THE URINARY SYSTEMMODULE 15

THE URINARY SYSTEM

• The main excretory system of the body.

• A continuous filtration system that maintains homeostasis by correcting imbalances in the composition of the blood.

• The primary means by which the body: 1. gets rid of toxins

2. controls the level of ions in the body

3. controls the amount of water in the body

4. controls the pH of the blood

5. controls red blood cell production

• Stores its waste, the urine, in a sac called the urinary bladder, until it can be emptied voluntarily.

ANATOMY OF THE URINARY SYSTEM

• The main organs of the urinary system are the kidneys.

• There are two kidneys, but the right kidney is lower than the left.

• Each kidney has a ureter that takes urine out of the kidney and into the urinary bladder.

• As urine collects, the urinary bladder can stretch because it is made of smooth muscle and its mucosa is made of stretchy stratified transitional epithelium.

• The urinary bladder empties through the urethra which leads outside the body.

THE PERITONEAL CAVITY

• The large organ-filled cavity anterior to the kidneys.

• Its outer membranous boundary is the parietal peritoneum . It is composed of a serous membrane that secretes a small amount of fluid into the cavity.

• Most of the abdominal organs (liver, spleen, pancreas and stomach) are housed in the peritoneal cavity.

• These organs are encased in another serous membrane called the visceral peritoneum. These two membranes lubricate the organs to reduce friction when they move.

THE KIDNEYS

• The kidneys lie behind the peritoneal cavity. As a result, they are called retroperitoneal organs.

• Unlike most of the abdominal organs that are housed within the peritoneal membranes, the kidneys do not move much when the body moves.

• Instead, they are held in place by a layer of adipose tissue called perirenal fat.

• If the kidneys moved when your body moves, the kidney’s ureters might get pinched, blocking the flow of urine into the bladder.

KIDNEY ANATOMY

• Your kidneys are bean-shaped and roughly the size of a clenched fist.

• Each kidney is surrounded by a renal capsule, which is composed of fibrous connective tissue.

• The interior is split into two regions: the outer region, called the cortex, and the inner region, called the medulla.

KIDNEY ANATOMY

• The medulla is composed of cone-shaped structures called renal pyramids.

• The bases of these pyramids form the boundary between the medulla and the cortex.

• The tips of the pyramids are called renal papillae. Urine formed here flows through minor calyces, major calyces, the renal pelvis, then the ureter.

THE NEPHRON

• The functional unit of the kidney is the nephron. There are about 1.3 million nephrons in each kidney.

• Nephrons extend into both the medulla and the cortex.

• The renal corpuscle is the beginning of the nephron in the cortex. It is a filter bed consisting of :• A capsule, called the Bowman’s capsule

• A network of capillaries called the glomerulus

FUNCTIONS OF THE URINARY SYSTEM

1. Urine formation

2. pH control

3. Blood pressure regulation

4. Stimulation of erythropoiesis

5. Activation of Vitamin D

6. Transport of urine

7. Storage and release of urine

URINE FORMATION: OVERALL SCHEME

• Desk-drawer analogy1. Leave stuff in drawer, pick out junk. OR

2. Dump out drawer, put back good stuff.

• That’s how kidneys clean the blood: Plasma, minus its proteins, is dumped into the nephron with the good and bad stuff.

• Then, the “good stuff” is pulled back into the blood vessel. The rest becomes urine.

• This desk-drawer approach to cleaning the blood takes place in four basic steps.

FOUR STEPS OF URINE FORMATION

1. Filtration

2. Reabsorption

3. Secretion

4. Water reabsorption

FILTRATION

• occurs within the renal corpuscle.

• Fluid leaves the capillaries, called the glomerulus, and passes through a filtration membrane to enter the lumen of the Bowman’s capsule.

• The filtration membrane is porous, but it keeps blood cells and proteins in the blood vessels so only the blood fluid and its nonprotein contents enter the nephron.

• This fluid is called filtrate, and it is like blood plasma, but without the proteins.

• The filtrate is now in the proximal tubule but is not yet urine.

