The Human Excretory The Human Excretory SystemSystem

There are certain There are certain organismsorganisms which lose the metabolic  which lose the metabolic

wastes through their body by diffusion. wastes through their body by diffusion. It It includesincludes sponges, coelenterates and echinoderms.  sponges, coelenterates and echinoderms.

- There are - There are flame cells flame cells which help in excretion and occur in which help in excretion and occur in flat wormsflat worms. .

- The excretory system is unique in - The excretory system is unique in round worms round worms which which contains contains canalscanals and  and tubetube. .

- AnnelidsAnnelids have have nephridianephridia as their excretory organ. as their excretory organ.- ArthropodsArthropods have have malphigian tubules malphigian tubules as their excretory as their excretory

organ. organ. - PrawnsPrawns have have green glands green glands for excretion and for excretion and vertebrates vertebrates

have have kidneyskidneys as their excretory organ.  as their excretory organ. 

The role ofThe role of excretory organ is to remove unwanted wastes  excretory organ is to remove unwanted wastes and regulate and regulate the waterthe water content in body and is known as  content in body and is known as Osmoregulation Osmoregulation

Animals are classified on the basis of type of removal Animals are classified on the basis of type of removal

of nitrogenous wasteof nitrogenous waste.. They can be ammonotelic, ureotelic, uricotelic and They can be ammonotelic, ureotelic, uricotelic and

aminotelic. aminotelic.

(a)(a) AmmonotelicAmmonotelic animals are those which excrete animals are those which excrete ammoniaammonia. It includes . It includes spongessponges, , ascarisascaris, liver fluke, , liver fluke, prawn, bony fish and prawn, bony fish and crocodilecrocodile,, frog frog etc. etc.

(b)(b) UreotelicUreotelic animals are those which excrete animals are those which excrete ureaurea. . It includes  It includes  earthwormearthworm, , mammalsmammals including man and including man and turtlesturtles etc. etc.

(c)(c) UricotelicUricotelic animals are those which excrete  animals are those which excrete uric aciduric acid. . InsectsInsects, crustaceans, , crustaceans, reptiles reptiles and and birdsbirds are included in it. are included in it.

(d)(d) AminotelicAminotelic animals are those which excrete animals are those which excrete amino amino acidsacids. . MolluscsMolluscs and and echinodermsechinoderms are included in this are included in this group. group.

Excretory Organ of the Human bodyExcretory Organ of the Human body

(1) SKIN(1) SKIN(2) LUNGS(2) LUNGS(3) LIVER(3) LIVER(4) KIDNEY(4) KIDNEY

((What is the Spleen?What is the Spleen? The human spleen is an organ that creates lymphocytes for the The human spleen is an organ that creates lymphocytes for the

destruction and recycling of old red-blood cells. The spleen is destruction and recycling of old red-blood cells. The spleen is also a blood reservoir. It supplies the body with blood in also a blood reservoir. It supplies the body with blood in emergencies such as a bad cut. The spleen is also the location emergencies such as a bad cut. The spleen is also the location where white blood cells trap organisms)where white blood cells trap organisms)

Excretory SystemExcretory SystemThe The kidneys kidneys regulate the amount of water, salts regulate the amount of water, salts and other substances in the blood. and other substances in the blood. The kidneys are fist-sized, bean shaped The kidneys are fist-sized, bean shaped structures that remove nitrogenous wastes structures that remove nitrogenous wastes (urine) and excess salts from the blood. (urine) and excess salts from the blood. The The ureters ureters are tubes that carry urine from the are tubes that carry urine from the pelvis of the kidneys to the urinary bladder. pelvis of the kidneys to the urinary bladder. The The urinary bladder urinary bladder temporarily stores urine temporarily stores urine until it is released from the body. until it is released from the body. The The urethra urethra is the tube that carries urine from is the tube that carries urine from the urinary bladder to the outside of the body. the urinary bladder to the outside of the body. The outer end of the urethra is controlled by a The outer end of the urethra is controlled by a circular muscle called a sphincter.circular muscle called a sphincter.

