UNIT V

DIGESTIVE AND URINARY SYSTEMS

 Digestive:

 Organs of Digestive system

 Digestion and Absorption.

 Urinary:

 Structure of Kidney and Nephron

 Mechanisms of Urine formation

 Regulation of Blood pressure by Urinary System

 Urinary reflex.

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 DIGESTIVE SYSTEM
Alimentary Canal Or The Gastrointestinal (GI) Tract
This is essentially a long tube through which food passes. It commences at
the mouth and terminates at the anus, and the various organs along its length have
different functions. The parts are:
a. Mouth
b. Pharynx
c. Esophagus
d. Stomach
e. Small intestine
f. Large intestine
g. Rectum and anal canal.
Accessory organs
Various secretions are poured into the alimentary tract, some by glands in
the lining membrane of the organs. They consist of:
h. Three pairs of salivary glands
i. The pancreas
j. The liver and biliary tract.
The organs and glands are linked physiologically as well as anatomically in
that digestion and absorption occur in stages, each stage being dependent upon the
previous stage or stages.

a. Oral cavity or mouth

 The oral cavity or mouth is responsible for the intake of food. It is lined by a stratified
squamous oral mucosa with keratin covering those areas subject to significant
Abrasion, such as the tongue, hard palate and roof of the mouth.
 Mastication refers to the mechanical breakdown of food by chewing and chopping
actions of the teeth. The tongue, a strong muscular organ, manipulates the food
bolus to come in contact with the teeth.
 It is also the sensing organ of the mouth for touch, temperature and taste using its
specialised sensors known as papillae.

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 Insalivation refers to the mixing of the oral cavity contents with salivary gland
secretions. The mucin (a glycoprotein) in saliva acts as a lubricant. The oral cavity
also plays a limited role in the digestion of carbohydrates.
 The enzyme serum amylase, a component of saliva, starts the process of digestion
of complex carbohydrates. The final function of the oral cavity is absorption of small

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 molecules such as glucose and water, across the mucosa. From the mouth, food
passes through the pharynx and oesophagus via the action of swallowing.
b. Pharynx
 The pharynx is divided for descriptive purpose into three parts, the nasopharynx,
oropharynx and laryngopharynx. The nasopharynx is important in respiration.
 The oropharynx and laryngopharynx are passages common to both the respiratory
and the digestive systems. Food passes from the oral cavity into the pharynx then to
the oesophagus below, with which it is continuous.

c. Oesophagus
 The oesophagus is a muscular tube of approximately 25cm in length and 2cm in
diameter. It extends from the pharynx to the stomach after passing through an
opening in the diaphragm.
 The wall of the oesophagus is made up of inner circular and outer longitudinal layers
of muscle that are supplied by the oesophageal nerve plexus. This nerve plexus
surrounds the lower portion of the oesophagus.
 The oesophagus functions primarily as a transport medium between compartments.

d. Stomach
 The stomach is a J shaped expanded bag, located just left of the midline between
the oesophagus and small intestine. It is divided into four main regions and has two
borders called the greater and lesser curvatures.
 The first section is the cardia which surrounds the cardial orifice where the
oesophagus enters the stomach.
 The fundus is the superior, dilated portion of the stomach that has contact with the
left dome of the diaphragm. The body is the largest section between the fundus and
the curved portion of the J.
 This is where most gastric glands are located and where most mixing of the food
occurs. Finally the pylorus is the curved base of the stomach.
 Gastric contents are expelled into the proximal duodenum via the pyloric sphincter.
The inner surface of the stomach is contracted into numerous longitudinal folds
called rugae. These allow the stomach to stretch and expand when food enters. The
stomach can hold up to 1.5 litres of material. The functions of the stomach include:
1. The short-term storage of ingested food.