REABSORPTION

• Reabsorption means moving molecules out of the nephrons and back into the blood.

• As blood passes through the many blood capillaries that run along the nephron, the useful molecules that blood needs are reabsorbed across the wall of the nephron.

• Reabsorption is controlled by a series of complex transport processes that ensure that useful molecules are reabsorbed to their proper levels.• E.g. Sodium and water

SECRETION

• Certain chemicals still in the blood must be removed and put into the nephron to be excreted in the urine.

• Harmful or excess molecules get secreted into the nephron.

• Compared to the amount of reabsorption that takes place in the kidneys, the amount of secretion is minimal.

• Nevertheless, secretion is vitally important• E.g. The kidneys can adjust the blood pH by secreting H+ ions if the

blood is too acidic.

WATER REABSORPTION

• In principle, water reabsorption could be grouped as a part of step 2, reabsorption.

• It is considered here as a separate step because of its complexity.

• The processes that occur in the nephron regulate the water volume in the body.

• The water enters the nephron, and then the blood reabsorbs exactly the amount of water it needs and leaves the rest.

• The remaining water is the principal component of urine.

RENAL BLOOD FLOW RATE

• An average adult has about 5 liters of blood, and that entire volume passes through the heart every minute.

• The blood flow to the kidneys is about 20% of that, so about one liter of blood flows through the kidneys every minute.

• The nephrons in the kidneys are performing this four-step process on about one liter of blood each minute, which equals about 1,400 liters of blood every day.

URINE FORMATION, STEP 1: GLOMERULAR FILTRATION• Filtration of the blood occurs in the glomerulus, a network

of capillaries within the renal corpuscle, so it is called glomerular filtration.

• The pores in the glomerular capillaries are called fenestraeand make them more porous than most other capillaries.

• The Bowman’s capsule is composed of a solid outer layer, the parietal layer and a porous inner layer, the visceral layer.

• The visceral layer is composed of podocytes, which are specialized cells that attach to the glomerular capillaries by means of small processes called foot processes.

THE FILTRATION MEMBRANE

• To leave the capillaries, the filtrate must pass through the filtration membrane.

• The filtration membrane is composed of the porous wall of the glomerular capillaries, the basement membrane of the glomerular capillaries, and the podocytes.

• Since the capillaries are so porous, they need to be held together. The podocytes act like netting to hold things together so that the filtration membrane can work.

• The filtration membrane keeps the proteins from leaving the capillaries because the proteins are too big to pass through the fenestrae.

GLOMERULAR FILTRATION RATE (GFR)

• The filtrate that leaves the Bowman’s capsule and enters the proximal tubule is blood plasma, minus the plasma proteins.

• The kidneys produce about 125 mL of filtrate every minute. This is called the glomerular filtration rate (GFR).

• The reason that the kidneys filter so much fluid so fast is that:

1. The filtration membrane is highly permeable.

2. There is relatively high blood pressure within the glomerular capillaries.

GLOMERULAR CAPILLARY PRESSURE (GCP)

• The pressure in glomerular capillaries is higher than that of normal capillaries (50 mmHg vs 25 mmHg).

• The efferent arteriole is narrower than the afferent arteriole, causing a backup of blood and increasing the pressure of the blood behind it.

• This increased glomerular capillary pressure (GCP) allows the glomerular filtration rate (GFR) to be high, because pressure is continually pushing filtrate out of the glomerulus.

• If the GCP goes down, the GFR will go down and vice versa.

FACTORS THAT AFFECT GCP

• The overall blood pressure in the body affects GCP. In addition, there are two factors that oppose the GCP:

1. Capsular pressure• The parietal layer of the Bowman’s capsule pushes back against

the glomerulus. It exerts about 10 mmHg against the GCP.

2. Colloid osmotic pressure• The water in the filtrate is attracted back to the higher

concentration of proteins in the capillaries. It measures about 30 mmHg.

• We need to worry about GCP because if the GCP decreases too much, the GFR will drop to zero. This results in renal shutdown, where no urine will be produced and the kidneys cannot clean the blood.

