13.4: Formation and Analysis of Urine

Urine formation is a three-step process that allows the kidneys to regulate blood composition while removing wastes and excess water from the body.

Master this section and you'll be able to

• Summarize the steps in urine formation
• Describe physical and chemical characteristics of urine
• Identify key urine test findings.  

Urine Formation

Urine is a waste byproduct that forms when the kidneys remove excess water and metabolic waste molecules from the blood. Although we often think of urine as the main output, the primary function of the renal system is actually to regulate blood volume and plasma osmolarity. Making urine is simply the most efficient way for the body to adjust water and solute levels while also removing wastes.

Urine formation relies on three key steps:

1. Filtration: Blood pressure forces water and small solutes out of the glomerulus and into the nephron.
2. Reabsorption: Useful substances such as water, glucose, amino acids, and ions move from the tubular fluid back into the bloodstream.
3. Secretion: Additional wastes, drugs, and excess ions are actively transported from the blood into the tubular fluid.

The three processes work together to maintain homeostasis and produce the final urine that leaves the body.

1) Filtration

Filtration begins when blood enters the afferent arteriole and flows into the glomerulus, a tuft of intertwined, fenestrated capillaries enclosed by the Bowman’s (glomerular) capsule. About 20% of the cardiac output becomes the filtration fraction, meaning this portion of the blood is routed to the kidneys for filtration while the remaining 80% continues through the systemic circulation.

The Bowman’s capsule surrounds the glomerulus and consists of two layers of simple squamous epithelium. The parietal layer forms the outer wall of the capsule, while the visceral layer lies directly on the glomerular capillaries and is composed of specialized cells called podocytes. Podocytes extend foot-like processes that create narrow filtration slits. As blood flows through the glomerulus, water and small solutes such as nitrogenous wastes pass through these slits to form the glomerular filtrate.

Selectivity of the filtration barrier is maintained by three components working together: the fenestrated capillary endothelium, the negatively charged basement membrane, and the podocyte filtration slits. This combination prevents large molecules (such as albumin), negatively charged proteins, and all blood cells from entering the filtrate. These non-filterable components remain in the bloodstream and exit the glomerulus through the efferent arteriole, which then branches into peritubular capillary network that supports oxygen delivery to kidney tissues and participates in reabsorption and secretion before draining into the venous system.

The Mechanisms of Filtration

Filtration at the glomerulus is driven mainly by pressure differences. The most important of these is glomerular hydrostatic pressure, which is the blood pressure inside the glomerular capillaries. This pressure pushes water and small solutes out of the capillaries and through the filtration slits of the Bowman's capsule.

Working against this pushing force is the osmotic pressure created by plasma proteins such as albumin. Because albumin stays in the blood, it pulls water back toward the capillaries. In other words, hydrostatic pressure pushes fluid out, and osmotic pressure pulls fluid in.

The balance between these two pressures determines the net filtration pressure, which is the actual force that drives filtration. When the pushing force of hydrostatic pressure is greater than the pulling force of osmotic pressure, filtrate moves into the glomerular capsule. These pressures, along with a few additional factors such as the resistance in the afferent and efferent arterioles, help determine the glomerular filtration rate (GFR)

The GFR is the amount of filtrate the kidneys produce each minute. Out of the roughly 5 liters of blood the heart pumps each minute, about 20% enters the kidneys for filtration. This produces about 125 mL of filtrate per minute in males and about 105 mL per minute in females (with normal ranges around these values). Over a full day, this adds up to about 180 liters of filtrate in males and 150 liters in females. This seems like a huge amount, and — luckily — we do not produce anywhere near that much urine. In fact, as the filtrate travels through the nephron’s tubules, the kidneys reabsorb almost all of it. Reabsorption is carefully regulated to help maintain stable blood volume, blood pressure, plasma osmolarity, and blood pH. Water, ions, and useful molecules move out of the tubules and back into the bloodstream through the peritubular capillaries, so they are not lost from the body. Because about 99% of the filtrate is reclaimed, only about 1 to 2 liters exit the body as urine each day.

2) Reabsorption

Reabsorption is the step in urine formation where the nephron takes back useful water, ions, and molecules into the bloodstream. As the filtrate flows through the PCT, loop of Henle, DCT, and CD, its composition changes because different parts of the nephron remove different substances. By the time the fluid reaches the collecting duct, most needed materials have been reclaimed, and a bit more secretion occurs before the final fluid leaves the kidney as urine.