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 2. Mechanical breakdown of food by churning and mixing motions.
3. Chemical digestion of proteins by acids and enzymes.
4. Stomach acid kills bugs and germs.
5. Some absorption of substances such as alcohol.
Most of these functions are achieved by the secretion of stomach juices by
gastric glands in the body and fundus. Some cells are responsible for secreting acid
and others secrete enzymes to break down proteins.

e. Small intestine
 The small intestine is composed of the duodenum, jejunum, and ileum. It averages
approximately 6m in length, extending from the pyloric sphincter of the stomach to
the ileo-caecal valve separating the ileum from the caecum.
 The small intestine is compressed into numerous folds and occupies a large
proportion of the abdominal cavity.
 The duodenum is the proximal C-shaped section that curves around the head of the
pancreas. The duodenum serves a mixing function as it combines digestive
secretions from the pancreas and liver with the contents expelled from the stomach.
 The start of the jejunum is marked by a sharp bend, the duodenojejunal flexure. It is
in the jejunum where the majority of digestion and absorption occurs.
 The final portion, the ileum, is the longest segment and empties into the caecum at
the ileocaecal junction.
 The small intestine performs the majority of digestion and absorption of nutrients.
Partly digested food from the stomach is further broken down by enzymes from the
pancreas and bile salts from the liver and gallbladder.
 These secretions enter the duodenum at the Ampulla of Vater. After further
digestion, food constituents such as proteins, fats, and carbohydrates are broken
down to small building blocks and absorbed into the body’s blood stream.
 The lining of the small intestine is made up of numerous permanent folds called
plicaecirculares. Each plica has numerous villi (folds of mucosa) and each villus is
covered by epithelium with projecting microvilli (brush border).
 This increases the surface area for absorption by a factor of several hundred. The
mucosa of the small intestine contains several specialised cells.
 Some are responsible for absorption, whilst others secrete digestive enzymes and
mucous to protect the intestinal lining from digestive actions.

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 f. Colon (large intestine)

 The colon is a 6-foot long muscular tube that connects the small intestine to the
rectum. The large intestine is made up of the cecum, the ascending (right) colon, the
transverse (across) colon, the descending (left) colon, and the sigmoid colon, which
connects to the rectum.
 The appendix is a small tube attached to the cecum. The large intestine is a highly
specialized organ that is responsible for processing waste so that emptying the
bowels is easy and convenient.
 Stool, or waste left over from the digestive process, is passed through the colon by
means of peristalsis, first in a liquid state and ultimately in a solid form. As stool
passes through the colon, water is removed.
 Stool is stored in the sigmoid (S-shaped) colon until a "mass movement" empties it
into the rectum once or twice a day.
 It normally takes about 36 hours for stool to get through the colon. The stool itself is
mostly food debris and bacteria. These bacteria perform several useful functions,
such as synthesizing various vitamins, processing waste products and food
particles, and protecting against harmful bacteria.
 When the descending colon becomes full of stool, or feces, it empties its contents
into the rectum to begin the process of elimination.
g. Rectum

 The rectum (Latin for "straight") is an 8-inch chamber that connects the colon to the
anus. It is the rectum's job to receive stool from the colon, to let the person know that
there is stool to be evacuated, and to hold the stool until evacuation happens.
 When anything (gas or stool) comes into the rectum, sensors send a message to the
brain. The brain then decides if the rectal contents can be released or not.
 If they can, the sphincters relax and the rectum contracts, disposing its contents. If
the contents cannot be disposed, the sphincter contracts and the rectum
accommodates so that the sensation temporarily goes away.
h. Anus

 The anus is the last part of the digestive tract. It is a 2-inch long canal consisting of
the pelvic floor muscles and the two anal sphincters (internal and external).

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 The lining of the upper anus is specialized to detect rectal contents. It lets you know
whether the contents are liquid, gas, or solid. The anus is surrounded by sphincter
muscles that are important in allowing control of stool.
 The pelvic floor muscle creates an angle between the rectum and the anus that
stops stool from coming out when it is not supposed to. The internal sphincter is
always tight, except when stool enters the rectum.
 It keeps us continent when we are asleep or otherwise unaware of the presence of
stool. When we get an urge to go to the bathroom, we rely on our external sphincter
to hold the stool until reaching a toilet, where it then relaxes to release the contents.

Accessory organs
Salivary glands
 Three pairs of salivary glands communicate with the oral cavity (Parotids,
Submandibular, Sublingual). Each is a complex gland with numerous acini lined
by secretory epithelium.
 The acini secrete their contents into specialised ducts. Each gland is divided into
smaller segments called lobes. Salivation occurs in response to the taste, smell or
even appearance of food.
 This occurs due to nerve signals that tell the salivary glands to secrete saliva to
prepare and moisten the mouth. Each pair of salivary glands secretes saliva with
slightly different compositions.

