My Science School

Inside the kidneys are thousands of tiny structures called glomeruli (one is called a glomerulus), working hard to filter toxic waste from the bloodstream. These tightly coiled clusters are the body’s smallest blood vessels (capillaries).

Each glomerulus works like a miniature sieve, transferring waste and excess water from the blood through the capillary wall into a renal tubule. This fluid drains away towards the bladder as urine.

The glomerulus receives its blood supply from an afferent arteriole of the renal arterial circulation. Unlike most capillary beds, the glomerular capillaries exit into efferent arterioles rather than venules. The resistance of the efferent arterioles causes sufficient hydrostatic pressure within the glomerulus to provide the force for ultrafiltration.

The glomerulus and its surrounding Bowman's capsule constitute a renal corpuscle, the basic filtration unit of the kidney.[2] The rate at which blood is filtered through all of the glomeruli, and thus the measure of the overall kidney function, is the glomerular filtration rate.

 

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If the kidneys become damaged or diseased, a dialysis machine can be used to filter the blood instead. This large machine takes up to four times longer than kidneys to clean the body’s blood – which shows what an efficient unit the kidneys are.

The dialysis machine acts as an artificial set of kidneys. Blood flows from the body to the machine, toxic waste and excess fluid are removed, and cleaned blood is returned.

Some kinds of acute kidney failure get better after treatment. In some cases of acute kidney failure, dialysis may only be needed for a short time until the kidneys get better.

In chronic or end stage kidney failure, your kidneys do not get better and you will need dialysis for the rest of your life. If your doctor says you are a candidate, you may choose to be placed on a waiting list for a new kidney.

In hemodialysis, an artificial kidney (hemodialyzer) is used to remove waste and extra chemicals and fluid from your blood. To get your blood into the artificial kidney, the doctor needs to make an access (entrance) into your blood vessels. This is done by minor surgery to your arm or leg.

 

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Each kidney has three layers – the outer layer (cortex), inner part (medulla), and central pelvis. Blood flows into the cortex for filtering. The medulla absorbs substances to return to the bloodstream. Waste is taken by tubes to the central pelvis, a collecting area where urine is emptied out into two tubes called ureters, and then passes to the bladder.

Multi-purpose organs

The two kidneys sit high in the back the abdomen. Each one is about the size of a fist, shaped like a bean, and surrounded by a protective layer of tissue.

Adrenal gland

The adrenal glands are small glands located on top of each kidney. They produce hormones that you can't live without, including sex hormones and cortisol. Cortisol helps you respond to stress and has many other important functions. Adrenaline released from this gland makes the heart beat faster in scary situations.

Renal artery

The renal arteries are responsible for carrying oxygen-rich blood to your kidneys, which in turn help the kidneys rid your body of waste and excess fluid. This artery carries blood into the kidney to be filtered.

Renal vein

The renal vein is an asymmetrically paired vessel that carries the deoxygenated blood from the kidney to the inferior vena cava. Cleaned blood is carried from the kidney by the renal vein.

Outer casing

The kidneys and adrenal glands are wrapped in a layer of fat and strong outer tissue. The outermost layer is a tough connective tissue layer called the renal fascia.

Central pelvis

Urine collects here, and is then sent to the bladder. The pelvis, which is shaped somewhat like a funnel that is curved to one side, is almost completely enclosed in the deep indentation on the concave side of the kidney, the sinus.

Renal cortex

The cortex is the outer part of the kidney. It contains the glomerulus and convoluted tubules.

The renal cortex is surrounded on its outer edges by the renal capsule, a layer of fatty tissue. Together, the renal cortex and capsule house and protect the inner structures of the kidney.

Renal medulla

This layer of the kidney absorbs water, making urine more concentrated. The renal medulla is split up into a number of sections, known as the renal pyramids. Blood enters into the kidney via the renal artery, which then splits up to form the interlobar arteries.

Left ureter

This is one of two tubes that carry urine down to the bladder. There are two ureters, one attached to each kidney. The upper half of the ureter is located in the abdomen and the lower half is located in the pelvic area. 

Nephrons

The kidneys contain tiny blood-filtering units called nephrons. Each nephron contains a glomerulus, a bundle of blood vessels surrounded by a capsule. As blood passes through the glomerulus, waste and excess water ooze into the capsule and are carried away by a tubule (tiny tube). Any useful substances, such as glucose, are absorbed by capillaries, while the waste is carried away to form urine.

 

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Your two kidneys filter and clean the blood by removing toxic chemicals like the heart, the kidneys are at work every second of every day, producing a continuous flow of clean blood.

As blood circulates it picks up waste substances produced by the body's cells. These would poison you if they were not removed from the body. The kidneys extract the toxins and excess water from the blood and process them to make urine. As well as cleaning the blood, the kidneys also release hormones, stimulate red blood cell production and keep the body's water content balanced.

