Urinary System

The Anatomy of The Kidneys

The kidneys both cleanse the blood of urea and nitrogenous waste from protein metabolism and regulate the water and electrolyte content.

This is an ongoing process that is both constant in its workings and great in the quantity of blood that is filtered.

For this reason the kidneys have a large renal artery and renal vein connecting them from the descending aorta and inferior vena cava respectively.

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This makes perfect sense as the blood that reaches the kidneys is newly oxygenated, thus providing plenty of oxygen to the many active cells of the kidney.  This blood has not long come from the liver, where it will contain nutrients, but more importantly urea.  Once the kidneys have filtered the blood plasma, returned obver 99% of the filtered water back to the blood supply and extracted around 50% of the urea and mixed it with excess water to form urine; it then needs to pass it on to the storage sac of the urinary system; the bladder.  It does this via two tubes called ureters.

The structure of the kidneys

In Chinese medicine, the kidneys are linked to the ears: An ear, it is said, looks like an upside-down fetus in the uterus and different parts of the fetus reflect the same parts of the human body.

For example, the earlobe, which looks like the head of a fetus, reflects problems in the face and head, such as toothache, acne and eye strain. Rubbing and kneading the earlobe, or letting blood from the lobe can help relieve these problems.
Kidneys are quite small considering their function – measuring 11x 7x 3 cm, the left kidney being slightly larger and placed slightly higher than the right one due to the fact that the liver on the right hand side forces the right kidney lower.

As with all organ, we see that the kidney is protected by a strong fibrous capsule that usually has a dense layer of fatty tissue covering it.

As we move inwards there is a lighter striated cortex, the renal cortex.  Here we have the renal columns where are found the interlobular arteries and veins, that ultimately supply the functional units of the kidneys.

The medulla, on the other hand, is the inner portion of an organ.  In the kidneys, the renal medulla contains triangular sections called renal pyramids.

These renal pyramids empty into a cup-like structure known as a calyx.

The calyces are the main collecting ducts of the kidneys and join together to form a large collecting reservoir called the renal pelvis, which then leads to the ureter.

The main filtration unit is the nephron.  There are millions of these.

The basic process a nephron uses to form urine.

The main functions of the kidneys are to cleanse the blood plasma and balance the electrolytes and body fluids.  The kidneys are able to produce urine which contains different amounts of water, elecrolytes and urea depending on what is required by the body to maintain the fluid level, electrolyte and acid base balances.

The kidneys do this through the processes of filtration, reabsorption and secretion.


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Filtration

Takes place in the renal corpuscle and is due to the hydrostatic pressure, thus forcing out blood plasma through the fenestrations of the capillaries and into the Bowmans capsule.  Here it is collected and directed to the 2nd part of the nephron; the proximal convoluted tubule.  the only substances not filtered out are those too large to fit through the fenestrations, namely the blood cells and plasma proteins.

As blood enters the glomerulus, due to how the capillaries are coiled up into small spaces – the hydrostatic pressure increases and is greater than the pressure in the bowman‘s capsule; therefore, blood plasma, urea, electrolytes and some glucose will be forcibly filtered out through the fenstrations.

If there is a drop in blood volume, there will also be a drop in blood pressure.  This will decrease the hydrostatic pressure and filtration rate.  The body responds via nervous and hormonal influences to effect a greater absorption of fluid out of the nephrons back into the body.

Vice versa

Any disease in the cardiovascular system will have an effect on the urinary system and vice versa.  By treating the kidneys you are having a positive effect on the heart.

Reabsorption

The 2nd phase of urine formation is reabsorbtion and it takes place in the Proximal Tubule.  It refers to the movement of nutrients and water back out of the kidneys and into the blood in the peritubular capillary.  If we didn’t do this we would lose too much from our urine.  It ttakes place in several places as the filtrate progresses through the nephron:

  • the first place is in the proximal tubule,
  • the 2nd in the loop of Henle and the
  • final place is in the distal tubes and collecting ducts.

The proximal convoluted tubule has epithelial cellsmaking up its walls and these cells have a brush border, which have the effect of greatly increasing the luminal (tubular)surface area.

In the proximal convoluted tubule:

  • Sodium is actively transported out of the tubular fluid and into the blood.
  • Glucose and amino acidshitch a ride‘ with sodium and passively move out of the tubule fluid by the sodium co transport system.
  • Negatively charged chlorine ions passively move into the blood due to the positive sodium ions, so as to balance up electrically the interstitial fluid
  • The movement of sodium and chloride ions out of the tubule fluid and into the blood plasma causes an osmotic imbalance, which causes water to follow and be reabsorbed back into the blood stream.
  • Half of the urea is reabsorbed into the blood. the remaining half travels on to the next part of the nephron – The Loop of Henle
  • around 2/3 original filtrate has now been reabsorbed.

In the Loop of Henle

  • reabsorbs water out of the tubule fluid and absorbs urea
  • reabsorbs sodium and chloride out of its ascending limb
  • by reabsorbing sodium and chlorine, it makes the tubule fluid dilute
  • reabsorption of sodium and chlorine into the interstitial fluid of the medulla maintains a a high osmotic pressure there.

In the distal tubules and collecting ducts

At this point the urine is very dilute and unless the body needs to lose excess water, then it needs to be reabsorbed.

ADH Anti diuretic hormone, released by the posterior pituitary gland when triggered by the hypothalamus (which measures blood volume).

ADH causes the distal tubule to become more permeable to water and due to the high osmotic pressure in the medulla, will rush out and the urine becomes more concentrated.

Aldosterone is released by the adrenal glands in response to a drop in blood pressure.  Aldosterone stimulates the distal tubules and collecting ducts into actively pumping out sodium and thus increasing osmotic pressure of the interstitial fluid, so water will move from the tubules into the medulla.

ANH: Atrial Natriuretic Hormone

Is antagonistic to ADH and aldosterone.  It causes the kidneys to lose excess water and therefore lower blood pressure.

Secretion

Tubular secretion refers to substances that move out of the blood into the tubular fluid.  For example, when ammonia plus carbon dioxide (urea) moves into the descending limb of the Loop of Henle by diffusion to form part of the urine to be secreted.  The Distal tubule is responsible for the resorption of sodium, water and secretion of hydrogen and potassium.

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