4.1 Homeostasis Flashcards
4.1 Homeostasis
Learning objectives:
• To understand the principle of homeostasis
• To be able to label the structures of a kidney
• To understand the role of the kidney
• Be able to describe the structure and function of a nephron
• To understand the mechanism of ultrafiltration
• To understand the mechanism of selective reabsorption
• To understand the role of the loop of Henle
• Be able to describe the mechanism of osmoregulation
• Be able to describe the effect of ADH
What is Homeostasis?
The maintenance of a constant internal environment
The Importance of Homeostasis in Mammals
Homeostasis in mammals is very important to:
- provide optimum conditions for enzyme function
- avoid osmotic problems in cells and body fluids
Principles of Homeostasis
The Principles of Homeostasis:
• The control system has a sensor (receptors) which monitor the factor being controlled. The monitor is usually the brain.
• If the receptors show deviation away from the set point, a corrective mechanism being about changes resulting the the regulation of this factor.
• This corrective mechanism involves a negative feedback system, which stops the corrective mechanism from over-correcting.
• Communication between receptors, monitor and control system can be via nervous (temperature) or hormonal (glucose) control.
The Mammalian Kidney
The principal functions of the kidneys are:
• Excretion - the removal of toxic nitrogenous waste products of metabolism (urea + creatine) from the blood
• Homeostasis - kidneys regulate the water content, ion composition and pH of body fluids; their role in the regulation of water content is called osmoregulation
The Kidney Nephron
The kidney nephron is the functions unit of the kidney. Each kidney nephron receives its own small supply of blood from extensive arteriole branches coming from the renal artery; each human kidney contains around 1 million nephrons.
Each kidney nephron is responsible for urine formation and this involves three basic processes:
- Ultrafiltration of blood
- Selective reabsorption of filtered materials
- Concentration of urine; the reabsorption of water back into the capillaries
Diagram of The Nephron
Function of The Nephron
Function of each nephron section:
• Glomerulus - Tightly coiled knot of capillaries. Site of Ultrafiltration.
• Proximal Convoluted Tubule (PCT) - First coiled section of a nephron. Main site of selective reabsorption.
• Loop of Henle - Consists of a descending limb, hairpin bend (within the medulla of kidney) and ascending limb. It creates a high concentration of salts in the medulla which allows water to be reabsorbed back into the blood.
• Distal Convoluted Tubule (DCT) and Collecting Duct - Remaining water will be reabsorbed back into the blood under the control of the hormone ADH.
- Ultrafiltration of the Blood
This is the filtration of plasma and substances below a certain size into the Bowman’s Capsule. Blood entering the glomerulus had a high hydrostatic pressure because:
- The short distance from the heart that the blood travels down via the aorta and into the renal artery before branching into the kidney’s arteriole.
- The afferent arteriole of each glomerulus is slightly wider than the efferent arteriole.
- Coiling capillaries in the glomerulus further contracts the blood flow therefore increasing g the pressure.
The Bowman’s Capsule and Glomerulus
Bowman’s Capsule
Blood entering the glomerulus is separated by space inside the Bowman’s capsule by the following 3 layers:
• Squamous endothelium of the cap Pilate which has thousands of pores (single layer: 1 cell thick)
• Basement membrane made of glycoprotein and collagen fibres (effective filter and molecular sieve)
• Podocytes in the wall of the Bowman’s capsule. Foot like processes provide porous gaps (filtration slits)
*The structure of the Bowman’s Capsule and Glomerulus aids the process of filtration
Diagram of the Bowman’s Capsule and Glomerulus
Structure and Function
Gaps in the Endothelium- Act as a filter allowing most molecules to pass through except large molecules such as RBC and WBC
Basement Membrane- Acts as and effective filter allowing only small molecules that are soluble to pass through, such as glucose and ions
Afferent Arteriole being wider than the efferent arteriole- Creates and ensures a high blood pressure is maintained in the glomerulus so that ultrafiltration can continue
Podocytes- Have extensions in two plains which mean that filtered material can easily pass through
Glomerulus Filtration diagram
What Substances are Filtered
Osmosis
*Ultrafiltration is linked to Osmosis
What is Osmosis?
Osmosis the NET movement of water molecules from a region of higher water potential to an area of lower water potential via a semi- permeable membrane.
In homeostasis you must be able to compare the water potential of blood with the water potential of the filtrate; with pure water having a water potential of 0 and solute solution having a water potential of -20.
Blood= more negative/ lower solute potential (due to plasma proteins) and a high pressure potential (due to high hydrostatic pressure).
Filtrate= less negative/ higher solute potential (due to lack of plasma proteins) and a lower pressure potential (due to hydrostatic pressure).
