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Glossary of Physiology Terms – H
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H zone
Definition:
The H zone is in the center of the A band where there is no overlap between the thick and the thin filaments. Therefore, in the H zone, the filaments consist only of the thick filament. The H zone becomes smaller as the muscle contracts and the sarcomere shortens. The center of the H zone is at the M line, which is also at the center of the sarcomere.

Related glossary terms/phrases:
A band
I band
M line
Z disk



Halitosis
Definition:
Bad, unpleasant, or offensive breath.



Hashimoto's disease
Definition:
A type of primary hypothyroidism, Hashimoto's disease is an autoimmune disorder, in which anti-thyroid antibodies destroy the ability of the thyroid gland to produce the thyroid hormones. Antibodies against thyroid peroxidase (TPO) and thyroglobulin (Tg) are often seen in this disorder.

Hashimoto's disease is also known as Hashimoto's thyroiditis.

Related glossary terms/phrases:
Hypothyroidism
Primary hypothyroidism



Hirsute
Definition:
Hairy. Particularly refers to excessive or unwanted hair on the face and body.

Related glossary terms/phrases:
Hirsutism



Hirsutism
Definition:
Abnormal, unwanted, and excessive hair growth on the face and body. Particularly used to refer to women with such a condition.

Related glossary terms/phrases:
Hirsute



Hodgkin cycle
Definition:
The Hodgkin cycle represents a positive feedback loop in neurons, where an initial membrane depolarization from the resting value (∼ −70 mV) to the threshold value (∼ −50 mV) leads to rapid depolarization of the membrane potential to approach the equilibrium potential for Na+ (VNa ≈ +60 mV). The voltage-gated Na+ channels of neurons are responsible for the Hodgkin cycle.

See the figure depicting the Hodgkin cycle.

See also:
Important Features of the Neuronal Action Potential



Hormone
Definition:
A chemical messenger molecule secreted (i.e., released) by endocrine cells within endocrine glands.

By definition, a hormone molecule is released into the bloodstream and travels throughout the body to find its target cells. Teget cells may be within the intravascular compartment (i.e., within blood vessels), but most hormones have target cells in tissues outside of the blood vessels.

Related glossary terms/phrases:
Endocrine cell
Endocrine gland



Hyperacousis
Definition:
Hyperacusis

See:
Hyperacusis



Hyperacusis
Definition:
A condition characterized by an inability to tolerate, or over-sensitivity to, sounds of moderate loudness encountered in daily life.

It is also spelled hyperacousis.

Other resources:
Hyperacusis (Wikipedia)



Hypercalcemia
Definition:
A total plasma calcium level that is greater than the normal range of 2.2 - 2.6 mM (9 - 10.5 mg/dL). The free calcium concentration in the plasma is approximately 1.5 mM (6 mg/dL), and the remaining amount is bound to plasma proteins.

Related glossary terms/phrases:
Hypocalcemia



Hyperglycemia
Definition:
Condition in which the plasma glucose concentration is abnormally high. In general, a fasting plasma glucose concentration higher than 110 mg/dL, or a postprandial plasma glucose concentration higher than 180 mg/dL is considered abnormally high.

Normal, fasting glucose concentrations range from 70 to 110 mg/dL.

Related glossary terms/phrases:
Hyperglycemic
Hypoglycemia
Hypoglycemic



Hyperglycemic
Definition:
Related to, or pertaining to hyperglycemia.

An agent that causes an increase in plasma glucose concentration (i.e., induces hyperglycemia).

Related glossary terms/phrases:
Hyperglycemia
Hypoglycemia
Hypoglycemic



Hyperphagia
Definition:
Excessive appetite or consumption of food. It may be associated with damage to the hypothalamus. Also referred to as polyphagia.



Hyperplasia
Definition:
An increase in the number of cells in a tissue. It is generally brought about through mitotic cell division of cells within a tissue.

It also refers to an enlargement of an organ or body part due to an increase in the number of cells within the organ or body part.

Related glossary terms/phrases:
Hypertrophy



Hyperpolarization
Definition:
Refers to a change in the value of the membrane potential, where the membrane potential becomes more negative than the resting membrane potential.

Related glossary terms/phrases:
Depolarization
Repolarization

See also:
Resting Membrane Potential - Introduction
Figure showing depolarization, repolarization, and hyperpolarization



Hyperpolarizing afterpotential
Definition:
Refers to the hyperpolarization phase of the action potential.

