Excretory System.

Introduction.

The excretory system is the system responsible for releasing toxic products from our body. These toxic products are, mainly, nitrogen aged chemical substances, such as ammonia, urea or uric acid. The excretory system is also related to the control of some ions and electrolytes that tend to be excessively built up, such as Na⁺, Cl^-, SO₄^2-, PO₄^3-, H⁺. All these products are dissolved in the urine at different concentrations, owing to the necessity of being eliminated by the body.

The excretory system plays an essential role in:

• Preserving the volume and composition of the circulating blood, controlling not only the amount of blood, but also its pH, concentration of electrolytes, etc.
• Controlling blood pressure, not only controlling the volume of blood, but also the diameter of the blood vessels by the action of the endocrine system and hormonal regulation related to the excretory system, called renin-angiotensin system.
• Other metabolic functions. The kidneys have some metabolic functions, such as the gluconeogenesis (production of glucose from simple organic substances produced during anabolic processes in the muscles), production of hormones (erythropoietin, for instance) and production of vitamins (vitamin D).

Anatomy of the Excretory System.

Components.

The excretory system is made up of kidneys and urinary ducts. The blood arrives in the kidneys through the renal arteries that come from the abdominal aorta. These arteries transport the blood to be filtrated. The filtrated blood exits the kidneys through the renal arteries that end up in the inferior vena cava. Kidneys are connected to the urinary bladder by the ureters. Finally, the urinary bladder is connected to the exterior by the urethra. 

Excretory System: Components.

Kidneys.

The kidneys are two red organs, with a shape similar to a bean, located in the waist, between the parietal peritoneum and the posterior wall of the abdomen. They are partly protected by the eleventh and twelfth ribs, although they are not perfectly symmetric, because the right kidney is lower than the left one due to the position of the liver. Each kidney is between ten and twelve centimetres long, between five and seven centimetres broad and two and a half centimetres thick.

Each kidney is covered by three layers. The inner one is fibrous and transparent and it is called renal capsule. The mid layer is called pararenal fat. Finally, the outer layer is called renal fascia and it is responsible for attaching the kidneys to other abdominal structures.

Analysing a longitudinal cut of the kidney, two clearly different regions can be found. The outer part of the kidney is called renal cortex. The inner part of the kidney is called renal medulla.

The renal medulla has between eight and eighteen conical-shaped structures called renal pyramids. These are the structures where the ducts that take the urine produced in the kidney join. There are lots of blood vessels too. The ducts and blood vessels give the pyramids a striped aspect. The base of each pyramidal structure contacts with the cortex, whereas the apical part is directed towards the opening of the kidney, located in their concave part. This opening of the kidney is called the hilium. The projections of cortex that can be found between the pyramids, separating them, are called renal columns. The hollow part of the kidney that can be found behind the hilium is called renal pelvis.

The hilium communicates the hollow cavity of the kidney with the exterior of the organ. The renal artery enters the kidney through the hilium, and the renal vein and ureter exit the kidney through the hilium.

The apical part of each renal pyramid is connected to a structure called minor calix. Several minor calixes connect to a structure called major calixes. These calixes receive the urine produced in the kidney that is transported through the ducts of the renal pyramids.

Finally, the major calixes are connected to the renal pelvis. And the renal pelvis is a structure similar to a funnel that collects the urine and pours it to the ureter. 

Anatomy of the Kidney.

The Nephron.

Nephrons are the functional subunits of the kidney. So these structures are not only the main component of the kidney, but also the structure responsible for the filtration of blood and production of urine. Each kidney is made up of more than one million nephrons, usually between one million and one and a half million.

Nephrons have two parts: the globular part and the tubular part.

The globular part is the beginning of the nephron and the place where the blood is filtrated. It has a group of tingled capillaries called glomerulus. These capillaries are surrounded by the Bowmans' capsule. The tubular part, called renal tubule, is a duct that transports the filtrated fluid that has been produced in the Bowmans' capsule. During its movement through this duct, the fluid is depurated, absorbing part of the water and some essential substances, such as sugar and ions. This process transforms the filtrated liquid it into urine.

The Bowmans' capsule is an epithelial structure that surrounds the net of capillaries called glomerulus. The blood from the capillaries is filtrated. The liquid enters the capsule, that is connected to the renal tubule . This liquid is the precursor of the urine.

The renal tubule has three parts. The first one, adjacent to the Bowmans' capsule, is called proximal convoluted tubule. The second part, just after the proximal convoluted tubule, is a U-shaped portion of duct called Loop of Henle. The third part is called distal convoluted tubule. It connects the Loop of Henle to the Collection Duct. The collection ducts are the main responsible for the striped structures of the renal pyramids. So the ducts end and spill their secretion in the renal calixes. 