Excretory SystemExcretory System

The KidneyThe KidneyEach kidney is composed of three Each kidney is composed of three sections: sections: – the outer (renal) cortex, the (renal) medulla the outer (renal) cortex, the (renal) medulla

(middle part) and the hollow inner (renal) (middle part) and the hollow inner (renal) pelvis. pelvis.

The The cortex cortex is where the blood is filtered. is where the blood is filtered.

The The medulla medulla contains the collecting ducts which contains the collecting ducts which carry filtrate (filtered substances) to the pelvis. carry filtrate (filtered substances) to the pelvis.

The The pelvis pelvis is a hollow cavity where urine is a hollow cavity where urine accumulates and drains into the ureter.accumulates and drains into the ureter.

The Parts of the Kidney

How blood is FilteredHow blood is Filtered

The filtering units of the kidneys are the The filtering units of the kidneys are the nephronsnephrons. . There are approximately one million nephrons in There are approximately one million nephrons in each kidney. each kidney. The nephrons are located within the cortex and The nephrons are located within the cortex and medulla of each kidney. medulla of each kidney. The tubes of the nephron are surrounded by The tubes of the nephron are surrounded by cells and a network of blood vessels spreads cells and a network of blood vessels spreads throughout the tissue. Therefore, material that throughout the tissue. Therefore, material that leaves the nephron enters the surrounding cells leaves the nephron enters the surrounding cells and returns to the bloodstream by a network of and returns to the bloodstream by a network of vessels. vessels.

Parts of the NephronParts of the Nephron

Each nephron consists of the following Each nephron consists of the following parts: parts: – 1) glomerulus ; 1) glomerulus ; – 2) Bowman’s capsule ; 2) Bowman’s capsule ; – 3) proximal tubule ; 3) proximal tubule ; – 4) loop of Henle ; 4) loop of Henle ; – 5) distal tubule ; 5) distal tubule ; – 6) collecting duct.6) collecting duct.

The GlomerulusThe Glomerulus

The The glomerulus glomerulus is a mass of thin-walled is a mass of thin-walled capillaries. capillaries. The The Bowman’s capsule Bowman’s capsule is a double-walled, is a double-walled, cup-shaped structure. cup-shaped structure. The The proximal tubule proximal tubule leads from the Bowman’s leads from the Bowman’s capsule to the Loop of Henle. capsule to the Loop of Henle. The The loop of Henle loop of Henle is a long loop which extends is a long loop which extends into the medulla. into the medulla. The The distal tubule distal tubule connects the loop of Henle to connects the loop of Henle to the collecting duct.the collecting duct.

How the kidney does its job!How the kidney does its job!Kidney ActionKidney ActionBlood enters Bowman’s capsule through a tiny Blood enters Bowman’s capsule through a tiny artery — (artery — (the renal arterythe renal artery). ). The artery branches to form a The artery branches to form a glomerulusglomerulus. . Blood pressure forces some blood plasma and Blood pressure forces some blood plasma and small particles into the surrounding capsule — small particles into the surrounding capsule — (this is called the (this is called the nephric filtratenephric filtrate). ). Large particles such as blood cells and proteins Large particles such as blood cells and proteins remain in capillaries. remain in capillaries. The nephric filtrate is pushed out of the capsule The nephric filtrate is pushed out of the capsule and into the and into the proximal tubule proximal tubule . . This is where reabsorption begins. This is where reabsorption begins.

Kidney action (cont)Kidney action (cont)

Only materials needed by the body are Only materials needed by the body are returned to bloodstream — for example, returned to bloodstream — for example, 99 % of water, all glucose and amino acids 99 % of water, all glucose and amino acids and many salts are reabsorbed. and many salts are reabsorbed. Osmosis , diffusion, and active transport Osmosis , diffusion, and active transport draw water , glucose , amino acids and draw water , glucose , amino acids and ions from filtrate into surrounding cells. ions from filtrate into surrounding cells. Small villi like projections help in the active Small villi like projections help in the active transport of glucose out of the filtrate and transport of glucose out of the filtrate and speed up the reabsorption process. speed up the reabsorption process. From here, these components return to From here, these components return to the bloodstream.the bloodstream.

