PHYSIOLOGY OF THE KIDNEYS

The Kidney: An Unsung Hero






Kegels: Life's events can weaken pelvic muscles. Pregnancy, childbirth, and being overweight can do it. Luckily, when these muscles get weak, you can help make them strong again.
Pelvic floor muscles are just like other muscles. Exercise can make them stronger. Women with bladder control problems can regain control through pelvic muscle exercises, also called Kegel exercises. Exercising your pelvic floor muscles for just 5 minutes, three times a day can make a big difference to your bladder control. Exercise strengthens muscles that hold the bladder and many other organs in place.
The part of your body including your hip bones is the pelvic area. At the bottom of the pelvis, several layers of muscle stretch between your legs. The muscles attach to the front, back, and sides of the pelvis bone.
Two pelvic muscles do most of the work. The biggest one stretches like a hammock. The other is shaped like a triangle. These muscles prevent leaking of urine and stool.

The Urinary System is a group of organs in the body concerned with filtering out excess fluid and other substances from the bloodstream. The substances are filtered out from the body in the form of urine. Urine is a liquid produced by the kidneys, collected in the bladder and excreted through the urethra. Urine is used to extract excess minerals or vitamins as well as blood corpuscles from the body. The Urinary organs include the kidneys, ureters, bladder, and urethra. The Urinary system works with the other systems of the body to help maintain homeostasis. The kidneys are the main organs of homeostasis because they maintain the acidic/basic balance and the water/salt balance of the blood.

Functions of the Urinary System

• One of the major functions of the Urinary system is the process of excretion.
  • Excretion is the process of eliminating, from an organism, waste products of metabolism and other materials that are of no use.
• The urinary system maintains an appropriate fluid volume by regulating the amount of water that is excreted in the urine (this is where the ADH is used, however the signal to send more or less comes from the posterior pituitary gland). Other aspects of its function include regulating the concentrations of various electrolytes in the body fluids and maintaining normal pH of the blood. Several body organs carry out excretion, but the kidneys are the most important excretory organ. The primary function of the kidneys are to maintain a stable internal environment (homeostasis) for optimal cell and tissue metabolism. They do this by separating urea, mineral salts, toxins, and other waste products from the blood. They also do the job of conserving water, salts, and electrolytes.
• At least one kidney must function properly for life to be maintained.
  • Healthy kidneys clean the blood by filtering out extra water and wastes and making hormones that keep your bones and blood healthy. When both of your kidneys fail, your body holds fluid, and your blood pressure rises. This causes harmful wastes to build up in your body.
  • Your body doesn't make enough red blood cells and you develop fatigue, nausea, and loss of appetite. When this happens, you need treatment (dialysis) to replace the work of your failed kidneys.

Five important roles of the kidneys are:

1. Regulation of plasma ionic composition. Ions such as sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphates are regulated by the amount that the kidney excretes.
2. Regulation of plasma osmolarity. The kidneys regulate osmolarity because they have direct control over how many ions and how much water a person excretes. (this is the same as number 1)
3. Regulation of plasma volume. Your kidneys are so important they even have an effect on your blood pressure. The kidneys control plasma volume by controlling how much water a person excretes. The plasma volume has a direct effect on the total blood volume, which has a direct effect on your blood pressure. Salt (NaCl) will cause osmosis (the diffusion of water into the blood) to occur.
4. Regulation of plasma hydrogen ion concentration (pH). The kidneys partner up with the lungs and they together control the pH. The kidneys have a major role because they control the amount of bicarbonate excreted or held onto. The kidneys help maintain the blood pH mainly by excreting hydrogen ions and reabsorbing bicarbonate ions as needed.
5. Removal of metabolic waste products and foreign substances from the plasma. One of the most important things the kidneys excrete is nitrogenous waste. As the liver breaks down amino acids it also releases ammonia. The liver then quickly combines that ammonia with carbon dioxide, creating urea which is the primary nitrogenous end product of metabolism in humans. The liver turns the ammonia into urea because it is much less toxic. We can also excrete some ammonia, creatinine and uric acid. The creatinine comes from the metabolic breakdown of creatinine phospate (a high-energy phosphate in muscles). Uric acid comes from the break down of nucleotides. Uric acid is insoluble and too much uric acid in the blood will build up and form crystals that can collect in the joints and cause gout.

