A&P Part Two: The Renal System!

Hello!
In this post I'll be chatting about the renal system! I know It's been a while since A&P but I thought I'd give us a break from A&P for a bit because it is a slightly hard to grasp at first!
But the renal system isn't so bad, so here goes...

The Renal System

The renal system (Also known as the Urinary system) is made up of the Kidneys whose main roles are the removal of wastes and the maintenance of the body's water balance.

Here are their other vital functions:

1. Control of the body's water balance. The amount of water in the body must be balanced against the amount of water which we drink and the amount we lose in urine and sweat etc.

2. Regulation of blood pressure via the renin-angiotensin-aldosterone system

3. Regulation of blood electrolyte balance - Na+, Ca2+, K+ etc.

4. Excretion of metabolic wastes such as urea, creatinine and foreign substances such as drugs and the chemicals we ingest with our food

5. Help in the regulation of the body’s acid base balance

6. Regulation of red blood cell production via the hormone erythropoietin

7. Help in the production of vitamin D

Indeed, this long list shows us how important the renal system is to the normal functioning of the body.


Structure of the renal system
The kidneys are large, bean shaped organs which lie on the dorsal side of the visceral cavity, roughly level with the waistline.
Blood is supplied to the kidneys by the renal arteries which branch off the aorta. The kidneys and are drained by the renal veins into the inferior vena cava. From the kidneys, urine passes to the urinary bladder via the ureters.
Urine is passed to the outside environment via the urethra (this is routed differently in males and females)- See Figure 1.


Macrostructure of the kidneys
The kidneys are protected by a tough fibrous coat called the renal capsule. Under the capsule, the arrangement of nephrons and capillaries in the kidney produce the appearance of distinct regions when viewed in longitudinal section.
The outer cortex region surrounds darker triangular structures called pyramids which collectively form the medulla. The inner part of the kidney, the renal pelvis, collects the urine draining from the nephron tubules and channels it into the ureter - (Figure 2 shows a sectioned view of the kidneys)

Microstructure of the kidneys
The basic functional unit of the kidney is the nephron. Each nephron is composed of a glomerular capsule, glomerulus, proximal convoluted tubule, loop of Henle and distal convoluted tubule. The renal corpuscle includes the glomerular capsule and the glomerulus. The renal tubule is the part of the nephron that directs the filtrate away from the glomerular capsule and includes the proximal convoluted tubule, loop of Henle, distal convoluted tubule and the collecting duct. The collecting duct is not considered part of the nephron as many nephrons drain into one collecting duct.
There are over one million nephrons in each human kidney and together they are responsible for the complex water regulation and waste elimination functions of the kidneys. The heads of the nephrons are in the cortical region and the tubular component then descends through the medulla and eventually drains into the renal pelvis - (Figure 3 Shows the arrangement of nephrons in the kidneys)

The key area of interface between the circulatory system and the tubular part of the kidney is the knot of glomerular capillaries in the Bowman's capsule. Those liquid parts of the blood that are able to cross through the filtration membrane of the capillaries pass into the Bowman's capsule and then into the tubular section of the nephron - (Figure 4 shows The Bowman's capsule and glomerulus). The filtration membrane only allows water to pass through it and small molecules that will dissolve in water such as waste (urea, creatinine etc.) glucose, amino acids and ions. Large proteins and blood cells are too large to be filtered and remain in the blood.

The filtered fluid or filtrate enters the proximal tubule and then into the loop of Henle which is the part of the nephron which dips in and out of the medulla. From the loop of Henle, the filtrate travels through the distal tubule and then into a common collecting duct which passes through the medulla and into the renal pelvis - See Figure 5.

Now to consolidate our knownledge of the nephrons components...

