Injoys
The kidneys regulate reabsorption and excretion to keep electrolytes in the blood within the normal range. The tubules return substances the body needs to the blood. This regulation involves hormonal signals from other organs.
This basic explanation of kidney function is based on an NIDDK guide reviewed in June 2018.
What Is Electrolyte Regulation by the Kidneys?
Electrolyte regulation by the kidneys balances the amount retained in the body with the amount released in urine. Electrolytes are electrically charged substances in body fluids. They include sodium, potassium, and calcium.
The balance of these substances is necessary for nerves and muscles to function normally. The body's ability to keep its internal conditions within a certain range is called homeostasis. The kidneys are the main organs responsible for the homeostasis of water and electrolytes. A basic explanation is available in the NIDDK guide to kidney function.
A researcher points to a kidney model illustrating electrolyte movement and reabsorption in the nephron.
Comparison of Reabsorption, Secretion, and Excretion
Reabsorption and secretion are processes in which substances move in opposite directions. The nephron is the basic functional unit of the kidney. A nephron consists of a glomerulus that filters the blood and a tubule that processes the filtrate.
| Process | Direction of substance movement | Meaning |
|---|---|---|
| Filtration | From the blood in the glomerulus to the space leading into the tubule | Filters water and small substances from the blood |
| Reabsorption | From inside the tubule toward the blood | Returns filtered substances to the body |
| Secretion | From the blood into the tubule | Adds substances to the fluid that will become urine |
| Excretion | Out of the body through urine produced by the kidneys | Ultimately removes substances from the body |
Not every filtered substance leaves the body. The tubules recover much of the filtered water and needed substances. The remaining fluid becomes urine. This process is described in the NIDDK explanation of nephrons.
Therefore, the amount of a substance excreted is not determined solely by the amount filtered. Reducing reabsorption can increase the amount remaining in the urine. For some substances, such as potassium, the amount secreted into the tubules is also regulated. The role of secretion is explained in OpenStax's overview of fluid regulation.
How Do the Kidneys Detect the Body's Condition?
The kidneys respond to hormones, nerve signals, and local sensing mechanisms. Hormones are chemical signals that travel through the blood. Changes in blood flow to the kidneys also provide cues that initiate regulation.
Describing this as an information network among organs is a metaphor for these physiological processes. It is not a system in which every organ directly reports a deficiency in each substance. Different signals work together to affect how the kidneys process substances.
Hormones from the Brain and Adrenal Glands
Hormones from the brain and adrenal glands change how the kidneys process water and electrolytes. Antidiuretic hormone signals the kidneys to increase water reabsorption. This hormone is produced in the hypothalamus of the brain. It is released into the blood from the back of the pituitary gland. Society for Endocrinology's explanation of antidiuretic hormone
Aldosterone from the adrenal glands increases sodium reabsorption. This hormone also promotes the excretion of potassium in urine. Even the same signal can direct different substances to be processed differently. Society for Endocrinology's explanation of aldosterone
Connections with the Heart and Liver
The heart and liver are also connected to the kidneys' regulation of body fluids. Natriuretic peptides released by the heart promote sodium excretion. This signal acts to reduce the amount of sodium and water retained in the body.
The liver produces a protein called angiotensinogen. Renin released by the kidneys is an enzyme that cleaves this protein. The pathway initiated by this reaction promotes sodium retention. This connection is described in the Society for Endocrinology's explanation of angiotensin.
Connections Among the Bones, Parathyroid Glands, and Intestines
The kidneys, bones, parathyroid glands, and intestines all contribute to the balance of calcium and phosphorus. The parathyroid glands are small endocrine glands in the neck. When blood calcium levels fall, secretion of parathyroid hormone increases.
Parathyroid hormone increases calcium reabsorption by the kidneys. It acts to reduce phosphate reabsorption. Phosphorus exists in the body mainly in the form of phosphate.
The kidneys also convert vitamin D into its active form. Active vitamin D helps the intestines absorb calcium. This connection is summarized in OpenStax's explanation of calcium and phosphate regulation.
Bones are also organs that produce hormones. FGF23, which is produced in the bones, is involved in regulating phosphorus metabolism. Related information is available in the NIDDK guide to mineral and bone disorder in kidney disease.
