Urinary system/ Digestive system

• Gastrointestinal tract (aka. alimentary canal, digestive tract) extends from the mouth to the anus.
• Accessory Organsrelease secretions into the alimentary canal- salivary glands, liver,gallbladder, and pancreas         →

Ingestion: taking in food 

Mechanical digestion and movement : Mechanical physically breaks down larger pieces into smaller portions , and two types of movement mix the contents of the lumen and propel it forward

Secretion: Fluid , digestive enzymes, acid, alkali, bile, and mucus are all secreted into the blood stream.  (Simple columnar cells secrete)

Chemical Digestion : The contents of the lumen are broken down into simpler molecules  via enzymes 

Absorption : Nutrient molecules pass across the mucosal layer of the GI tract and into the blood or lymph.

Elimination- Undigested material is eliminated from the body.→

Mucosa: Innermost layer of simple columar epithelium  with goblet cells forming a protective barrier . It is the site of secretion and absorption, all nutrients must cross the mucosa to enter the blood.

Submucosa : Lies deeps to the mucosal layer and consists of dense CT with blood and lymph vessels, lymph nodules, nerves

• Muscular layer: 2-3 layers of smooth muscle : inner circular layer and outer longitudinal layer (plus middle oblique in some areas along digestive tract
•           
•               Sphincters -specialized circular smooth muscle  layer  that regulates the movement of contents through the GI  tract.

Serosa : the outer most layer composed of visceral peritoneum. The serosa protects underlying tissues and secretes serous fluid for purposes of lubrication. →

1. Peristalsis propels food forward. peristaltic movement involves a ring of contraction along the wall of the tube. as the right of contraction occurs , the muscular wall just ahead of the ring relaxes- phenomenon called receptive relaxation, and thus  pushes food forward. 


2. Segmentation mixes food. Contraction and relaxation occurs randomly , which "mixes" contents within the lumen, food particles are pressed against the mucosa, enabling the body to absorb their nutrients.

The autonomic nervous system maintains muscle tone and regulates the strength, rate , and velocity of muscular contractions. In the GI tract, the postganglionic fibers are organized into nerve plexuses: ( Sympathetic -fight or flight, parasympathetic - rest and digest) 

• Parasympathetic control :
• Sumucosal plexus controls secretions in submucosa

Myenteric plexuscontrols motility, control muscle→

Entrance to the GI tract: Lined by stratified squamous epithelium . The mouth receives food and begins mechanical  digestion by breaking up solid particles into smaller pieces, via mastication and mixing them with saliva to form a bolus for swallowing 

The mouth function as an organ of speech and taste, its surrounded by the lips, cheeks, tongue, an palate

Main function is to catch bacteria that come in when breathing

• palatine
• pharyngeal
• Lingual

Bone like for mastication 

20 deciduous teeth are replaced by 32 permanent 

Root canal- death of tooth nerve caused by cutting off blood supply to tooth 

Dental caries (cavities)- demineralization of enamel and dentine by bacterial action

Periodontitis- bacteria invade bone surrounding tooth and dissolve bone away from tooth→

thick muscular organ, line by mucous membrane, Its posterior region is anchored to the hyoid bone. under surface is connected to the frenulum. has papillae projections that contain taste buds.

It mixed food particles with saliva during chewing moves positions food below the teeth for chewing, and moves food toward the pharynx during swallowing.

saliva is continuously produced, digestion, defense, cleanse teeth

• parotid gland
• submandibular
• sublingual 

saliva is composed of primarily water and also contains:

• salivary amylase: begins carb digestion
• Mucin- mucus like protein that holds food particles together
• Lysozyme-inhibits bacterial growth
• Bicarbonate HCO3 maintain PH between 6.5-7.5 the range which salivary amylase is mose effective. 

salivary glands are comprised of two cells types:

Serous cells produce a watery fluid containing amylase, 

Mucous cells produce mucous

Stratified columnar epithelium , lies behind the trachea 

Cardiac sphincter- is located at the terminal end of the esophagus. It prevents acidic stomach contents from backing up into the esophagus.

Primary function carries solid food and liquids to the stomach. when a bolus of food reaches the esophagus, a reflex is initiated by the stimulation of stretch receptors in the esophagus. A wave of smooth muscle contraction (peristalsis) moves food into the stomach. ⇢

Located in the upper left quadrant of the abdomen. It begins just distal to the cardiac sphincter. thick J shaped organ . It size varies according to physical body size and the amount of food consumption. Its internal structure has rugae (gross folds) , which disappear as  the stomach fills. 

