The Genitourinary System / The Kidneys - Basic Version

submitted by Dr. Gary Farr Last Updated July, 17, 2003

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What are the Kidneys?

Renal System Anatomy

The renal system consists of the renal arteries, renal veins, and the kidneys.

Renal Arteries

The renal arteries are two large blood vessels that branch off from the sides of the major artery of the lower trunk (abdominal aorta). Each kidney receives blood from a renal artery. Before entering the kidney, each renal artery divides into four or five branches that lead to the fatty tissue surrounding the kidney (the capsule), the tubes that conduct urine from the kidneys to the bladder (the ureters), and nearby tissues and muscles. Many people possess another renal artery, which is a lower offshoot of the abdominal aorta. This vessel, known as the inferior renal artery, supplies blood to the lower part of the kidney. Renal arteries channel blood to the kidneys; the kidneys filter out waste products from the blood.

Renal Veins

The renal veins of the kidneys arise from three locations: beneath the capsule, the area around the tubules, and the network around the conical masses that make up the inner portion of the kidney (pyramids of Malphigi). The renal veins carry filtered blood to the major vein of the lower trunk (inferior vena cava).

Kidneys

Each kidney is composed of about one million microscopic “filtering packets” called glomeruli. The glomeruli remove uremic waste products from the blood. Each glomerulus connects to a long tube, called the tubule. Urine made by the glomerulus moves down the tubule. Together, the glomerulus and the tubule form a unit called a nephron. Each nephron connects to progressively larger tubular branches, until it reaches a large collection area called the calyx. The calices form the funnel-shaped portion of the upper ureter (renal pelvis). Urine moves from the renal pelvis to the ureters, the large tubes that connect the kidney to the bladder.

Bladder

The bladder, or urinary bladder, is a hollow, muscular, balloon-shaped sac that stores urine. The bladder is located behind the pubic bone and it sits within and is protected by the hip bone (pelvis). The bladder is held in place by ligaments (bands of tough, fiberlike tissue) that connect it to the pelvis and to other organs. In a healthy person, the bladder can store up to 16 ounces (2 cups) of urine for 2 to 5 hours. Nerves in the bladder signal the body when it is time to urinate.

The narrow, internal opening of the urethra within the bladder is called the bladder neck. In this region, the bladder’s muscular fibers help the sphincter muscles to close the urethra. The bladder serves as a reservoir that holds urine until it contracts and expels the urine out of the body via the urethra.

Urethra

The urethra is the tube that passes urine from the bladder out of the body. In women, the urethra is approximately 4 centimeters long. It is composed of smooth (involuntary, nonstriated) muscle fibers, sphincter (encircling, striated) muscle fibers, a layer of elastic tissue, and collagen tissue. The urethra is lined with mucous membrane. The female urethra starts at the bladder neck and exits the body directly in front of the opening of the vagina (female reproductive canal).

In men, the urethra is roughly 8 to 9 inches in length. It extends from the bladder neck to the end of the penis. The male urethra is composed of three portions: prostatic, membranous, and spongy. The prostatic portion is the widest part of the tube and passes through the prostate gland. Its walls are made up of fibrous tissue, muscle fibers, and tiny glandular openings that connect to the prostate. The membranous portion is approximately three-quarters of an inch long and lies between the triangular ligaments of the pelvis. The spongy portion is the longest part of the urethra and extends through the penis to the glans (tip of the penis). The corpus spongiosum is the lower area of the penis that surrounds and protects the urethra.

Kidney Functions

Filtration

The kidneys’ main function is to remove toxins (uremic wastes) that accumulate in the blood as a result of the body’s metabolism. The body continuously uses digested elements from foods and stored nutrients to perform normal bodily functions. The by-products of nutrient metabolism and cell function are filtered from the blood by the kidneys, which excrete (discharge) wastes as urine. Every day, approximately 200 liters of blood flow to the kidneys where 2 liters of waste are filtered out.