URINE FORMATION, STEP 2: REABSORPTION

• Once the filtrate enters the proximal tubule, the next step of urine formation, reabsorption, occurs.

• The blood must reabsorb the useful molecules and leave the excess or waste molecules in the filtrate, to be disposed as part of the urine.

• The efferent arteriole leaves the glomerulus and then forms capillaries that wrap around the tubules of the nephron. These are called peritubular capillaries.

• As the filtrate passes through the tubules of the nephron, substances that the blood needs pass across the wall of the tubule and back into the capillaries.

REABSORPTION EFFICIENCY

• To make reabsorption efficient, the walls of the nephron tubes are thin.

• In the proximal tubule, the wall is composed of simple cuboidal epithelium to make room for cellular machinery to facilitate reabsorption.

• The walls of the nephron tubules have a lot of surface area to increase the rate of reabsorption.

• To accomplish this, cells have a “brush border” on the inside wall of the tubule.

REABSORPTION PATHWAY

• Most of the reabsorption occurs immediately, in the proximal tubule (65% of water and 100% of glucose).

• In addition to the proximal tubule, some nephrons have a loop of Henle, which is composed of simple squamousepithelium.

• All nephrons have a distal tube, which is composed of simple cuboidal epithelium.

• Once the filtrate passes through the tubules, it enters the collecting duct, which is made of simple columnarepithelium.

• When it reaches the end of the collecting duct, the final product is called urine.

ACTIVE REABSORPTION

• Active reabsorption requires ATP and a carrier.

• All of the nutrients and minerals that your body needs are actively reabsorbed.

• Thus, glucose, amino acids, water-soluble vitamins (Vit C and B vitamins), and minerals (sodium ion, potassium ion, calcium ion, etc.) are all actively reabsorbed.

• These must have a carrier that takes them across the nephron epithelium, and the epithelial cells must expend energy to make that happen.

• Proteins that leak through the filtration barrier are too big for carrier molecules, so are reabsorbed by pinocytosis.

TUBULAR MAXIMUM (T-MAX)

• Substances that are actively reabsorbed via a carrier have a tubular maximum (T-max) that limits how much of it can be reabsorbed in the nephron.

• If a substance must be actively transported but cannot find a carrier, it is automatically a waste product because it cannot get back into the blood.

• If the T-max of a substance is low, the kidney is designed to limit the amount of that substance in the blood. If the T-max is high, a high amount of that substance will be allowed in the blood.

PASSIVE REABSORPTION

• Does not require ATP.

• Water is reabsorbed this way. Water needs no carrier because it is a tiny molecule. Also, osmotic pressure attracts the water back into the peritubular capillaries.

• Urea is a waste product, but due to its small size it diffuses back into the peritubular capillaries along its concentration gradient.

• Chloride ions are another example of passive reabsorption. As positive sodium ions are actively reabsorbed into the peritubular capillaries, they attract negative chloride ions, which “hitch a ride” back into the blood.

URINE FORMATION, STEP 3: SECRETION

• Waste products that did not get filtered are specifically sent into the nephron.

• Secretion happens mostly in the distal tubule.

• Some drugs, when broken down by the liver, can be secreted into the nephron.

• The two major substances that are secreted are potassium ions (K+) and hydrogen ions (H+).

• Secretion of H+ is a powerful control of pH. More H+ makes a solution more acidic, while less H+ makes a solution more basic.

• If the pH of the blood in the peritubular capillaries starts to get too low, H+ is secreted into the nephron.

URINE FORMATION, STEP 4: REABSORPTION OF WATER

• The kidneys must regulate how much water is reabsorbed and how much is released as urine.

• The interstitial fluid deep in the kidneys is much more concentrated than normal interstitial fluid.

• Typical interstitial fluid has a solute concentration of 300 mOsm/kg. Blood plasma and interstitial fluid from the kidney’s cortex also have a concentration of about 300 mOsm/kg.

