Chapter 1 - Homeostasis and the Kidney Flashcards
(164 cards)
1
Q
What is mammalian tissue essentially made up of?
A
A collection of cells bathed in a fluid medium or ‘extracellular’ fluid (tissue fluid).
2
Q
Mammalian tissue is essentially made up of a collection of cells bathed in a fluid medium or ‘extracellular’ fluid (tissue fluid). The composition of this fluid (and …
A
Consequentially the blood due to the permeable nature of the capillary walls)
3
Q
Mammalian tissue is essentially made up of a collection of cells bathed in a fluid medium or ‘extracellular’ fluid (tissue fluid). The composition of this fluid (and consequentially the blood due to the permeable nature of the capillary walls) must be ….
A
Keep constant
4
Q
Mammalian tissue is essentially made up of a collection of cells bathed in a fluid medium or ‘extracellular’ fluid (tissue fluid). The composition of this fluid (and consequentially the blood due to the permeable nature of the capillary walls) must be kept constant in terms of factors such as …
A
Water content Ion content Temperature pH Oxygen levels
5
Q
Mammalian tissue is essentially made up of a collection of cells bathed in a fluid medium or ‘extracellular’ fluid (tissue fluid). The composition of this fluid (and consequentially the blood due to the permeable nature of the capillary walls) must be kept constant in terms of factors such as water and ion content, temperature, pH and oxygen levels, …
A
Irrespective of the external conditions outside the body.
6
Q
What is homeostasis?
A
Homeostasis is the maintenance of constant or steady state conditions within the body.
7
Q
Most homeostatic responses have how many basic features?
A
Three
8
Q
Describe the three basic features that most homeostatic responses have
A
• A control system with sensors (receptors) which provides information allowing the monitoring of the factor being controlled.
- The receptors can be in the brain or localised throughout the body.
- However, the monitor (control centre) is usually in the brain.
• If the receptors show a departure from normal levels (the set point) for the factor being controlled, for example temperature, then a corrective mechanism brings about the changes required to return the factor to its normal level.
- For example, if mammals overheat, the corrective measures can include swearing and the vasodilation of capillaries in the skin.
• The corrective mechanism involves a negative feedback system.
- Negative feedback occurs as the return of the factor being controlled to its normal level (set point) causes the corrective measures to be turned off.
- This prevents over-correction.
- In our example of temperature regulation, the stimulation of the sweat glands and the degree of vasodilation of blood capillaries is reduced as blood (body) temperature returns to normal.
9
Q
In a homeostatic response, communication occurs between …
A
The sensors/receptors and the monitor (and between the monitor and the effectors that bring about the corrective response).
10
Q
Communication between the sensors/receptors and the monitor (and between the monitor and the effectors that bring about the corrective response) can be by …
A
Nervous or hormonal control.
11
Q
Give an example of a factor which is primarily under nervous control
A
Temperature
12
Q
Give an example of a factor which is under hormonal control
A
Blood glucose levels
13
Q
For what reasons is homeostatic control of mammalian body systems essential?
A
- Providing the optimum conditions for enzyme reactions in terms of pH and temperature.
- Avoiding osmotic problems in cells and in body fluids.
14
Q
Homeostatic control of mammalian body systems is essential for many reasons including:
- providing the optimum conditions for enzyme reactions in terms of pH and temperature.
- avoiding osmotic problems in cells and in body fluids.
While mammals have …
A
Complex and effective homeostatic controls
15
Q
Homeostatic control of mammalian body systems is essential for many reasons including:
- providing the optimum conditions for enzyme reactions in terms of pH and temperature.
- avoiding osmotic problems in cells and in body fluids.
While mammals have complex and effective homeostatic controls, many other animals have …
A
Simpler controls that are less able to keep the internal environment constant.
16
Q
Homeostatic control of mammalian body systems is essential for many reasons including:
- providing the optimum conditions for enzyme reactions in terms of pH and temperature.
- avoiding osmotic problems in cells and in body fluids.
While mammals have complex and effective homeostatic controls, many other animals have simpler controls that are less able to keep the internal environment constant, for example, …
A
The body temperature of insects usually varies with the external environment.
17
Q
Homeostatic control of mammalian body systems is essential for many reasons including:
- providing the optimum conditions for enzyme reactions in terms of pH and temperature.
