The urinary system

This process will produce waste products

- If accumulate in the cells, these products could reach toxic concentration and threaten homeostasis
- These waste products must be excreted from the cells
- What is excretion? Is it different from elimination?
– Excretion: Removal of metabolic wastes
– Elimination: the discharge of undigested or unabsorbed food from the digestive tract


Learning objectives:
– Identify the principal metabolic waste product and the organs that excrete them
– Label a diagram of the urinary system and give the function of each structure
– Describe a nephron and give the functions of the following structures:
Bowman’s capsules
Glomerulus
Renal tubules
Collecting ducts
Afferent arterioles
Efferent arterioles
– Label a diagram of a nephron
– Trace a drop of filtrate from glomerulus to urethra, listing in sequence each structure through which it passes
– Describe the process of urin formation and give the composition of urine
– Summarize the regulation of urine volume including the role of ADH
– Summarize the functions of the kidney in maintaining homeostasis
– Describe the process of urination

Introductions

In order to live properly, cells of our body carry on metabolic activities
This process will produce waste products
If accumulate in the cells, these products could reach toxic concentration and threaten homeostasis
These waste products must be excreted from the cells
What is excretion? Is it different from elimination?
– Excretion: Removal of metabolic wastes
– Elimination: the discharge of undigested or unabsorbed food from the digestive tract
The principal of metabolic wastes:
– Water
– Carbon dioxide
– Wastes that contain nitrogen
Amino acids and nucleic acids contain nitrogen
If excess amino acids are broken down, the nitrogen-containing amino group is removed
The amino group is chemically converted to amonia and then to urea.
Nucleic acid is broken down to uric acids
Both urea and uric acids are transported from the liver to the kidney by the circulatory system
Metabolic wastes are excreted mainly by urinary system
Other systems involved in the disposal of metabolic wastes are:
– The skin
Via the sweat glands
Only 5-10% secreted through the skin
Sweat contains: the same substances as urine (water, salt, and nitrogen wastes) but much more dilute
– The lungs
CO2 and water (in the form of water vapor)
– Digestive system
Liver secretes bile pigments (leave the body with the feses)

The urinary system

The principal of the urinary system consists of:
– Kidney
A pair of kidneys
Remove waste from the blood
Produce urine
– Urinary bladder
The site storage for urine (temporary)
– Ureters
Conduct urine from the kidneys to the urinary bladder
– Urethra
Discharge urine from the body
This is a single urethra

Kidneys

There are two of them
Located behine the peritoneum lining the abdominal cavity (described as retroperitoneal)
They are located exactly near the posterior body just below the diapgragm
They are protected by the lower ribs
Each kidney receives blood from a renal artery and is drained by a renal vein
Their shape is like a large, dark red lima bean (about the size of a fist)
The kidneys are connected with the blood vessels and urethra at the hilus
They are covered by a strong capsule of connective tissue called renal capsule
The kidney consists of:
Outer renal cortex
Inner renal medula
Contais between 5 – 18 renal pyramids (triangular structures)
The tip of each pyramid is called a renal papilla
Each renal papilla extend into a small tube called a calyx
Each papilla has pores (the openings of collecting ducts through which urine passes into the calyx)
These calyces join to form a large cavity called renal pelvis
As urine produced, it flows through the renal pelvis and into the ureter

The Nephrons

These are microscopic units of Kidney
Each kidney contains > 1 million nephrons
These structures filter the blood and produce urine
The main structure of each nephron:
– Renal corpuscle
– Renal tubule
– Renal corpuscle
The site of blood filtration
Consists of glomerulus (a network of capillaries)
This glomerulus is surrounded by Bowman’s capsule (cuplike structure) (Figure 16.4)
The blood flows into the glomerulus through a small afferent arteriole and leave it through efferent arteriole
This arteriole conduct blood to a second set of capillaries called peritubular capillaries
Bowman’s capsule has an opening in its bottom
This opening is the way through which the filtrate passes into the renal tubule
– Renal tubule
Tube through which the filtrate flows
Along this long tube, substances needed by the body are returned to the blood
Wastes, excess water, and others not needed by the body pass into the collecting duct as urine
The first part of the renal tubule is called proximal convulated tubule
This continous with loop of Henle
This loop continous with distal convulated tubule
Urine from this distal convulated tubule of several nephron drain into a collecting duct
Part of distal convulated tubule curves upward and contact the afferent arteriole
The cells that make this contact form juxtaglomerular apparatus (Fig. 16.5)
This structure secretes enzyme renin (activator of a hormone called angiotensin that regulate blood pressure)
Note that:
– The renal corpuscle, the proximal convoluted tubule, and distal covoluted tubule of each nephron are located within the renal cortex
– Loop of Henle dip down into the medulla

The process of urine production

This process involves three processes:
– Glomerular filtration
– Tubular reabsorption
– Tubular secretion

Glomerular filtration

This is the first step of urine production
The process itself:
– The diameter of afferent arteriole is larger than that of efferent arteriole
– Resulting in the volume of blood enters the glomerulus is more rapidly than it can leave
– This cause the blood pressure in the glomerular capillaries become higher than other capillaries
– This pressure will force plasma and dissolved substances out of the capillaries and into the Bowman’s capsule
– Therefore, the filtrate consists of blood plasma that concains small dissolved molecules
– This can be considered that the glomerular filtration is not a selective process
– Some useful substances present in the filtrate:
Glucose
Amino acids
Salts
– Blood cells and proteins are too large to pass through the wall of the capillaries and the capsule
– If those two components present in the urine, it means that a problem occurs with the glomerular filtration