Hormones and Types

A hormone is a type of chemical signal. They are a means of communication between cells.
The endocrine system produces hormones that are instrumental in maintaining homeostasis and regulating reproduction and development. A hormone is a chemical messenger produced by a cell that effects specific change in the cellular activity of other cells (target cells). Unlike exocrine glands (which produce substances such as saliva, milk, stomach acid and digestive enzymes), endocrine glands do not secrete substances into ducts (tubes). Instead, endocrine glands secrete their hormones directly into the surrounding extra cellular space. The hormones then diffuse into nearby capillaries and are transported throughout the body in the blood.
The endocrine and nervous systems often work toward the same goal. Both influence other cells with chemicals (hormones and neurotransmitters). However, they attain their goals differently. Neurotransmitters act immediately (within milliseconds) on adjacent muscle, gland, or other nervous cells, and their effect is short-lived. In contrast, hormones take longer to produce their intended effect (seconds to days), may affect any cell, nearby or distant, and produce effects that last as long as they remain in the blood, which could be up to several hours.

Secretion of Hormones The endocrine system has assistance from the kidney's when releasing hormones. Renin is released by the kidneys. Renin leads to the secretion of aldosterone which is released from the adrenal cortex. Aldosterone promotes the kidneys to reabsorb the sodium (Na+) ions. The kidneys also secrete erythropoietin when the blood doesn't have the capacity to carry oxygen. Erythropoietin stimulates red blood cell production. The Vitamin D from the skin is also activated with help from the kidneys. Calcium (Ca+) absorption from the digestive tract is promoted by vitamin D.

Antagonistic Hormones
Maintaining homeostasis often requires conditions to be limited to a narrow range. When conditions exceed the upper limit of homeostasis, specific action, usually the production of a hormone is triggered. When conditions return to normal, hormone production is discontinued. If conditions exceed the lower limits of homeostasis, a different action, usually the production of a second hormone is triggered. Hormones that act to return body conditions to within acceptable limits from opposite extremes are called antagonistic hormones. The two glands that are the most responsible for homeostasis are the thyroid and the parathyroid glands.
The regulation of blood glucose concentration (through negative feedback) illustrates how the endocrine system maintains homeostasis by the action of antagonistic hormones. Bundles of cells in the pancreas called the Islets of Langerhans, contain two kinds of cells, alpha cells and beta cells. These cells control blood glucose concentration by producing the antagonistic hormones insulin and glucagon.
Beta cells secrete insulin. When the concentration of blood glucose raises after eating, beta cells secret insulin into the blood. Insulin stimulates the liver and most other body cells to absorb glucose. Liver and muscle cells convert glucose to glycogen, for short term storage, and adipose cells convert glucose to fat. In response, glucose concentration decreases in the blood, and insulin secretion discontinues through negative feedback from declining levels of glucose.

Alpha cells secrete glucagon. When the concentration of blood glucose drops such as during exercise, alpha cells secrete glucagon into the blood. Glucagon stimulates the liver to release glucose. The glucose in the liver originates from the breakdown of glycogen and the conversion of amino acids and fatty acids into glucose. When blood glucose levels return to normal, glucagon secretion discontinues through negative feedback.

Another example of antagonistic hormones occurs in the maintenance of Ca2+ concentration in the blood. Parathyroid hormone (PTH) from the parathyroid glands increases Ca2+ in the blood by increasing Ca2+ absorption in the intestines and reabsorption in the kidneys and stimulating Ca2+ release from bones. Calcitonin (CT) produces the opposite effect by inhibiting the breakdown of bone matrix and decreasing the release of calcium in the blood.


