The Urinary System



The Urinary System & Excretion Text Reference Pg 408 - 435

The Urinary System & Excretion

Functions of the Urinary System

1) The kidneys excrete metabolic (nitrogenous) wastes to rid the body of toxic

substances

• There are 3 metabolic wastes the kidney excretes in urine, and all three are classified as nitrogenous wastes

A) Urea

• By-product of amino acid metabolism

• Breakdown of amino acids in the liver releases ammonia, which the liver chemically combines with carbon dioxide to form urea

B) Creatinine

• Creatine phosphate is a high-energy phosphate reserve in muscle tissues when it is broken down the metabolic waste creatinine is produced

C) Uric Acid

• Results from the breakdown of nucleic acids, particularly those with thymine or adenine

• Uric acid is not water soluble and so if too much accumulates in the blood crystals form and collect in the joints, producing the painful condition called gout

2) The kidneys regulate blood pressure by controlling water-salt balance of the blood

• Blood volume is directly influenced by with the salt balance of the body

• Concentrations of NaCl in the blood influence water to move by osmosis to and from the tissues and blood vessels.

High [salts] in blood = ↑ blood volume = ↑ blood pressure

Low [salts] in the blood = ↓ blood volume = ↓ blood pressure

• The kidneys also regulate the levels of several other important ions in the blood such as potassium ions (K+), bicarbonate ions (HCO3-), and calcium ions (Ca2+)

3) The kidneys regulate the acid-base balance of the blood

• In order for a person to remain healthy, blood pH should be approx 7.4

• The kidneys monitor and help control blood pH by excreting hydrogen ions (H+) and reabsorbing bicarbonate ions (HCO3-) as needed to maintain a steady blood pH of 7.4

4) The kidneys secrete hormones and other substances that are involved with a variety

of important physiological processes.

• E.g. Erythropoietin – stimulates RBC production when oxygen levels in the blood are low

• E.g. Renin – a substance secreted by the adrenal glands that sit on top of the kidneys that stimulates the release of aldosterone, a hormone that promotes the reabsorption of Na+ ions by the kidneys

• The kidneys also helps activate vitamin D from the skin, so calcium can be absorbed from the digestive tract

Anatomy of the Urinary System

Kidneys (2)

• The kidneys are 2 bean-shaped organs that lie deep in the lumbar region of the body cavity on either side of the spine

• Averages approximately the size of a large bar of soap in an adult

• A fatty encasement attaches each kidney to the posterior body wall and cushions them from blows to the back

• Atop each kidney there is an adrenal gland – an endocrine gland that is functionally unrelated to the kidneys

Ureters (2, one for each kidney)

• Muscular tube-like organs that transport urine from the kidneys to the bladder

• Peristaltic contractions cause urine to enter the bladder in spurts every 1-5 minutes

Urinary Bladder

• Temporary storage reservoir for urine

• The bladder has folds in its mucosa called rugae (much like the stomach) that give it expansive properties

• A moderately full bladder contains approx 500 mL of urine, but it can hold more than double that if necessary

Urethra

• Tube-like organ that passes urine from the bladder to the outside of the body

• Females - only 3-4 cm long.

• Males – 20 cm long and plays a role in both the urinary and reproductive systems –it carries semen as well as urine out of the body

• There are two sphincters where the urethra exits the bladder

o Internal sphincter = located at the opening of the urethra

o External sphincter = composed of skeletal muscles that can be voluntarily controlled.

Internal Anatomy of the Kidneys

• A frontal section through a kidney reveals that it has 3 distinct regions:

1) Renal Cortex – Outer layer (dips down in between renal pyramids of renal medulla).

• The cortex is hypotonic

2) Renal Medulla – Middle region

• Consists of specialized cone-shaped tissue masses called renal pyramids

• The pyramids appear to be striped because they are formed almost entirely of parallel bundles of microscopic urine-collecting tubules

3) Renal Pelvis – Inner Region

• Is a central space, or cavity, that is continuous with a ureter

• Branching extensions of the renal pelvis called renal calyces collect urine that drains continuously from the collecting ducts of nephrons and dumps it into the renal pelvis.

• From the pelvis urine drains into the ureter so it can be transported to the bladder

Renal Blood Supply

• Because the kidneys continuously cleanse the blood and adjust its composition, it is not surprising that they receive 20-25% of total cardiac output each minute (~1.2 L / min).

• The kidneys receive unfiltered blood rich in metabolic wastes directly from the heart via the abdominal aorta which branches into the left and right renal arteries

• Each kidney is composed of over 1 million nephrons (aka renal tubules) and each nephron has its own blood supply.

