Anatomy Terms Starting With H

Heart is the hollow, four-chambered muscular organ that pumps blood through the pulmonary and systemic circulatory circuits by alternating contraction and relaxation of its chambers.

The right side receives deoxygenated blood from the body and pumps it to the lungs; the left side receives oxygenated blood from the lungs and pumps it to the body at higher pressure. Three concentric layers form the heart wall: the outer epicardium, the thick muscular myocardium, and the smooth inner endocardium. At rest, cardiac output averages approximately 5 liters per minute; during intense exercise, output can exceed 25 liters per minute in trained athletes, driven by increases in both heart rate and stroke volume.

Hematopoiesis is the continuous process by which hematopoietic stem cells in the bone marrow proliferate and differentiate to produce all cellular components of the blood throughout an organism's lifetime.

All blood cells, including erythrocytes, platelets, granulocytes, monocytes, and lymphocytes, arise from a common pluripotent hematopoietic stem cell through a hierarchy of progressively restricted progenitor cells. In the fetus, this process shifts from the yolk sac to the liver and spleen before the bone marrow becomes the primary site around birth. Production rates are regulated by cytokines including erythropoietin, thrombopoietin, and granulocyte colony-stimulating factor, all of which are manipulated clinically to treat anemia, thrombocytopenia, and neutropenia.

Hemoglobin is the iron-containing tetrameric protein inside erythrocytes that binds, transports, and releases oxygen cooperatively, and also carries carbon dioxide and buffers blood pH.

Each hemoglobin molecule consists of four globin subunits, each holding a heme group with a central iron atom capable of binding one oxygen molecule, giving the tetramer a maximum capacity of four oxygen molecules. Binding of the first oxygen molecule shifts the protein’s conformation and increases affinity for subsequent oxygen molecules, producing the characteristic sigmoidal oxygen-dissociation curve. At the lower oxygen tension and higher carbon dioxide concentration of metabolically active tissues, this cooperative mechanism promotes oxygen unloading precisely where demand is greatest.

Humerus is the single long bone of the upper arm, articulating proximally with the scapula at the glenohumeral joint and distally with the radius and ulna at the elbow.

The proximal humerus bears a rounded head that fits into the shallow glenoid cavity of the scapula, forming the most mobile joint in the body. Two bony projections, the greater and lesser tubercles, flank the anatomical neck and anchor the four rotator cuff muscles that stabilize this inherently loose articulation. The radial nerve winds around the humeral shaft in the spiral groove, making it vulnerable to injury in midshaft fractures, a complication that produces wrist drop from loss of extensor muscle innervation.

Hydrothermal vent is an opening in the seafloor where geothermally heated, mineral-rich water flows out from fractures in Earth's oceanic crust, supporting chemosynthesis-based ecosystems independent of sunlight.

Vent fluids discharge at temperatures exceeding 400 degrees Celsius at depth, where pressure prevents boiling, and carry reduced compounds such as hydrogen sulfide, methane, and ferrous iron leached from surrounding rock. Chemosynthetic bacteria and archaea oxidize these compounds to fix carbon dioxide into organic molecules, forming the base of a food web that requires no photosynthesis. Giant tube worms (Riftia pachyptila) grow to over 2 meters in length and harbor dense colonies of sulfur-oxidizing bacteria within a specialized organ called the trophosome, which supplies up to 90 percent of the worm’s energy.

Black smoker vents precipitate iron and metal sulfides that build chimney structures reaching several meters in height and host thermophilic and piezophilic microorganisms adapted to extreme heat and pressure.

Hypothalamic-pituitary axis is the neuroendocrine control system in which the hypothalamus releases peptide hormones into the hypophyseal portal circulation to regulate the synthesis and secretion of pituitary hormones that govern peripheral endocrine glands.

The hypothalamus continuously monitors circulating hormone levels, temperature, osmolarity, and metabolic signals, then secretes specific releasing or inhibiting hormones, such as corticotropin-releasing hormone and gonadotropin-releasing hormone, into a short portal blood vessel network connecting it to the anterior pituitary. These signals arrive at the anterior pituitary in concentrations roughly 100 times higher than they would reach through general circulation, allowing precise control with very small hormone quantities. The anterior pituitary responds by releasing tropic hormones, including ACTH, TSH, LH, and FSH, into systemic blood, where they stimulate the adrenal cortex, thyroid, and gonads.

Negative feedback from the target gland hormones then suppresses both hypothalamic and pituitary secretion, forming a self-regulating loop that keeps hormone levels within narrow physiological ranges.