Structure of kidney

The kidneys belong to the excretory system. They are reddish, bean-shaped organs situated one on each side of the vertebral column, just below the ribs. Often, the left kidney is positioned an inch higher than the right kidney. Each kidney is about 4-5 inches long and about two inches thick, weighing 4-6 grams in the average adult. Because of the presence of many blood vessels, the kidneys are colored a dark reddish-brown. The kidney is divided into an outer cortex, middle medulla and inner pelvis, 20% of blood pumped by heart goes through the kidneys.
The medulla is composed of 8 - 15 renal pyramids separated by renal columns. It is also stripped in appearance due to the microscopic tubules and blood vessels.
The kidneys are retroperitoneal, meaning that they are behind the abdominal cavity.

The Position of the kidneys

Nephron

A nephron is the basic structural and functional unit of the kidney. The name nephron comes from the Greek word (nephros) meaning kidney. Its chief function is to regulate water and soluble substances by filtering the blood, reabsorbing what is needed and excreting the rest as urine. Nephrons eliminate wastes from the body, regulate blood volume and pressure, control levels of electrolytes and metabolites, and regulate blood pH. Its functions are vital to life and are regulated by the endocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.
Each nephron has its own supply of blood from two capillary regions from the renal artery. Each nephron is composed of an initial filtering component (the renal corpuscle) and a tubule specialized for re-absorption and secretion (the renal tubule). The renal corpuscle filters out large solutes from the blood, delivering water and small solutes to the renal tubule for modification.

Here is a video about the Nephrons:

Adrenal glands are a pair of ductless glands located above the kidneys. Through hormonal secretions, the adrenal glands regulate many essential functions in the body, including biochemical balances that influence athletic training and general stress response. The glucocorticoids include corticosterone, cortisone, and hydrocortisone or cortisol. These hormones serve to stimulate the conversion of amino acids into carbohydrates which is a process known as gluconeogenesis, and the formation of glycogen by the liver. They also stimulate the formation of reserve glycogen in the tissues, such as in the muscles. The glucocorticoids also participate in lipid and protein metabolism. The cortex of the adrenal gland is known to produce over 20 hormones, but their study can be simplified by classifying them into three categories: glucocorticoids, mineralcorticoids, and sex hormones.

They are triangular-shaped glands located on top of the kidneys. They produce hormones such as estrogen, progesterone, steroids, cortisol, and cortisone, and chemicals such as adrenaline (epinephrine), norepinephrine, and dopamine. When the glands produce more or less hormones than required by the body, disease conditions may occur.

The adrenal cortex secretes at least two families of hormones, the glucocorticoids and mineral corticoids. The adrenal medulla secretes the hormones epinephrine (adrenalin) and norepinephrine (noradrenalin).

Adrenal Cortex: The hormones made by the Adrenal Cortex supply long-term responses to stress. The two major hormones produced are the Mineral Corticoids and the Glucocorticoids. The Mineral Corticoids regulate the salt and water balance, leading to the increase of blood volume and blood pressure. The Glucocorticoids are monitoring the ACTH, in turn regulating carbohydrates, proteins, and fat metabolism. This causes an increase in blood glucose. Glucocorticoids also reduce the body's inflammatory response.

Cortisol is one of the most active glucocorticoids. It usually reduces the effects of inflammation or swelling throughout the body. It also stimulates the production of glucose from fats and proteins, which is a process referred to as gluconeogenesis.