• Blood enters the kidney via the renal artery that branches into an afferent arteriole which leads to a “knot” of capillaries called the glomerulus

• The efferent arteriole takes non-filterable blood components to the peritubular capillary network, a tangled network of capillaries that surrounds the rest of the nephron

• In the nephron wastes are filtered out of the blood and clean blood is returned to systemic circulation via the left and right renal veins which converge with the inferior vena cava and then the heart

Path of Blood through the Kidney:

Renal Artery (

The Nephron (Renal Tubule)

• Each kidney is made up of over 1 million structural and functional tubules called nephrons

• Nephrons carry out the processes that form urine while simultaneously removing waste products from blood plasma and aiding in maintaining water, salt, and pH balance

• There are several parts to each nephron, and each has a specialized function in processing filtrate and forming urine:

Urine Formation

• Nephrons form urine in a 3 step process.

1) Pressure Filtration (glomerular filtration)

2) Tubular Reabsorption (water, salts, ions, nutrients)

3) Tubular Secretion / Excretion (H+ ions, Drugs, excess salts)

1) Glomerular Filtration (aka Pressure Filtration)

• Each nephron is closely associated with a specialized “knot” of capillaries called the glomerulus. The capillaries which make up the glomerulus are specialized for filtration

• ~ 1.2 L of blood passes through the glomeruli each minute, and so they filter out our entire plasma volume more than 60 times per day!

• The glomerulus differs from all other capillary beds in the body in that it is both fed by an arteriole - afferent arteriole and drained by an arteriole - efferent arteriole

• Blood pressure within the glomerular capillaries forces filterable blood components (small molecules such as ions, nutrients, and water) through pores into the Bowman’s capsule (glomerular capsule)

• Because blood pressure is the driving force behind glomerular filtration, blood pressure has a direct affect on urine formation

• The Bowman’s capsule (glomerular capsule) is the beginning of a nephron. It is a cup-like structure that surrounds and receives filtrate from the glomerulus.

• Blood in the glomerulus has 2 portions:

1) Filterable Components – Continue on through the nephron as filtrate

• Small molecules such as water, glucose, amino acids, and nitrogenous wastes can be filtered from the blood into the renal tubule

2) Non-Filterable Components – Returned to circulation via the efferent arteriole.

• Large molecules such as plasma proteins, blood cells, and platelets are too large to be filtered into the renal tubule and usually stay in the blood

|Filterable Components |Non-Filterable Blood Components |

|Water |Formed Elements (blood cells & platelets) |

|Nitrogenous Wastes |Plasma Proteins |

|Nutrients | |

|Salts (NaCl, K+, Ca2+, etc.) | |

2) Tubular Reabsorption

Proximal Convoluted Tubule (PCT)

• Tubular reabsorption begins in the proximal convoluted tubule (PCT), a portion of the nephron which lies within the renal cortex

• The PCT is specialized for reabsorption - it contains many microvilli and mitochondria

• As sodium ions are pumped out of the PCT, approximately 65% of water present in the filtrate follows passively down its concentration gradient by osmosis and is absorbed back into the bloodstream via the peritubular capillary network

• 100% of glucose and amino acids are also reabsorbed from the PCT and end up back in blood plasma

• Therefore, “normal” urine should not contain any glucose or amino acids and glucose levels are the same in the renal artery as in the renal vein

|Components Reabsorbed Back Into Peritubular Capillaries from PCT |Non-Filterable Components |

| |(Continue on through the nephron) |

|Lots of Water (65% reabsorbed) |Some Water |

|Nutrients (amino acids, glucose etc.) | Nitrogenous Wastes |

|Required Salts (NaCl, K+, Ca2+, etc.) |Excess salts |

• In the PCT, filtrate can be divided into 2 portions:

Loop of Henle - Water & Sodium Reabsorption

• Additional water and NaCl is also reabsorbed from the Loop of Henle

• The hypertonicity in the medulla facilitates the reabsorption of water (by osmosis)from the descending loop (downward loop) – the ascending loop is NOT permeable to water

• The hypertonic environment of the medulla is established in a large part, by the passive and active transport of NaCl out of the ascending loop (upward loop) and also by a small amount of urea that leaks out of the collecting duct

• The table below summarizes the reabsorption values of several substances from the nephron.

|Substance |Amount Filtered / Day |Amount Excreted / Day |Reabsorption |

|Water |180 L |1.8 L |99.0 % |

|Sodium |630 g |3.2 g |99.5 % |

|Glucose |180 g |0.0 |100.0 % |

|Urea |54 g |30.0 g |44.0 % |

3) Tubular Secretion

Distal Convoluted Tubule (DCT)

• Excess H+ , K+, NaCl, nitrogenous wastes, and medications like Penicillin that are in the blood within the peritubular capillaries are actively transported from the blood into the DCT

• Most tubular secretion depends largely on the body’s needs at any given time and is regulated by hormones (more on this later)