- avoiding osmotic problems in cells and in body fluids.
While mammals have complex and effective homeostatic controls, many other animals have simpler controls that are less able to keep the internal environment constant, for example, the body temperature of insects usually varies with the external environment.
How do many species of less complex animals avoid large swings in body conditions?
A
By living in an environment where the external environment is relatively constant, such as the sea.
18
Q
Homeostatic control of mammalian body systems is essential for many reasons including:
- providing the optimum conditions for enzyme reactions in terms of pH and temperature.
- avoiding osmotic problems in cells and in body fluids.
While mammals have complex and effective homeostatic controls, many other animals have simpler controls that are less able to keep the internal environment constant, for example, the body temperature of insects usually varies with the external environment.
Why do many species of less complex animals live in environments where the external environment is relatively constant?
A
To avoid large swings in body conditions.
19
Q
Draw a flow diagram showing the general principles of homeostatic control
A
Textbook page 6
20
Q
Name a major homeostatic organ in mammals
A
The kidney
21
Q
Name the two very important functions of the kidney
A
- Excretion
2. Osmoregulation
22
Q
What is excretion?
A
Excretion is the removal of the toxic waste of metabolism
23
Q
What is the main toxic waste product excreted by the kidneys?
A
Urea
24
Q
Name the toxic waste products excreted from the kidneys
A
Urea
Creatinine
25
What is urea?
A nitrogenous waste produced during the breakdown of excess amino acids (and nucleic acids) in the liver.
26
What is creatinine?
A waste product produced from the breakdown of creatine phosphate (a molecule important in ATP synthesis) in muscles, ie it is a toxic byproduct of muscle metabolism.
27
What is osmoregulation?
A homeostatic process that controls the water potential of body fluids.
28
How do the kidneys help regulate the water potential of the blood?
Through controlling both the volume and concentration of urine produced.
29
- The structure of the urinary (excretory) system
Traditionally the body system including the kidneys, ureters, bladder and urethra has been called the excretory system. However, this is perhaps not the best term as ...
Excretion is also carried out by other parts of the body (for example, CO2 is excreted from the lungs); consequently, many textbooks now refer to it as the urinary system.
30
Draw a diagram showing the urinary (excretory) system
Textbook page 7
31
Blood travelling through the aorta and renal artery reaches the kidney at ...
The high pressures required for filtration.
32
Blood travelling through the aorta and renal artery reaches the kidney at the high pressures required for filtration. In essence, the kidney operates as ...
A complex filter, keeping useful products in the blood and eliminating excretory products and excess water.
33
Why can the kidney be referred to as a complex filter?
As it keeps useful products in the blood and eliminates excretory products and excess water.
34
Filtered blood leaves the kidneys via ...
The renal vein
35
Filtered blood leaves the kidneys via the renal vein whereas the excretory products and excess water ...
Pass into the ureter as urine, which takes it to the bladder for storage.
36
How is the release of urine controlled?
Sphincter muscles in the base of the bladder control the release of urine, which exits the body through the urethra.
37
The sphincter muscles are in a constant state of ...
Contraction
38
The sphincter muscles are in a constant state of contraction, however they ...
Relax during urination to allow the release of urine via the urethra.
39
A section through a kidney shows that it contains how many main zones (regions) of tissue?
Two
40
Name the two main zones (regions) of tissue that make up a kidney
Cortex
| Medulla
41
- Kidney structure
| What is the cortex?
The cortex is the outer dark region immediately under the thin covering layer (capsule).
42
- Kidney structure
| What is the medulla?
The medulla is the inner lighter region. The medulla is subdivided into a number of pyramids whose apices extend down into a large central cavity called the renal pelvis.
43
What is the functional unit of the kidney?
The nephron
44
How many nephrons are there in each kidney?
Over one million
45
The functional unit of the kidney is the nephron. There are over one million nephrons in each kidney. Each nephron operates as what?
An individual filter.
46
Where does each individual nephron originate and end?
The cortex.
47
What part of the nephron extends down into the medulla?
The long central region (the loop of Henlé)
48
Many nephrons join with ...
A collecting duct.
49
Many nephrons join with a collecting duct, which ...
Also extends down through the medulla.
50
- The structure of the nephron
| The nephron originates as ...