Aldosterone is one example of a mineralcorticoid. It signals the tubules in the kidney nephrons to reabsorb sodium while secreting or eliminating potassium. If sodium levels are low in the blood, the kidney secretes more renin, which is an enzyme that stimulates the formation of angiotensin from a molecule made from the liver. Angiotensin stimulates aldosterone secretion. As a result, more sodium is reabsorbed as it enters the blood.Aldosterone, the major mineralcorticoid, stimulates the cells of the distal convoluted tubules of the kidneys to decrease re-absorption of potassium and increase re-absorption of sodium. This in turn leads to an increased re-absorption of chloride and water. These hormones, together with such hormones as insulin and glucagon, are important regulators of the ionic environment of the internal fluid.
The renin-angiotensin-aldosterone mechanism can raise blood pressure if it tends to drop. It does this in two ways. Angiotensin is a vasoconstrictor, decreasing the diameter of blood vessels. As vessels constrict, blood pressure increases. In addition, as sodium is reabsorbed, the blood passing through the kidney becomes more hypertonic. Water follows the sodium into the hypertonic blood by osmosis. This increases the amount of volume in the blood and also increases the blood pressure.

Adrenal Medulla The hypothalamus starts nerve impulses that travel the path from the bloodstream, spinal cord, and sympathetic nerve fibers to the Adrenal Medulla, which then releases hormones. The effects of these hormones provide a short-term response to stress.
Excessive secretion of the glucocorticoids causes Cushing's syndrome, characterized by muscle atrophy or degeneration and hypertension or high blood pressure. Under secretion of these substances produces Addison's disease, characterized by low blood pressure and stress.
Epinephrine and norepinephrine produce the "fight or flight" response, similar to the effect from the sympathetic nervous system. Therefore, they increase heart rate, breathing rate, blood flow to most skeletal muscles, and the concentration of glucose in the blood. They decrease blood flow to the digestive organs and diminish most digestive processes.

Glomerulus

The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. The glomerular blood pressure provides the driving force for fluid and solutes to be filtered out of the blood and into the space made by Bowman's capsule. The remainder of the blood not filtered into the glomerulus passes into the narrower efferent arteriole. It then moves into the vasa recta, which are collecting capillaries intertwined with the convoluted tubules through the interstitial space, where the reabsorbed substances will also enter. This then combines with efferent venules from other nephrons into the renal vein, and rejoins with the main bloodstream.

Because glomerular capillaries are extremely permeable and have an extensive surface area, this modest net filtration pressure produces an extraordinarily large volume of filtrate. The glomerular filtration rate (GFR) is the volume of filtrate produced by both kidneys per minute. The GFR averages 115 ml per minute in women and 125 ml per minute in men. This is equivalent to 7.5 L per hour or 180 L per day (about 45 gallons)! Since the total blood volume averages about 5.5 L, this means that the total blood volume is filtered into the urinary tubules every 40 minutes. Most of the filtered water must obviously be returned immediately to the vascular system, or a person would literally urinate to death within minutes.

Formation of Urine

Filtration: Blood enters the afferent arteriole and flows into the glomerulus. Blood in the glomerulus has both filterable blood components and non-filterable blood components. Filterable blood components move toward the inside of the glomerulus while non-filterable blood components bypass the filtration process by exiting through the efferent arteriole. Filterable Blood components now take on plasma like form called glomerular filtrate. A few of the filterable blood components are water, nitrogenous waste, nutrients and salts (ions). Nonfilterable blood components include formed elements such as blood cells and platelets along with plasma proteins. The glomerular filtrate is not the same consistency as urine, as much of it is reabsorbed into the blood as the filtrate passes through the tubules of the nephron.

Reabsorption: Within the peritubular capillary network, molecules and ions are reabsorbed back into the blood. Sodium Chloride reabsorbed into the system increases the osmolarity of blood in comparison to the glomerular filtrate. This re-absorption process allows water (H2O) to pass from the glomerular filtrate back into the circulatory system.
Glucose and various amino acids also are reabsorbed into the circulatory system. These nutrients have carrier molecules that claim the glomerular molecule and release it back into the circulatory system. If all of the carrier molecules are used up, excess glucose or amino acids are set free into the urine. A complication of diabetes is the inability of the body to reabsorb glucose. If too much glucose appears in the glomerular filtrate it increases the osmolarity of the filtrate, causing water to be released into the urine rather than reabsorbed by the circulatory system. Frequent urination (polyuria) and unexplained thirst (polydipsia) are warning signs of diabetes, due to water not being reabsorbed.
Glomerular filtrate has now been separated into two forms: Reabsorbed Filtrate and Non-reabsorbed Filtrate. Non-reabsorbed filtrate is now known as tubular fluid as it passes through the collecting duct to be processed into urine.

