Longitudinal section of a kidney. The functional units of the kidneys are the nephrons. The nephrons consist of a coiled renal tubule and a vascular network of peritubular capillaries to filter waste, macromolecules and ions from the blood and form the urine. Each kidney consists of approximately 1 million nephrons.

The kidney is composed of an enormous number of minute tubules called nephrons or uriniferous tubules or renal tubules or just kidney tubules. These nephrons are the structural as well as functional units of the kidney. These tiny, tubular structures that stretch across both regions perpendicular to the surface of the kidney and in each kidney, there are one million of these structures. Each of them is about 4 – 6cm long and the total length of tubules together is more than 60km. This great length provides a huge surface for reabsorption of usable substances specially water, as the contents move through them. Blood flowing through kidneys per minute is about one liter.

Renal physiology is the study of kidney function, while nephrology is the medical specialty concerned with kidney diseases. Diseases of the kidney are diverse, but individuals with kidney disease frequently display characteristic clinical features. Common clinical conditions involving the kidney include the nephritic and nephrotic syndromes, renal cysts, acute kidney injury, chronic kidney disease, urinary tract infection, nephrolithiasis, and urinary tract obstruction.

Various cancers of the kidney exist; the most common adult renal cancer is renal cell carcinoma. Cancers, cysts, and some other renal conditions can be managed with removal of the kidney, or nephrectomy. When renal function, measured by glomerular filtration rate, is persistently poor, dialysis and kidney transplantation may be treatment options. Although they are not severely harmful, kidney stones can be a pain and a nuisance. The removal of kidney stones includes sound wave treatment to break up the stones into smaller pieces, which are then passed through the urinary tract. One common symptom of kidney stones is a sharp pain in the medial/lateral segments of the lower back. Creatinine and creatinine clearance tests measure the level of the waste product creatinine in blood and urine. These tests tell how well the kidneys are working. A sodium test checks how much sodium (an electrolyte and a mineral) is in the blood.

A closer look of Bowman's capsule The Bowman's capsule performs the first step in the filtration of blood to form urine. It allows liquids and small particles of solid to pass through but prevents larger structures (e.g. blood cells) from passing through.

The parts of the nephron are as follows:

Bowman’s capsule: This closed end at the beginning of the nephron is located in the cortex. It is a thin – walled (single – cell thick epithelium) cup, something like a hollow ball pressed deep on one side. Its hollow internal space continues into the tubule. The outer concavity of the cup lodges a knot – like mass of blood capillaries, called glomerulus. The Bowman’s capsule and the glomerulus together are called Malpighian capsule or just renal capsule.

Proximal convoluted tubule or proximal tubule: The first twisted region of the tubule after the Bowman’s capsule in the the cortex (‘proximal’ means nearer, i.e. nearer to the Bowman’s capsule).

Loop of Henle: A long, hairpin loop after the proximal tubule, it extends from the cortex down into the medulla and back. Middle U – shaped part (Loop of Henle) is shaped like a hair – pin and runs in medulla to turn back and to re – enter the cortex to continue into the next convoluted region of the tubule.

Distal convoluted tubule or distal tubule: This second twisted portion of the nephron after the loop of Henle is located in the cortex (“distal” means farther, i.e. farther or away from Bowman’s capsule). It opens into a collecting duct. The collecting duct receives the contents of many kidney tubules and pours it as urine in the pelvis of the kidney.

Collecting duct: This long straight portion after the distal tubule that is the open end of the nephron extends from the cortex down through the medulla.

Model of water and solute reabsorption Kidney reabsorption ensures the body gets the small molecules it needs back from the filtrate.

The filtrate passes through the renal tubule where much of the water is reabsorbed together with the useful substances. However reabsorption is limited to the extent that the normal concentration of the blood is not disturbed. The small molecules that the body needs such as ions, glucose and amino acids get reabsorbed from the filtrate back into the blood. Specialized proteins called transporters are located on the membranes of the various cells of the nephron and these transporters grab the small molecules from the filtrate as it flows by them. Each transporter grabs only one or two types of molecules.

For example, glucose is reabsorbed by a transporter that also grabs sodium. Transporters are concentrated in different parts of the nephron. For example, most of the Na transporters are located in the proximal tubule, while fewer ones are spread out through other segments. Some transporters require energy, usually in the form of adenosine triphosphate which is called active transport, while others are passive. Water gets reabsorbed passively by osmosis in response to the buildup of reabsorbed Na in spaces between the cells that form the walls of the nephron. Other molecules get reabsorbed passively due to solvent drag when they are caught up in the flow of water. Reabsorption of most substances is related to the reabsorption of Na, either directly, via sharing a transporter, or indirectly via solvent drag, which is set up by the reabsorption of Na.