Stores foods, chemical breakdown of proteins , regulates delivery. It empties in 2-6 hours ,chyme leaves the stomach via the pyloric sphincter to enter  duodenum 

Internally, it is lines by simple squamous epithelium with goblet cells.(produce protective mucus forms gastric glands)

Secretes Gastric Juice , PH 1.6- 2.6

Columnar epithelial lining contain million of gastric pits leading to gastric glands , which are composed of three types of cells 

Mucous neck cells - produce mucus, which protects the stomach mucus

• Chief cells - secrete pepsinogen, an inactive form of enzyme pepsin →
•  pepsin digest large proteins to small chains of amino acids.

Parietal cells secrete prehydrochloric acid, which is converted to hydrochloric acid (HCI) and intrinsic factor . HCI makes the stomach contents very acidic: kills bacteria, breaks down CT in meat , and activates pepsin (enzyme), intrinsic factor aids in the absorption of vit B12 in the small intestine.

Three stages of gastric secretions 

Cephalic phase: secretion of gastric juice before food enter the stomach. initiated by the sight, smell,taste or thought of food

Gastric phase: distention of the stomach and the release of more gastric juice initiated by distention of the stomach , which stimulates gastrin secretion, and activates CNS and local reflexes  that promote gastric secretion 

Intestinal phase: Movement of feed into the small intestine. initiated by acidic chyme, which enter the duodenum and stimulates neuronal reflexes an the secretion of hormones  that induce and then inhibits gastric secretions .

parasympathetic innervation stimulates the release of gastric juice

Gastrin is produced in the stomach and functio s to increase the secretory activity of gastric glands

Somatostatin - us produced in te stomach and functions to inhibit acid secretion 

Cholecystokinin, which is produced by the small intestine, inhibits gastric secretions and decrease gastric motility→

• When stomach is empty, tonic muscle contractions keep it small. after eating a meal , tonic contractions cease and the stomach relaxes and stretches to accommodate the food →
• allows for increase in volume.

• Mixing waves mix the stomach contents with stomach secretions to form chyme.
• A stomach takes 4 hrs to empty. Peristalsis moves chyme into the duodenum. →

Gastrin and stretching of the stomach stimulate peristaltic contraction. 

The enterogastric reflex is a reflex involving the small intestine wall and inhibits peristalsis in the stomach to slow down movement of food into the duodenum   →

The small intestine begins after the pyloric sphincter and extends to the ileocecal junction.

Nearly 90% of absorbable nutrients are absorbed in in the small intestine: water , monosaccharides, amino acids, fat molecules, sodium, potassium, chloride, nitrate, and bicarbonate.

Three subdivisions : 1st Duodenum -receives bile, via the common bile duct from the liver and gallbladder, and pancreatic juice via the pancreatic duct from the pancreas. Sphincter of oddi controls emptying form the ducts.

Second Portion: Jejunum

3rd portion : Ileum- Joins large intestine at the ileocaecal Junction .

The luminal surface is arranged for maximum surface area: ( increase absorption)

a.circular folds , villi, Microvilli →

1-2 Liters secreted per day- PH 7.4 to 7.8 (neutral) 

Brunners glands are mucus secreting glands located in the submucosa of the proximal portion of the duodenum. mucus(alkaline) protects against digestive enzymes and stomach acids. 

Digestive enzymes for carbohydrates,lipids, proteins, and nucleic acids are bound to the intestinal wall.

Hormones-control secretion of pancreas and gallbladder→

Distention of intestinal wall stimulates the parasympathetic reflexes that cause intestinal secretions .

Stomach contents entering the small intestine stimulate the duodenal mucous glands to release mucus

Direct contact with chyme chemically and mechanically stimulates the goblet cells and intestinal glands to secrete their products

Stretch of smooth muscles, local reflexes, and parasympathetic nervous system  stimulate contractions 

segmental contractions mix intestinal contents , peristaltic contractions move materials distally.

Distention of the cecum initiates a reflex that inhibits contractions . →

Begins at the cecum and ends at the anus

Absorbs water, electrolytes, and vitamins, 

Performs last stages of chemical digestion through the actions of normal flora 

compaction of intestinal contents into feces and eliminate semisolid waste.

