Urinary System

 

1.          List the functions of the kidneys.

The kidneys maintain the purity and constancy of fluids in our internal environment. They filter gallons of fluid from the bloodstream, ridding the body of metabolic wastes, toxins, and excess ions, while returning needed substances to the blood.

 

Major Functions:

a.         Eliminating nitrogenous wastes, toxins, and drugs from the body.

b.         Regulating the volume and chemical makeup of the blood.

c.         Maintaining the balance between water and salts and between acids and bases.

 

Other Functions:

a.         Producing renin to help regulate blood pressure and kidney function.

b.         Producing the hormone erythropoietin (stimulates red blood cell production).

c.         Metabolizing vitamin D to its active form.

 

2.         List the organs of the urinary system.

kidneys, ureters, urinary bladder, urethra

 

3.         Describe the kidney structure and how urine flows from kidney to outside the body.

Kidneys are bean-shaped and lie in a retroperitoneal position in the superior lumbar region. They receive some protection from the lower rib cage. The right kidney is crowded by the liver and lies slightly lower than the left. The medial surface contains a cleft (hilus). The ureters, renal blood vessels, lymphatics, and nerves enter or exit the kidney at the hilus.

 

Three layers of supportive tissue surrounds each kidney:

a.         transparent renal capsule - closest, provides a strong barrier that prevents infection from surrounding regions from spreading to the kidneys.

b.         adipose capsule - middle layer, fatty tissue which helps protect kidneys and hold them in place.

c.         renal fascia - dense fibrous connective tissue, anchors kidneys to surrounding structures.

 

Internal anatomy:

a.         cortex - outer region, contains the nephrons

b.         medulla - middle region, contains the renal pyramids

c.         pelvis - flat, funnel-shaped tube continuous with the ureter, branching sections form the major calyces, each of which subdivides further

 

The calyces collect urine, which drains from the papillae (tips of renal pyramids) and empties into the pelvis. Urine then flows through the pelvis and into the ureter which transports it to the bladder to be stored. The walls of the calyces, pelvis, and ureter contract rhythmically and propel the urine along by peristalsis.

 

4.         Describe the structure and general function of the nephron.

Each kidney contains over one million tiny blood processing units called nephrons which carry out the process of urine formation.

 

Parts of the nephron and processes that occur:

a.         glomerulus - tuft of capillaries; filtration

b.         Bowman's capsule - enlarged, cup-shaped capsule which surrounds the glomerulus; collects filtrate

c.         proximal convoluted tubule - tubular reabsorption

d.         loop of Henle - sodium and water balance

e.         distal convoluted tubule - tubular secretion

 

Each region of the tubule (proximal convoluted, loop of Henle, and distal convoluted) has a special filtrate-processing function as listed above.

 

5.         Describe the general process of urine formation.

The filtrate reaching the distal convoluted tubule empties into the collecting tubules, each of which receives filtrate from many nephrons. The collecting tubules run through the medullary pyramids. As the collecting tubules approach the renal pelvis, they fuse to form larger papillary ducts which deliver urine into the minor calyces via the papillae of the pyramids. The kidneys process about 47 gallons of fluid daily. Of this amount, only about 1.5 liters actually leaves the body as urine. The process of urine formation and adjustment of blood composition involves three processes: glomerular filtration, tubular reabsorption, and tubular secretion.

 

6.         Describe glomerular filtration. (fenestrated capillaries, net filtration pressure, three factors of GFR and which is most important, regulation of GFR by macula densa cells)

Glomerular filtration is the beginning of urine formation. It is a passive, non-selective process in which fluids and solutes are forced through a membrane by hydrostatic pressure. The glomerulus has special characteristics which make it an efficient filter: (1) fenestrated capillaries and very permeable to water and solutes (which allows free passage of everything except blood cells and plasma proteins) and (2) the glomerular blood pressure is higher resulting in a higher filtration pressure.

 

Net Filtration Pressure is the pressure that favors the forming of renal filtrate from plasma. Its numerical value is the result of opposing forces - those that would force fluid into the Bowman's capsule against those that would force fluid back into the glomerulus. The glomerular hydrostatic pressure (55 mm Hg) is the chief force pushing water and solutes out of the glomerulus into the capsule. It is opposed by forces that drive fluids back into the glomerulus: (1) glomerular osmotic presssure (30 mm Hg) and (2) capsular hydrostatic pressure (15 mm Hg). Thus the net filtration pressure is 10 mm Hg.

 

Glomerular Filtration Rate - GFR

GFR is the amount of fluid filtered from the blood into the capsule each minute. Factors governing the filtration rate at the capillary beds are:

            (1)        total surface area available for filtration

            (2)        filtration membrane permeability

            (3)        net filtration pressure

 

GFR is directly proportional to net filtration pressure because the capillaries are exceptionally permeable and have a huge surface area. So therefore, net filtration pressure is the limiting factor. Normal GFR in both kidneys in adults is approximately 120 ml/min. A change in any of the pressures acting at the filtration membrane changes the NFP and thus the GFR. An increase in arterial blood pressure in the kidneys increases GFR, whereas dehydration inhibits filtrate formation.

 

Regulation of Glomerular Filtration:

The kidney has an autoregulatory system that can maintain a constant GFR despite some fluctuations in systemic arterial blood pressure. The proper reabsorption of water and solutes depends upon the rate at which filtrate flows through the renal tubules.