The Bowmans' capsules are mainly located in the renal cortex. The tubular part of the nephrons, however, are located in the renal medulla, and above all in the renal pyramids. There are many blood capillaries surrounding the renal tubules. The capillaries that surround the convoluted tubules are called peri tubular vessels, whereas the capillaries that surround the loop of Henle are called vasa recta. This capillary network absorbs water, mineral salts and other important substances from the liquid transported by the tubular system.

There are two types of nephrons. The most abundant ones, around 80%, are called cortical nephrons. They have the glomerulus in the most superficial part of the cortex. The second kind of nephrons are called yuxtabasal. They are less abundant than the cortical nephrons, around the 20% and their glomerulus are located close to the modular zone. Yuxtabasal nephrons have a longer loop of Henle, so that they can produce urine with different amounts of water. They are responsible for controlling the amount of urine produced and how concentrated this urine is.

Nephron

Filtration, absorption and production of urine.

The Bowmans' capsule filtrated plasma from the blood. The plasma exits the capillaries and enters the capsule. This liquid can not be released as urine, because it has a large amount of water. The fluid moves from the capsule to the tubular part of the nephron, where more than 95% of water must return from the  duct to the blood.This absorption takes place throughout all the duct. Besides, some ions must also return from the duct to the bold, such as Na⁺, K⁺, Cl^-, PO₄^3-, CO₃^2-. Nutrients, such as glucose or amino acids are also re-absorbed.

The absorption stars in the proximal convoluted tubule. Nearly all the glucose and nutrients a absorption in this part of the duct. The absorption of many substances, such as Na⁺, requires energy consumption.

The loop of Henle is the place is the part of the duct where the amount of water of the urine is finely controlled, so it is also the place where the amount of urine produced and the concentration of several substances such as urea or uric acid are also regulated. This control is related to the movement of ions from the duct to the capillaries.

The distal convoluted tubule is the part of the duct where the concentration of substances in the urine is finally adjusted. If the urine produced in the loop of Henle is, for instance, excessively diluted, it can be concentrated in the distal convoluted tubule.

The main hormone that controls this process is the aldosterone. It is released by the adrenal renal gland. It raises the absorption of Na⁺ in the nephron. This leads to an increase of water in the nephron, so the amount of urine produced is also increased. 

Another hormone involved in the control of filtration is called anti diuretic hormone (ADH). This hormone increases the permeability of the tubule. This process reduces the amount of urine produced by the kidney.

The tubules also releases other ions, like K⁺, that tend to accumulate in our body. The control of other ions, such as H⁺ and HCO₃^-, is related to the regulation of the blood pH.

It is clear that a variable amount of water must be released in the urine. If the amount of water in our body excessively drops, the concentration of minerals rises. Then, our body needs water, so that the amount of some hormones, like ADH, rises. This it triggers the sensation of thirst and increases the absorption of water in the kidney, in order to increase the amount of internal water and reduce the concentration of minerals.

Urinary Ducts.

Ureters.

The urine that has been produced in the nephrons is transported by the collection ducts to the renal calixes. The renal calixes end up in the renal pelvis. The renal pelvis is connected to the duct that exits the kidney. This duct is called ureter.

The ureter is a twenty five centimetres long duct that connects the kidney with the urinary bladder. The ureters enter in the urinary bladder in a transversal way, so that when the urinary bladder is full of water, its dilation closes the end of the ureters, blocking the flow of urine from the ureters to the urinary bladder of from the urinary bladder to the ureters, with no other occlusive structure required.

The urine moves through the ureters due to the effect of three different forces: the hideous tatie pressure, that is higher in the upper part of the duct where the urine is produced; the gravity force, because the kidney is usually above the urinary bladder; and the peristaltic movements of the duct wall.

Urinary Bladder.

It is a muscular hollow organ, located in the pelvis, just in front of the rectum in males and behind the vagina and under the womb in females. Its morphology changes depending on the amount of urine stored. It is flat and collapsed when it is empty, spherical when it has some amount of urine and pear shaped when it is totally filled.

It can support between 700 and 800 millilitres of urine, although when the amount of liquid raises up to 400 millilitres the sensor for tension of its wall start to send signals to the brain that trigger the urination 

Two sphincters separate the urinary bladder from the urethra. The first one is called external urethral sphincter and it has voluntary control. The second one, called internal urethral sphincter, doesn't have voluntary control. The contraction of the urinary bladder is also absolutely involuntary.

Urethra.

The urethra is the duct that connects the urinary bladder with the exterior. It is different in males and females. In females it is a 4 centimeters long duct that opens up to the vagina. In males, however, it is a 20 centimetres long duct that crosses the prostate, the urogenital diagram and the penis, opening to the exterior in its vertex.