Dissociation occurs in oxygen-depleted tissues: oxygen is released and haemoglobin is reformed. The haem groups also bind other inorganic molecules. The haem groups monoxide. This binds more strongly than oxygen and competes with it (hence its toxicity). In vertebrates, haemoglobin is contained in the red blood cells (erythocytes).

Haemolysis. Breakdown of haemoglobin from red blood corpuscles owing to damage to surface membrane.

Haemorrhage. An escape of blood from the blood vessels due to a wound or disease.

Hair.
(1) (Bot). (Trichome). Single-celled or many-celled out-growth from an epidermal Cell. Functions are various, e.g. absorbing secretory; lowering transpiration rate
(2) (Zool). Characteristic of mammal. Each hair consists of numerous cornified epidermal cells; grows at its base in hair-follicle where active cell-division occurs.

Haploid. Having a single set of unpaired chromosomes in each nucleus. Characteristic of gametes; of some Sporzoa; of somatic nuclei of parthenogenetically produced males of some animals, e.g. bees; and of gametophytes and many spores of algae, fungi, Bryophyta, and vascular plants.

Hapten. A substances that is able to combine with specific antibodies but does not produce them unless attached to a large Carrier Molecule.

Hatscheck’s pet. A mucusgland in the oral hood of Amphioxus.

Haversain canals. Narrow tube blood-vessels and nerves, which ramify throughout bone, communicating with its surface and marrow. Sheets (lamellae) of bone, and bone cells, are arranged concentrically around the canals.

Hay Fever.
An allergic reaction to atmospheric dust and pollen Hay fever causes watery eyes, sneezing etc. due to inflammation of the mucous membranes of the eyes and nose.