(i) Parotids
 The parotid glands are large, irregular shaped glands located under the skin on the
side of the face. They secrete 25% of saliva. They are situated below the zygomatic
arch (cheekbone) and cover part of the mandible (lower jaw bone).
 An enlarged parotid gland can be easier felt when one clenches their teeth. The
parotids produce a watery secretion which is also rich in proteins. Immunoglobins
are secreted help to fight microorganisms and a-amylase proteins start to break
down complex carbohydrates.

(ii) Submandibular
 The submandibular glands secrete 70% of the saliva in the mouth. They are found in
the floor of the mouth, in a groove along the inner surface of the mandible.

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 These glands produce a more viscid (thick) secretion, rich in mucin and with a
smaller amount of protein. Mucin is a glycoprotein that acts as a lubricant.

(iii) Sublingual
 The sublinguals are the smallest salivary glands, covered by a thin layer of tissue at
the floor of the mouth.
 They produce approximately 5% of the saliva and their secretions are very sticky
due to the large concentration of mucin. The main functions are to provide buffers
and lubrication.

i) Liver

 The liver is a large, reddish-brown organ situated in the right upper quadrant of the
abdomen. It is surrounded by a strong capsule and divided into four lobes namely
the right, left, caudate and quadrate lobes.
 The liver has several important functions. It acts as a mechanical filter by filtering
blood that travels from the intestinal system. It detoxifies several metabolites
including the breakdown of bilirubin and oestrogen.
 In addition, the liver has synthetic functions, producing albumin and blood clotting
factors. However, its main roles in digestion are in the production of bile and
metabolism of nutrients.
 All nutrients absorbed by the intestines pass through the liver and are processed
before traveling to the rest of the body. The bile produced by cells of the liver, enters
the intestines at the duodenum.
 Here, bile salts break down lipids into smaller particles so there is a greater surface
area for digestive enzymes to act.

j) Gall bladder

 The gallbladder is a hollow, pear shaped organ that sits in a depression on the
posterior surface of the liver’s right lobe. It consists of a fundus, body and neck. It
empties via the cystic duct into the biliary duct system.
 The main functions of the gall bladder are storage and concentration of bile. Bile is a
thick fluid that contains enzymes to help dissolve fat in the intestines. Bile is
produced by the liver but stored in the gallbladder until it is needed.

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 Bile is released from the gall bladder by contraction of its muscular walls in response
to hormone signals from the duodenum in the presence of food.

k) Pancreas

 Pancreas is a lobular, pinkish-grey organ that lies behind the stomach. Its head
communicates with the duodenum and its tail extends to the spleen. The organ is
approximately 15cm in length with a long, slender body connecting the head and tail
segments.
 The pancreas has both exocrine and endocrine functions. Endocrine refers to
production of hormones which occurs in the Islets of Langerhans. The Islets produce
insulin, glucagon and other substances and these are the areas damaged in
diabetes mellitus.
 The exocrine (secretrory) portion makes up 80-85% of the pancreas and is the area
relevant to the gastrointestinal tract.It is made up of numerous acini (small glands)
that secrete contents into ducts which eventually lead to the duodenum.
 The pancreas secretes fluid rich in carbohydrates and inactive enzymes such as
trypsinogen, chymotrypsinogen . Secretion is triggered by the hormones released by
the duodenum in the presence of food.
 Pancreatic enzymes include carbohydrases, lipases, nucleases and proteolytic
enzymes that can break down different components of food. These are secreted in
an inactive form to prevent digestion of the pancreas itself. The enzymes become
active once they reach the duodenum.

REGULATORY MECHANISMS OF DIGISTION

Neural and endocrine regulatory mechanisms work to maintain the optimal
conditions in the lumen needed for digestion and absorption. These regulatory
mechanisms, which stimulate digestive activity through mechanical and chemical
activity, are controlled both extrinsically and intrinsically.

Neural Controls
 The walls of the alimentary canal contain a variety of sensors that help regulate
digestive functions. These include mechanoreceptors, chemoreceptors, and
osmoreceptors, which are capable of detecting mechanical, chemical, and
osmotic stimuli, respectively.