Kidney action (cont)Kidney action (cont)

The filtrate reaches the end of the The filtrate reaches the end of the proximal tubule and the fluid is isotonic proximal tubule and the fluid is isotonic with the surrounding cells. with the surrounding cells.

Glucose and amino acids have been Glucose and amino acids have been removed from the filtrate. removed from the filtrate.

The filtrate then moves to the The filtrate then moves to the loop of loop of Henle Henle whose primary function is to whose primary function is to remove water from the filtrate by osmosis. remove water from the filtrate by osmosis.

Kidney action (cont)Kidney action (cont)

There is in increase in sodium There is in increase in sodium concentration in the cells of the loop of concentration in the cells of the loop of Henle as we move from the area of the Henle as we move from the area of the cortex to the inner pelvis of the kidney.cortex to the inner pelvis of the kidney.

This causes water to be drawn from the This causes water to be drawn from the filtrate. filtrate.

These high levels of sodium are a result of These high levels of sodium are a result of active transport and results in increased active transport and results in increased concentration of filtrate (due to water loss).concentration of filtrate (due to water loss).

Why doesn’t water on outside go Why doesn’t water on outside go back in?back in?

The water that left the descending loop The water that left the descending loop cannot enter the ascending loop because cannot enter the ascending loop because it is impermeable to water. it is impermeable to water. As the water concentration in the filtrate As the water concentration in the filtrate decreases, the chlorine concentration decreases, the chlorine concentration increases and causes it to diffuse out of increases and causes it to diffuse out of the tubule. the tubule. This process is helped by the electrical This process is helped by the electrical attraction of chlorine to sodium. .attraction of chlorine to sodium. .

Water (cont)Water (cont)

The filtrate moves to the The filtrate moves to the distal tubule distal tubule where tubular secretion occurs. where tubular secretion occurs. Active transport is used to pull hydrogen Active transport is used to pull hydrogen ions, creatinine, drugs such as penicillin ions, creatinine, drugs such as penicillin out of the blood and into the filtrate. out of the blood and into the filtrate. Fluid from a number of nephrons moves Fluid from a number of nephrons moves from the distal tubules to a common from the distal tubules to a common collecting duct collecting duct which carries what can which carries what can now be called now be called urine urine to the to the renal pelvisrenal pelvis..

NOTENOTE

Since the kidneys control what leaves Since the kidneys control what leaves and what remains in the nephrons, they and what remains in the nephrons, they maintain the levels of water, ions and maintain the levels of water, ions and other materials nearly constant and other materials nearly constant and within the limits to maintain within the limits to maintain homeostasishomeostasis..

Function of the Kidney

The principal function of the kidney is to filter blood in order to remove cellular waste products from the body.

At any given time, 20 % of blood is in the kidneys. Humans can function with one kidney.

If one ceases to work, the other increases in size to handle the workload.

The kidney has other functions but it is usually associated with the excretion of cellular waste such as :

1) urea (a nitrogenous waste produced in the liver from the breakdown of protein. It is the main component of urine) ;

2) uric acid (usually produced from breakdown of DNA or RNA) and

3) creatinine (waste product of muscle action).

All of these compounds have nitrogen as a major component.

The kidneys are more than excretory organs.

They are one of the major homeostatic organs of the body.

They control water pH, secrete erythropoietin (a hormone that stimulates red blood cell production) and activate vitamin D production in the skin.

That is why a doctor can tell so much from a urine sample.

Disorders of the Excretory System

Our excretory system is very important in maintaining homeostasis. There are certain conditions which can affect the excretory system. We will discuss some of these common disorders.

