Function of Kidney
The most important function of the kidney is to eliminate waste products produced by metabolism and assists in homeostasis of the body by regulating the volume and composition of body fluids.
Kidneys have functions in:
- Filtration of the blood plasma and returning the filtered blood plasma to the rest of the circulatory system
- Regulation of our blood volume, blood pressure and body fluid’s osmolarity
- Calcitriol synthesis, a form of vitamin D that regulates calcium levels in the body
- Detoxification by getting rid of toxic substances and free radicals
- Gluconeogenesis, a metabolic pathway that generates glucose from lactose, glycerol and some amino acids
- Secretion of the enzyme renin which activates our body’s mechanisms that control blood pressure
- Secretion of erythropoietin, a hormone that controls red blood cell count and their role in carrying oxygen molecules
Here are the basic functions of each nephron sections:
1. Ultrafiltration in the Glomerulus (Glomerular filtration)
It is also called ‘filtration under pressure’, which is the first step in urine formation. Hydrostatic pressure is built up when blood enters the glomerular capillary which has a much less diameter than that of the arteriole. By that, water and small solute molecules squeeze out of the capillaries into the capsular space whereas molecules with larger molecular sizes, for example red blood cells and platelets are left behind in the blood. Glomerular filtration produces about 125 cm3 of glomerular filtrate per minute in humans.
Glomerular filtration takes place through three layers:
(a) Fenestrated endothelium of the capillary: Thin layer of endothelium with numerous pores of around 7-9 nm in diameter. They are much more permeable than any other endothelium in our body, although they are capable of excluding red blood cells from the filtrate. (b) Basement membrane: A 200-300 nm thick structure where glomerular endothelium sits on. It consists of a meshwork of negatively charged fibres. Any molecules larger than 8nm in molecular size or some small molecules with negative electrical charge will not be able to pass through this layer.
(c) Epithelium of the renal capsule: Made up of special cells called podocytes that are highly modified for filtration. Foot-like projections (pedicels) on podocytes wrap around capillaries and interdigitate with each other. They fit loosely between each other to form slit pores or filtration slits. Slit pores of about 30nm are again negatively charged, excluding the passage of large anions.
2. Selective Reabsorption in the Proximal Convoluted Tubule
The volume and composition of filtrate is altered by secretion and reabsorption in the PCT. Selective reabsorption is the second phase in urine formation. PCT carries glomerular filtrate from the renal capsule to the loop of Henle. PCT are suitable for selective reabsorption because they have microvilli and basal channels that provide large absorptive surface area and numerous mitochondria that provide ATP for active transport. Around 70% of the glomerular filtrate is reabsorbed in the PCT, including glucose, amino acids, vitamins, hormones, ions (eg. HCO3-) and water.
One of the most important functions of PCT is the reabsorption of NaCl (salt) and water.Figure below shows the selective reabsorption of Na+ in the PCT.
(1)High Na+ concentration in lumen causes Na+ reabsorption to PCT cells, by diffusing down a concentration gradient.
(2) Na+ are active transported to basement membrane via Na+/K+ pump.
(3)Na+ diffuse through basement membrane to blood capillary.
(4)Water tend to follow Na+ by osmosis.
3. The loop of Henle acts as a countercurrent multiplier
The loop of Henle functions to conserve water and creating a high salt concentration in the renal medulla. The descending limb is permeable to water and other solutes. Ascending limb on the other hand, is impermeable to water but highly premeable to Na+ and Cl-.
‘Countercurrent’ refers to the fluid flowing in the opposite directions of the two sides of the nephron loop whereas ‘multiplier’ is termed so because it multiplies the concentration deep in the renal medulla.
The whole process starts from the thin ascending limb and is driven by the Na+/K+ pumps, resulting in Na+, K+ and Cl‑ accumulating in the interstitial fluid of renal medulla, creating an osmotic gradient that allows us to produce concentrated urine. Figure below shows the simplified version of the process.
(1) Na+ and Cl- are actively transported out of the ascending loop of Henle via Na+/K+ pump.
(2) This raises the concentration of Na+ and Cl-in the tissue fluid of medulla.
(3) H20 is lost from descending loop of Henle via osmosis. Loss of H20 concentrates Na+ and Cl- in the descending loop of Henle
(4) Na+ and Cl- diffuse out of this concentrated solution in the lower part of the ascending loop of Henle
4. Distal Convoluted Tubule
The DCT is another important site of secretion and reabsorption. It regulates NaCl and K+ concentration of body fluids by controlling the amount of K+ secretion and Na+ reabsorption from the filtrate. It also regulates pH by controlling H+ secretion and HCO3- reabsorption. Other molecules that are secreted from the blood vessels into the tubule include ammonia and uric acid.
5. Collecting Duct
The collecting duct connects nephrons to the ureter and carries filtrate to the renal pelvis from renal medulla. The high salt concentration formed by the countercurrent multiplier allows water to be reabsorbed from the collecting ducts (a process called osmosis) into the hyperosmotic interstitial fluid.
In the inner medulla, collecting duct becomes more permeable to urea. This causes diffusion of some urea out of the duct, into the interstitial fluid. Together with NaCl, they contribute to the high osmolarity of the interstitial fluid in the renal medulla, which enables us to excrete urine that is hyperosmotic to our body fluids.
- The following movie shows an animated representation of the function of nephron.