Special guest: Normal bacterial flora lives in the large intestine and thrives on ingestible materials that pass though the colon . Microorganisms are responsible for the bulk of feces produced.

(abx wipe out good bacteria , and give you diarrhea)

Luminal surface is lined by simple columnar epithelium up to the anal canal , where it becomes stratified squamous epithelium

Tenia coli, Haustra, Epiploic appendages 

• Cecums
• appendix
• colon rectums
•  
• anal canal-  which has two sphincters 

Internal anal sphincter- smooth muscle- involuntary

External anal sphincter- skeletal muscle- voluntary

Mucus provides protection to the intestinal lining (simple columnar epithelium 0

HCO3 is secreted by epithelial cells 

Na+ is absorbed by active transport and H2O  is absorbed by osmosis  →

Segmental movements mix the colons contents

mass movements are strong peristaltic contractions  that occur 3-4/day

Defecation:  elimination of feces 

The defecation reflex is triggers by holding a deep breath and contracting the abdominal wall muscles. The actions of the defection reflex are to increase internal abd pressure and force feces into the rectum peristaltic waves are triggered and anal sphincters relax

Voluntary activity, via contraction of puborectalis  and external anal sphincter, regulates movement.→

It comprised of two portions , each having different functions:

1. Endocrine Portion - endocrine cells are organized into clusters called islets of Langerhans (pancreatic islets) which secrete glucagon and insulin. 

Glucagon: stimulates breakdown of  glycogen into glucose

• Insulin: promotes formation of glycogen from glucose, increase movement of glucose into cells and decrease blood glucose . 
• →

Cells arranged into acini, which secrete a mixture of fluid and digestive enzymes called pancreatic juice . ZYMOGEN GRANULES - are granules that store pancreatic enzymes. The acini connect to a duct system that eventually forms the pancreatic duct, which empties into the duodenum →

water- main constituent

Bicarbonate- Neutralizes gastric acidity

• enzymes:
• Amylase- →carbs- simple sugars

Lipase→Lipids - glycerol and fatty acids


Trypsin→protein- amino acids

Nucleases→Nucleic acids- phosphate and nucleotides

Functions :

1. Produces bile: bile contains bile salts , bile pigments, cholesterol, neutral fats , Phospholipids, and electrolytes 

Bile Neutralizes gastric acidity and emulsifies  fats/lipids. emulsification dramatically aids lipase action. 

Lack of bile salt result in poor lipid absorption and vitamin deficiencies.

2. Manufactures plasma proteins, such as albumin and fibrinogen, from amino acids 

3. stores glucose as glycogen after a meal, and converts glycogen to glucose between meals.

4. stores fat-soluble vitamins  (A,D,E, and K) and iron

5. synthesizes and stores some lipids

6. Detoxification: Inactivates many chemicals, including alcohol, hormones, drugs, and poisons

7.Converts ammonia, a toxic waste product of metabolism, into less toxic urea

8. Destroys worn-out RBCs and bacteria

Lipids- Bile- Emulsification- small lipids droplets

Lobes of the liver are made up of many functional units called lobules.  A lobule consist of hepatocytes, which are arranged in branching, interconnected plates around a central vein.

The liver has larger, endothelium-lined spaces called sinusoids, through which blood passes. Sinusoids contain macrophages (kupffers cells ). Which destroy worn out WBCs and RBC, bacteria and other foreign pathogens in the venous blood draining  from the GI tract.

Bile canliculi are canals within hepatic lobules that receive secretion from hepatic cells .

Carries nutrient-rich blood directly from the digestive organs to the liver, via the hepatic portal vein. Therefore, the liver is ideally located to begin processing and storing nutrients for the body just as soon as digestion and absorption have begun.

concentrates the bile 

Hollow pear shaped organ, situated on the undersurface of the liver

Function: Stores and concentrates bile

Hepatopancreatic sphincter (of Oddi)- controls the emptying of the bile duct and pancreatic duct into the duodenum

A. Excretion- removal of waste from the body fluid

• Urine formation begins at the nephron 
• The kidneys filter the entire blood plasma volume more than 60 times per day. 

Elimination (Urination/ Micturition) -The Process of urine elimination.