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 For example, these receptors can sense when the presence of food has caused
the stomach to expand, whether food particles have been sufficiently broken
down, how much liquid is present, and the type of nutrients in the food (lipids,
carbohydrates, and/or proteins).
 Stimulation of these receptors provokes an appropriate reflex that furthers the
process of digestion. This may entail sending a message that activates the
glands that secrete digestive juices into the lumen, or it may mean the stimulation
of muscles within the alimentary canal, thereby activating peristalsis and
segmentation that move food along the intestinal tract.
 The walls of the entire alimentary canal are embedded with nerve plexuses that
interact with the central nervous system and other nerve plexuses—either within
the same digestive organ or in different ones. These interactions prompt several
types of reflexes.
 Extrinsic nerve plexuses orchestrate long reflexes, which involve the central and
autonomic nervous systems and work in response to stimuli from outside the
digestive system. Short reflexes, on the other hand, are orchestrated by intrinsic
nerve plexuses within the alimentary canal wall.
 These two plexuses and their connections were introduced earlier as the enteric
nervous system. Short reflexes regulate activities in one area of the digestive
tract and may coordinate local peristaltic movements and stimulate digestive
secretions.
 For example, the sight, smell, and taste of food initiate long reflexes that begin
with a sensory neuron delivering a signal to the medulla oblongata. The response
to the signal is to stimulate cells in the stomach to begin secreting digestive juices
in preparation for incoming food.
 In contrast, food that distends the stomach initiates short reflexes that cause cells
in the stomach wall to increase their secretion of digestive juices.

Hormonal Controls

 A variety of hormones are involved in the digestive process. The main digestive
hormone of the stomach is gastrin, which is secreted in response to the presence
of food. Gastrin stimulates the secretion of gastric acid by the parietal cells of the
stomach mucosa.

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 Other GI hormones are produced and act upon the gut and its accessory organs.
Hormones produced by the duodenum include secretin, which stimulates a
watery secretion of bicarbonate by the pancreas; cholecystokinin (CCK), which
stimulates the secretion of pancreatic enzymes and bile from the liver and
release of bile from the gallbladder; and gastric inhibitory peptide, which inhibits
gastric secretion and slows gastric emptying and motility.

 These GI hormones are secreted by specialized epithelial cells, called

endocrinocytes, located in the mucosal epithelium of the stomach and small
intestine. These hormones then enter the bloodstream, through which they can
reach their target organs.

Digestion and absorption both can be performed in small intestine.

Digestion
i) Digestion of proteins:
 Pancreatic juices such as Trypsinogen and chymotrypsinogen are inactive
enzyme precursors activated by enterokinase, an enzyme in the microvilli,

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  which converts them into the active proteolytic enzymes trypsin and
chymotrypsin. These enzymes convert polypeptides to tripeptides, dipeptides
and amino acids.
 It is important that they are produced as inactive precursors and are activated
only upon their arrival in the duodenum, otherwise they would digest the
pancreas.

ii) Digestion of carbohydrates:

Pancreatic amylase converts all digestible polysaccharides (starches) not acted
upon by salivary amylase to disaccharides.
iii) Digestion of fats:
 Lipase converts fats to fatty acids and glycerol. To aid the action of lipase, bile
salts emulsify fats, i.e. reduce the size of the globules, increasing their surface
area.
 Chemical digestion associated with enterocytes Alkaline intestinal juice (pH
7.8–8.0) assists in raising the pH of the intestinal contents to between 6.5 and
7.5. The enzymes that complete chemical digestion of food at the surface of the
enterocytes are:
 Peptidases
 Lipase
 Sucrase,
 Maltase and
 Lactase.
 Peptidases such as trypsin break down polypeptides into smaller peptides and
amino acids. Peptidases are secreted in an inactive form from the pancreas (to
prevent them from digesting it) and must be activated by enterokinase in the
duodenum.
 The final stage of breakdown of all peptides to amino acids takes place at the
surface of the enterocytes. Lipase completes the digestion of emulsified fats to fatty
acids and glycerol in the intestine.
 Sucrase, maltase and lactase complete the digestion of carbohydrates by converting
disaccharides such as sucrose, maltose and lactose to monosaccharides at the
surface of the enterocytes.
Absorption