Rate of filtration

Each minutes, 25% of the cardiac output is delivered to the kidneys
So, every 4 minutes, the kidney receives a volume of blood equal to the total volume of blood circulates in the body
Or this is equal to 180 L of filtrate produced per day
Common senses suggests that we could not excrete 180 L urine per day
If we were loosing fluid that quickly, dehydration would become a life threatening problem within a few moment

Tubular reabsorption

About 99% of the filtrate will be returned to the blood by tubular reabsorption
This leaves only 1.5 L to be excreted as urine within 24 hours period
This process is the job of the renal tubules and collecting duct
This process will prevent dehydration to occur
This process is a highly selective process
– Waste, surplus salts, and excess water are kept as part of the filtrate
– Glucose, amino acids, and other needed substances are reabsorbed into the blood
A few substances are actively secreted from the blood in the peritubular capillaries into the renal tubules
This tubular secretion is important in regulating the potassium and hydrogen ion concentration in the blood
Some toxic substances and certain drugs (penicillin) are remove from the body by this tubular secretion


Composition of urine

The composition of filtrate that reaches the ureter has been carefully adjusted
– The useful materials have been returned to the blood while waste and excess materials have been cleared from the blood
The adjusted filtrate is then called urine.
The urine composed of:
– App. 96% water
– 2.5% nitrogen waste (mainly urea)
– 1.5% salts
– Traces of other substances
Healthy urine is sterile
However, it rapidly decomposes when exposed to bacterial action, forming ammonia and other product
The ammonia causes diaper rush in infants

The role of ADH to regulate the urine volume

The volume of urine produced by the kidney is proportionally related to the amount of water intake
In this case:
– Water is absorbed from the digestive tract into the blood
– Excess water is removed from the blood by the kidneys
– By regulating urine volume, the body maintains a steady volume and composition of blood
– The kidneys receive information about the state of the blood indirectly (Fig. 16.7)
In the case of low fluid intake, the body begins to dehydrate
This will causethe volume of blood decrease, and the conc. of dissolved salt is greater
This results in an increase in the osmotic pressure of the blood
This changes will cause the specialized receptors in the brain, heart, and in certain blood vessels to respond
Besides that, the posterior of pituitary glands responds by releasing antidiuretic hormone (ADH)
ADH serve as chemical messenger carrying information from the brain to the distal convoluted tubules and collecting ducts of the kidney
The effect of this signal is to cause the wall of ducts to become more permeable to water.
Therefore, more water is reabsorbed into the blood
This will cause the volume of the blood to increase and the homeostasis of fluid volume is restored
Only a small amount of concentrated urine is produced

– In the case of large amount of water intake
The blood become dilluted and its osmotic pressure falls
Release of ADH by the pituitary gland decreases
This reduces the amount of water reabsorbed from the distal convoluted tubules and collecting ducts
As a result, a large volume of dilluted urine is produced

Possible dissorders:
– Not sufficient of ADH produced
Water is not efficiently reabsorbed from the ducts
This results in large volume of urine produced
This condition is called diabetes insipidus
A person with this dissorder may excrete up to 25 quarts of urine per day
He/she must drink almost continually to offset this fluid loss
– It is clear that ADH regulates excretion of water by the kidney
– Salts excretion is regulated by hormones (mainly aldosteron produced by the adrenal glands

Diuretics

These are chemicals compounds contained in coffee, tea, alcoholic beverage
These can increase the volume of urine by inhibiting the reabsorption of water
Some inhihit secretion of ADH
Some act directly on the tubules in the kidneys

The role of kidneys in maintaing homeostasis

Their functions include:
– Excretion of metabolic wastes, such as water, urea, and uric acid
– Disposal of excess water and salts
– Regulation of acid-base (pH level of the blood and body fluids. Acid and base that are not needed are excreted in the urine
– Secretion of regulatory substances
The kidney secretes the enzyme renin
This enzyme is important in regulating blood pressure
Kidney also secretes a hormone (erythropoietin) that regulates the production of blood cells

Transport of urine to the bladder

Urine passes from kidneys through the paired ureter
– These are ducts about 25 cm long
– Conduct the kidneys to the urinary bladder
The urine is forced along through the ureter by peristaltic contraction
Once reach the bladder, it is temporary stored there
Urinary Bladder:
– It is lined with a mucous membrane
– The membrane has folds called rugae
– With this rugae, the bladder has the capacity of 800 ml of urine
The urine leaves the body through the urethra
Urethra:
– A duct leading to the outside of the body
– In male, it is lengthy and passes through the prostate gland and the penis
– In female, it is short and transport the urine only
Its opening is just above the opening into the vagina
Because it is short, it is more susseptible to bacterial infection than male urethra

Urination or micturation

The process of emptying the bladder and expelling urine
The process:
– When the volume of the bladder reach 300 mL, special nerve endings in the bladder wall are stimulated
– These receptors send send neural messages to the spinal cord
– This initiates a urination reflex
– This reflex contract the bladder wall and also relaxes the internal urethral sphincter (a ring of smooth muscle at the upper end of the urethra)
– This stimulate a concious desire to urinate
At appropriate time and place, the external urethral sphincter is voluntarily relaxed
This allows urination to occur
This voluntary control cannot occur in immature nervous system
That’s why most babies under the age of 2 automatically urinate everytime the bladder fills