• The kidneys maintain a normal blood pH of 7.4 excreting hydrogen ions (H+) from the DCT and producing more bicarbonate ions(HCO3-)

o If blood is too acidic: H+ is secreted from the blood into filtrate and HCO3- is absorbed

o If blood is too basic: the above does not occur

Collecting Duct

• The DCTs of several nephrons join to form one collecting duct

• For the most part, the collecting duct is impermeable (UNLESS ADH is present)

• drains the filtrate, now called urine, into calyces and the renal pelvis which is continuous with a ureter

• Peristalsis propels urine through the ureter and into the bladder

[pic]

Composition of Urine

• ~ 95% water, 5% solutes (urea, uric acid, creatinine, salts, water soluble vitamins, bicarbonate, H+, +Hetc.)

• Its yellow color comes from a pigment called urochrome which results from the breakdown of hemoglobin

• Except in cases of kidney or urinary tract infection (UTI), urine is virtually sterile and nearly odorless

• Normal urine is usually slightly acidic and has a pH of 6.

Micturition (Peeing)

• Micturition is also called voiding or urination. It is the act of emptying the bladder.

• When about 200 mL of urine accumulates in the bladder, stretch receptors send sensory nerve impulses to the spinal cord. In response to the stimulus, motor nerve impulses are sent from the spinal cord to the bladder. This causes the bladder to contract and sphincters to relax, so urination can occur.

• Contractions of the bladder become frequent and urgent and we can make the conscious decision to empty the bladder (or not) thanks to voluntary control over the external urethral sphincter.

• If you ignore the initial urge to pee, the bladder will collect another 200-300 mL and the reflex occurs again.

• Eventually, the urge to void becomes irresistible, and an individual usually gives in to the urge when the volume of urine in the bladder exceeds 500 mL.

Aldosterone “Anti-Pee Hormone”

• The hormone aldosterone which is secreted by the adrenal cortex is used to regulate the reabsorption of sodium at the distal convoluted tubule

i. Aldosterone promotes the excretion of K+ and reabsorption of Na+.

ii. Release of aldosterone is set in motion by the kidneys themselves

iii. When blood volume and thus blood pressure is not enough for glomerular filtration, the juxtaglomerular apparatus (region of contact between the afferent arteriole and DCT) secrete renin.

iv. Renin stimulates a chemical chain reaction in the body that ends up stimulating the adrenal cortex to release aldosterone.

v. Aldosterone causes reabsorption of Na+ and thus water into the blood, increasing the blood pressure.

Antidiuretic Hormone (ADH) “Anti-Pee Hormone”

• An antidiuretic is a substance that inhibits or prevents the formation of urine

• ADH prevents wide swings in water balance within the body. This helps the body avoid dehydration and water overload.

• When solutes in the blood become too concentrated (high solute concentration in the blood = low blood volume = low blood pressure) because of excessive sweating or dehydration, specialized receptors called osmoreceptors trigger the posterior pituitary gland (which stores the hormone ADH produced by the hypothalamus) to release ADH.

• ADH causes the walls of the collecting duct to become more permeable to water

Diuretics

• Diuretics are chemicals that increase the flow of urine.

• E.g. Alcohol – Inhibits the secretion of ADH, as well as water-reabsorption from the nephron which ultimately results in dehydration.

• E.g. Caffeine – Increases glomerular filtration rate and decreases tubular reabsorption of Na+

• E.g. Anti-hypertensive drugs (blood pressure medication) – Inhibit active transport of Na+ at the loop of Henle or at the DCT. Consequently water reabsorption decreases and so does blood volume and blood pressure.

-----------------------

Excretion is an essential process in all forms of life where metabolic waste products and other non-essential materials are eliminated from the body. Excretion should not be confused with defecation; defecation, the elimination of feces from the body via the digestive tract, is not an example of excretion because the undigested food and bacteria that make up feces are simply undigested food material and not metabolic wastes.

You may not have realized it, but we have already learned about one key excretory process - the formation and elimination of carbon dioxide. Recall that carbon dioxide molecules are a byproduct of cellular respiration (a metabolic process which produces ATP), which is eventually excreted or exhaled from the lungs.

Our skin and liver are also organs with important excretory functions: the skin eliminates metabolic wastes like urea and lactic acid through sweating, and the liver excretes bile pigments by way of the digestive system and produces urea from toxic ammonia waste that accumulates from the breakdown of proteins.

To summarize, there are 4 key organs from 4 different body systems that are involved with the excretion of metabolic wastes from the body.

1) Kidneys (Urinary System)

2) Lungs (Respiratory System)

3) Liver (Digestive System)

4) Skin (Integumentary System)

In this unit, we will focus our study on the urinary system and another key excretory process, the formation of urine in the kidneys.

Did You Know?