They release more urine if you have drunk a lot and less if you are dehydrated.

 

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The iliac crest of the male pelvis rises higher than the iliac crest of the female pelvis, and there is a greater distance between the anterior superior iliac spines of the female pelvis when compared to the male pelvis. This usually gives women a curvier appearance in the hip region, as compared to the average male. The female pelvic cavity is also going to be shallower, whereas the male pelvic cavity is deeper.

Although the female pelvis is wider than the typical male pelvis, these bones are thinner and lighter than the denser, rougher bones of the male pelvis.

The pelvic brim of the female is generally larger than the male’s inlet, which facilitates childbirth. The male inlet is smaller and heart-shaped, and a trick to remembering that is to remember the following phrase: the way to a man’s heart is through his pelvis!

The sacrum of the female is shorter, wider, and has a greater curve, whereas the male sacrum is thinner, longer, and is less curved. The coccyx bone of the male curves more toward the front of the body in comparison to the female’s coccyx.

 

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Pelvic girdle

The pelvis is made up of two hip bones, one on each side. Each hip bone has three parts - the ilium, ischium, and pubis. They connect to the sacrum - the lower  part of the spine - to form a ring like shape, called the pelvic girdle.

Spine

The spine, or backbone is a column of bones that runs from the pelvis to the neck. The spine is classified into 4 distinct areas.

The cervical area consists of 7 bony parts in the neck. The thoracic spine consists of 12 bony parts in the back area. The lumbar spine consists of 5 bony segments in the lower back area; 5 sacral* bones; and 4 coccygeal* bones (the number of coccygeal bones can vary from 3 to 5). (* By adulthood, the 5 sacral vertebrae fuse to form 1 bone, and the 4 coccygeal vertebrae fuse to form 1 bone.)

Sacrum

At the base of the spine, this triangular-shaped bone connects the two hip bones. It consists of the last four or five vertebrae that by adulthood, fuse together to form a single bone. Located just above the coccyx and wedged between the right and left iliac bones (hip bones), the sacrum forms the back wall of the pelvis. The coccyx, commonly referred to as the tailbone, is the very bottom segment of the vertebral column. The right and left iliac bones are joined together in the front by the symphysis pubis.

Ilium

The largest bone in the pelvis is the ilium, or hip bone. One on each side connects the muscles used to stand and walk. The body of the ilium forms the superior part of the acetabulum (acetabular roof). Immediately above the acetabulum, the ilium expands to form the wing (or ala).

Fixed joint

These bones are held firmly together by strong ligaments. The relatively fixed joint between the sacrum and the lumbar spine is ideal for bearing the load of the upper body. At the most ventral area of the pelvis, there is a fibrocartilaginous joint in the pubic symphysis.

Pelvic cavity

The pelvic cavity is formed by three bilateral pairs of bones (pubis, ilium and ischium) and two posteriorly located bones (sacrum and coccyx). The intestines and bladder are contained here, surrounded by the protective pelvis.

Pubis

The pubis is one of the two smallest bones in the pelvis. In vertebrates, the pubic bone is the most forward-facing (ventral and anterior) of the three main bones making up the pelvis. The left and right pubic bones are each made up of three sections, a superior ramus, inferior ramus, and a body.

Pubic symphysis

This strong cartilage joint connects the two pubis bones together. It is located in front of and below the urinary bladder. In males, the suspensory ligament of the penis attaches to the pubic symphysis.

Coccyx

Below the sacrum is the Coccyx, or tailbone all that remains of the tail of our distant ancestors. Although the tailbone is considered vestigial (or no longer necessary) in the human body, it does have some function in the pelvis. For instance, the coccyx is one part of a three-part support for a person in the seated position. Weight is distributed between the bottom portions of the two hip bones (or ischium) and the tailbone, providing balance and stability when a person is seated.

Hip joint

The ball shaped top of the thighbone sits in this hollow creating the ball and socket hip joint. The adult os coxae, or hip bone, is formed by the fusion of the ilium, the ischium, and the pubis, which occurs by the end of the teenage years. The 2 hip bones form the bony pelvis, along with the sacrum and the coccyx, and are united anteriorly by the pubic symphysis.

Ischium

The lowest bone in the pelvis the ischium carries all the weight when the body is sitting down. Situated below the ilium and behind the pubis, it is one of these three bones whose fusion creates the hip. The superior portion of this bone forms approximately one third of the acetabulum.

Holes

Small holes in the bones are for nerves and blood vessels.