A cup-shaped Bowman's capsule.
51
The Bowman's capsule is also referred to as ...
The renal capsule.
52
Each Bowman's capsule is supplied with ...
Blood from an afferent arteriole (a branch of the renal artery)
53
Draw a simple diagrammatic cross section of a kidney showing the main zones of tissue
Textbook page 7
54
Each Bowman's capsule is supplied with blood from an afferent arteriole (a branch of the renal artery) and the blood leaves through ...
An efferent arteriole
55
What is present within the Bowman's capsule?
Within the 'cup' of the capsule, the arteriole branches to form a tightly coiled knot of capillaries called the glomerulus - capillaries which subsequently unite before forming the efferent arteriole.
56
What happens to the efferent arteriole after it leaves the Bowman's capsule?
It branches to form a capillary network (the vasa recta) that remains closely associated with the rest of the nephron.
57
Draw a diagram of a nephron
Textbook page 7
58
In the nephron itself, what comes after the Bowman's capsule?
The Bowman's capsule extends into a coiled tube called the proximal convoluted tubule.
59
What does 'proximal' mean?
First
60
What does 'convoluted' mean?
Coiled
61
In the nephron itself, what comes after the proximal convoluted tubule?
The proximal convoluted tubule extends into the loop of Henlé which dips down into the medulla of the kidney.
62
Name the two parts of the loop of Henlé
The descending limb
| The ascending limb
63
In the nephron itself, the Bowman's capsule extends into a coiled tube called the proximal convoluted tubule (proximal = first; convoluted = coiled). The proximal convoluted tubule extends into the loop of Henlé which dips down into the medulla of the kidney. The descending part of the loop is, unsurprisingly, called ...
The descending limb
64
In the nephron itself, what comes after the descending limb of the loop of Henlé?
The loop of Henlé then bends sharply and returns back up through the medulla (the ascending limb) to reach the cortex again.
65
In the nephron itself, what comes after the loop of Henlé?
The distal convoluted tubule
66
In the nephron itself, the Bowman's capsule extends into a coiled tube called the proximal convoluted tubule (proximal = first; convoluted = coiled). The proximal convoluted tubule extends into the loop of Henlé which dips down into the medulla of the kidney. The descending part of the loop is, unsurprisingly, called the descending limb. The loop of Henlé then bends sharply and returns back up through the medulla (the ascending limb) to reach the cortex again. At this stage it becomes ...
The distal convoluted tubule
67
In the nephron itself, what comes after the distal convoluted tubule?
The distal convoluted tubule (and the distal convoluted tubules from many other nephrons) join a collecting duct.
68
What is the role of a collecting duct?
The collecting ducts converge at the base of the renal pelvis and empty their contents (now called urine) into the ureter which takes the urine to the bladder.
69
Kidney (and nephron) function involves two main processes:
Ultrafiltration
| Reabsorption
70
Give a definition of ultrafiltration relevant to the urinary (excretory) system
The filtration of plasma and substances below a certain size into the Bowman's capsule (nephron) under high pressure.
71
Give a definition of reabsorption relevant to the urinary (excretory) system
As ultrafiltration is based purely on molecular size (and not whether products are useful or not), it is essential that filtered useful products are selectively reabsorbed back into the bloodstream from the nephron.
72
Why does blood entering the glomerulus have a high hydrostatic pressure?
1. The short distance from the heart that the blood travels down the aorta and into the renal artery before branching into the kidney arterioles.
2. The fact that the afferent arteriole of each glomerulus is wider than its efferent arteriole.
3. The coiling of the capillaries in the glomerulus further restricts blood flow therefore increasing pressure.
73
The high hydrostatic pressure of blood entering the glomerulus forces small components out of the blood. Name some of these components.
```
Glucose
Amino acids
Salts
Water
Urea
```
74
What is the role of the high hydrostatic pressure in the glomerulus?
Forces the smaller components in the blood out of the capillaries and into the Bowman's capsule.
75
Large components of the blood remain in the glomerular capillaries. Name some of these components.
Blood cells
| Plasma proteins
76
Why do larger components of the blood, such as blood cells and plasma proteins, remain in the glomerular capillaries?
They are too large to pass through the small pores of the squamous (flattened) endothelium that makes up the glomerular capillaries within the glomerulus.