Urinary bladder and urethra are responsible for elimination.

The kidney is a reddish brown, bean-shaped organ with smooth surface . It is enclosed  in a tough, fibrous renal capsule.

Kidneys are retroperitoneal organs, situated between vertebral levels , embedded in a perirenal fat pad  and capped by the adrenal capsule .

Renal cortex-outermost region

Renal medulla- innermost region- comprise the renal pyramids and renal columns, renal columns contain blood vessels an nerves 

Renal papillae - apex of the pyramids, empty into the calyces

•  The calyces funnel urine into the renal pelvis, which continues as the ureter

The nephron is the microscopic functional unit of the kidneys

Two parts: The renal corpuscle +Renal tubule 

Renal corpuscle- is essentially a filtering unit , which is formed by The  :

Glomerulus-a tangled cluster of capillaries , which filters fluid and small solutes form the blood

Bowmans (glomerular)  capsule - a surrounding thin-walled, sac like structure, which receives the filtrate from the glomerulus . The capsule is composed of two layers of epithelial ells : visceral and parietal layer, cells of the visceral layer interdigitate with one another ot form filtration slits →

Proximal convoluted tubule (PCT)  -portion of renal tubule leading away from Bowmans capsule.   (Has Dirty lumen)

Primary Function is to reabsorb water and solutes from filtrate. 

Nephron Loop (loop of henle)- hairpin loop dipping into medulla 

Distal convoluted tubule (DCT)-  The nephron loop returns near its origin where it becomes the distal convoluted tubule. 

Collecting duct (Collecting tubule)

Distal convoluted tubules merge to form a collecting duct, which passes through the medullary pyramid.

Collecting duct empty into the minor calyx through an opening in the renal papilla.

The ascending limb of the nephron passes between the afferent and efferent arterioles  of it own glomerulus and make contact with them. at the point of contact, the epithelial cell of the ascending limb form a structure called the macula densa


close by in the wall of the afferent arteriole near the glomerulus, are large vascular, smooth muscle cells, called Juxtaglomerular cells  

Juxtaglomerular cells + Macula densa= Juxtaglomerular apparatus 

The juxtaglomerular apparatus regulates renin secretions 

Under normal resting conditions , the large renal arteries deliver one - fourth of the total cardiac out put to the kidneys each minute.

1. Renal artery is the main blood supply to the kidney.

2. The renal artery divides into five Segmental arteries.

3. Each segmental artery branches into interlobar arteries.

Interlobar arteries branch into arcuate arteries, which arch over the bases on the medullary pyramids.

5. Arcuate arteries  branch into interlobular arteries- which pass through the renal cortex 

(Interlobular arteries give rise to afferent arterioles)→

1. Afferent arterioles arise from interlobular arteries. Afferent arterioles have larger diameters , compared to other arterioles . afferent arterioles give rise to tangle cluster of capillaries called glomeruli (aka glomerular capillaries)

2.Glomerular capillaries -are in direct contact with renal tubules . they give rise to second set of arterioles , the efferent arterioles 

3. Efferent arterioles  have smaller diameters  efferent arterioles carry blood to  a second network of capillaries , peritubular capillaries 

4.  Peritubular capillaries - Surround the renal tubules and contribute to the exchange of substances within the tubule lumen. 

5.Blood from the peritubular capillaries  flows into smaller veins , which ultimately coverage to form the renal vein.

Pressure inside of the glomerular capillary is relatively high (Hydrostatic pressure) due to the smaller diameter of the efferent  arteriole.

Recall:↓ diameter ↑ resistance


Greater resistance in the efferent arteriole causes a back up of blood in the glomerular capillary, thus, resulting in a higher pressure (compared to the pressure inside capillaries elsewhere in the body)

Pressure types and source:- Hydrostatic pressure -BP[- Need increased pressure for filtration .

Significance: Hydrostatic pressure -outward, it forces water and small molecules out of the capillary

• How can pressure inside the glomerulus be further altered? 
• 1. change in diameter of the afferent arteriole
• 2. change in diameter of the efferent arteriole.

Pressure inside the glomerular capillary can be altered to accommodate fluctuations in BP.

• Key idea: Pressure in the glomerulus can be altered by increasing /decreasing the diameter of the arterioles '
• ↑ Diameter- resistance decreases.
• ↓Diameter- resistance increase .