Reabsorption happens in several ways, depending on the substance and the nephron segment:

• Passive diffusion: molecules move from a higher concentration to a lower concentration across the cell membrane without using energy.
• Active transport: membrane pumps use ATP to move substances like sodium ions against their concentration gradients.
• Cotransport: substances move together across the membrane, such as sodium pulling glucose or water with it. This helps water follow solutes that are actively reabsorbed.

The overall goal is simple: the nephron reabsorbs what the body needs and leaves behind what will become urine.

3) Secretion

Secretion is the final adjustment step in urine formation. During this step, certain substances leave the blood in the peritubular capillaries and enter the nephron’s tubules so they can be eliminated from the body. This process works in the opposite direction of reabsorption and allows the kidneys to fine tune the composition of the blood by removing excess ions, wastes, and some medications.

Most secretion uses active transport, and some substances can also move by passive diffusion. Only a limited number of substances are normally secreted, and they are mainly wastes the body needs to remove. These include potassium ions, hydrogen ions, ammonium ions, creatinine, urea, some hormones, and certain drugs such as penicillin.

As an example, the kidneys help regulate the body’s pH by secreting hydrogen ions (H⁺) and ammonium ions (NH₄⁺) from the blood into the tubular fluid. Removing these acidic ions helps prevent the blood from becoming too acidic. This process also helps the body conserve sodium bicarbonate (NaHCO₃), an important buffer that stabilizes blood pH.

Blood pH is normally kept between 7.35 and 7.45, while urine is usually more acidic, with a typical pH around 6.0. This difference reflects the kidneys’ role in removing excess acid from the body.

Although the respiratory system is the main regulator of pH by adjusting CO₂ levels, the kidneys provide an important secondary line of defense. By secreting H⁺ and NH₄⁺, the kidneys fine-tune pH balance and help maintain homeostasis.

 

Once reabsorption and secretion are complete, the remaining fluid in the collecting duct becomes urine. Urine contains water that has not been reabsorbed and therefore represents a way for the body to lower blood volume by allowing more water to leave in the urine. Urea is the other major component of urine and provides a safe way for the body to remove nitrogen, which comes from protein metabolism. Urine also contains various ions and other waste products. Red blood cells and glucose are not normally present in urine, and their presence may indicate problems such as injury to the glomerulus or diabetes mellitus.

 

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Because its composition reflects many internal body processes, urine is often used in diagnosing disease conditions. Physical and chemical features of urine can reveal important information about health, and these are commonly evaluated through urinalysis, a routine clinical test.

Basic Physical Characteristics of Urine.

Characteristic	Normal Findings	What Influences It	Possible Abnormal Causes
Color	Yellow to amber	Hydration level, diet	Dehydration, blood in urine (kidney damage or disease)
Smell	Mild odor when fresh	Time since voiding, diet	Sweet or fruity smell in diabetes, strong ammonia smell in aged urine
pH	4.6 to 8 (average about 6)	Diet, acid base balance	Persistent high or low pH may indicate metabolic imbalance or disease
Density (Specific Gravity)	1.001 to 1.035	Hydration, solute concentration	Very high values may reflect dehydration or excess solutes; very low values may indicate kidney problems
Turbidity	Clear or slightly cloudy	Time since voiding	Cloudiness due to infection, increased cells, or obstruction

Changes in any of these physical characteristics may indicate disease or metabolic imbalance. Although these changes may appear minor on their own, they can be early signs of conditions such as diabetes mellitus or glomerular damage.

Normal Chemical Composition of Urine

Urine is an aqueous solution of greater than 95% water, with a minimum of these remaining constituents, in order of decreasing concentration:

• Urea 9.3 g/L.
• Chloride 1.87 g/L.
• Sodium 1.17 g/L.
• Potassium 0.750 g/L.
• Creatinine 0.670 g/L.
• Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites).

Formed in the kidneys and stored in the bladder, urine is sterile (free of harmful microbe). As it passes through the urethra, however, it picks up bacteria that live on the epithelial lining. Many of these bacteria are normal residents of the lower urinary tract and are usually harmless.

