Summary
Highlights
The adrenal glands, also known as suprarenal glands, are pyramid-shaped organs located on top of the kidneys. The adrenal cortex has three main layers: the Zona Glomerulosa, Zona Fasciculata, and Zona Reticularis. The inner part is the Adrenal Medulla, which consists of neural tissue. This video specifically focuses on the Zona Glomerulosa.
The Zona Glomerulosa secretes aldosterone. The most potent stimulus for aldosterone release is Angiotensin II, which is produced in response to low blood pressure. Renin, secreted by kidney cells in low blood pressure, converts angiotensinogen (from the liver) into Angiotensin I. Then, Angiotensin Converting Enzyme (ACE) in the lungs converts Angiotensin I into Angiotensin II. Angiotensin II then binds to a G-protein coupled receptor on the Zona Glomerulosa cell, activating a stimulatory pathway that leads to aldosterone production.
Aldosterone is a steroid hormone synthesized from cholesterol, unlike protein hormones. The pathway involves several steps: cholesterol is converted into pregnenolone, then progesterone, then 11-deoxycorticosterone, corticosterone, and finally aldosterone. This pathway requires multiple enzymes, including 21-hydroxylase, which plays a critical role. The stimulatory pathways initiated by Angiotensin II and ACTH activate protein kinase A, which phosphorylates key enzymes in this synthesis pathway.
Another significant stimulus for aldosterone secretion is low blood sodium levels (hyponatremia) or high blood potassium levels (hyperkalemia). Adrenocorticotropic Hormone (ACTH) is a weaker stimulus, usually implicated in stressful situations. The primary inhibitor of aldosterone synthesis is Atrial Natriuretic Peptide (ANP), which is secreted when blood pressure is high. ANP activates a G inhibitory pathway, causing potassium efflux, hyperpolarizing the cell, and significantly reducing aldosterone synthesis.
Aldosterone circulates in the blood, bound to proteins like transcortin or albumin, and targets cells in the distal convoluted tubule of the kidneys. Being a steroid hormone, it passes through the cell membrane and binds to an intracellular receptor, activating specific genes. This leads to the transcription of mRNA and the translation of proteins, including sodium-potassium pumps (Na+/K+ ATPase), sodium channels, and potassium channels. These proteins increase sodium reabsorption into the blood and potassium excretion into the urine. This ultimately increases blood volume and blood pressure, counteracting the initial low blood pressure stimulus.
Aldosterone effectively raises blood pressure and sodium levels while lowering potassium levels. Angiotensin II is the most powerful stimulus, followed by imbalances in sodium and potassium. ACTH is a weak stimulus. High blood pressure triggers ANP release, which inhibits aldosterone, preventing further increases in blood pressure. The effects of ACTH are more pronounced on other adrenal cortex layers, which will be discussed in future videos.