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 Absorption of nutrients
 Absorption of nutrients from the small intestine through the enterocytes occurs by
several processes, including diffusion, osmosis, facilitated diffusion and active
transport.
 Water moves by osmosis; small fat soluble substances, e.g. fatty acids and glycerol,
are able to diffuse through cell membranes; while others are generally transported
inside the villi by other mechanisms.
 Monosaccharides and amino acids pass into the blood capillaries in the villi. Fatty
acids and glycerol enter the lacteals and are transported along lymphatic vessels to
the thoracic duct where they enter the circulation.
 A small number of proteins are absorbed unchanged, e.g. antibodies present in
breast milk and oral vaccines, such as poliomyelitis vaccine. Other nutrients such as
vitamins, mineral salts and water are also absorbed from the small intestine into the
blood capillaries.
 Fat soluble vitamins are absorbed into the lacteals along with fatty acids and
glycerol. Vitamin B12 combines with intrinsic factor in the stomach and is actively
absorbed in the terminal ileum.
 The surface area through which absorption takes place in the small intestine is
greatly increased by the circular folds of mucous membrane and by the very large
number of villi and microvilli present.
 It has been calculated that the surface area of the small intestine is about five times
that of the whole body surface. Large amounts of fluid enter the alimentary tract
each day. Of this, only about 1500 mL is not absorbed by the small intestine, and
passes into the large intestine.

URINARY SYSTEM
The urinary system is the main excretory system and consists of the
following structures:
● Two kidneys, which secrete urine.
● Two ureters that convey the urine from the kidneys to the urinary bladder.
● The urinary bladder, which collects and stores urine.
● The urethra through which urine leaves the body.
Structure of kidney:

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 Structure of kidney
 Kidneys are bean-shaped organs, about 11 cm long, 6 cm wide, 3 cm thick and
weigh 150 g. The kidneys lie on the posterior abdominal wall, one on each side of
the vertebral column, behind the peritoneum and below the diaphragm.
 The right kidney is usually slightly lower than the left, probably because of the
considerable space occupied by the liver.
i) Renal Hilus:
The renal hilus is an indentation near to the centre of the concave area of the
kidney. This is the area of the kidney through which the ureter leaves the kidney and
the other structures including blood vessels (illustrated), lymphatic vessels, and
nerves enter/leave the kidney.
ii) Renal capsule:
The renal capsule is a smooth, transparent, fibrous membrane that
surrounds, encloses, and protects the kidney. Each kidney has it's own renal
capsule (outer layer), which helps to maintain the shape of the kidney as well as
protecting it from damage. The renal capsule is itself surrounded by a mass of fatty

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tissue that also helps to protect the kidney by damage by cushioning it in cases of
impact or sudden movement.
iii) Renal cortex:
The renal cortex is the outer part of the kidney and has a reddish colour
(shown as very pale brown above). It has a smooth texture and is the location of the
Bowman's Capsules and the glomeruli, in addition to the proximal and distal
convoluted tubules and their associated blood supplies.
iv) Renal medulla:
The renal medulla is the inner part of the kidney. "Medulla" means "inner
portion". This area is a striated (striped) red-brown colour.
v) Renal pyramids:
There are approx. 5 - 18 striated triangular structures called "Renal
Pyramids" within the renal medulla of each kidney. The appearance of striations is
due to many straight tubules and blood vessels within the renal pyramids.
vi) Renal pelvis:
The renal pelvis is the funnel-shaped basin (cavity) that receives the urine
drained from the kidney nephrons via the collecting ducts and then the (larger)
papillary ducts..
vii) Renal artery:
The renal artery delivers oxygenated blood to the kidney. This main artery
divides into many smaller branches as it enters the kidney via the renal hilus. These
smaller arteries divide into vessels such as the segmental artery, the interlobular
artery, the arcuate artery and the interlobular artery. These eventually separate into
afferent arterioles, one of which serves each nephron in the kidney.
viii) Renal vein:
The renal vein receives deoxygenated blood from the peritubular veins
within the kidney. These merge into the interlobular, arcuate, interlobular and
segmental veins, which, in turn, deliver deoxygenated blood to the renal vein,
through which it is returned to the systemic blood circulation system.
ix) Interlobular artery:
The interlobular artery delivers oxygenated blood at high pressure to the
glomerular capillaries.

x) Interlobular vein:

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 The interlobular vein receives deoxygenated blood (at lower pressure) that it
drains away from the glomerular filtration units and from the Loops of Henle.
xi) Kidney nephron:
Kidney nephrons are the functional units of the kidneys. That this, it is the
kidney nephrons that actually perform the kidney's main functions. There are approx.
a million nephrons within each kidney.
xii) Collecting Duct (Kidney):
The collecting duct labeled in the diagram above is part of the kidney
nephron (shown much enlarged). The distal convoluted tubules of many nephrons
empty into a single collecting duct.
Many such collecting ducts unite to drain urine extracted by the kidney into
papillary ducts, then into a minor calyx, then the major calyx (at the centre of the
kidney), and finally into the ureter through which the urine leaves the kidney en-route
to the urinary bladder.
xiii) Ureter:
The ureter is the structure through which urine is conveyed from the kidney
to the urinary bladder.