UTI (Urinary Tract Infection)

Is a very common disorder. If the bladder has become infected, it is known as cystitis. If the urethra is infected., it is called urethritis.

UTI is more common in women than men because of differences in anatomy.

Symptoms include painful urination burning sensation), frequent urination (even if no urine present) and bloody or brown urine.

This can lead to chills, fever, nausea, vomiting and upper abdomen tenderness.

Kidney InfectionsResult when an infection reaches the kidneys and becomes known as pyelonephritis.

Common causes can be infection from elsewhere in the body or obstruction of the prostate gland (usually in older men).

For children, infection can be caused by the tube that drains urine from the kidneys and the bladder.

NOTE :

If left untreated, all UTI’s can lead to permanent kidney damage and possible kidney failure.

The general treatment is by antibiotics. A person needs to maintain good personal hygiene when eliminating wastes (liquid and solid forms) from the body.

Also, a person should drink lots of water.

Kidney StonesAre also a common kidney disorder. They form when chemicals in the urine precipitate out and form crystals. The most common crystals are from calcium oxalate, while others could be from uric acid and cystine. Kidney stones are more common in men than women and can reoccur at any time. Factors such as recurrent urinary bladder infections, insufficient water intake and consumption, low levels of physical activity, and too much Vitamin C and D intake can lead to kidney stones. One of the best ways to decrease the occurrence of stones is to drink lots of water and to change your dietary habits.

Kidney Stones (cont)Kidney Stones (cont)

Symptoms include severe back or abdomen pain, blood in the urine, nausea and vomiting. Diagnosis involves a complete medical examination, including X-rays. Treatment may vary from letting the stones pass through the urinary tract to ultrasound shock (or lithotripsy) to disintegrate the stones to a small size that can be passed through the urinary tract. Real large stones require surgery for removal.

Diseases and Conditions of the KidneyDiseases and Conditions of the KidneySome of the most common kidney diseases and conditions are: Some of the most common kidney diseases and conditions are: polycystic kidney disease,nephrosis, lupus nephritis, diabetic polycystic kidney disease,nephrosis, lupus nephritis, diabetic nephropathy,rhabdomyolysis, kidney stones, and renal tubular nephropathy,rhabdomyolysis, kidney stones, and renal tubular acidosis.acidosis.Polycystic kidney diseasePolycystic kidney disease (PKD) is an inherited disease  (PKD) is an inherited disease characterized by the growth of numerous cysts filled with fluid in characterized by the growth of numerous cysts filled with fluid in the kidneys.the kidneys.Nephrosis is a kidney disease characterized by lesions of the Nephrosis is a kidney disease characterized by lesions of the epithelial lining of the renal tubules. The lesions cause a epithelial lining of the renal tubules. The lesions cause a disturbance in the filtration function of the kidney. As a result, disturbance in the filtration function of the kidney. As a result, large amounts of protein is found in the urine.large amounts of protein is found in the urine.Lupus nephritis is an inflammation of the kidney caused by Lupus nephritis is an inflammation of the kidney caused by systemic lupus erythematosus (SLE), a disease of the immune systemic lupus erythematosus (SLE), a disease of the immune system.system.Diabetic Nephropathy is a kidney condition that occurs only in Diabetic Nephropathy is a kidney condition that occurs only in people with diabetes mellitus. It results in progressive damage people with diabetes mellitus. It results in progressive damage to the small filtering units of the kidney (glomeruli). About 20-to the small filtering units of the kidney (glomeruli). About 20-30% of people with diabetes develop diabetic nephropathy.30% of people with diabetes develop diabetic nephropathy.Glomerulonephritis is a type of kidney disease caused by Glomerulonephritis is a type of kidney disease caused by inflammation of the internal kidney structures.inflammation of the internal kidney structures.