It is also referred to as undershoot.

See figure.

Related glossary terms/phrases:
Hyperpolarization

See also:
Important Features of the Neuronal Action Potential



Hyperthyroidism
Definition:
Hyperthyroidism refers to a pathophysiological condition in which the thyroid gland produces and releases abnormally high levels of the thyroid hormones (T3 and T4).

The symptoms of hyperthyroidism may include high metabolic rate, weight loss, nervousness, excess heat production, tachycardia, and tremor.

Two main forms of hyperthyroidism exist: (1) Primary hyperthyroidism, and (2) Secondary hyperthyroidism.

In primary hyperthyroidism, the thyroid gland produces high levels of the thyroid hormones, either as a result of a secretory tumor of the thyroid gland, or under the control of thyroid stimulating immunoglobulins (such as in Graves' disease).

Secondary hypothyroidism is caused by high levels of thyroid stimulating hormone (TSH) produced by the anterior pituitary gland. TSH then stimulates the thyroid gland to produce excessive amounts of the thyroid hormones.

Related glossary terms/phrases:
Hypothyroidism
Euthyroidism



Hypertrophy
Definition:
An increase in the size of a cell such as muscle. It is generally brought about through the addition of cellular components.

It also refers to an enlargement of an organ or body part due to an increase in the size of the cells within the organ or body part.

Related glossary terms/phrases:
Hyperplasia



Hypocalcemia
Definition:
A total plasma calcium level that is less than the normal range of 2.2 - 2.6 mM (9 - 10.5 mg/dL). The free calcium concentration in the plasma is approximately 1.5 mM (6 mg/dL), and the remaining amount is bound to plasma proteins.

Related glossary terms/phrases:
Hypercalcemia



Hypoglycemia
Definition:
Condition in which the plasma glucose concentration is abnormally low. In general, a plasma glucose concentration lower than 70 mg/dL is considered abnormally low.

Normal, fasting glucose concentrations range from 70 to 110 mg/dL.

Related glossary terms/phrases:
Hypoglycemic
Hyperglycemia
Hyperglycemic



Hypoglycemic
Definition:
Related to, or pertaining to hypoglycemia.

An agent that causes a reduction in plasma glucose concentration (i.e., induces hypoglycemia).

Related glossary terms/phrases:
Hypoglycemia
Hyperglycemia
Hyperglycemic



Hypophysectomy
Definition:
Surgical removal of the pituitary gland (hypophysis).

Related glossary terms/phrases:
Hypophysis



Hypophysis
Definition:
Pituitary gland

Related glossary terms/phrases:
Adenohypophysis
Neurohypophysis



Hypopituitarism
Definition:
Underactivity of the anterior pituitary gland characterized by reduced secretion of anterior pituitary hormones.



Hypothyroidism
Definition:
Hypothyroidism refers to a pathophysiological condition in which the thyroid gland does not produce sufficient amounts of the thyroid hormones (underactive thyroid), leading to abnormally low levels of the thyroid hormones (T3 and T4).

The symptoms of hypothyroidism may include reduced metabolic rate, chronic fatigue, weight gain, myxedema, and depression.

Three main forms of hypothyroidism exist: (1) Primary hypothyroidism, (2) Secondary hypothyroidism, and (3) Tertiary hypothyroidism.

In primary hypothyroidism, the thyroid gland itself is incapable of producing normal levels of the thyroid hormones.

Secondary hypothyroidism is caused by low levels of thyroid stimulating hormone (TSH). TSH, produced by the anterior pituitary gland, is required to stimulate the thyroid gland to produce the thyroid hormones.

Tertiary hypothyroidism is caused by low levels of thyrotropin releasing hormone (TRH). TRH, produced by the hypothalamus, is required to stimulate the anterior pituitary gland to produce TSH which, in turn, is required to stimulate the thyroid gland to produce the thyroid hormones.

Related glossary terms/phrases:
Primary hypothyroidism
Secondary hypothyroidism
Tertiary hypothyroidism

Hyperthyroidism
Euthyroidism



Hypouresis
Definition:
Oliguria

See:
Oliguria

Related glossary terms/phrases:
Polyuria

Other resources:
Oliguria (Wikipedia)



Hypovolemia
Definition:
A state of decreased total blood volume caused by blood loss, reduction in plasma volume, or other causes.









Posted: Monday, October 18, 2010
Last updated: Sunday, February 7, 2016