Homeostasis and the Kidney

Overview

Homeostasis refers to the maintenance of a relatively stable internal environment in organisms, despite changes in external conditions.
The kidney plays a crucial role in homeostasis by regulating the balance of water and electrolytes in the body, and removing waste products from the bloodstream.

The kidney consists of an outer cortex and an inner medulla encased within a tough capsule.
Blood is supplied to the kidneys via the renal artery and leaves via the renal vein.
Each kidney is made up of approximately a million filtering units called nephrons.

Each nephron consists of a Bowman’s capsule, a glomerulus, a proximal convoluted tubule, a loop of Henle, a distal convoluted tubule, and a collecting duct.
The nephron is the functional unit of the kidney where the filtration, reabsorption, and secretion of substances occur.

Filtration occurs at the glomerulus and Bowman’s capsule. The process is driven by high blood pressure and leads to the formation of glomerular filtrate.
Filtrate is mostly water but also contains glucose, amino acids, ions, and urea. Red blood cells and large proteins remain in the blood.

Reabsorption predominantly happens in the proximal convoluted tubule and removes useful substances (glucose, amino acids, ions) from the filtrate back into the bloodstream.
Reabsorption of water in the Loop of Henle plays a crucial role in water balance in the body and is regulated by the hormone ADH.
Secretion actively transports unwanted molecules from the bloodstream into the nephron lumen for excretion, this final adjustment can include excess ions or drugs.

The kidneys are regulated by various hormones including antidiuretic hormone (ADH), aldosterone, and Atrial natriuretic peptide (ANP).
ADH, secreted by the pituitary gland, increases the permeability of the collecting duct, and thus promotes water reabsorption and concentrated urine when the body is dehydrated.
Aldosterone, released from the adrenal glands, stimulates sodium reabsorption and potassium excretion in the distal convoluted tubule and collecting duct.
ANP, released by the heart, reduces the reabsorption of sodium in the nephron to lower blood pressure.

Imbalanced homeostasis can lead to kidney diseases such as Chronic Kidney Disease (CKD), characterised by the gradual loss of kidney function over time.
Dialysis mimics the filtering function of the kidneys for those with kidney failure. Haemodialysis filters the blood outside the body, and Peritoneal Dialysis involves filtering the blood inside the body using the peritoneal membrane as a filter.
Kidney transplants offer a long-term solution to kidney failure but are limited by availability, rejection risks and the need for life-long immunosuppressant use.