77
How are the glomerular capillaries specialised for the function of ultrafiltration?
Composed of a single layer of squamous (flattened) endothelial cells containing small pores (and is therefore porous).
78
What type of cell is the Bowman's capsule lined with?
Podocytes
79
What are podocytes?
Specialised cells that line the Bowman's capsule.
Have extensions in two planes that allow filtered material to pass through easily.
Support delicate glomerular capillaries to prevent them from bursting due to high hydrostatic pressures involved.
80
What is the effective filter in the Bowman's capsule?
The basement membrane
81
Describe the structure of the basement membrane
Extracellular matrix (gel) formed of many different substances including proteins. It is effectively a molecular sieve.
82
What structure present in the Bowman's capsule determines which components of the blood enter the Bowman's capsule itself?
Basement membrane
83
Where is the basement membrane situated?
The basement membrane is situated on the outside of the fenestrated endothelial lining of the glomerular capillaries, separating the capillaries and podocytes of the Bowman's capsule.
84
What are diuretics?
Drugs that cause large amounts of urine to be produced.
85
What are the glomerular capillaries composed of?
A single layer of squamous (flattened) endothelial cells, containing small pores, and an outer basement membrane.
86
What is the effective filter in the Bowman's capsule?
The basement membrane
87
What structure present in the Bowman's capsule prevents the blood cells and plasma proteins from leaving the blood?
The basement membrane
88
Draw a diagram showing the site of ultrafiltration
Textbook page 9
89
Draw a diagram showing a glomerular capillary
Textbook page 9
90
In effect, each of the three layers separating the blood in the capillary and the inside of the Bowman's capsule is specialised. In what way?
Through either being porous (capillary endothelial cells and podocytes) or through acting as a filter (basement membrane).
91
What is the glomerular filtrate?
The substances that pass through the basement membrane and enter the Bowman's capsule.
92
What is the name given to the substances that pass through the basement membrane and enter the Bowman's capsule?
The glomerular filtrate
93
Compare and contrast the composition of the glomerular filtrate and the blood present in the glomerular capillaries
Similar except the glomerular filtrate lacks plasma proteins and blood cells that are too large to penetrate the basement membrane.
94
Describe the cells that line the nephron
Apart from the podocytes lining the Bowman's capsule, the remaining epithelial cells lining the nephron (and the collecting duct) are cuboidal (cube-shaped) epithelial cells.
The podocytes themselves are a special type of epithelial cell.
95
- The filtration force
| Ultrafiltration is a necessary process in ...
Kidney function
96
What are the forces involved in filtration in the kidneys?
1. Hydrostatic pressure
| 2. Water potential gradient
97
- The filtration force
Ultrafiltration is a necessary process in kidney function. However, the hydrostatic pressure forcing through water and small molecules is ...
Not the only force involved.
98
- The filtration force
Ultrafiltration is a necessary process in kidney function. However, the hydrostatic pressure forcing through water and small molecules is not the only force involved. In terms of water potential, it is important to compare ...
The forces on each side of the membrane
99
- The filtration force
Ultrafiltration is a necessary process in kidney function. However, the hydrostatic pressure forcing through water and small molecules is not the only force involved. It is important to compare the forces on each side of the membrane in terms of what factor?
Water potential
100
- The filtration force
Ultrafiltration is a necessary process in kidney function. However, the hydrostatic pressure forcing through water and small molecules is not the only force involved. In terms of water potential, it is important to compare the forces on each side of the membrane. What conditions must be met for filtration to occur?
The water potential within the glomerular capillaries (blood plasma) must exceed the water potential within the Bowman's capsule (glomerular filtrate), ie the glomerular filtrate must have a more negative water potential.
101
How many components does water potential have?
Two
102
What are the two components which make up water potential?
Solute potential
| Pressure potential
103
Compare the hydrostatic pressure (pressure potential) between the glomerular capillaries (blood) and the glomerular filtrate within the nephron
Pressure potential of blood is much greater than the hydrostatic pressure (back pressure) created by the filtrate in the nephron.
104
Compare the solute potential between the glomerular capillaries (blood) and the glomerular filtrate within the nephron
* Plasma proteins greatly contribute to solute potential in the blood in the glomerular capillaries.
* Plasma proteins are absent from filtrate.