1. Regulates blood volume and BP

2. Regulates plasma (ions) charged particles

3. Help stabilize blood PH

4. Conserves nutrients 

5. Assists liver in Detoxifying the blood

The main function of the nephrons and collection ducts is to control the composition of body fluids and remove waste from the blood. The product is urine, which is excreted form the body. Urine contain wastes  , excess water , and electrolytes

The cardiovascular system and urinary system are linked by three processes that contribute to urine formation: 

1. Glomerular filtration - physically , both systems are linked at the renal corpuscle.

• 2. Tubular reabsorption -
• 3 Tubular secretion - physiologically ,both systems are linked at the renal tubule and peritubular capillary. 

• Urinary Excretion =Glomerular Filtration - Tubular Reabsorption + Tubular secretion 

First step: Glomerular filtration- (blood to filtrate) During glomerular filtration , blood pressure forces water and small solutes out of the glomerular capillary across a filtration membrane . Large particles , such as proteins , are restrained with in the glomerular capillary. water and solutes that are small enough to pass through the filtration membrane  are collected by Bowmans capsule. 

Second step: Tubular Reabsorption -(Filtrate to blood)- water and solutes are reabsorbed from the glomerular filtrate into the peritubular capillary blood. It is a selective process .

Reabsorption involves active transport mechanism (transporters in the membrane of the renal epithelial cells ) and passive transport mechanism (diffusion and osmosis)

If reabsorption did not occur, most constituents of the plasma would be rapidly lost in the urine. 

Third step : Tubular secretion (Blood to filtrate)- Tubular secretion is essentially a back up process for filtration . a few substance are secreted from the peritubular capillary blood into tubular fluid. as with reabsorption, the secretion mechanism involve transporters in the membranes of the epithelial cells lining the nephron.

Recall :The cardiovascular system and urinary system are physically linked at the renal corpuscle, where the glomerular capillary comes in direct contact with Bowmans capsule to form a selective filtration barrier.

Fluid and small particle in the glomerular capillary are pushed through the filtration barrier. The resultant filtrate (filtered solutes + fluid)  is then collected by bowmans capsule.

Glomerular filtration is driven by Hydrostatic pressure (i.e blood pressure): fluid and smaller particles are pushed out of the glomerular capillary , while the larger particles of blood , such as protein remain trapped inside the capillary. 

Result: Glomerular filtrate is mostly water and the same solutes as in blood plasma , minus larger proteins.

→

Driving force of filtration: - Glomerular hydrostatic pressure → results from blood pressure- Recall glomerular capillary pressure i high.


Forces opposing Filtration : Glomerular osmotic pressure is largely created by the presence of plasma proteins - Hydrostatic pressure in Bowman's capsule 


The net filtration pressure is the net effect of the above forces . Net filtration pressure is normally positive and this favors glomerular filtration

Net filtration pressure=Driving forces of filtration- Forces opposing filtration. 

• (See diagram pic in notes)

In an average adult, plasma is filtered through the glomeruli about 60 times per day . Not all plasma is excreted as urine. Instead most of the fluid that passes through the renal tubules is reabsorbed and reenters the plasma .

GFR IS directly proportional to the net filtration

Factors that affect the glomerular hydrostatic pressure , glomerular osmotic pressure , or hydrostatic pressure in Bowman's capsule also affect the rate of filtration. :forces favoring filtration in the glomerular capillaries always predominate. 

The volume of plasma the kidneys filter also depends on the surface areaof the glomerular capillaries →

Glomerular hydrostatic pressure is the most important factor determining net filtration pressure and GFR

Glomerular hydrostatic pressure is determined by three variables, each of which is under physiologic control:

• 1. Arterial blood pressure 
• 2. Afferent arteriolar resistance
• 3. Efferent arteriolar resistance

• In Afferent arteriole
• Vasoconstriction ↓net filtration -↓GFR
• Vasodilation↑ net filtration - ↑ GFR

• In EFFERENT ARTERIOLE :  Blood leaves glomerulus
• Vasoconstriction ↑ net filtration- ↑GFR
• Vasodilation ↓ net filtration - ↓ GFR

• Conditions that lower that plasma osmotic pressure increase GFR
• ex:decreased plasma proteins (because less force will be used to pull water back into glomerular capillary meaning and increase in net filtration.)→

An increase in capsular hydrostatic pressure opposes glomerular filtration so GFR MAY significantly decrease .

ex: obstruction along the Urinary tract (stones or enlarge prostate.)