Factors that effect reabsorption: Two major factors which affect the reabsorption process are concentration of small molecules in the filtrate and rate of flow of the filtrate. The higher the concentration of molecules, more molecules can be reabsorbed. Flow rate affects the time available for the transporters to reabsorb molecules. To get an idea of the quantity of reabsorption across the nephron, let’s look at the sodium ion (Na) as an example:

1. Proximal tubule: reabsorbs 65 percent of filtered Na. In addition, the proximal tubule passively reabsorbs about 2/3 of water and most other substances.
2. Loop of Henle: reabsorbs 25 percent of filtered Na.
3. Distal tubule: reabsorbs 8 percent of filtered Na.
4. Collecting duct: reabsorbs the remaining 2 percent only if the hormone aldosterone is present.

Thus, the lumen wall of the epithelial cells of the proximal tubule is like the lumen wall of the small intestine ‐ both are bordered with millions of microvilli to increase surface area. The role of these epithelial cells is to reabsorb ions, nutrients, and water and transport them to the blood vessels nearby.

Loop of henle‐ Scheme of renal tubule and its vascular supply. (Loop of Henle visible center‐left.) In the kidney, the loop of Henle (or Henle’s loop or ansa nephroni) is the portion of a nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. Its main function is to create a concentration gradient in the medulla of the kidney. By means of a countercurrent multiplier system, which utilizes electrolyte pumps, the loop of Henle creates an area of high urine concentration deep in the medulla, near the collecting duct. Water present in the filtrate in the collecting duct flows through aquaporin channels out of the collecting duct, moving passively down its concentration gradient. This process reabsorbs water and creates a concentrated urine for excretion.

Your kidneys have the ability to conserve or waste water. For example, if you drink a large glass of water, you will find that you will have the urge to urinate within an hour or so. In contrast, if you do not drink for a while, such as overnight, you will not produce much urine and it will usually be very concentrated (i.e. darker). How does your kidney know the difference? The answer to this question involves two mechanisms:

1. The structure and transport properties of the loop of Henle in the nephron.
2. The anti – diuretic hormone (ADH), also called vasopressin, secreted by the pituitary gland.

The loop of Henle has a descending limb and an ascending limb. As filtrate moves down the loop of Henle, water is reabsorbed, but ions (Na,Cl) are not. The removal of water serves to concentrate the Na and Cl in the lumen. Now, as the filtrate moves up the other side (ascending limb), Na and Cl are reabsorbed, but water is not. What these two transport properties do is, set up a concentration difference in NaCl along the length of the loop, with the highest concentration at the bottom and lowest concentration at the top. The loop of Henle can then concentrate NaCl in the medulla. The longer the loop, the bigger the concentration gradient. This also means that the medulla tissue tends to be saltier than the cortex tissue.

Now, as the filtrate flows through the collecting ducts, which go back down through the medulla, water can be reabsorbed from the filtrate by osmosis. Water moves from an area of low Na concentration (high water concentration) in the collecting ducts to an area of high Na concentration (low water concentration) in the medullary tissue. If water is removed from the filtrate at this final stage the urine gets concentrated. The loop of Henle sets up the Na concentration gradient across the medulla, allowing for water to be reabsorbed from the collecting ducts and ADH allows the water to pass through those collecting ducts.

Kidney maintain acid-base balance Kidneys regulate acid–base loads by adjusting hydrogen ion (H⁺) secretion and bicarbonate (HCO₃) filtration in response to elevated carbon dioxide (CO₂) or HCO₃.

Now, the acid/base balance of our blood changes in response to many things including:

1. Diet: Diets rich in meat provide acids to the blood when digested. In contrast, diets rich in fruits and vegetables make our blood alkaline because they are rich in bicarbonates.
2. Exercise: exercising muscles produce lactic acid that must be eliminated from the body or metabolized.
3. Breathing: high altitude causes rapid breathing that makes our blood alkaline. In contrast, certain lung diseases that block the diffusion of oxygen can cause the blood to be acidic.