BUN and Creatinine

The concentration in the blood (blood level) of blood urea nitrogen (BUN), known as urea, and creatinine (Cr) can be measured by routine laboratory tests. BUN and creatinine levels indicate the general function of the kidneys. BUN is a metabolic by-product of protein-rich food such as meat, poultry, and certain vegetables. BUN is filtered out of the blood by the kidneys and excreted in the urine. Creatinine is continuously generated by normal cell metabolism within the muscles. Creatinine is also filtered out of the blood by the kidneys and excreted in the urine.

The amounts of BUN and creatinine in the blood are equal to the amount excreted by the kidneys. The blood levels of BUN and Cr remain unchanged unless there is sudden deterioration of renal (i.e., kidney) function. If the kidneys are suddenly unable to function, BUN and Cr increase daily. This condition is known as acute renal failure. Chronic renal failure is a condition distinguished by a gradual increase in BUN and Cr over a long period of time.

Measurement of Kidney Function

When renal function decreases, blood levels of Cr and BUN increase because the kidneys are unable to clean the blood effectively. Factors not related to the kidneys also impact BUN and Cr levels. Creatinine, in particular, is affected by age, sex, weight, and muscle mass.

Renal function is measured to evaluate the rate at which both kidneys are able to clean the blood. To measure renal function, a 24-hour urine sample must be collected. It is of important that the 24-hour sample is complete (i.e., no urine is missing), true renal function will be underestimated.

The amount of Cr in the urine sample is compared to the blood level of Cr. This figure is known as creatinine clearance (CrCl), the rate at which both kidneys clean the blood. The normal CrCl is about 90 to 130 milliliters per minute (mL/min). Many people gradually lose renal function as they age. Alternative renal function measurements rely on tables or formulas that take into consideration age, body weight, sex, and blood creatinine.

Some health care facilities in the United States offer the Glofil-125 assay to evaluate renal function. Sodium iothalamate I-125 (a safe radiopharmaceutical) is injected into the skin, and blood and urine samples are obtained to determine renal function. The test is easy to perform, is more sensitive than blood creatinine measurements, and provides results within 2 to 3 hours.

Why it is important to measure kidney function

Measurements of renal function determine the severity of kidney impairment. It is important to monitor renal function over time to document the rate of deterioration or improvement with treatment. Patients often undergo yearly evaluations.

Regulation of Electrolytes

The kidneys regulate levels of sodium, potassium, calcium, phosphorus, chloride, bicarbonate, and magnesium in the blood. These are salts that conduct electricity (called electrolytes). They are essential for proper cell metabolism, muscle coordination, heart function, nerve function, and fluid absorption and excretion. How much of each chemical is excreted in the urine is determined by the individual’s diet. If a person consumes a large amount of potassium, the kidneys excrete a large amount of potassium in the urine to keep the blood level within a normal range. If a person limits dietary potassium, the kidneys excrete very little to maintain a normal concentration.

Regulation of Fluid Volume

The kidneys help to regulate the pH (acidity) of the blood and are the primary determinants of fluid volume within the body. The kidneys conserve or excrete fluid to keep the fluid volume within a normal range. If a person drinks 15 glasses of water over several hours, the kidneys detect the extra fluid within the body and excrete a large amount of dilute urine to prevent edema (swelling). If a person takes in very little fluid, the kidneys retain as much fluid as possible and the urine becomes very concentrated to prevent dehydration.

Hormone Production

Hormones are substances that are produced by organs and circulate to another part of the body to produce their effects. The kidneys produce three important hormones: erythropoetin (EPO), active vitamin D (vitamin D1,25), and renin.

Erythropoetin (EPO) is made by the kidneys and travels to the bone marrow to stimulate red blood cell production. Patients with renal failure are unable to produce adequate EPO and become anemic (i.e., have a low red blood cell count).

The hormone active vitamin D (vitamin D1, 25) is converted into its strongest form in the kidneys. This hormone plays a crucial role in bone metabolism and regulates the body’s metabolism of calcium and phosphorus.

Renin helps to control blood vessel stability and to regulate blood pressure. It is formed in specialized cells within the kidneys. Renin converts angiotensinogen (a blood protein) into angiotensin I, which is broken down by an enzyme to form angiotensin II (a potent blood vessel constrictor that can increase blood pressure).

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