Hormones from the Heart, Kidney, and Liver
You already know that hormones are chemical messengers released by endocrine glands — the pituitary, thyroid, adrenal, and so on. But here is a twist: some organs that are not primarily endocrine glands also secrete hormones. The heart, kidney, and liver are classic examples. They produce these substances only when the body needs them, acting as local sensors that turn into global regulators.
Why would the heart need to make a hormone?
Think about blood pressure. If your blood volume rises too high, your heart gets stretched more than normal. That stretch is a danger signal — it means the heart is working too hard. The heart's response is to release a hormone that tells the kidneys to dump salt and water, lowering blood volume and pressure. That hormone is Atrial Natriuretic Factor (ANF).
ANF⟶Kidneys: increase Na+ and water excretion⟶↓ Blood volume, ↓ Blood pressure
ANF is produced by the atrial wall of the heart when it is stretched by high blood volume. It acts directly on the kidney to promote natriuresis (sodium loss) and diuresis (water loss). It also opposes the action of aldosterone (a hormone that retains sodium) and ADH (which retains water). So ANF is a natural blood-pressure-lowering hormone — the heart's own safety valve.
A common mistake is to think ANF raises blood pressure because it comes from the heart. The opposite is true: it lowers pressure by reducing blood volume.
The kidney's hormone: Erythropoietin (EPO)
The kidney monitors oxygen levels in the blood. When oxygen is low (hypoxia) — due to anaemia, high altitude, or blood loss — the kidney releases erythropoietin. EPO travels to the bone marrow and stimulates it to produce more red blood cells. More RBCs mean more oxygen-carrying capacity, correcting the hypoxia.
Low O2⟶Kidney releases EPO⟶Bone marrow ↑ RBC production⟶↑ O2 delivery
EPO is a classic example of a negative feedback loop: once oxygen levels return to normal, EPO secretion stops.
EPO is abused by some athletes to boost endurance (blood doping). It is dangerous because too many RBCs make blood thick, increasing the risk of stroke and heart attack.
The liver's hormones: mostly indirect
The liver does not have a single famous hormone like ANF or EPO, but it produces several important ones:
- Angiotensinogen — an inactive protein that the kidney's renin converts into angiotensin I, which then becomes angiotensin II (a powerful vasoconstrictor that raises blood pressure). So the liver provides the raw material for blood pressure regulation.
- Thrombopoietin — stimulates platelet production from megakaryocytes in bone marrow.
- Insulin-like growth factor (IGF-1) — secreted in response to growth hormone; mediates many of growth hormone's effects on bone and muscle.
The liver also produces hepcidin, a hormone that controls iron absorption from the gut — too much hepcidin causes anaemia, too little causes iron overload.
Gastrointestinal hormones (a quick note) …