Mechanisms that regulates RBF are closely linked to the control  of GFR and the excretory functions of the kidneys 

The mechanism for changing blood flow is changing arteriolar resistance . In the kidney, this is accomplished by changing afferent arteriolar resistance and /or efferent arteriolar resistance→

Overview : changes in arterial pressure have some influence on renal blood flow. How ever , the kidneys have effective mechanism for maintaining renal blood flow (RBF) and GFR relatively constant over a wide range of mean renal arterial pressure  through a process called  autoregulation . 


Autoregulation occurs through mechanisms that are completely intrinsic to the kidneys . The ANS is not involved.

Significance: Autoregulatory mechanisms a) prevent large changes in GFR and b) prevent renal excretion of water and solutes that would otherwise occur with changes in blood pressure. 

• Mechanism: the only way to maintain constant RBF  in the face of changing pressure is by varying the resistance  of arterioles. Thus, as renal arterial pressure ↑
• or ↓resistance in the arterioles must ↑ or ↓
•  proportionally

↑ BP→vasoconstric the afferent arteriole →Net filtration  pressure ↓GFR ↓= Urine formation ↓

• ↓BP →vasoconstric the efferent arteriole →
• Net filtration↑- GFR↑= Urine formation ↑

strong activation of the sympathetic nervous system can constrict the renal arterioles  and decrease RBF and GFR

fyi: The sympathetic seems to be most important in reducing  GFR during severe acute disturbances , such as those elicited by defense reaction, brain ischemia or hemorrhage

norepinephrine, epinephrine an endothelin, constrict afferent and efferent arterioles and decrease GFR

Angiotensin II is a potent vasoconstrictor of afferent and efferent arterioles, however, the efferent arteriole is more sensitive to angiotensin II and thus raises glomerular hydrostatic pressure , while reducing RBF

Decreased arterial blood pressure activates the renin- angiotensin system, High levels of angiotensin II and increased sympathetic nerve activity constrict afferent and efferent arterioles  and decrease RBF and GFR

The Juxtaglomerular apparatus secretes renin in response to stimulation from the sympathetic nerves, renal artery hypotension detected by renal  baroreceptors, and ↓ na+ in the distal tubules.


once in the blood stream, renin reacts with angiotensinogen (plasma protein) to for angiotensin I


an enzyme , angiotensin- converting enzyme (ACE) on capillary endothelial cells (particularly in the lungs), rapidly converts angiotensin I to angiotensin II

Angiotensin II has numerous effects 

It constricts resistance vessels through out the body , ↑ PVR and  ↑BP


Adrenal cortex →aldosterone → ↑sodium and fluid retention by kidneys 


Posterior pituitary → Antidiuretic hormone (ADH)  ↑fluid retention by kidneys


stimulate thirst centers in the Hypothalamus

Enhances sympathetic Function

RAS is controlled through negative feedback ,all the effects ultimately work to Raise BP to normal levels

During tubular reabsorption , water and nutrients(in filtrate) are reabsorbed from the proximal convoluted tubule (PCT) back into the Peritubular capillary. Its a selective process

Since the PCT  and peritubular capillary are not physically connected , substances are passed between the both structures through active and passive transport mechanisms.

Tubular reabsorption  is controlled by epithelial cells that make up the renal tubules and collecting ducts: cells of the PCT has microvilli, which greatly increase the surface area exposed to the glomerular filtrate and enhance reabsorption. →

Sodium reabsorption occurs via sodium- pump mechanism (active transport)- uses energy

Cl- PO43 and HCO3  Reabsorption: Chloride , phosphate and bicarbonate, plus other anions , passively accompany the movement of NA+ due to the electromechanical attraction between oppositely charged particles . active transport may reabsorb some of the ions, such as hco and PO directly

Water reabsorption occurs via osmosis , and is closely associated with the active reabsorption of Na+. Thus as na+ reabsorption increases, water reabsorption increases , and vise versa

Glucose reabsorption occurs via active transport - carrier molecules. carrier molecules transport only a certain number of molecules in a given time because the number of carries is limited. 

usually all the glucose within the glomerular filtrate is reabsorbed because there are enough carrier molecules to transport it. when the plasma glucose concentration reaches the renal plasma threshold (critical level)  More glucose molecules are in the filtrate than the active transporters can handle. as a result, some glucose remain in the filtrate and is excreted in the urine. 

