The Urinary System

The Urinary System consists of a number of organs in the body. These organs include

Kidney
Ureter
Bladder, and
Urethra.

Use the image below to identify these organs.

Functions of the Kidney and the Renal System

Excretion of metabolic waste products and foreign chemicals.
Regulation of water and electrolyte balance.
Regulation of body fluid osmolality and electrolyte concentration
Regulation of arterial pressure
Regulation of acid-based balance
Secretion, metabolism and excretion of hormones
Gluconeogenesis

1. Excretion of Metabolic Waste Products and Foreign Chemicals

This is the major function of the kidney and other parts of the urinary system. Some of the waste products include:

Uric acid produced from nucleic acids.
Urea produced from amino acids
Creatinine produced from muscle creatine
Products of hemoglobin breakdown such as bilirubin
Metabolites of various hormones

2. Regulation of Water and Electrolyte Balance

It is very essential that water and electrolyte intake must match the excretion. This is to maintain water and electrolyte level in the body at optimal level for efficient and effective body function.
Humans take in water and electrolytes such as sodium, hydrogen, magnesium, calcium, phosphate irons, chloride etc

3. Regulation of Arterial Pressure

The kidney contributes to the short term regulation of arterial pressure through the secretion of renin which leads to the formation of angiotensin 11, a vasoactive substance
In the long term, the regulation of sodium and water in the body contributes to regulation of arterial pressure.

4. Regulation of Acid-Based Balance

1. The kidney together with the lungs and body fluid buffers regulate acid-base balance by
  excreting acids and regulating the body buffer store.
2. Certain types of acids can only be eliminated through the kidney. Eg Sulphuric acid and phosphoric acid generated from protein metabolism

5. Regulation of Erythrocyte Production

Kidney is the major source of erythropoietin which stimulate the production of red blood cell in hypoxia
People with severe kidney disease or those on hemodialysis suffer severe anemia due to decreased erythropoietin production.

Regulation of 1, 25-dihydroxy Vitamin D3 Production (Calcitriol).

This vitamin is important for normal calcium deposit in the bone and calcium reabsorption by the gastrointestinal tract
Calcitriol plays important role in calcium and phosphate regulation.

6. Glucose Synthesis

Gluconeogenesis occurs during prolonged fasting in the kidney.
It is the synthesis of glucose from amino acids and other precursors during prolonged fasting
The ability of the kidney to engage in glucose synthesis during prolonged fasting equates to that of the liver

General Organization of the Kidney and Urinary Tract

There are two kidney in the human body located at the posterior wall of the abdomen right outside the peritoneal cavity.
Each kidney is bean shaped, reddish brown and the size of a clenched fist weighing about 150gram.
There is an indented or concaved side of the kidney called the hilum. It is through this region that the renal artery, vein, lymphatics, nerve supply and ureter pass through to the inside of the kidney.

BISECTED KIDNEY

A bisected kidney will reveal:

An outer covering called the renal capsule
This is followed by the renal cortex
The renal cortex links to the renal medulla which houses the renal pyramid that projects to the renal pelvis of the ureter through the papilla
The renal pelvis collects urine from the papilla through the minor calyx which projects to the minor calyx.

Blood Supply To The Kidney

RENAL BLOOD SUPPLY

The renal artery arises from the aorta. There may be one or two renal arteries supplying each kidney
The renal artery enters the kidney through the helium and then branches progressively to form the:

Interlobar arteries,
Arcuate arteries,
Interlobular
Arteries
Afferent arterioles
Glomerular capillaries where larger amount of fluid and solutes (except
plasma proteins) are filtered to begin urine formation.