Our kidneys behave much like a water purification plant that keeps the city’s water supply drinkable and free of wastes. The kidneys pressure filter nearly 200 L of plasma and convert it to 1-1.18 L of urine daily. This provides a way to remove toxins, metabolic wastes, and excess ions from the body while returning needed substances to the blood. If our kidneys malfunction, it doesn’t take long before our body fluids become contaminated and full of toxic wastes.

Homeostatic Imbalance

Are you one of the 40% of women that suffer from urinary tract infections? Because the female’s urethra is very short and its opening lies close to the anus, improper toilet habits (wiping back to front after defecation) can easily carry fecal bacteria into the urethra. Most urinary tract infections occur in sexually active females, because intercourse drives bacteria from the vagina and external genital region toward the bladder. The use of spermicides magnifies the potential for a woman to get a bacterial urinary tract infection because it kills helpful bacteria, allowing infectious fecal bacteria such as E.coli to colonize the vagina. Because the urethra is continuous with the rest of the urinary system, infection and inflammation can quickly spread to the bladder and even the kidneys.

CORTEX

(hypotonic)

MEDULLA

(hypertonic)

The production of urine is vital to the health of the body. Most of us have probably never thought of urine as being valuable, but we could not survive if we did not produce it and eliminate it. Urine is composed of water, excess salts (electrolytes), and various nitrogenous waste products (urea, uric acid, creatinine, ammonia) that are filtered out of the blood. Remember, as the blood flows through the body, wastes resulting from the metabolism of food molecules in cells are deposited into the bloodstream, and this waste must be disposed of.

Homeostatic Imbalance

In diabetes mellitus, excess glucose occurs in the blood because liver and muscle cells have failed to store it as glycogen. The kidneys cannot reabsorb all the glucose in the filtrate (there are a limited amount of glucose carrier proteins), and so it appears in the urine. The presence of glucose in the urine increases its osmolarity, and therefore, less water is reabsorbed back into circulation. Frequent urination and increased thirst are experienced by diabetics because this water ends up staying with the filtrate and it ends up being eliminated from the body in urine.

Did You Know?

Every second approximately 125 mL of blood is pressure-filtered by the Bowman’s capsule. If the filtrate that results wasn’t treated, and water was not reabsorbed we would produce 180 L of urine per day, and lose many vital nutrients along with it. This is why the nephron has specialized regions for nutrient and water reabsorption.

Homeostatic Imbalance

On occasion, calcium, magnesium, or uric acid salts in urine crystallize and precipitate in the renal pelvis forming kidney stones. Most kidney stones are less than 5 mm and pass through the urinary tract without causing any problems. However, larger stones can obstruct a ureter and block urine drainage. This increases pressure within the kidney and causes excruciating pain. Most kidney stones can be treated using lithotripsy, a procedure which uses ultrasonic shock waves to shatter the crystals. The pulverized sand-like remnants can then be eliminated painlessly in the urine. Predisposing conditions of kidney stones include frequent bacterial infections of the urinary tract, urine retention, high levels of blood calcium, and alkaline urine.

Osmoreceptors detect increased [solute] in blood and send an impulse to the hypothalamus.

Posterior pituitary secretes ADH into the bloodstream.

More water is reabsorbed from collecting duct.

Increased blood volume and blood pressure.

Decreased urine output (water retention).

Decreased [solute] in blood.

Osmoreceptors stop sending impulses to the hypothalamus.

ADH release is stopped.

Water reabsorption decreases.

Negative Feedback Loop

Homeostatic Imbalance

One result of ADH deficiency is diabetes insipidus. People who suffer from this condition experience the output of huge amounts of urine and intense thirst. Diabetes insipidus can be caused by a blow to the head that damages the hypothalamus or the posterior pituitary, the source gland for ADH.

Homeostatic Imbalance

Patients with renal failure must have wastes and excess ions filtered from their blood using a dialysis machine at least once or twice a week. A dialysis machine is designed to remove wastes from the blood when the concentration of wastes and salts gets too high in the blood or when the blood gets too acidic from the accumulation of H+. During hemodialysis a patient is hooked up to a machine where their blood flows through a system of tubes composed of semi-permeable membranes. Dialysis fluid (diastylate) has a composition and temperature similar to blood except that the concentration of wastes within it is very low. The diastylate runs through the machine in a counter-current (opposite) direction to the patient’s blood flow so that fresh diastylate can maintain favourable concentration gradients that will promote the diffusion of unwanted substances out of the patient’s blood. In this way diastylate is used to extract unwanted toxic substances from the blood, or to add substances to the blood such as bicarbonate ions if the blood is too acidic.

h™(ohù'«5?h™(oh hå5?hœk[?]RH– jh™(o INCLUDEPICTURE "" \* MERGEFORMATINET [pic]

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download