* Filtrate has a less negative (more positive) solute potential.
* Difference in solute potential opposes filtration, but this effect is insignificant compared to the differences in hydrostatic pressure across the basement membrane.
* A difference which very strongly promotes filtration.
* Net filtration pressure causes fluid to move from the glomerular capillaries into the Bowman's capsule.
105
Read the worked example on page 10 of the textbook
Do you understand it?
106
- Reabsorption
| Useful blood products temporarily lost to the glomerular filtrate are ...
Reabsorbed back into the blood
107
- Reabsorption
Useful blood products temporarily lost to the glomerular filtrate are reabsorbed back into the blood, mainly as the filtrate passes along ...
The proximal convoluted tubule
108
- Reabsorption
Useful blood products temporarily lost to the glomerular filtrate are reabsorbed back into the blood, mainly as the filtrate passes along the proximal convoluted tubule. What substances are selectively reabsorbed back into the blood?
Glucose
Amino acids
Some salts
109
- Reabsorption
Useful blood products temporarily lost to the glomerular filtrate are reabsorbed back into the blood, mainly as the filtrate passes along the proximal convoluted tubule. Glucose and amino acids - small enough to pass through the basement membrane but too valuable to be lost in the urine - are selectively reabsorbed by what processes?
Facilitated diffusion
| Active transport
110
- Reabsorption
Useful blood products temporarily lost to the glomerular filtrate are reabsorbed back into the blood, mainly as the filtrate passes along the proximal convoluted tubule. Glucose and amino acids - small enough to pass through the basement membrane but too valuable to be lost in the urine - are selectively reabsorbed by facilitated diffusion and active transport. Why is the term selectively reabsorbed used?
As toxic substances such as urea are not actively reabsorbed but (mainly) remain in the filtrate.
111
Why are useful blood products, such as glucose and amino acids, selectively reabsorbed?
As they are small enough to pass through the basement membrane but too valuable to be lost in the urine.
112
- Reabsorption
| How is the osmotic gradient required for the reabsorption of water created?
* The reabsorption of glucose, amino acids and some salts reduces the solute potential in both the (reabsorbing) epithelial cells of the tubule and the blood in the capillaries.
* Osmotic effect created causes over 70% of the water in the filtrate to re-enter the blood capillaries passively by osmosis.
113
Small plasma proteins which may have passed through the basement membrane in the glomerular filtrate are reabsorbed by what process?
Pinocytosis
114
How does urea re-enter the blood from the glomerular filtrate?
Although urea (being a toxic metabolic waste) is not selectively reabsorbed, some urea passes from the nephron back into the blood by diffusion.
115
Although urea (being a toxic metabolic waste) is not selectively reabsorbed, some urea passes from the nephron back into the blood by diffusion. How much urea can diffuse back into the blood?
Theoretically up to 50%
116
The glucose and amino acids can be absorbed back into the blood from the glomerular filtrate by facilitated diffusion provided ...
The concentration gradient permits
117
The glucose and amino acids can be absorbed back into the blood from the glomerular filtrate by facilitated diffusion provided the concentration gradient permits. Active transport is necessary ...
To ensure that all the glucose is reabsorbed from the nephron back into the capillary network.
118
Why do the epithelial cells of the proximal convoluted tubule need to be highly adapted for their role?
As they have high levels of metabolic activity and continually carry out energy-demanding processes such as active transport.
119
The epithelial cells of the proximal convoluted tubule have high levels of metabolic activity and continually carry out energy-demanding processes such as active transport. Consequently, ...
They are highly adapted for this role.
120
By the time the filtrate reaches the end of the proximal convoluted tubule, it will ...
Have no glucose or amino acids present as they will all have been reabsorbed.
121
How does some of the urea re-enter the blood along the length of the proximal convoluted tubule?
By diffusion
122
What happens to the concentration of urea along the length of the proximal convoluted tubule?
Despite the fact that some urea diffuses back into the blood along the length of the proximal convoluted tubule, the concentration of urea in the filtrate increases along its length due to the reabsorption of water into the blood.
123
At the end of the proximal convoluted tubule, the filtrate is ...
Isotonic with the blood plasma
124
What will happen to the filtrate once it reaches the end of the proximal convoluted tubule?