The distal end of the glomerulus coalesces to form the efferent arterioles which leads to the second capillary network the peritubular capillaries that surrounds the renal tubules.
The renal circulation is unique in that it has two capillary beds, the glomerular and the peritubular capillaries which are arranged in series and separated by the efferent
arterioles which help regulate hydrostatic pressure in both sets of capillaries
The renal circulation is unique in that it has two capillary beds, the glomerular and the peritubular capillaries which are arranged in series and separated by the efferent arterioles which help regulate hydrostatic pressure in both sets of capillaries.
High hydrostatic pressure (about 60mm Hg) in the glomerular capillaries causes rapid filtration, whereas a much lower hydrostatic pressure in the peritubular capillaries (about 13 mmHg) permits rapid reabsorption.
By adjusting the resistance of the afferent and efferent arterioles the kidney can regulate the hydrostatic pressure in both glomerular and peritubular capillaries
thereby changing the rate of glomerular filtration, tubular reabsorption or both in response to body homeostatic demand
The peritubular capillaries empty into the vessels of the venous system which runs parallel to the arteriolar vessels and progressively form the interlobular vein, the
arcuate vein, interlober vein and renal vein, which leaves the kidney beside the renal artery and ureter.

The Nephron

The nephron is the functional unit of the kidney
Its duty or function is to regulate water and solutes by filtering the blood, reabsorb what it needs and excrete
the rest
Each kidney contains about 1 million nephrons, each capable of forming urine

A nephron is divided into two namely:

The glomerulus through which large amount of fluid are filtered from blood.
A long tubule in which filtered fluid is converted into urine on its way to the renal pelvis.

Glomerulus

The glomerulus contains a network of branching and anastomosing glomerular capillaries that when compared with other capillaries, have high hydrostatic pressure (about 60mmltg)
The glomerular capillaries are covered by epithelial cells and the total glomerulus is encased in a Bowman’s capsule.
Fluid filtered from the glomerular capillaries flow into the Bownian’s capsule and then into the proximal tubule which lies in the cortex of the kidney
From the proximal tubule it flows into the loop of Henle which deeps into the medulla
Each loop consists of descending and ascending limb
The walls of the descending limb and the lower end of the ascending limb are called the thin segment of the loop of Henle.
After the ascending limb has returned partway to the cortex its wall becomes much thicker and it is referred to as the thick segment of the ascending limb
Finally, the filtered urine move from the ascending limb of the loop of Henle into the connecting tubule, cortical connecting tubule, medullar connecting tubule and then into the connecting ducts then discharged into the ureter

Roles of Some Functional Parts of the Nephron

1. Macula Densa:

At the point that the distal tubule traverses the afferent and efferent arterioles near the glomerus is a short segment which is
actually a plague in its wall, known as macula densa. It plays an important role in controlling nephron function.

2. Ddistal Tubule

Beyond the mucula densa fluids enter the distal tubule which also lies in the cortex like the proximal tubule.

3. Juxtaglomerular Apparatus

A group of cells located between the afferent and efferent arterioles are known as a junxtaglomerular cells. The juxta glomerular cells together with another specialized cell in that area called Macula densa are known as juxta glomerular apparatus.
It is in the juxtaglomerular apparatus that the hormone renin is formed. Rennin is normally formed in response to decreased blood pressure in the afferent arterioles, decreased sodium chloride in the distal convoluted tubule and sympathetic nerve stimulation of receptors on the juxtaglomerular cells. Rennin is therefore needed in the formation of Argiotensin I and Argiotensin II which stimulate secretion of aldosterone by the adrenal cortex

Glomerular Capsule or Bowman’s Capsule:

Bowman’s capsule (also called the glomerular capsule) surrounds the glomerular capillaries and is composed of visceral (simple squamous epithelial cells) inner layers and parietal (simple squamous epithelial cells) outer layers
The visceral layer lies just beneath the thickened glomerular basement membrane and is made of podocytes which send foot processes over the length of the glomerus.
Foot processes interdigitate with one another forming filtration slits that, in contrast to those in the glomerular
endothelium, are spanned by diaphragms.
The size of the filtration slits restricts the passage of large molecules (e.g red blood cells and platelets)
In addition, foot processeses have a negatively-charged coat (glycocalyx) that limits the filtration of negatively charged molecules, such as albumin. This action is called electrostatic repulsion.
The parietal layer of Bowman’s capsule is lined by a single layer of squamous epithelium between the visceral and parietal layers in Bowman’s space, into which the filtrate enters after passing through the podocyte’s filtration slits
Unlike the visceral layer, the parietal layer does not function in filtration, rather, the filtration barrier is formed by three components; the diaphragms of the filtration slits, the thick glomerular basement membrane and the glycocalyx secreted by podocytes
99% of glomerular filtrate will ultimately be reabsorbed.
The process of filtration of the blood in the Bowman’s capsule is ultrafiltration (or glomerular filtration) and the normal rate of filtration is 125 ml/min, equivalent to ten times the blood volume daily.
Measuring the glomerular filtration rate (GFR)is a diagnostic test of kidney function.
A decreased GFR may be a sign of renal failure.
Conditions that can effect GFR include; arterial pressure, afferent arteriole constriction, plasma protein concentration and colloid osmotic pressure.
Any protein that are roughly 30 kilodaltons or under can freely pass through the membrane.
Although, there is some extra hindrance for negatively charged molecules due to the negative charge of the basement membrane and the podocytes. Any small molecules such as water, glucose, salt (NaCl), amino acides and urea pass freely into Bowman’s space but cells, platelets and large proteins do not.
As a result, the filtrate leaving the Bowman’s capsule is very similar to blood plasma in composition as it passes into the proximal convoluted tubule.
Together, the glomerulus and Bowman’s capsule are called the renal corpuscle.

Proximal Convoluted Tubule (Pct)

The proximal tubule can be anatomically divided into two segments; proximal convoluted tubule and the proximal straight tubule.
Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries, including approximately two-thirds of the filtered salt and water and all filtered organic solutes (primarily glucose and amino acids)
This is driven by sodium transport from the lumen into the blood by the Na+/K+ATPase in the basolateral membrane of the epithelial cells
Much of the mass movement of water and solutes occurs in between the cells through the tight junctions, which in this case are not selective.
The solutes are absorbed isotonically, in that the osmotic pressure of the fluid leaving the proximal tubule is the same as that of the initial glomerular filtrate.
However, glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary active transport through cotransport channels driven by the sodium gradient out of the nephron

Loop of Henle:

The loop of Henle (sometimes known as the nephron loop) is a U-shaped tube that consists of a decending limb
and ascending limb
It begins in the cortex, receiving filtrate from the proximal convoluted tubule, extends into the medulla, and then
returns to the cortex to empty into the distal convoluted tubule. Its primary role is to concentrate the salt in the
interstitium, the tissue surrounding the loop

Descending Limb

The descending limb is permeable to water but completely impermeable to salt, and thus only indirectly contributes to the concentration of the
interstitium
As the filtrate descends deeper into the hypertonic interstitium of the renal medulla, water flows freely out of the descending limb by osmosis until the tonicity of the filtrate and interstitium equilibrate. Longer descending
limbs allow more time for water to flow out of the filtrate, so longer limbs make the filtrate more hypertonic than
shorter limbs

Ascending Limb:

Unlike the descending limb, the ascending limb of Henle’s loop is impermeable to water, a critical feature of the countercurrent exchange mechanism employed by the loop
The ascending limb actively pumps sodium out of the filtrate, generating the hypertonic interstitium that drives
countercurrent exchange
In passing through the ascending limb, the filtrate grows hypotonic since it has lost much of its sodium content. This hypotonic filtrate is passed to
the distal convoluted tubule in the renal cortex.

Distal Convoluted Tubule (DCT)

The distal convoluted tubule is similar to the proximal convoluted tubule in structure and function
Cells lining the tubule have numerous mitochondria, enabling active transport to take place by the energy
supplied by ATP
Much of the ion transport taking place in the distal convoluted tubule is regulated by the endocrine system
In the presence of parathyroid hormone, the distal convoluted tubule reabsorbs more calcium and excretes more phosphate
When aldosterone is present, more sodium is reabsorbed and more potassium excreted
Atrial natriuretic peptide causes the distal convoluted tubule to excrete more sodium
In addition, the tubule also secretes hydrogen and ammonia to regulate PH
After traveling the length of the distal convoluted tubule, only 3% of water remains and the remaining salt content is negligible
97% of the water in the glomerular filtrate enters the convoluted tubules and collecting ducts by osmosis.