Kidney: Excretion & Osmoregulation
Each kidney contains approximately one million microscopic filtering units called nephrons, which are responsible for filtering blood and forming urine. The nephron is the functional unit of the kidney.
The kidney itself is divided into three main regions: the outer cortex, the inner medulla, and the central renal pelvis. Different parts of the nephron are located in these regions.
The main components of a nephron include the Bowman's capsule and glomerulus (located in the cortex), the Proximal Convoluted Tubule (PCT) (cortex), the Loop of Henle (medulla), the Distal Convoluted Tubule (DCT) (cortex), and the collecting duct (medulla).
The renal pelvis is a funnel-shaped cavity that collects urine from multiple collecting ducts and channels it into the ureter for transport to the bladder.
Ultrafiltration is the initial step in urine formation, occurring in the glomerulus, a capillary network encased within the Bowman's capsule. Blood enters the glomerulus under high pressure due to the afferent arteriole (leading into the glomerulus) having a wider lumen than the efferent arteriole (leading away).
This high hydrostatic pressure forces small molecules from the blood plasma through a specialized filter membrane into the Bowman's capsule. This membrane acts as a sieve, allowing water, glucose, urea, and ions to pass through.
Larger molecules, such as proteins, and blood cells are too large to pass through the filter and remain in the blood. This ensures that essential components are not lost during the initial filtration step.
The fluid collected in the Bowman's capsule after ultrafiltration is called glomerular filtrate. It contains water, glucose, urea, and various ions, but is essentially protein-free.
Following ultrafiltration, the glomerular filtrate flows into the Proximal Convoluted Tubule (PCT), where selective reabsorption occurs. This process recovers useful substances from the filtrate back into the bloodstream.
All of the glucose present in the glomerular filtrate is reabsorbed in the PCT. This is a critical step because glucose is an essential energy source for cellular respiration and must be conserved.
Glucose reabsorption occurs primarily through active transport, which moves glucose against its concentration gradient from the filtrate into the surrounding capillaries. This process requires significant energy, which is supplied by numerous mitochondria within the PCT cells.
Other useful substances, such as some ions and amino acids, are also selectively reabsorbed in the PCT, while waste products like urea remain largely in the filtrate to be excreted.
The final stage of urine formation involves the reabsorption of water, which is crucial for osmoregulation and maintaining the body's fluid balance. While some water is reabsorbed in the Loop of Henle, the majority of water reabsorption occurs in the collecting duct.
Water moves from the filtrate in the collecting duct back into the bloodstream by osmosis, driven by the osmotic gradient created in the kidney medulla. The amount of water reabsorbed is precisely controlled by the hormone Antidiuretic Hormone (ADH).
ADH is produced by the hypothalamus and released by the pituitary gland. Its primary action is to increase the permeability of the collecting duct walls to water. More ADH leads to more water reabsorption and concentrated urine, while less ADH results in less water reabsorption and dilute urine.
This regulation operates as a negative feedback loop: osmoreceptors in the hypothalamus detect changes in blood water potential. If water potential is too low (dehydration), more ADH is released; if too high (overhydration), less ADH is released, adjusting urine volume accordingly.
Normal urine is primarily composed of urea (a nitrogenous waste product from protein metabolism), excess ions (e.g., sodium, potassium), and excess water. The color and quantity of urine can vary significantly based on physiological conditions.
The concentration of urine is influenced by several factors: water intake (high intake leads to large volumes of pale, dilute urine), temperature (higher temperatures increase sweating, reducing water available for urine, leading to smaller volumes of dark, concentrated urine), and exercise (increased sweating also leads to concentrated urine).
The presence of certain substances in urine can indicate underlying health issues. For example, glucose in urine (glycosuria) suggests high blood glucose levels, often associated with diabetes, as the PCT's reabsorption capacity is overwhelmed.
Similarly, protein in urine (proteinuria) can indicate damage to the glomerulus, as large protein molecules should normally be retained in the blood during ultrafiltration. High blood pressure can sometimes lead to such glomerular damage.