It will be isotonic with the blood plasma
125
How are the cuboidal epithelial cells lining the proximal convoluted tubule adapted for their role?
* Cell surface membrane contains protein carrier molecules for selective reabsorption (facilitated diffusion and active transport) of, for example, glucose and amino acids.
* Microvilli increase surface area for reabsorption.
* Numerous mitochondria provide ATP for active transport.
* Infolding of membrane further increases surface area.
* Capillaries lie close to cells lining the proximal convoluted tubule.
126
Draw a diagram of some of the cuboidal epithelial cells that make up the proximal convoluted tubule
Textbook page 11
127
Where in the nephron does further regulation of blood composition take place?
In the distal convoluted tubule
128
What happens in the distal convoluted tubule?
Further regulation of blood composition takes place
129
In what way does further regulation of blood composition take place in the distal convoluted tubule?
The pH and ionic composition of the blood in the capillaries surrounding the tubule are adjusted and some toxic substances, for example, creatinine (a byproduct from muscle metabolism), are secreted from the blood into the filtrate for disposal.
130
How do the kidneys carry out osmoregulation?
Through controlling the water balance of the blood (by controlling the volume and concentration of urine produced); consequently the water content of the tissue fluid and the cells is also controlled.
131
Where does water regulation take place in the nephron?
The collecting duct
132
Where in the nephron can reabsorption of water back into the blood take place?
```
Proximal convoluted tubule (passive)
Descending limb of the loop of Henlé (passive)
Distal convoluted tubule (regulated by ADH)
Collecting duct (regulated by ADH)
```
133
Where in the nephron is most of the water reabsorbed back into the blood?
The proximal convoluted tubule
134
What is the downside of water reabsorption back into the blood from the nephron via the proximal convoluted tubule and descending limb of the loop of Henlé?
The process is passive and the exact amount of water reabsorbed back into the blood cannot be controlled.
135
How is the reabsorption of water back into the blood via the collecting duct controlled?
By varying the permeability of the collecting duct walls
136
Where in the nephron does the fine control of water balance take place?
In the collecting duct (where the permeability of the collecting duct walls to water can be varied)
137
The collecting duct is where the water regulation takes place. Although most water is reabsorbed in the proximal convoluted tubule (and some from the descending limb of the loop of Henlé), the process is passive and the exact amount of water reabsorbed back into the blood cannot be controlled. However, reabsorption in the collecting ducts can be controlled by varying the permeability of the collecting duct walls - this is where the fine control of water balance takes place. What hormone is crucial in this process?
The antidiuretic hormone (ADH)
138
What does ADH stand for?
Antidiuretic hormone
139
Why is the antidiuretic hormone (ADH) crucial in the process of osmoregulation?
As it can control the degree of permeability of the collecting duct walls.
140
Where is ADH produced in the body?
In the hypothalamus (part of the brain just above the junction with the spinal cord)
141
Where in the body is ADH stored?
Secreted from hypothalamus into the posterior lobe of the pituitary body where it is stored.
142
Where in the body is the solute potential of the blood monitored?
The solute potential of the blood is monitored by osmoreceptors (specialised cells) in the hypothalamus.
143
For what reasons can blood become too concentrated?
Blood can become too concentrated, ie a more negative solute potential, for many reasons:
• Sweating after exercise or on a hot day
• Not drinking enough water
• Eating a very salty meal
144
What happens if the blood becomes too concentrated?
1. Solute potential of the blood becomes more negative and this is detected by the osmoreceptors in the hypothalamus.
2. Posterior lobe of the pituitary body releases more ADH into the blood.
3. This causes the walls of the DCTs and the collecting ducts to become more permeable - special channel proteins (aquaporins) open which helps make the walls of the collecting ducts more permeable.
4. Therefore more water is reabsorbed from the collecting ducts back into the blood.
5. The net result is that the solute potential of the blood returns to normal (becomes less negative) and a smaller volume of more concentrated (hypertonic) urine is produced.
6. This process exemplifies negative feedback. As the blood concentration changes it sets in train a process that returns the solute potential back to normal; as the blood concentration returns to normal the release of ADH reduces, returning to normal levels.
145
For what reasons can blood become too dilute?
Blood can become too dilute, ie a more positive solute potential, for many reasons:
• Drinking a hypotonic liquid
146
What happens if the blood becomes too dilute?