The net difference is that blood water potent is greater than the filtrate water potential therefore the fluid moves from the capillaries to the Bowman’s Capsule.
Note: Water Potential= solute potential + pressure potential
Worked Example of Water Potential difference between Blood and Filtrate
- Selective Reabsorption
Useful substances are also removed by ultrafiltration but they are needed by the body so they are then reabsorbed back into the blood. This is called selective reabsorption as only certain substances are reabsorbed, such as salt, glucose, amino acids and sodium.
The process is selective as toxic substances such as urea are not being actively reabsorbed into the blood and remains in the filtrate.
Adaptions of the Proximal Convoluted Tubule
The PCT is adapted for selective reabsorption by consisting of cuboidal epithelial cells which contain:
•Microvilli ~ increases surface area for reabsorption
•Mitochondria ~ increased number of mitochondria to increase the release of ATP for active transport
•Infolding of Membrane ~ further increases surface area for selective reabsorption
•Cell Surface Membrane ~ contains protein carrier molecules to aid the movement for facilitated diffusion and active transport
• Capillaries of the vasa recta lie close to the cells of the PCT therefore shorter diffusion distance
Reabsorption in the PCT
As the filtrate travels along the PCT, 80% of the water is reabsorbed by osmosis into the adjacent blood capillaries. Because of this, ions follow partly by diffusion and partly by active transport.
Glucose, salt and amino acids are completely reabsorbed by active transport from the filtrate into the vasa recta capillaries and any small plasma proteins are reabsorbed through the basement membrane by pinocytosis. This lowers the solute potential of the cuboidal epithelium and blood capillaries resulting in an osmotic gradient that is responsible for the bulk of water being reabsorbed by osmosis back into the blood capillaries.
By the end of the proximal convoluted tubule, the filtrate is isotonic with the plasma.
Substance and Method of Reabsorption
Terms and their Meanings
Isotonic: Same/equal water potential
Hypertonic: More negative/ lower water potential
*(e for less)
Hypotonic: Less negative/ higher water potential
*(o for more)
Loop of Henlé
Process of the countercurrent multiplier takes place here, where filtrate move up and down the loop of Henlé at the same time.
This part of the nephron allows mammals to produce hypertonic urine and plays an important role in water reabsorption from the collecting ducts.
• The isotonic renal fluid leaves the PCT and enters the descending limb, the hairpin bend and the ascending limb that runs in the opposite directing of the descending limb;
• The purpose of the Loop of Henlé is to create a gradient of hypertonicity (increasing solute potential) within the interstitial tissue of the medulla that becomes increasingly concentrated on moving from the cortex end towards the pelvis
• This creates a more negative solute potential in the medulla, which allows water to be drawn continuously out of the collecting ducts by osmosis
As the filtrate is flowing in opposite directions up and down down Loop of Henlé it creates an increasingly salty/ high solute potential in the medulla.
Note:
-The longer the Loop of Henlé the more water can be reabsorbed into the blood, which is far better for species that need to reserve their water. This is seen in desert animals etc.
-Cuboidal epithelial cells in the ascending limb have high concentrations of mitochondria to meet the energy needs required for the active transport of salt (NaCl)
The Loop of Henle
Distal Convoluted Tubule and Collecting Duct
This is the site of fine control of salt, water and pH balance of the blood. It is at this point that toxic substances, such as creatinine, are secreted into the filtrate. The hormonal control of the permeability of the DCT is linked to the collecting duct.
Formation of Concentrated Urine - through Osmoregulation
The concentration and volume of the urine, excreted by the mammalian kidneys is determined by the amount of water reabsorbed from the distal convoluted tubules and collecting ducts.
• These are variably permeable to water depending upon the presence or absence of the hormone ADH (anti-diuretic hormone)
•The volume and concentration of the urine that we extreme is dependent upon the water concentration (osmotic potential) of the blood which, in turn, determines the presence or absence of the hormone ADH and the subsequent permeability of the collecting duct walls.
ADH Present vs. Absent
How does the body know to produce ADH?
Key:
- Hypothalamus
- Osmoreceptor Cells
- In the hypothalamus of the brain osmoreceptors can detect the solute potential of blood
- When the water content of the blood is low, water moves out of the osmoreceptor cells by osmosis which reduces their volume- this triggers nerve cells in the hypothalamus to produce ADH in the pituitary gland
- ADH is then carried in the blood to the kidney where it causes the tubule walls to become more permeable and so more water is reabsorbed into the blood
- When the water level in the blood rises, the osmoreceptors are no longer stimulated and so they stop stimulating nerve cells and ADH is no longer released
- The collecting duct now becomes less permeable to water.
This control is an example of NEGATIVE FEEDBACK
Osmoregulation Summary Diagram