• As you travel down into the medulla, the concentration of solutes in the interstitial fluid increases to 1200 mOsm/kg, mainly because of urea.

LOOP OF HENLE

• Only about 10% of nephrons contain loops of Henle. These are the nephrons that concentrate urine through reabsorption of water.

• The renal corpuscle and the proximal tubule of the nephron are in the cortex. The loop of Henle goes down deep into the medulla where the concentration of solutes is 1200 mOsm/kg and then it rises again to the cortex.

• The collecting duct then goes all the way down into the medulla and to a minor calyx.

• The concentration of solutes increases steadily from the cortex to the medulla.

ANTIDIURETIC HORMONE (ADH)

• A hormone produced by the posterior pituitary and controlled by the hypothalamus.

• Enables the body to conserve or get rid of water, depending on the body’s needs.

• When the blood contains too much water, ADH production is limited, and lots of urine is produced.

• When the body is low on water, the amount of ADH released increases, and less urine is produced.

• ADH makes the distal tubule and collecting duct permeable to water.

STORAGE AND RELEASE OF URINE

• Storage of urine happens in the urinary bladder.

• The mucosa of the bladder is made of transitional stratified epithelium so it can stretch without tearing.

• You have control over when you urinate because there is a ring of skeletal muscle around the external end of the urethra called the external urinary sphincter.

• If nervous system damage occurs, you can lose control of the external urinary sphincter, causing incontinence.

BLOOD PRESSURE CONTROL

• Since the kidneys determine the amount of water in the blood, they affect the blood pressure.

• A drop in blood pressure or a decrease in blood sodium ion levels can be detected in the afferent arteriole by the juxtaglomerular cells.

BLOOD PRESSURE CONTROL• When the juxtaglomerular cells detect a decrease in blood

pressure or sodium, they respond by releasing the enzyme renin.

• Renin activates angiotensinogen into angiotensin I, which is further activated into angiotensin II in the lungs.

• Angiotensin II has four functions:1. Increases vasoconstriction2. Increases thirst3. Increases salt appetite4. Causes the release of aldosterone

• If the blood pressure is too high, the atria produce atrial natriuretic hormone, which inhibits sodium reabsorption in the nephrons.

ACID-BASE BALANCE IN THE BODY

• The normal pH range for blood is 7.35 to 7.45 and is under tight control.

• If the pH of the blood falls below 7.35, it is called acidosis.

• If the pH of the blood rises above 7.45, it is called alkalosis.

• Prolonged gastric vomiting can lead to alkalosis due to loss of stomach acid.

• Diarrhea can lead to acidosis due to loss of bicarbonate from the intestine.

• Kidney dysfunction and respiratory dysfunction can also lead to pH imbalances in the blood.

CONTROL OF ACID-BASE BALANCE

There are three ways that the body controls acid-base balance:

1. The respiratory system- release CO2

2. The kidneys- secrete H+ into the filtrate

3. Buffer systems- a mixture of an acid and a base that resists changes in pH.

BUFFER SYSTEMS

• A buffer solution contains a weak acid and a weak base.

• pH is a measure of the amount of free H+ in solution.

• If acid is added, the weak base neutralizes the acid before it can lower the pH much.

• If a base is added, the weak acid neutralizes it before it can raise the pH much.

• A buffer resists changes in pH, but it cannot prevent them.

• Buffers act quickly, but they are the least effective means of acid-base regulation in the body.

BUFFER SYSTEMSThere are three buffer systems in the body:

1. The bicarbonate buffer system- composed of carbonic acid (H2CO3) and the bicarbonate ion (HCO3

-).• Found in the extracellular fluids.

2. The phosphate buffer system- composed of dihydrogen phosphate (H2PO4

-) and monohydrogen phosphate (HPO42-)

• Found inside cells and in the nephron tubules.

3. The Protein buffer system- proteins are single-molecule buffers because they are composed of amino acids. The amino group is a weak base while the acid group is a weak acid.• Found inside cells and the blood plasma.

THE BICARBONATE BUFFER