1. Solute potential of the blood becomes more positive and this is detected by osmoreceptors in the hypothalamus.
2. Posterior lobe of the pituitary body releases less ADH into the blood.
3. This causes the walls of the DTCs and the collecting ducts to become less permeable - special channel proteins (aquaporins) close which helps make the walls of the collecting ducts less permeable.
4. Therefore less water is reabsorbed from the collecting ducts back into the blood.
5. The net result is that the solute potential of the blood returns to normal (becomes less positive) and a larger volume of more dilute (hypotonic) urine is produced.
6. This process exemplifies negative feedback. As the blood concentration changes it sets in train a process that returns the solute potential back to normal; as the blood concentration returns to normal the release of ADH increases, returning to normal levels.
147
What is the role of the loop of Henlé?
Enables mammals to produce a hypertonic urine and plays a significant role in water reabsorption from the collecting ducts.
148
What part of the nephron enables mammals to produce a hypertonic urine and plays a significant role in water reabsorption from the collecting ducts?
The loop of Henlé
149
Describe the structure of the loop of Henlé
* The walls of the descending limb are thin and permeable to water.
* The ascending limb walls are much thicker and impermeable to water.
150
What is the role of the ascending limb?
* Secretes Na+ ions and Cl- ions into the medulla
* This process involves active transport of these ions.
* Consequently, sodium chloride (salt) builds up in the interstitial fluid in the medulla, creating a very negative solute potential.
* As a result of the ions leaving the ascending limb, the filtrate in it becomes progressively more dilute and is hypotonic by the time it reaches the top of the ascending limb.
151
What is the role of the descending limb?
* The net result of the very negative water potential in the medulla, caused by the high concentration of sodium chloride, and the permeability of the descending limb walls, is that water is osmotically removed along the length of the descending limb.
* Therefore, the filtrate in the descending limb becomes progressively more concentrated with distance down the limb (aided by sodium and chloride ions entering the descending limb by diffusion) until at the very bottom it is hypertonic to the blood.
152
At what point in the loop of Henlé is the filtrate
a) isotonic
b) hypotonic
c) hypertonic
With the blood
a) Top of descending limb
b) Top of ascending limb
c) Bottom of loop of Henlé
153
What does the ascending limb secrete?
Na+ and Cl- ions
154
Draw a diagram showing the osmotic changes that take place along the length of the loop of Henlé. Your diagram should also include a collecting duct.
Textbook page 13
155
What is the main function of the loop of Henlé?
To create the very concentrated interstitial fluid in the medulla, a feature which facilitates the osmotic removal of water from the collecting ducts (which also pass through the medulla).
156
How are cuboidal epithelial cells in the ascending limb adapted for their role?
Rich in mitochondria which provide ATP necessary for pumping sodium and chloride ions into the medulla
157
What happens to water that leaves the descending limb ?
Water that leaves the descending limb by osmosis enters the capillary network (vasa recta) and is removed from the medulla, therefore having little effect on the solute potential of the interstitial fluid.
158
The osmotic differences between the descending and ascending limbs at any one level are small but the cumulative effect over the length of the limbs (depth of medulla) is significant. This, together with the filtrates in the limbs travelling in opposite directions, is why the process is described as ...
The countercurrent multiplier effect
159
Describe the process known as the countercurrent multiplier effect
The osmotic deferences between the descending and ascending limb at any one level are small but the cumulative effect over the length of the limbs (depth of medulla) is significant. The filtrate in the limbs travel in opposite directions.
160
The longer the loop of Henlé, the more water that can be reabsorbed. Why is this?
A longer loop allows the medulla to have an even more negative water potential.
161
There is positive correlation between ...
The length of the loop of Henlé and the ability to reabsorb water and concentrate the liquid in the collecting duct (liquid that will become urine) and conserve water in a species.
162
The kidneys are a very effective filter but are equally effective in ...
Osmoregulation
163
How often does all the blood in the circulatory system pass through the kidneys?
Every five minutes
164
The kidneys are a very effective filter but are equally effective in osmoregulation. All the blood in the circulatory system passes through the kidneys every five minutes. What happens to water that is filtered out of the blood?
Over 99% of the filtered water is reabsorbed with less than 1% ending up as urine.