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 STRUCTURE OF NEPHRON:

 Nephron is the functional unit of the Kidney. The kidney contains about 1–2 million
functional units, the Nephrons, and a much smaller number of collecting ducts. The
collecting ducts transport urine through the pyramids to the calyces, giving the
pyramids their striped appearance.
 The collecting ducts are supported by connective tissue, containing blood vessels,
nerves and lymph vessels. The nephron is essentially a tubule closed at one end
that joins a collecting duct at the other end. The closed or blind end is indented to
form the cup-shaped glomerular capsule (Bowman’s capsule).
 Continuing from the glomerular capsule, the remainder of the nephron is about 3 cm
long and described in three parts:

• The proximal convoluted tubule.

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 • The medullary loop (loop of henle).
• The distal convoluted tubule, leading into a collecting duct.
 The collecting ducts unite, forming larger ducts that empty into the minor calyces.
The kidneys receive about 20% of the cardiac output. After entering the kidney at the
hilum, the renal artery divides into smaller arteries and arterioles.
 In the cortex an arteriole, the afferent arteriole, enters each glomerular capsule and
then subdivides into a cluster of tiny arterial capillaries, forming the glomerulus.
Between these capillary loops are connective tissue phagocytic mesangial cells,
which are part of the monocyte–macrophage defence system.
 The blood vessel leading away from the glomerulus is the efferent arteriole. The
afferent arteriole has a larger diameter than the efferent arteriole, which increases
pressure inside the glomerulus and drives filtration across the glomerular capillary
walls.
 The efferent arteriole divides into a second peritubular (meaning ‘around tubules’)
capillary network, which wraps around the remainder of the tubule, allowing
exchange between the fluid in the tubule and the blood stream .
 This maintains the local supply of oxygen and nutrients and removes waste
products. Venous blood drained from this capillary bed eventually leaves the kidney
in the renal vein, which empties into the inferior vena cava.
 The walls of the glomerulus and the glomerular capsule consist of a single layer of
flattened epithelial cells. The glomerular walls are more permeable than those of
other capillaries.
 The remainder of the nephron and the collecting duct are formed by a single layer of
simple squamous epithelium . Renal blood vessels are supplied by both sympathetic
and parasympathetic nerves.
 The presence of both divisions of the autonomic nervous system controls renal
blood vessel diameter and renal blood flow independently of auto regulation.

KIDNEY AND THE MECHANISM OF URINE FORMATION

Different parts of the nephron are responsible for various functions of Kidney.
Fluid filtered from the blood enters the Bowman’s capsule then flows into the
proximal tubule, down the descending limb of the loop of Henle, then up the

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 ascending limb, into the distal tubule and then the connecting and finally the
collecting tubule.
There are three stages to urine formation :
 Glomerular filtration
 Tubular reabsorption
 Tubular secretion.

 Glomerular Filtration

This occurs when fluid from the glomerular capillaries pass into the
Bowman’s capsule. This is fairly non-selective meaning that almost all of the
substances in the the blood except cells and plasma proteins as well as the
substances bound to these proteins enter the nephron.
 Tubular Reabsorption

During this phase, all parts of the tubule act to return essential substances
out of the nephron so that it is not lost in the urine. It is a highly selective process in
that the tubules carefully “choose” what will be returned to the body and what will be
passed out with the urine. This transfer of substances is known as Tubular
Reabsorption and may involve both active and passive mechanisms.

Some of the substances pass through the space between the epithelial cells
while others through the cells itself. In this way, the substances are returned back to
the body either by being “dumped” into the tissue of the kidney outside of the
nephron or returned directly into the bloodstream.
 Tubular Secretion

Just as substances that enter through the glomerulus are removed from the
nephron and returned to the body, many substances are drawn from the body and
“dumped” into the tubules. Acids, alkalines, certain ions, toxins and drugs are
secreted into the tubules and this process is known as tubular secretion.

In this manner it can be rapidly passed out with the urine independent of
glomerular filtration and in a more selective manner.

