine that acts on the gastrointestinal tract and pancreas. Many hormones, such as vasopressin (antidiuretic hormone) and aldosterone act on the kidneys. Hormones that target bone are calcitonin and parathyroid hormone. Hormones that target gonads include follicle stimulating hormone and luteinizing hormone.

The hormones secreted by the alpha and beta cells of the pancreas are glucagon and insulin, respectively. These hormones are associated with the metabolism glucose. Glucagon increases blood glucose levels, while insulin does the opposite. Follicle-stimulating hormone is responsible for egg or sperm development. Tropic hormones from the hypothalamus are what release or inhibit pituitary hormones. Lastly, thymosin and thymopoietin are what regulate blood calcium and phosphate levels.

Adipose tissue secretes leptin, adiponectin, and resistin which affect food intake, metabolism, and reproduction. The thymus gland is responsible for secreting thymosin and thymopietin. Ovaries and the placenta secrete estrogen and progesterone. The adrenal medulla secretes epinephrine and norepinephrine.

The adrenal medulla produces epinephrine and norepinephrine, which cause the fight or flight response. The thyroid gland produces triiodothryonine and thyroxine, which are used for metabolism, growth, and development. The ovaries and placenta produce estrogen and progesterone, which are used for fetal/maternal development and egg production. Lastly, the stomach and small intestines produce gastrin and secretin, which are used to assist digestion and nutrient absorption.

The adrenal cortex secretes androgens, cortisol, and aldosterone. The adrenal medulla is what secretes epinephrine. The kidney secretes erythropoietin. Lastly, the pancreas is what secretes insulin.

Steroid hormones are synthesized from cholesterol. The amine hormones are synthesized from the amino acid tyrosine. Cortisol is an example of a steroid hormone, and is not a precursor for the synthesis of other steroid hormones.

The secretion of hormones of the anterior pituitary is stimulated or inhibited by releasing and/or inhibiting factors released from the hypothalamus into the hypophyseal portal system.

The following are appropriate matches of adrenal layers to their corresponding secretory products: zona glomerulosa; aldosterone, zona fasciculata; cortisol and minor amount of androgens, zona reticularis; androgens, medulla; catecholamines. The layers of the adrenal glands, from superficial to deep are: capsule, zona glomerulosa, zona fasciculata, zona reticularis, medulla.

Follicle-stimulating hormone (FSH) stimulates the release of estrogen. Estrogen then thickens the endometrial lining during the proliferative phase and stimulates the release of luteinizing hormone (LH) on the 14th day of the cycle. This causes the luteal surge that results in the development of the corpus luteum.

Testosterone release is triggered by luteinizing hormone during spermatogenesis, which does not occur in females.

Human chorionic gonadotropin is used by over-the-counter pregnancy tests to detect fertilization and is secreted after fertilization occurs to maintain high levels of progesterone. When fertilization occurs, progesterone causes a pause in the menstrual cycle; follicle-stimulating hormone will only trigger the estrogen spike if progesterone levels decline enough. With progesterone levels elevated, the corpus luteum is maintained rather than transitioning into the corpus albicans.

Normal inspiration typically involves the flattening (contraction) of the diaphragm in order to increase the volume of the thoracic cavity, and can be done unconsciously. In order to increase the amount of inhaled air, other muscles such as the external intercostals and the sternocleidomastoids are included by conscious control. Both of these muscles aim to raise and expand the thoracic cavity in order to assist in inhalation.

The rectus abdominis is involved in the opposite action of forced exhalation. The rectus abdominis aims to decrease the volume of the thoracic cavity by contracting. This assists in forced exhalation.

Unconscious breathing is controlled by the pons and the medulla oblongata, both of which are parts of the brain stem. This unconscious breathing can be consciously controlled by using the cerebral cortex, which manages most voluntary actions.

It helps to remember that the brain stem is responsible for unconscious control of the body: breathing, heart rate, blood pressure, etc. It is the addition of the cerebral cortex that allows humans to have conscious control over actions, such as breathing, and override the unconscious controls. For example, the cerebral cortex is used to consciously stop breathing when diving underwater.

At rest the diaphragm is slightly curved superiorly such that it makes this sort of shape: When it contracts, it flattens out, with the middle of the muscle being pulled down until the muscle is roughly horizontal. Remembering that the diaphragm separates the thoracic and abdominal cavities, if it contracts, it physically increases the volume of the thoracic cavity. Now, remembering your fluid physics, an increase in volume is accompanied with a decrease in pressure. We know that high pressure flows to low pressure spontaneously. The atmospheric pressure is now higher than the intrapleural (or thoracic cavity) pressure, causing air to flow into the lungs.

Note that the external intercostals aid in inspiration and the internal intercostals aid in expiration.

Tidal volume is, by definition, the amount of air inspired/expired during normal breathing. The maximum volume of air that can be inspired after a normal expiration is the inspiratory capacity. The maximum volume of air that can be expired after a maximal inspiration is the vital capacity. The volume of air still in the lungs after a maximal expiration is the residual volume. The maximum volume of air that can be inspired after a normal inspiration is the inspiratory reserve volume.

During a respiratory cycle, the diaphragm contracts and moves downward. When this occurs the pressure in the alveoli falls. This pulls air into the lungs. At the same time external intercostals muscles contract, raising ribs and sternum and enlarges the cavity even more. During exhalation the diaphragm relaxes (moves up) and air is foced out of the body. A hiccup is a muscular spasm of the respiratory muscles including the diaphragm. A pneumothorax is a "hole" in the lungs that causes air to accumulate in the pleural space.