4.1 Homeostasis

Question 1

4.1 Homeostasis

Answer 1

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

Question 2

What is Homeostasis?

Answer 2

The maintenance of a constant internal environment

Question 3

The Importance of Homeostasis in Mammals

Answer 3

Homeostasis in mammals is very important to:

• provide optimum conditions for enzyme function
• avoid osmotic problems in cells and body fluids

Question 4

Principles of Homeostasis

Answer 4

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.

Question 5

The Mammalian Kidney

Answer 5

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

Question 6

The Kidney Nephron

Answer 6

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:

1. Ultrafiltration of blood
2. Selective reabsorption of filtered materials
3. Concentration of urine; the reabsorption of water back into the capillaries

Question 7

Diagram of The Nephron

Answer 7

Question 8

Function of The Nephron

Answer 8

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.

Question 9

1. Ultrafiltration of the Blood

Answer 9

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:

1. 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.
2. The afferent arteriole of each glomerulus is slightly wider than the efferent arteriole.
3. Coiling capillaries in the glomerulus further contracts the blood flow therefore increasing g the pressure.

Question 10

The Bowman’s Capsule and Glomerulus

Answer 10

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

Question 11

Diagram of the Bowman’s Capsule and Glomerulus

Answer 11

Question 12

Structure and Function

Answer 12

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

Question 13

Glomerulus Filtration diagram

Answer 13

Question 14

What Substances are Filtered

Answer 14

Question 15

Osmosis

Answer 15

*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

Question 16

Worked Example of Water Potential difference between Blood and Filtrate

Answer 16

Question 17

2. Selective Reabsorption

Answer 17

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.

Question 18

Adaptions of the Proximal Convoluted Tubule

Answer 18

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

Question 19

Reabsorption in the PCT

Answer 19

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.

Question 20

Substance and Method of Reabsorption

Answer 20

Question 21

Terms and their Meanings

Answer 21

Isotonic: Same/equal water potential

Hypertonic: More negative/ lower water potential
*(e for less)

Hypotonic: Less negative/ higher water potential
*(o for more)

Question 22

Loop of Henlé

Answer 22

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)

Question 23

The Loop of Henle

Answer 23

Question 24

Distal Convoluted Tubule and Collecting Duct

Answer 24

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.

Question 25

Formation of Concentrated Urine - through Osmoregulation

Answer 25

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.

Question 26

ADH Present vs. Absent

Answer 26

Question 27

How does the body know to produce ADH?

Answer 27

Key:
- Hypothalamus
- Osmoreceptor Cells

1. In the hypothalamus of the brain osmoreceptors can detect the solute potential of blood
2. 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
3. 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
4. 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
5. The collecting duct now becomes less permeable to water.

This control is an example of NEGATIVE FEEDBACK

Question 28

Osmoregulation Summary Diagram

Answer 28