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Mechanism of urine formation:

The basic function of the nephron is to filter blood and remove waste
substances while retaining essential substances for various biochemical processes.
In the process, the nephron can also influence the pH (acid-base balance) of the
blood, regulate blood pressure, maintain the blood volume and control the level of
electrolytes in the body fluids.
The functions of the nephron can be discussed with regards to each part :
Bowman’s capsule
 Collects the incoming fluid from the glomerular capillaries.
Proximal tubule
 Sodium, chloride, water, glucose and amino acids are reabsorbed (removed from
the tubules).
 Organic acids and bases like bile salts, oxalate and urate are secreted into the
proximal tubule.
Loop of Henle

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 Water is reabsorbed mainly in the descending limb and thin segment of the
ascending limb.
 Sodium, calcium, chloride, magnesium and potassium are actively reabsorbed in
the thick segment of the ascending limb.
Distal tubule
 Controls the blood flow through the glomerular capillaries and glomerular filtration
of the nephron to which it belongs.
 Sodium, potassium and chloride reabsorption.
Collecting tubule
 Sodium, potassium and chloride reabsorption.
 Hydrogen ion secretion.

Role of Kidney in Acid base regulation

The Role of the Kidneys in Acid-Base Balance. Thekidneys help maintain
the acid–base balance by excreting hydrogen ions into the urine and reabsorbing
bicarbonate from the urine.

The kidney is responsible for the maintenance of an important sector of

the regulation of acid-base balance, i.e., that related to the maintenance of the
alkaline reserve of the body and to excretion of fixed acids. Renal Regulation
of Acid-Base Balance.

The renal regulation of the body’s acid-base balance addresses the metabolic
component of the buffering system. Whereas the respiratory system (together with
breathing centers in the brain) controls the blood levels of carbonic acid by
controlling the exhalation of CO2, the renal system controls the blood levels of
bicarbonate. A decrease of blood bicarbonate can result from the inhibition of
carbonic anhydrase by certain diuretics or from excessive bicarbonate loss due to
diarrhea.

Blood bicarbonate levels are also typically lower in people who have
Addison’s disease (chronic adrenal insufficiency), in which aldosterone levels are
reduced, and in people who have renal damage, such as chronic nephritis. Finally,
low bicarbonate blood levels can result from elevated levels of ketones (common in

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 unmanaged diabetes mellitus), which bind bicarbonate in the filtrate and prevent its
conservation.

Bicarbonate ions, HCO3–, found in the filtrate, are essential to the bicarbonate
buffer system, yet the cells of the tubule are not permeable to bicarbonate ions. The
steps involved in supplying bicarbonate ions to the system are seen in Figure 3 and
are summarized below:

 Step 1: Sodium ions are reabsorbed from the filtrate in exchange for H + by an
antiport mechanism in the apical membranes of cells lining the renal tubule.
 Step 2: The cells produce bicarbonate ions that can be shunted to peritubular
capillaries.

 Step 3: When CO2 is available, the reaction is driven to the formation of carbonic
acid, which dissociates to form a bicarbonate ion and a hydrogen ion.

 Step 4: The bicarbonate ion passes into the peritubular capillaries and returns to the
blood. The hydrogen ion is secreted into the filtrate, where it can become part of new
water molecules and be reabsorbed as such, or removed in the urine.

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 Conservation of Bicarbonate in the Kidney.
Tubular cells are not permeable to bicarbonate; thus, bicarbonate is
conserved rather than reabsorbed. Steps 1 and 2 of bicarbonate conservation are
indicated.

It is also possible that salts in the filtrate, such as sulfates, phosphates, or

ammonia, will capture hydrogen ions. If this occurs, the hydrogen ions will not be
available to combine with bicarbonate ions and produce CO 2. In such cases,
bicarbonate ions are not conserved from the filtrate to the blood, which will also
contribute to a pH imbalance and acidosis.

The hydrogen ions also compete with potassium to exchange with sodium in
the renal tubules. If more potassium is present than normal, potassium, rather than
the hydrogen ions, will be exchanged, and increased potassium enters the filtrate.

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 When this occurs, fewer hydrogen ions in the filtrate participate in the
conversion of bicarbonate into CO2 and less bicarbonate is conserved. If there is
less potassium, more hydrogen ions enter the filtrate to be exchanged with sodium
and more bicarbonate is conserved.

Chloride ions are important in neutralizing positive ion charges in the body. If
chloride is lost, the body uses bicarbonate ions in place of the lost chloride ions.
Thus, lost chloride results in an increased reabsorption of bicarbonate by the renal
system.