FYI glucose is needed for cellular respiration 

Tubular secretion is essentially  back up process for glomerular filtration.

During tubular secretion, water and particles from the peritubular capillary are secreted into the distal convoluted tubule for eventual elimination from the body. Tubular secretion is essentially reabsorption in reverse. 

Similar to tubular reabsorption , the DCT and peritubular capillary are physiologically connected: Substances are passed between the two structures through active and passive transport mechanisms .

Major processes :

Disposing of certain drugs and metabolites, such as penicillin.

Regulates blood PH:H+ is actively secreted throughout the entire renal tubule and eliminate from the body. Note that the tubular secretion of H+ is linked to tubular reabsorption of HCO3

Eliminates nitrogen waste products: Nitrogen is formed as a byproduct  of metabolism. Nitrogen is transported in the blood in the form of NH+4, Urea, and creatine; and is ultimately eliminated by the kidneys.

Urea also plays a role in the counter count exchange systems of the nephrons , which allows for reabsorption of water and critical ions from excreted urine. the mechanism is important for preventing water loss: maintaining blood pressure , and maintaining suitable Na+ in the blood plasma

Eliminating excess K+- potassium is critical for the normal functioning of smooth muscle , skeletal muscle, cardiac muscle, and nerves, Relatively small changes in intracellular K+ can lead to tremendous changes in the plasma K+

composition of concentration of urine

Urine Composition reflects the filtration, reabsorption, and secretion activities of the nephrons. It also reflects the volume of water and solutes that the kidney must eliminate from the body or retain in the internal environment to maintain homeostasis. 

Composition varies with metabolic and hormonal events under way:

• water content 93-97%
• Specific gravity : 1.003-1.030
• ph: 6.0 slightly acidic d/t H+ during tubular reabsorption
• Color: clear yellow
• Bacterial content: sterile

• Major organic waste products:
• Urea-most abundant , produced by the breakdown of amino acids .

Creatine -produced by breakdown of creatine phosphate (associated with muscle contraction)

Uric acid- formed by the recycling of RNA

Urine volume:0.6-2.5 L a day, factors such as fluid intake , environmental temp, emotional conditions, respiratory rate, and body temp influence the urine volume.→

Kidneys are capable of producing urine that is either more dilute or more concentrated than plasma, The kidneys can conserve water when it is in short supply and get rid of it when there is too much

Loop of henle modifies urine water content

Antidiuretic hormone (ADH, vasopressin) is secreted by the posterior pituitary gland.

ADH inhibits diuresis (urine output), and thus conserves water in the body. 

Urine volume is regulated by controlling the water permeability of the collecting ducts(and to a lesser extent, the renal tubule)

• Producing concentrated Urine: Conserving water 
• The formation of concentrated urine requires ADH.

Key: in the presence of ADH most of the water is reabsorbed from the collecting duct, leaving a small volume of concentrated urine to be excreted.

• Producing Dilute Urine: Excreting Excess Water 
•  
• kEY:in the absence of ADH ..the collecting duct is impermeable to water. thus water remain trapped inside the collecting duct and is bound for excretion as urine

If water is reabsorbed into the body, what happens to urine concentration ? it reduces the amount of water in urine so its more concentrated

If water is reabsorbed into the body the blood pressure will go up .

Ureters

Each ureter is a tubular organ(25cmWhich begins as the funnel- shaped renal pelvis .

Ureters pass downward retroperitoneally to reach the bladder within the pelvic cavity

The presence of urine in the renal pelvis initiates peristaltic waves. which move urine along the length of the ureter.The frequency of construction keeps  pace with the rate of urine formation. 

• wall of the Ureter
• Mucous coat : Transitional epithelium
• Muscular coat: Smooth muscle fibers in circular and longitudinal bundles 
• Fibrous coat- Outer connective tissue layer.→

The urinary bladder is a hollow, distensible muscular organ, situated in the pelvic cavity it varies in size and shape depending on its fullness.