Secretion: Some substances are removed from blood through the peritubular capillary network into the distal convoluted tubule or collecting duct. These substances are hydrogen ions, creatinine, and drugs. Urine is a collection of substances that have not been reabsorbed during glomerular filtration or tubular secretion.

Diuretics

A diuretic (colloquially called a water pill) is any drug that elevates the rate of bodily urine excretion (diuresis). Diuretics also decrease the extracellular fluid (ECF) volume, and are primarily used to produce a negative extracellular fluid balance. Caffeine, cranberry juice and alcohol are all weak diuretics. In medicine, diuretics are used to treat congestive heart failure, liver cirrhosis, hypertension and certain kidney diseases. Diuretics alleviate the symptoms of these diseases by causing sodium and water loss through the urine. As urine is produced by the kidney, sodium and water – which cause edema related to the disease – move into the blood to replace the volume lost as urine, thereby reducing the pathological edema. Some diuretics, such as acetazolamide, help to make the urine more alkaline and are helpful in increasing excretion of substances such as aspirin in cases of overdose or poisoning. The antihypertensive actions of some diuretics (thiazides and loop diuretics in particular) are independent of their diuretic effect. That is, the reduction in blood pressure is not due to decreased blood volume resulting from increased urine production, but occurs through other mechanisms and at lower doses than that required to produce diuresis. Indapamide was specifically designed with this is mind, and has a larger therapeutic window for hypertension (without pronounced diuresis) than most other diuretics. Chemically, diuretics are a diverse group of compounds that either stimulate or inhibit various hormones that naturally occur in the body to regulate urine production by the kidneys. Alcohol produces diuresis through modulation of the vasopressin system.

Urinary Tract Infections

The second most common type of bacterial infections seen by health care providers is UTI's (urinary tract infections). Out of all the bacterias that colonize and cause urinary tract infections the most common is Escherichia coli (E-coli). In the hospital indwelling catheters and straight catheterizing predispose the opportunity for urinary tract infections. Therefore, UTIs are the most common nosocomial (hospital acquired) infection. In females there are three stages in life that predispose urinary tract infections, that is menarche, manipulation between intercourse, and menopause. However, a small percentage of men and children will get urinary tract infections. In men it is usually due to the prostate gland growth which usually occurs in older age men. In children it can occur 3% to 5% in girls and 1% in boys, uncircumcised boys it is more common than circumcised ones to have a urinary tract infections. In girls it may be the result of onset of toilet training. Some predispositions for getting urinary tract infection include family history and urinary tract anomalies. In neonates urinary tract infections is most common when bacteria is present.

Kidney Stones
Kidney stones, also known as renal calculi are made up of proteins and crystals that grow until they break loose and pass into the urine collection system. Small stones that are anchored in place are usually not noticed, but large stones get stuck in the calyces or pelvis and cause an obstruction in the flow of urine. When a stone breaks loose and passes through the ureters it causes steady increasing pain. The pain often becomes so intense that the patient requires narcotic drugs. The majority of kidney stones are composed of calcium oxalate crystals, but some may can also be composed of crystals of calcium phosphate, uric acid or cystine. These are normally found in the urine in a supersaturated state, from which they can crystallize for a variety of reasons. The stones can be removed surgically or broken up by a non-invasive procedure called shock-wave lithotripsy.