 

When massive amounts of filtrate are formed and flow is very rapid, needed substances are not adequately reabsorbed and are lost. When filtrate is scant and flows very slowly, nearly all of it is reabsorbed, including most of the wastes. For optimal processing, the diamter of the afferent arterioles are regulated.

 

Special cells called macula densa cells are located in the walls of the distal convoluted tubules. When filtrate is flowing too slowly, these cells stimulate vasodilation of the afferent arterioles. When filtrate is flowing too rapidly, these cells stimulate vasoconstriction, hindering blood flow into the glomerulus, and decreasing GFR.

 

7.         Describe tubular reabsorption. (differences of filtrate and urine, process of reabsorption, what is reabsorbed, active sodium cotransport, passive diffusion of water by osmosis and other solutes via solvent drag)

Filtrate and urine are different. Filtrate is formed as soon as the water and its solutes leave the glomerular capillary and enter the Bowman's capsule. Filtrate contains everything that blood plasma does except blood proteins; but by the time filtrate has moved into the collecting ducts, it has lost most of its water, nutrients, and essential ions. When filtrate exits the collecting ducts into the calyces, it is called urine. What remains consists mostly of metabolic waste and unneeded substances.

 

Most of the tubule contents are quickly reclaimed and returned to the blood in a process called tubular reabsorption. This primarily happens in the proximal convoluted tubule. In healthy kidneys virtually all organic nutrients such as glucose and amino acids are completely reabsorbed.

 

Active Tubular Reabsorption: Substances reclaimed by active reabsorption are usually moving against a concentration gradient. Substances actively reabsorbed include glucose, amino acids,  vitamins, and most ions. These substances are usually transported together with sodium on carrier molecules. Sodium ions are abundant in the filtrate and the bulk of the energy used for active transport is devoted to their reabsorption. When the carrier molecules are saturated, excesses of that substance are excreted in the urine. This happens in hyperglycemic individuals when they have a high blood glucose level.

 

Passive Tubular Reabsorption: Sodium movement (by active transport) establishes a strong osmotic gradient, and water moves by osmosis out of the filtrate (to go back into the blood). As water leaves the tubules, the relative concentration of substances still present in the filtrate increases dramatically, and they too begin to follow their concentration gradients out of the tubule. This is called solvent drag. Aldosterone stimulates the active transport of sodium out of the tubule back into the blood and thus secondarily increases water reabsorption as well. Renin (produced by the kidney) leads to production of angiotensin which stimulates aldosterone secretion which leads to water and sodium reabsorbing as described. This increases blood volume and blood pressure. Some waste substances are not reabsorbed or are incompletely reabsorbed, such as nitrogenous end products of protein and nucleic acid metabolism (urea, creatine, uric acid).

 

8.         Describe the process of tubular secretion.

Primarily occurs in the distal convoluted tubule. Tubular secretion is important for:

a.         disposing of substances not already in the filtrate (drugs)

b.         eliminating undesirable substances that have been reabsorbed by passive processes (urea and uric acid)

c.         ridding the body of excess potassium ions

d.         controlling pH (if too low, secrete hydrogen ions)

 

9.         Describe the loop of Henle. (permeability characteristics of descending and ascending)

The loop of Henle occurs between the proximal and distal convoluted tubules and is where sodium and water are selectively reabsorbed in order to keep the solute load of body fluids constant (at the right osmolarity). This is done by regulating urine concentration and volume.

 

The descending part of the loop is impermeable to sodium and freely permeable to water. In the tissue fluid outside of the descending loop, the sodium concentration gradually increases. This sodium outside of the tubule causes water to leave the tubule by osmosis. As the water leaves the tubule, the sodium concentration in the tubule increases. The ascending part of the loop is permeable to sodium and impermeable to water. Now sodium leaves the tubule by diffusion and the contents of the tubule becomes more dilute.

 

10.        Describe the formation of dilute and concentrated urine.

Dilute: Tubular filtrate is diluted while traveling through the ascending part of the loop of Henle, so all the kidney needs to do to secrete dilute urine is to allow the filtrate to continue on its course to the reanl pelvis. When ADH is not being released, that is what happens - the distal and collecting tubules remain impermeable to water and no further water reabsorption occurs.

 

Concentrated: ADH does this by causing the distal and collecting tubules to be permeable to water, and water passes into the interstitial spaces and is reabsorbed into the blood, increasing blood volume. ADH is released more or less continuously, but its release is enhanced with excessive water loss or reduced blood volume or pressure.

 

Diuretics: Diuretic drugs are used for hypertension or the edema of congestive heart failure. They increase urine flow by inhibiting sodium ion reabsorption, thus inhibiting water reabsorption, since water follows sodium.

 

11.        Describe the structure and function of the ureter, bladder, and urethra.

Ureters - slender tubes that convey urine from the kidneys to the bladder. During bladder filling or emptying the bladder compresses and closes the distal ends of the ureters. The ureters play an active role in urine transport. As the urine enters the renal pelvis, peristaltic waves initiated there force urine into the ureter, distending it. Distention of the ureter stimulates its contraction, which propels urine into the bladder.

 

Bladder - smooth, collapsible, muscular sac located retroperitoneally on the pelvic floor. Has openings for ureters and the urethra. Very distensible due to transitional epithelium.

 

Urethra - thin-walled muscular tube that drains urine from the bladder floor and conveys it out of the body.