Component's, their description and functions

Glomerular (Bowman) capsule
• The start of the nephron.
• It is a double-walled chamber that looks as if the wall of the nephron had been pushed in on itself.
• The walls of the glomerular capsule are thin, but only allow water and small ions to pass through.
• Filtrate (water and small molecules) which is similar to blood plasma passes into the capsular space of the glomerular capsule.
• The glomerular capsule continues as the proximal convoluted tubule (PCT).
• Function: Filtration

Glomerulus
• A tiny capillary network that lies within a glomerular capsule.
• The glomerulus receives blood at high pressure from a tiny branch of the renal artery, called the afferent arteriole.
• The filtered blood (blood cells, proteins and large molecules) leaves the glomerulus via the efferent arteriole which goes on to form a capillary plexus around the PCT, before draining into a tiny branch of the renal vein.
• Function: Filtration

Proximal convoluted tubule (PCT)
• Originating from the glomerular capsule the PCT is a highly twisted and coiled tubule that descends through the cortex.
• It is the part of the nephron responsible for most of the reabsorption of the filtrate.
• Water, glucose, amino acids and salts are reabsorbed from the PCT back into the bood.
• Drugs, toxins and solutes such as bicarbonate, hydrogen and potassium ions and urea are secreted into the PCT.
• It continues as the loop of Henle.
• Function: Reabsorption & Secretion

Loop of Henle
• A tubule with a long hairpin turn, its descending limb enters the medulla, where it makes a 180 degree turn so that its ascending limb enters the cortex.
• Salts are reabsorbed from the loop of Henle into the medulla of the kidney (making the medulla very salty compared to the filtrate).
• It ends in the cortex as the distal convoluted tubule (DCT).
• Function: Reabsorption

Distal convoluted tubule (DCT)
• A highly coiled tubule located in the cortex and surrounded by capillaries.
• Salts such as sodium are actively absorbed from the DCT under the control of a hormone called aldosterone.
• Hydrogen and potassium ions are actively secreted into the DCT to regulate pH.
• The rate of absorption and secretion in the DCT are controlled by hormones.
• It empties into the collecting tubule (CT).
• Function: Active Secretion

Collecting tubule(CT)
• They pass through the medulla forming the pyramids of the kidneys.
• Bicarbonate, potassium and hydrogen ions, are secreted into the CT to regulate pH.
• Water and salts are reabsorbed from the urea in the CT under the control of two hormones (one of them being anti-diuretic hormone that increases the CT permiability to water).
• Each CT opens into a minor calyces at the apex of the renal pyramid.
• From here urine flows via funnel-like calyces into the pelvis of the kidney.
• Function: Reabsorption, Secretion & Transport


Understanding the functions of the components

Filtration

Filtration at the glomerulus is under pressure as the afferent arteriole is so close to the abdominal aorta. The fluid that passes through the wall of the glomerular capsule into the nephron is called the glomerular filtrate and is similar in composition to plasma. Blood and protein cannot pass into the filtrate but small waste molecules can.
Interesting fact I found: 600 ml of blood will pass through the glomerulus each minute, 125 ml of which will be absorbed into the nephron as glomerular filtrate!

Reabsorption

The tubule of the nephron functions to reabsorb most of the glomerular filtrate. The cells of the tubule reabsorb vital nutrients and water back into the blood, while retaining the waste products that the body needs to eliminate. The plexus formed by the efferent arteriole (from the glomerulus) passes closely to the proximal convoluted tubule, allowing direct transfer into the blood. In the loop of Henle the filtrate is further concentrated. Water is absorbed by osmosis, being transported down its concentration gradient.
The amount of water reabsorbed is controlled by an anti-diuretic hormone (OMG Endocrine system, guys!!) secreted by the posterior lobe of the pituitary gland. The amount of salts reabsorbed is controlled by aldosterone secreted by the cortex of the suprarenal glands. These hormones are increased or decreased according to the needs of the body.

Active secretion

During active secretion, wastes that were not initially filtered out of the blood in the glomerular capsule such as ammonia and certain drugs and toxins are removed from the capillaries into the distal convoluted tubule.


I think that's all i'll type about renal for today, guys! There's so much to go through and i'm pretty sure that i've barely scratched the surface!! O_o
So watch out for A&P Part Three: The Respiratory system coming very soon!!

Bye!

Emily