Urea is the main nitrogenous waste in urine. It is produced in the liver from ammonia and carbon dioxide. Ammonia is highly toxic to cells, so converting it to urea makes it much safer to transport in the blood and to eliminate from the body.

Abnormal Types of Urine

There are several conditions that can cause abnormal substances to appear in urine or cause urine to have abnormal characteristics. Many of these are named with the suffix -uria, which means “related to urine.” Some common examples are:

• Proteinuria: Protein in the urine, often due to leaky or damaged glomeruli in the kidneys.

• Oliguria: An abnormally small volume of urine, often caused by shock or kidney damage.

• Polyuria: An abnormally large volume of urine, often seen in diabetes and some other hormonal or kidney disorders.

• Dysuria: Painful or uncomfortable urination, often caused by urinary tract infections or inflammation.

• Hematuria: Red blood cells in the urine, which can result from infection, kidney stones, or other injury to the urinary tract.

• Glycosuria: Glucose (sugar) in the urine, usually due to high blood glucose in diabetes that exceeds the kidneys’ ability to reabsorb it in the proximal convoluted tubule.

 

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Urinalysis

A urinalysis (UA) is a common diagnostic test that examines the physical, chemical, and microscopic characteristics of urine. It helps identify many medical conditions because urine contains measurable clues about how the body is functioning.

A standard urinalysis may evaluate substances such as glucose, proteins, ketones, and red or white blood cells, as well as properties like pH and specific gravity. Some parts of the test use urine dipsticks, which change color to indicate the presence or concentration of certain substances. Other parts require examining a small urine sample under a light microscope.

Clinicians typically order either a routine urinalysis or a routine and microscopy (R&M) urinalysis. A routine urinalysis includes the physical and chemical tests only. An R&M urinalysis adds microscopic analysis and may include culturing bacteria found in the urine. Because of this, R&M is particularly useful when evaluating possible urinary tract infections (UTIs).

Test Strip Urinalysis

Test strip urinalysis uses small reagent strips that change color when they come into contact with specific substances in urine. It is the most common method used in routine urinalysis because it is fast, inexpensive, and easy to interpret. A single strip can screen for several important findings, including:

• Leukocytes – White blood cells in the urine, called leukocyturia, often suggest inflammation or infection in the urinary tract.

• Nitrites – Certain bacteria convert nitrates into nitrites, so nitrituria is another indicator of a possible urinary tract infection.

• Proteins – Protein in the urine is called proteinuria (or albuminuria/microalbuminuria). This may point to increased permeability of the glomeruli or early kidney damage.

• Blood – The presence of red blood cells, called hematuria, can occur with infection, kidney stones, or injury.

• pH – Test strips can quickly determine urine acidity or alkalinity. Abnormal urine pH can help identify metabolic acidosis, alkalosis, or certain types of infections.

Urine Microscopy

The numbers and types of cells and/or material, such as urinary casts, can yield a great detail of information and may suggest a specific diagnosis. A urinary cast is any tiny structure found in urine that consists of multiple molecules or cells bound together.

Casts form within the nephron when abnormal cells and molecules are filtered from blood, and are excreted as the bound structures in urine. Microscopy can identify casts in urine and use them to diagnose kidney diseases, by characterizing symptoms such as:

• Red blood cell casts are associated with glomerulonephritis, vasculitis, or malignant hypertension.
• White blood cell casts are associated with acute interstitial nephritis, exudative glomerulonephritis, or severe pyelonephritis.
• Epithelial cell casts are associated with toxin-induced, acute tubular necrosis, hepatitis, and cytomegalovirus.
• (Heme) granular casts are associated with acute tubular necrosis, and are often composed of proteins, especially antibodies.
• Hyaline casts are associated with dehydration; it is the most common type of cast.
• Bacterial casts are associated with urinary tract infection; the cast may be cultured in order to identify the causative organism of the cast.

   

Figure \(\PageIndex{1}\): Light-Microscope View of Urine Sediment. This sample is from a patient with a urinary tract infection. Numerous white blood cells (WBCs) appear as round, granular cells scattered throughout the field, indicating inflammation within the urinary tract. Small bacterial clusters are also present, confirming active infection. Together, the high WBC count and visible bacteria are characteristic findings in a UTI.