Pyelonephritis is an infection of the kidney and the ureters.Pyelonephritis is an infection of the kidney and the ureters.Rhabdomyolysis is a disorder involving injury to the kidney. It Rhabdomyolysis is a disorder involving injury to the kidney. It has been linked to the drug crestor.has been linked to the drug crestor.A kidney stone is a hard mass developed from crystals that A kidney stone is a hard mass developed from crystals that separate from the urine and build up on the inner surfaces of separate from the urine and build up on the inner surfaces of the kidney. Kidney stones occur in roughly one in 10 people in the kidney. Kidney stones occur in roughly one in 10 people in the United States. Once a person gets more than one stone, the United States. Once a person gets more than one stone, others are likely to develop.others are likely to develop.Renal tubular acidosis (RTA) is a disease that occurs when the Renal tubular acidosis (RTA) is a disease that occurs when the kidneys fail to excrete acids into the urine, which causes a kidneys fail to excrete acids into the urine, which causes a person's blood to remain too acidic.person's blood to remain too acidic.Location of the KidneyLocation of the KidneyTo look at the kidney in relation to other organs in the body, To look at the kidney in relation to other organs in the body, visit the digestive systemvisit the digestive systemKidneys and DiabetesKidneys and DiabetesDiabetes is the most common cause of kidney failure, Diabetes is the most common cause of kidney failure, accounting for more than 40 percent of new cases. High blood accounting for more than 40 percent of new cases. High blood pressure, or hypertension, is a major factor in the development pressure, or hypertension, is a major factor in the development of kidney problems in people with diabetes.of kidney problems in people with diabetes.

Organ Donations Save Lives! Organ Donations Save Lives! 

Organ donations are vital because they provide necessary lifesaving Organ donations are vital because they provide necessary lifesaving measures for individuals who need the organs to survive. The following will measures for individuals who need the organs to survive. The following will detail some information regarding organ donations and highlight procedures detail some information regarding organ donations and highlight procedures where organ donation may be necessary.where organ donation may be necessary.

What is organ donation?What is organ donation? Organ donation is the removal of specific tissues of the human body from a Organ donation is the removal of specific tissues of the human body from a

person who has recently died, or from a living donor, for the purpose of person who has recently died, or from a living donor, for the purpose of transplanting them into other persons. Anyone can choose to be an organ transplanting them into other persons. Anyone can choose to be an organ donor. However, individuals who are under the age of 18 need parental donor. However, individuals who are under the age of 18 need parental consent in order to be an organ donor. If you decide to be an organ donor, consent in order to be an organ donor. If you decide to be an organ donor, you can place an 'organ donor' sticker on your driver’s license and/or on an you can place an 'organ donor' sticker on your driver’s license and/or on an organ donor card.organ donor card.Organ donation proceduresOrgan donation procedures

When an individual elects to have their organs donated after they die, their When an individual elects to have their organs donated after they die, their organs will be matched to possible recipients on the organ donation registry. organs will be matched to possible recipients on the organ donation registry. If there is a match, the organs will be distributed to the people who have the If there is a match, the organs will be distributed to the people who have the highest priority on the list.highest priority on the list.

Types of organs that can be donatedTypes of organs that can be donated The types of organs which can be donated include The types of organs which can be donated include

lungs, liver, pancreas, intestines, heart, kidneys, and skin.lungs, liver, pancreas, intestines, heart, kidneys, and skin.

Considerations Regarding Organ DonationConsiderations Regarding Organ Donation Organ donation is a serious matter. It is an option that should be examined Organ donation is a serious matter. It is an option that should be examined

closely and discussed with various individuals. It is very important to discuss closely and discussed with various individuals. It is very important to discuss organ donation wishes with family members. It is important that family organ donation wishes with family members. It is important that family members are told about a decision to become an organ donor.members are told about a decision to become an organ donor.Those who wish to become organ donors should also discuss the issue with Those who wish to become organ donors should also discuss the issue with medical health professionals or representatives at the organ donor registry medical health professionals or representatives at the organ donor registry so that they know exactly what they are signing up for. Having as much so that they know exactly what they are signing up for. Having as much information as possible will lead the individual to make an informed decision information as possible will lead the individual to make an informed decision regarding their organ donation selection.regarding their organ donation selection.Organ donation is neededOrgan donation is needed

Organ donation is something which is always in demand. There are many Organ donation is something which is always in demand. There are many individuals on waiting lists throughout the world who need organ individuals on waiting lists throughout the world who need organ replacements as soon as possible. Therefore, if one is considering organ replacements as soon as possible. Therefore, if one is considering organ donation it is helpful to know that their election to do so will help a great deal donation it is helpful to know that their election to do so will help a great deal of individuals who currently need organ transplants to survive.of individuals who currently need organ transplants to survive.

