Hormones
Hormones act as chemical messengers that relay signals to target cells via the circulatory or lymphatic systems. These hormones are secreted by endocrine glands into the bloodstream or lymphatic fluid and travel throughout the body. Despite circulating widely, hormones only affect cells that display specific receptors for them. These target cells possess receptors either on their surface or within the cell, designed to recognise and bind to particular hormones.
When a hormone binds to its receptor, it triggers a series of cellular changes. This binding can activate or inhibit various cellular processes, such as altering gene expression, regulating enzyme activity or modifying the permeability of cell membranes. For example, insulin binds to receptors on muscle and liver cells, promoting glucose uptake and metabolism, while adrenaline binds to receptors on heart cells, increasing heart rate and contraction strength.
The sensitivity of a cell to a hormone is directly related to the number of receptors it displays for that hormone. Cells can upregulate or downregulate receptor numbers in response to hormonal levels, adjusting their sensitivity. For instance, in conditions of high hormone levels, cells may reduce the number of receptors to prevent overstimulation. Conversely, low hormone levels can lead to increased receptor numbers, enhancing the cell’s response.
This dynamic regulation of receptor availability ensures that hormonal signals are receptive to the body’s needs, allowing precise control over physiological processes.