QUESTIONS:

1. The hormone that directly increases the re-absorption of water by the kidneys is:
a. ADH
b. aldosterone
c. PTH
d. ANP

2.While reading a blood test I notice a high level of creatinine, I could assume from this that
a. There is a possibility of a UTI
b.There is a possibility of diabetes
c. There is a possibility of kidney failure
d. There is nothing wrong, this is normal

3.Direct control of water excretion in the kidneys is controlled by
a. Anti-diuretic hormone
b. The medulla oblongata
c. Blood plasma
d. Sodium amounts in the blood

4. What type of specialized junction connects epithelial cells lining the renal tubules?
a. gap junctions
b. tight junctions
c. desmosomes
d. intercalated disks
e. slit pores

5. What are the most common infections acquired in a hospital?
a. Staphylococcus
b. Cytomegalovirus (CMV)
c. Urinary tract infections
d. both A and C
e. None of the above

6.What are some of the main differences between the male and female urinary systems?
a. Length of the Urethra
b. Distance between the openings of the Urethra and the anus
c. Type of skin lining the Urethra
d All of the above
e. a and b

7. The external urethral sphincter is....
a. Voluntary
b. involuntary
c. none existent
d. a very weak muscle

8.What is the minimum amount of urine the body needs to produce a day to be able to excrete
metabolic waste products?
a. 700 mL
b. 200 mL
c. 400 mL
d. none of the above

9. Most reabsorption of water and solutes occurs in the:
a. proximal tubule.
b. descending limb of the loop of Henle.
c. ascending limb of the loop of Henle.
d. distal tubule.
e. collecting duct.

10. The kidneys perform the following:
a. regulation of plasma ionic composition.
b. regulation of plasma volume.
c. regulation of plasma osmolarity.
d. removal of metabolic waste products.
e. all of the above

11. BUN test stands for:
a. Bones Urine Nickle
b. Blood Urine Nitrogen
c. Blood Urea Nitrogen
d. Belly Underwear Nickleback
e. Bones Urea Nitrogen

12. The function of the loop of the nephron in the process of urine formation is
a. reabsorption of water.
b. production of filtrate.
c. reabsorption of solutes.
d. secretion of solutes.

13. Which of the following materials would not normally be filtered from the blood at the
glomerulus?
a. water
b. glucose
c. sodium ions
d. urea
e. protein

14. By what process are most molecules secreted from the blood into the tubule?
a. osmosis
b. diffusion
c. active transport
d. facilitated diffusion

15. The presence of ADH (antidiuretic hormone) causes an individual to excrete
a. sugars
b. less water
c. more water
d. Both a and c

16. About 65% of the glomerular ultrafiltrate is reabsorbed in?
a. the proximal tubule
b. the distal tubule
c. the loop of henle
d. the collecting duct

17. The appearances of glucose in the urine
a. occurs normally
b. indicates the presence of kidney disease
c. occurs only when the transport carriers for glucose become saturated
d. is a result of hypoglycemia

18. Reabsorption of water through the tubules occurs by?
a. osmosis
b. active transport
c. facilitated diffusion
d. all of the above

19. After my physical my physician told me I needed to drink more water. To be a compliant patient, I drank three 16 ounce bottles. After some time I felt the urge to urinate. Thanks to my handy dandy physiology book I know that the part of the urinary bladder that actually eliminates the urine is the:
a. detrusor muscle
b. trigone
c. rugae
d. transitional epithelium

20. Creatinine is a nitrogenous waste product that comes from energy metabolism in:
a. the brain
b. the liver
c. the lungs
d. the muscles

21. If the body has too much alkaline, the kidneys will excrete more of these ions in urine:
a. calcium ions
b. bicarbonate ions
c. hydrogen ions
d. potassium ions

22. High serum levels in the urine that makes the urine dark red, or cola colored and causes muscle pain are symptoms of....
a. heart failure
b. incontinence
c. rhabdomyolosis
d. acute mountain sickness
e. Ondine's curse