11.3 The Kidney and Osmoregulation

Excretion

The removal from the body of the waste products of metabolic activity

Two key functions of excretory system

* remove nitrogenous waste
* remove excess water

Removing nitrogenous waste

Nitrogenous wastes are produced from the breakdown of nitrogen-containing compounds like amino acids and nucleotides

* Nitrogenous wastes are toxic to the organism and hence excess levels must be eliminated from the body

Nitrogenous waste in aquatic animals

NH3 - highly toxic but very water soluble and hence can be effectively flushed by animals in aquatic habitats

Nitrogenous waste in mammals

urea - less toxic than ammonia and can be stored at high concentrations

Nitrogenous waste in reptiles and birds

Uric acid - requires more energy to make then urea and ammonia but is relatively non-toxic and requires even less water flush

Osmoconformers vs osmoregulators

* osmoconformer: maintain internal conditions that are equal to osmolarity of environment
* By matching internal osmotic conditions to the environment, osmoconformers minimise water movement in and out of cells
* Osmoregulator: Keep body’s osmolarity constant
* osmoregulation is a more energy-intensive process, it ensures internal osmotic conditions are always tightly controlled

Excretory system in insects

Malpighian tubules

Malpighian tubules

* Malpighian tubules branch off from the intestinal tract and actively uptake nitrogenous wastes and water from the hemolymph


* The tubules pass these materials into the gut to combine with the digested food products
* Solutes, water and salts are reabsorbed into the hemolymph at the hindgut, whereas nitrogenous wastes (as uric acid) and undigested food materials are excreted via the anus

Names of arteries and veins from kidney

Renal artery and renal vein

Specialized structure kidney

Nephron - filter blood and eliminate waste

Tube kidney

Ureter, transports urine to bladder from kidney

Blood in renal vein (after kindey)

* lower urea (removed by nephron to form urine)
* less water and solutes/ions
* less glucose and oxygen
* more co2

Components of nephron and functions

* Bowman’s capsule – first part of the nephron where blood is initially filtered (to form filtrate)
* Proximal convoluted tubule – folded structure connected to the Bowman’s capsule where selective reabsorption occurs
* Loop of Henle – a selectively permeable loop that descends into the medulla and establishes a salt gradient
* Distal convoluted tubule – a folded structure connected to the loop of Henle where further selective reabsorption occurs

Glomerulus

Capillary tuft within Bowman’s capsule

What is each nephron connected to?

A collecting duct which will feed into renal pelvis

Nephron three stages of filtering and reabsorption

1. Ultrafiltration - blood is filtered out of the glomerulus at Bowman’s capsule to form filtrate
2. Selective reabsorption - usable materials are reabsorbed in convoluted tubules (both proximal and distal)
3. Osmoregulation - The loop of Henle establishes a salt gradient, which draws water out of the collecting duct.

Structure of the Bowman’s Capsule

* As the blood moves into the kidney via afferent arterioles it enters a knot-like capillary tuft called a glomerulus
* This glomerulus is encapsulated by the Bowman’s capsule, which is comprised of an inner surface of cells called podocytes
* Podocytes have cellular extensions called pedicels that wrap around the blood vessels of the glomerulus
* Between the podocytes and the glomerulus is a glycoprotein matrix called the basement membrane that filters the blood

Basement membrane

* where blood is filtered
* Lies between glomerulus and bowman’s capsule
* Glomerular blood vessels are fenestrated (have pores) which means blood can freely exit the glomerulus
* The podocytes of the Bowman’s capsule have gaps between their pedicels, allowing for fluid to move freely into the nephron
* Consequently, the basement membrane functions as the __sole__ filtration barrier within the nephron

Hydrostatic pressure

**Ultra**filtration involves blood being forced at *high pressure* against the basement membrane, optimising filtration

* This high hydrostatic pressure is created in the glomerulus by having a wide afferent arteriole and a narrow efferent arteriole
* This means it is easy for blood to enter the glomerulus, but difficult for it to exit – increasing pressure within the glomerulus
* Additionally, the glomerulus forms extensive narrow branches, which increases the surface area available for filtration
* The net pressure gradient within the glomerulus forces blood to move into the capsule space (forming filtrate)

Ultrafiltration

The first of three processes by which metabolic wastes are separated from the blood and urine is formed

Selective reabsorption

Second of the three processes by which blood is filtered and urine is formed

* It involves the reuptake of useful substances from the filtrate and occurs in the convoluted tubules (proximal and distal)
* The majority of selective reabsorption occurs in the *proximal* convoluted tubule, which extends from the Bowman’s capsule

Features of convoluted tubule

* microvilli - increase surface area for material absorption for the filtrate
* single cell thick & connected by tight junctions
* Create a thin tubular surface with no gaps
* lot of mitochondria as reabsorption involves active transport

Active transport in reabsorption

* Substances are actively transported across the apical membrane (membrane of tubule cells facing the tubular lumen)
* Substances then passively diffuse across the basolateral membrane (membrane of tubule cells facing the blood) 

What materials are reabsorbed?