The kidney can correct any imbalances by removing excess acid (hydrogen ion) or bases (bicarbonate) in the urine and restoring the bicarbonate concentration in the blood to normal. The kidney cells produce a constant amount of hydrogen ion and bicarbonate because of their own cellular metabolism (production of carbon dioxide). Through a carbonic anhydrase reaction similar to the red blood cells, hydrogen ions get produced and secreted into the lumen of the nephron. Also, bicarbonate ions get produced and secreted into the blood. In the lumen of the nephron, filtered bicarbonate combines with secreted hydrogen ions to form carbon dioxide and water (carbonic anhydrase is also present on the luminal surface of the kidney cells).

Whether the kidney removes hydrogen ions or bicarbonate ions in the urine depends upon the amount of bicarbonate filtered in the glomerulus from the blood relative to the amount of hydrogen ions secreted by the kidney cells. If the amount of filtered bicarbonate is greater than the amount of secreted hydrogen ions, then bicarbonate will be lost in the urine. Likewise, if the amount of secreted hydrogen ion is greater than the amount of filtered bicarbonate, then hydrogen ions will be lost in the urine (i.e. acidic urine).

Food sources rich in alkaline diet Leafy green, allium, and cruciferous vegetables are key parts of alkaline diet.

Similarly when we take alkaline diet like fruit or vegetable, bicarbonate is added to the blood from the diet and combines with hydrogen ion to form carbon dioxide and water. This reaction reduces the hydrogen ion concentration and increases the pH.

The increased bicarbonate concentration increases the amount of bicarbonate filtered in the glomerulus. The filtered bicarbonate exceeds the amount of hydrogen ion secreted by the kidney cell and excess bicarbonate is lost in the urine.

The amount of bicarbonate secreted from the kidney cells into the blood was equal to the hydrogen ions secreted into the lumen and less than the filtered load of bicarbonate from the blood – – therefore, the blood has a net loss of bicarbonate.

This process continues to lose bicarbonate in the urine and reduce the bicarbonate in the blood until the concentrations of hydrogen (pH) and bicarbonate ions in the blood are restored to normal.

Constituents of Urine Urine contains: 96% water, 4% dissolved organic and inorganic wastes (mainly urea which is the end product of protein metabolism).

The normal human urine consists of about 95% water and 5% of solid wastes dissolved in it. The percentage of the solid wastes may slightly vary according to the food taken and according to the time after taking food but usually these are approximately (in grams per litre of urine) as follows: Besides the normal constituents, the urine may pass out certain hormones and also certain medicines like the antibiotics and the excess vitamins.

Physical properties of urine

The color of urine is usually clear yellow (due to pigment urochrome) but the color varies with the diet. Stored urine turns turbid due to the sedimentation of salts and mucus. Volume is about 1 to 1.5 liters per day but varies. pH is typically in the range: 5 to 8 and is slightly acidic (pH = 6). Protein diet makes it more acidic while vegetable diet makes it alkaline. Specific gravity is about 1.003 to 1.035. Abnormal constituents in urine such as blood, excess glucose, albumin and bile pigments indicate certain disorders. The blood passes with urine, due to infection in urinary tract, kidney stone or tumor. Excess glucose passes with urine, due to diabetes mellitus (sugar diabetes). Albumin is high as permeability of Bowman’s capsule membrane increases due to high blood pressure, or due to bacterial infection. Bile pigments may be present due to anemia, hepatitis (jaundice) or due to liver cirrhosis.

Diuretics Diuretics act by increasing the excretion by the kidneys of sodium in the urine. Diuretics are medicines that aid the elimination of sodium (salt) and water from the body.

People with chronic high blood pressure (hypertension) often take a class of drugs called diuretics to control their blood pressure. Diuretics reduce Na reabsorption from the lumen of the nephron. Water reabsorption is also reduced.

Therefore, Na and water are lost in the urine, which increases urine flow. The decreased reabsorption of Na and water from the nephron reduces blood volume, thereby reducing blood pressure. Liquid diets such as tea, coffee and alcohol that increase the formation of urine are called ‘diuretics’.

Diuretics (often called water pills) are drugs that cause the body to rid itself of excess fluids and sodium through urination. Excess fluid in the body can lead to high blood pressure. This helps to relieve the heart's workload and also decreases the buildup of fluid in the lungs and other parts of the body, such as the ankles and legs. Different diuretics remove fluid at varied rates and through different methods. They are used to treat high blood pressure, congestive heart failure and some congenital heart defects.