 

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The pelvis is a large bowl-shaped group of bones at the base of the abdomen. This section of the skeleton is made up of several bones fused and linked together. It surrounds and protects the soft organs inside the lower abdomen. The pelvis has many functions. It supports the intestines and bladder and the space in the middle allows waste from the intestines and bladder to leave the body In women, the pelvis supports the uterus as it expands to hold a growing baby and also provides the baby's route out of the body during childbirth. Many muscles in the back, abdomen, and legs are anchored to the pelvis, helping to keep the body upright. The pelvis allows us to stand walk, and run without falling over.

The muscles of the pelvic floor wrap around and control the opening of your bladder and rectum. When there is an increase in abdominal pressure (for example when you cough, sneeze, laugh or jump), these muscles contract around your urethra and anus to prevent leakage. Equally as important, these muscles have to relax and lengthen to allow us to urinate or have bowel movements easily.  

 

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After hours of mechanical and chemical digestion, food has been reduced into chyme. As particles of food become small enough, they are passed out of the stomach at regular intervals into the small intestine, which stimulates the pancreas to release fluid containing a high concentration of bicarbonate. This fluid neutralizes the gastric juices, which can damage the lining of the intestine and result in duodenal ulcer. Other secretions from the pancreas, gallbladder, liver, and glands in the intestinal wall help in digestion.

When food particles are sufficiently reduced in size and composition, they are absorbed by the intestinal wall and transported to the bloodstream. Some food material is passed from the small intestine to the large intestine. In the large intestine, bacteria break down any proteins and starches in chyme that were not digested fully in the small intestine.

When all of the nutrients have been absorbed from chyme, the remaining waste material changes into semisolids that are called feces. The feces pass to the rectum, to be stored until ready to be discharged from the body during defecation.

 

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At every stage of digestion, food is pushed through the system by a powerful muscle action called peristalsis. The muscles lining the intestine contract then relax in a rhythmic, wave-like action. As the intestine walls are squeezed together, the food is forced further along the tube.

The job of your large intestine is to absorb water, minerals, and some of the remaining nutrients from your food. It will change the leftover waste into a bowel movement. This is also called stool. Your rectum stores the stool until you feel the need to have a bowel movement. Muscles of your rectum then push the stool through your anus and out of your body.

 It takes about 36 hours for food to move through the entire colon. All in all, the whole process — from the time you swallow food to the time it leaves your body as feces — takes about two to five days, depending on the individual.

 

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Up, across, and down

The large intestine is a wide tube that goes around the small intestine. The tube goes up. across the abdomen, then down again. It has a lumpy appearance because of the way the muscles in its wall contract.

Transverse colon

Passing just below the stomach, this is the middle part of the large intestine.  It extends between the right and left colic (splenic) flexures, spanning the right hypochondriac, epigastric and left hypochondriac regions of the abdomen. 

Ascending colon

On the right side of the abdomen, this section of the intestine rises from the caecum. The ascending colon is retroperitoneal and it is connected to the posterior abdominal wall by the Toldt’s fascia. 

Descending colon

The descending colon extends between the left colic flexure and sigmoid colon. It travels through the left hypochondriac region, left flank and left iliac fossa. This section passes down the left side of the abdomen.

lleocaecal valve

A valve stops waste flowing back into the small intestine. An ileocecal valve regulates the passage of intestinal contents from the small into the large intestine. 

Caecum

The caecum receives chyme from the small intestine. The cecum is intraperitoneal with various folds and pockets (retrocecal peritoneal recesses) surrounding it.

Appendix

The vermiform appendix is a blind lymphoid pouch located in the right iliac fossa which arises from the cecum. These two parts of the large intestine are connected by the meso-appendix. This small tube may help digestion by storing "friendly" gut bacteria.

Sigmoid colon

The S-shaped sigmoid colon travels from the left iliac fossa until the third sacral vertebra (rectosigmoid junction). The sigmoid colon contracts forcefully pushing the faeces (waste) into the rectum

Rectum

The rectum is the last section of the large intestine. The typical characteristics of the large intestine (taenia coli, haustra, epiploic appendages) change or even terminate at the rectum. The roles of the rectum include temporary storage of fecal matter and defecation.

Anus

This is where waste leaves the body as faeces. The anal canal forms the terminal part of the gastrointestinal tract. It extends from the anorectal junction to the anus.

Gut bacteria

Trillions of bacteria live in the large intestine. Most are either harmless or actively help to complete digestion by processing the remaining nutrients that could not be digested by enzymes. However, some microorganisms that enter the digestive system can cause illness.

 

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The large intestine is the final stage of the digestive system. It's where most of the water, and the last few nutrients, are taken from the chyme that enters from the small intestine.

It then moves unusable waste out of the body. Most nutrients have been taken from the food before it gets to the large intestine - but there is still vital work for it to do Here, trillions of bacteria help to break the remaining food down into valuable nutrients. The large intestine is wider than the small intestine but it's not nearly as long.