* Mineral ions and vitamins are actively transported by protein pumps and carrier proteins respectively
* Glucose and amino acids are co-transported across the apical membrane with sodium (symport)
* Water follows the movement of the mineral ions passively via osmosis

Osmoregulation

The third of three processes by which blood is filtered and urine is formed

* Osmoregulation is the control of the water balance of the blood, tissue or cytoplasm of a living organism

Two key events in osmoregulation

1. The loop of Henle establishes a salt gradient (hypertonicity) in the medulla
2. Anti-diuretic hormone (ADH) regulates the level of water reabsorption in the collecting duct

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1. Establishing a salt gradient (osmoregulation)

* The function of the loop of Henle is to create a high solute (hypertonic) concentration in the tissue fluid of the medulla
* The *descending* limb of the loop of Henle is permeable to **water** but __not__ salts
* The *ascending* limb of the loop of Henle is permeable to **salts** but __not__ water
* This means that as the loop descends into the medulla, the interstitial fluid becomes more salty and hypertonic
* Additionally, the ^^vasa recta blood^^ network that surrounds the loop of Henle flows in the opposite direction (counter-current)
* This means that salts released from the ascending limb are drawn down into the medulla, further establishing a salt gradient

2. Water reabsorption (osmoregulation)

* As the collecting duct passes through the medulla, the hypertonic conditions of the medulla will draw water out by osmosis
* The amount of water released from the collecting ducts to be retained by the body is controlled by anti-diuretic hormone (ADH)
* ADH is released from the posterior pituitary in response to dehydration (detected by osmoreceptors in the hypothalamus)
* ADH increases the permeability of the collecting duct to water, by upregulating production of aquaporins (water channels)
* This means less water remains in the filtrate, urine becomes concentrated and the individual urinates less (i.e. anti-diuresis)
* When an individual is suitably hydrated, ADH levels decrease and less water is reabsorbed (resulting in more dilute urine)

Interstitial fluid

Fluid surrounding the nephron

Dehydration

* loss of water from body
* individuals will experience thirst and excrete small quantities of heavily concentrated urine
* blood pressure will drop
* The individual will become lethargic and experience an inability to lower body temperature (due to lack of sweat)
* Severe cases of dehydration may cause seizures, brain damage and eventual death

Overhydration

* Overhydration is a less common occurrence that results when an over-consumption of water makes body fluids hypotonic
* Individuals will produce excessive quantities of clear urine in an effort to remove water from the body
* The hypotonic body fluids will cause cells to swell (due to osmotic movement), which can lead to cell lysis and tissue damage
* Overhydration can lead to headaches and disrupted nerve functions in mild cases (due to swelling of cells)
* In severe cases, overhydration may lead to blurred vision, delirium, seizures, coma and eventual death

Relation between loop of Henle and water needs

Animals in desert environments = longer loop of Henle (juxtamedullary nephrons)

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Animals in moist environments = short loop of Henle (cortical nephrons)

How can you test for diseases using urine?

Testing for:

* glucose - indicator for diabetes (high glucose = incomplete reabsorption)
* proteins - high quantities of protein may indicated disease or hormonal condition
* blood cells - indicate a variety of diseases, including certain infections and cancer
* drugs/toxins - Many drugs pass through the body into urine and can be detected

Hemodialysis

Kidney dialysis involves the external filtering of blood in order to remove metabolic wastes in patients with kidney failure 

Blood is removed and pumped through a dialyzer, which has two key functions that are common to biological membranes:

* It contains a porous membrane that is *semi-permeable* (restricts passage of certain materials)
* It introduces fresh dialysis fluid and removes wastes to maintain an appropriate *concentration gradient*

Kidney transplant

Hemodialysis ensures continued blood filtering, but does not address the underlying issue affecting kidney function

The best long-term treatment for kidney failure is a kidney transplant:

* The transplanted kidney is grafted into the abdomen, with arteries, veins and ureter connected to the recipient’s vessels
* Donors must typically be a close genetic match in order to minimise the potential for graft rejection
* Donors can survive with one kidney and so may commonly donate the second to relative suffering kidney failure