Organ Transplants. What You Should Know!Organ Transplants. What You Should Know! Transplants are the best possible treatment for most people with organ Transplants are the best possible treatment for most people with organ

failure. A transplant is the replacement of a patient's organ(s) with a normal failure. A transplant is the replacement of a patient's organ(s) with a normal organ(s) from someone--called a donor--who has died or is living.organ(s) from someone--called a donor--who has died or is living.

The donor's organ(s) is completely removed and quickly transported to the The donor's organ(s) is completely removed and quickly transported to the patient. Sometimes the patient may be in a different state or country. Organs patient. Sometimes the patient may be in a different state or country. Organs are cooled and kept in a special solution while being taken to the patient.are cooled and kept in a special solution while being taken to the patient.

Some of the body's replaceable parts are listed below.Some of the body's replaceable parts are listed below.

Bone Marrow:Bone Marrow:Bone marrow is a spongy tissue found inside bones. The bone marrow in Bone marrow is a spongy tissue found inside bones. The bone marrow in the breast bone, skull, hips, ribs and spine contains stem cells that produce the breast bone, skull, hips, ribs and spine contains stem cells that produce the body's blood cells.the body's blood cells.Transplants of blood-producing bone marrow cells are used to treat Transplants of blood-producing bone marrow cells are used to treat conditions such as leukemia,multiple myeloma, immune deficiency conditions such as leukemia,multiple myeloma, immune deficiency disorders, lymphoma, and breast cancer.disorders, lymphoma, and breast cancer.

CorneaCorneaThe cornea is the clear "window" in front of the eye. It allows allows light to The cornea is the clear "window" in front of the eye. It allows allows light to enter into the eye.The first cornea transplant was performed in 1905. enter into the eye.The first cornea transplant was performed in 1905. Corneas can be preserved for up to 10 days before transplant.Corneas can be preserved for up to 10 days before transplant.

CellsCells Transplanting cells taken from aborted fetal tissue is sometimes Transplanting cells taken from aborted fetal tissue is sometimes

performed to try to improve degenerative disease such performed to try to improve degenerative disease such as macular degeneration, parkinson disease, and huntington as macular degeneration, parkinson disease, and huntington disease.disease.HeartHeart

A healthy heart is obtained from a donor who has suffered brain A healthy heart is obtained from a donor who has suffered brain death but remains on life-support. The healthy heart is death but remains on life-support. The healthy heart is transported in a special solution that preserves the organ.transported in a special solution that preserves the organ.KidneyKidney

Kidney transplants are most successful when the kidney comes Kidney transplants are most successful when the kidney comes from a living relative. Kidney transplants are often options for from a living relative. Kidney transplants are often options for people with polycystic kidney disease and kidney failure.people with polycystic kidney disease and kidney failure.LiverLiver

A liver transplant is necessary when a disease causes the liver A liver transplant is necessary when a disease causes the liver to stop working. The most common reason for liver to stop working. The most common reason for liver transplantation in adults is cirrhosis and liver cancer. Cirrhosis transplantation in adults is cirrhosis and liver cancer. Cirrhosis is a disease in which healthy liver cells are killed and replaced is a disease in which healthy liver cells are killed and replaced with scar tissue.with scar tissue.