The large intestine has 3 primary functions: absorbing water and electrolytes, producing and absorbing vitamins, and forming and propelling feces toward the rectum for elimination. By the time indigestible materials have reached the colon, most nutrients and up to 90% of the water has been absorbed by the small intestine. The role of the ascending colon is to absorb the remaining water and other key nutrients from the indigestible material, solidifying it to form stool. The descending colon stores feces that will eventually be emptied into the rectum. The sigmoid colon contracts to increase the pressure inside the colon, causing the stool to move into the rectum. The rectum holds the feces awaiting elimination by defecation.

 

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This magnified image of a scan shows a cut away in the deeply folded lining of the duodenum, the first section of the small intestine. The folds are called villi, and these greatly increase the area of the lining - creating a larger surface through which food nutrients can be absorbed.

Nutrients from food are absorbed by microvilli on the surface of the villi, shown here as a green, fur-like layer. Nutrients then pass through a layer of cells (shown in blue-green) before being carried away by blood vessels in in the middle.

Their function is to increase the surface area of the small intestinal wall for absorption of the digested food. These projections absorb the protein molecules and help in the transfer of the proteins to all cells and tissues. Many blood vessels are present within these villi, that help in the absorption of digested food and carry it to the bloodstream. Later, from the bloodstream, the absorbed food is delivered to each and every cell of the body.

 

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Bundled up

The small intestine is at the front of the lower abdomen, surrounded by the large intestine and other organs. Although it's very long - more than 6 m (20 ft) - the small intestine fits into this space because it is bundled up in a series of loops and coils.

Duodenum

The first part of the small intestine is where bile and enzymes are added to the chyme to help break it down. It extends from the pyloric sphincter of the stomach, wraps around the head of the pancreas in a C-shape and ends at duodenojejunal flexure. The duodenum has four parts: superior (duodenal bulb/ampulla), descending, horizontal and ascending. 

Jejunum

This is the middle section of the small intestine, where most of the digestion and absorption of food takes place. The jejunum is entirely intraperitoneal as the mesentery proper attaches it to the posterior abdominal wall.

Ileum

The final, longest section of the small intestine absorbs some nutrients. It is found in the lower right quadrant of the abdomen, although the terminal ileum can extend into the pelvic cavity. The ileum terminates at the ileal orifice (ileocecal junction) where the cecum of the large intestine begins.

lleocaecal valve

The ileocecal valve is a sphincter muscle situated at the junction of the ileum (last portion of your small intestine) and the colon (first portion of your large intestine). Its function is to allow digested food materials to pass from the small intestine into your large intestine. Chyme from the small intestine passes through here to the large intestine.

Looking inside

A cross-section through part of the small intestine shows the muscles that help to push food along its length. The lining is covered with millions of tiny finger-like projections called villi (a single projection is a villus).

 

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The small intestine is the longest part of the digestive system. It's where most of the digestive process takes place, releasing the nutrients in food so that they can be used to fuel the body's cells.

By the time food reaches the small intestine, the stomach has turned it into a liquid called chyme. This chyme is squirted into the duodenum, the first part of the small intestine, along with bile from the gallbladder and enzymes from the pancreas, which break the chyme down even more. Finally, when most of the food has been broken down into simple nutrients, these pass through the walls of the small intestine and into the bloodstream. The remaining food progresses to the next stage - the large intestine.

As a person grows the small intestine increases 20 times in length from about 200 cm in a newborn to almost 6 m in an adult. The length of the small intestine is approximated by three times the length of the infant, or height of the child or adult.

The duodenum is about 25 cm (10 inches) long; the jejunum is about 2.5 m (8 feet) long and the ileum is about 3.6 m (12 feet) long.

 

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Ligaments are made out of connective tissue that has a lot of strong collagen fibers in it. They are found in different shapes and sizes in the body. Some look like pieces of string, others look like narrow or wide bands. There are arch-shaped ligaments, too.

Ligaments often connect two bones together, particularly in the joints: Like strong, firmly attached straps or ropes, they stabilize the joint or hold the ends of two bones together. This ensures that the bones in the joint don’t twist too much or move too far apart and become dislocated.

But there are also some ligaments that aren’t connected to bones. For instance, some make sure that internal organs are kept in place. A typical example is the womb, which is kept in the right position in the pelvis by ligaments. Ligaments may also connect two or more organs to each other. For instance, the liver, intestine and stomach are held in place by ligaments in the abdominal cavity. These ligaments often have sensitive structures like blood vessels or gland ducts running through them. The strong connective tissue in the ligaments protects these structures and prevents them from bending, twisting or tearing.

 

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