Females: The bladder contacts the anterior walls of the uterus and vagina

Males : the bladder lies anterior to the rectum 

Rugae(folds) are present inside the bladder, except at a smooth, triangular area call the trigone

• Orifices of the ureters are situated posteriorly to the base of the trigone.
• Internal Urethral orificeis situated anteriorly at the apex of the trigone→

• Mucous coast: Transitional epithelium 
• Submucous coat:CT with elastic fibers. Found  everywhere , except for the trigone. 
• Muscular coat:Bundles of interlaced smooth muscle (detrusor muscle). Innervated by the ANS 
• Males :detrusor muscle surrounds the neck of the bladder to form the internal urethral sphincter
• Females:detrusor muscle follows the urethra, but does not form a true sphincter. (In females muscle , fibers are oriented in a longitudinal manner at the urethra, as opposed to circular)

Adventitial coat:  fibrous connective tissue forming the outermost coat 

Parietal peritoneum blankets the superior surface of the bladder.→

Filling of the bladder- smooth muscle allows bladder to stretch (due to accommodation)

Autonomic innervation of the bladder

sympathetic : ( F OR F)-want to hold the urine. relaxation of the detrusor muscle (relaxes bladder wall) contracts internal sphincter

Parasympathetic: (Rest and digest) contraction of the detrusor muscle (contracts bladder wall) relaxation of internal urethral sphincter.

Thin walled muscular tube draining urine from bladder to the outside of the body.

Females-The urethra  4 cm long , passes from the bladder and courses below the symphysis pubis, where it is surrounded by the compressor urethrae muscle (skeletal muscle)

The urethra terminates as the external urethral orifice at the vestibule. The external urethral orifice is anterior the vaginal opening and posterior to the clitoris. 

Paraurethral glands (skene's glands) secrete mucus and are associated with the lower end of the urethra. Each drains via duct that opens into the lateral margin of the external urethral orifice. 

• Males Duel Function: carries semen from testes and urine from bladder.
•  
• It is divided into three sections 

Prostatic Urethra - passes from the bladder through the prostate gland. The ejaculatory and prostatic duct join the prostatic urethra . 

Membranous Urethra- passes through the deep perineal pouch, surrounded by the sphincter urethrae muscle  (skeletal muscle)

Penile (spongy) Urethra 15cm-  It passes through the corpus spongiosum of the penis and terminates as the external urethral orifice at the glans →

Internal Urethral sphincter

Formed by detrusor muscle of the bladder

Unconsciously controlled by the ANS - keeps urethra close when there is little pressure inside the bladder. 

External Urethral sphincter

Formed by Skeletal muscle in the deep perineal pouch of both sexes.

Males:- In males , the external urethral sphincter forms a true ring around membranous urethra. Thus , it is called the sprinter urethrae

Female:  In females, the external urethral sphincter , know as the compressor urethra, only partially encircles the urethra as it passes through the deep perineal pouch. thus contraction of the muscle produces compresses the urethra

Significance- The compressor urethrae is structurally weaker and more subject to damage caused by stretching of the perineum, thus females are more prone to urinary incontinence

Consciously controlled by the somatic nervous system m via pudendal nerve =( shame full areas)

Mean  bladder capacity in male adults varies around 400 500 + is pain caused by tension in the wall, leading to the urgent desire to void. pain is referred to the cutaneous areas supplied by spinal segments supplying the bladder

Micturition,the act of emptying the bladder . it consist of storage phase and voiding phase.

• Storage phase -as the bladder fills with urine, distention of the bladder wall initiates storage reflexes . Afferent impulses from stretch receptors in the bladder are sent to the spinal cord resulting in the following.
• 1.
• activation of sympathetics: To detrusor muscle and internal urethral sphincter 

2. Activation of somatic efferents: via duodenal nerve  to external urethral sphincter.

3. additionally, activation of somatic efferents is further stimulated by the storage center within the pons , thus providing conscious control.

• Micturition Reflex- the micturition reflex is initiated either by further bladdera distension  more than 500 , which increases afferent impulses to the pontine micturition center, or by the input from higher brain center initiating voluntary micturition.
• 1. afferent impulses from stretch receptors in the bladder are sent to the pons.

2. The pontine micturition center is activated and results in the following 

a. Activation of parasympathetics,(to detrusor muscle and internal urethral sphincter)

b. inhibition of sympathetics

inhibition of somatic efferents .(Relaxes External urethral sphincter).