LungLung A lung transplant is necessary when a disease causes A lung transplant is necessary when a disease causes

the lung to stop working. Lung transplants are often the lung to stop working. Lung transplants are often performed with a heart transplant.performed with a heart transplant.PancreasPancreas

Pancreas transplants give the patient a chance to Pancreas transplants give the patient a chance to become independent of insulin injections. If the patient become independent of insulin injections. If the patient is diabetic and needs a kidney transplant, the kidney is diabetic and needs a kidney transplant, the kidney and pancreas transplant may be performed at the and pancreas transplant may be performed at the same time.same time.IntestineIntestine

Most recipients of small intestine transplants are Most recipients of small intestine transplants are between the ages of 1 and 5 or 18 and 34.between the ages of 1 and 5 or 18 and 34.StomachStomach

Stomach transplants are uncommon. When they are Stomach transplants are uncommon. When they are performed, they are usually done as part of a multi-performed, they are usually done as part of a multi-organ transplant along with the liver, intestine or organ transplant along with the liver, intestine or pancreaspancreas

EYEEYE

What is an Eye?What is an Eye?An eye is a round-shaped organ that works with the brain to An eye is a round-shaped organ that works with the brain to

provide us with vision. The shape of the eye is maintained by provide us with vision. The shape of the eye is maintained by the pressure of the aqueous humor. The aqueous humor is the the pressure of the aqueous humor. The aqueous humor is the fluid that fills the front chamber of the eye.fluid that fills the front chamber of the eye.Function of the EyeFunction of the Eye

The main function of the eye is to work with the brain to provide us The main function of the eye is to work with the brain to provide us with vision. The eye and brain translate light waves into a with vision. The eye and brain translate light waves into a sensation we call vision.sensation we call vision.Eye PartsEye Parts

The eye has many parts. Some of the main parts are listed and The eye has many parts. Some of the main parts are listed and described below.described below.lenslens– The transparent crystalline lens of the eye is located immediately behind The transparent crystalline lens of the eye is located immediately behind

the iris.the iris.

corneacornea– The cornea is a transparent dome which serves as the outer window of The cornea is a transparent dome which serves as the outer window of

the eye. The cornea is the most powerful structure focusing light entering the eye. The cornea is the most powerful structure focusing light entering the eye.the eye.

retinaretina– The retina is the innermost layer of the eye. It is composed of nerve tissue which The retina is the innermost layer of the eye. It is composed of nerve tissue which

senses the light entering the eye.senses the light entering the eye.– The retina sends impulses through the optic nerve back to the brain, which The retina sends impulses through the optic nerve back to the brain, which

translates the impulses into images that we see.translates the impulses into images that we see.– There are 4 types of light-sensitive receptors found in the retinaThere are 4 types of light-sensitive receptors found in the retina

rodsrodscones that absorb long-wavelength light (red)cones that absorb long-wavelength light (red)cones that absorb middle-wavelength light (green)cones that absorb middle-wavelength light (green)cones that absorb short-wavelength light (blue)cones that absorb short-wavelength light (blue)

pupilpupil– The pupil is the hole in the center of the eye where light passes through.The pupil is the hole in the center of the eye where light passes through.

irisiris– The iris is the colored part of the eye. It is a thin diaphragm composed mostly of The iris is the colored part of the eye. It is a thin diaphragm composed mostly of

connective tissue and smooth muscle fibers. The iris lies between the cornea and connective tissue and smooth muscle fibers. The iris lies between the cornea and the crystalline lens.the crystalline lens.

optic nerveoptic nerve– The optic nerve is a continuation of the axons of the ganglion cells in the retina. It The optic nerve is a continuation of the axons of the ganglion cells in the retina. It

acts like a cable connecting the eye with the brain.acts like a cable connecting the eye with the brain.– The optic nerve is also called the cranial nerve II.The optic nerve is also called the cranial nerve II.

sclerasclera– The sclera is the white, opaque portion of the eye. It provides protection and The sclera is the white, opaque portion of the eye. It provides protection and

serves as an attachment for the extra-ocular muscles which move the eye.serves as an attachment for the extra-ocular muscles which move the eye.