Disorders of the…
 Acid-Base Balance: Ketoacidosis Diabetic acidosis, or ketoacidosis, occurs
most frequently in people with poorly controlled diabetes mellitus. When
certain tissues in the body cannot get adequate amounts of glucose, they
depend on the breakdown of fatty acids for energy. When acetyl groups break
off the fatty acid chains, the acetyl groups then non-enzymatically combine to
form ketone bodies, acetoacetic acid, beta-hydroxybutyric acid, and acetone,
all of which increase the acidity of the blood. In this condition, the brain isn’t
supplied with enough of its fuel—glucose—to produce all of the ATP it
requires to function.
 Ketoacidosis can be severe and, if not detected and treated properly, can
lead to diabetic coma, which can be fatal. A common early symptom of
ketoacidosis is deep, rapid breathing as the body attempts to drive off
CO2 and compensate for the acidosis. Another common symptom is fruity-
smelling breath, due to the exhalation of acetone. Other symptoms include
dry skin and mouth, a flushed face, nausea, vomiting, and stomach pain.
Treatment for diabetic coma is ingestion or injection of sugar; its prevention is
the proper daily administration of insulin.
 A person who is diabetic and uses insulin can initiate ketoacidosis if a dose of
insulin is missed. Among people with type 2 diabetes, those of Hispanic and
African-American descent are more likely to go into ketoacidosis than those of
other ethnic backgrounds, although the reason for this is unknown.

Regulation Of Blood Pressure

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 Kidneys are to maintain blood pressure is through the regulation of the volume of
blood in the body. The kidneys are able to reduce blood volume by reducing the
reabsorption of water into the blood and producing watery, dilute urine.
 When blood pressure becomes too low, the kidneys can produce the enzyme renin
to constrict blood vessels and produce concentrated urine, which allows more
water to remain in the blood.
 The renin-angiotensin system or RAS regulates blood pressure and fluid balance in
the body. When blood volume or sodium levels in the body are low, or blood
potassium is high, cells in the kidney release the enzyme, renin.
 Renin converts angiotensinogen, which is produced in the liver, to the hormone
angiotensin I.
 An enzyme known as ACE or angiotensin-converting enzyme found in the lungs
metabolizes angiotensin I into angiotensin II. Angiotensin II causes blood vessels to
constrict and blood pressure to increase. Angiotensin II stimulates the release of the
hormone aldosterone in the adrenal glands, which causes the renal tubules to retain
sodium and water and excrete potassium.
 Together, angiotensin II and aldosterone work to raise blood volume, blood pressure
and sodium levels in the blood to restore the balance of sodium, potassium, and
fluids. If the renin-angiotensin system becomes overactive, consistently high blood
pressure results.

Urinary reflex (or) Micturition Reflex

 Micturition is the physiological term for urination or voiding. It results from an
interplay of involuntary and voluntary actions by the internal and external urethral
sphincters.
 When bladder volume reaches about 150 mL, an urge to void is sensed but is easily
over ridden. Voluntary control of urination relies on consciously preventing relaxation
of the external urethral sphincter to maintain urinary continence.
 As the bladder fills, subsequent urges become harder to ignore. Micturition is a
result of stretch receptors in the bladder wall that transmit nerve impulses to the
sacral region of the spinal cord to generate a spinal reflex.
 The resulting parasympathetic neural outflow causes contraction of the detrusor
muscle and relaxation of the involuntary internal urethral sphincter. At the same

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 time, the spinal cord inhibits somatic motor neurons, resulting in the relaxation of the
skeletal muscle of the external urethral sphincter.
 The micturition reflex is active in infants but with maturity, children learn to override
the reflex by asserting external sphincter control, thereby delaying voiding (potty
training).
 Nerves involved in the control of urination include the hypogastric, pelvic, and
pudendal . Voluntary micturition requires an intact spinal cord and functional
pudendal nerve arising from the sacral micturition center.
 Since the external urinary sphincter is voluntary skeletal muscle, actions by
cholinergic neurons maintain contraction (and thereby continence) during filling of
the bladder.
 At the same time, sympathetic nervous activity via the hypogastric nerves
suppresses contraction of the detrusor muscle. With further bladder stretch, afferent
signals traveling over sacral pelvic nerves activate parasympathetic neurons.
 This activates efferent neurons to release acetylcholine at the neuromuscular
junctions, producing detrusor contraction and bladder emptying.

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