Homeostasis

Homeostasis is the maintenance of constant or steady conditions within the body.

Homeostasis’ Is Important

• Osmotic Processes - Require constant temp, pH, oxygen levels, water and ion content.
• Enzymes Reactions - Require constant temp and pH

A Homeostasis’ Control Uses

• Receptors - Which monitor the factor being controlled.
  
• Corrective Mechanism - Causes a corrective change to occur.
• Negative Feedback - stops corrective action to prevent over correction.

Can be nervous system controlled or hormonal and animals with simpler systems live in relatively constant conditions 

The Kidney

Functions

• Osmoregulation -  Controls the water potential of body fluids by controlling urine produced.
  
• Excretion - Removes toxic waste like urea.

Structure 

The kidney has a few main sections

• The Cortex - The outer dark region 
• The Medulla - The inner lighter region. 
• The Nephrons - Functional unit of the kidney. 
  

The medulla is subdivided into a number of pyramids that extend down into a large central cavity called the pelvis

The Urinary System

Structure

The urinary system has five main structures

• Kidneys.
• Ureter.
• Bladder.
• Sphincter Muscle
• Urethra.

Process

• Kidneys have blood entering from the renal artery at high pressure for filtration.
• The filtered blood leaves the kidney via the renal vein. 
• The excretory products and excess water pass into the ureter as urine.  
• Urine is then stored in the bladder.
• The sphincter muscle then controls its release through the urethra.

Nephron Structure

The nephron consists of 5 areas that are adapted either for ultrafiltration or reabsorption of important molecules. 

• Bowman’s Capsule - Ultrafiltration. 

• Proximal Convoluted Tubule - Most Reasborption. 

• The Loop of Henlé - Reasborption. 

• Distal Convoluted Tubule - Reasborption. 

• Collecting Duct - Reasborption.

The nephron originates and ends in the cortex, with the central region called the loop of Henlé extending down into the medulla. Many nephrons join with a collecting duct, which also extends down the medulla.

Bowman's Capsule

Bowman's capsule is the first part of the nephron it contains the glomerulus and its function is ultrafiltration. 

Ultrafiltration the filtration of plasma and substance below a certain size into the Bowman’s capsule.

As the filtration process is only looking to remove urea and other toxic elements the BC has three layers to further filter the glomerular filtrate.

• The capillary wall which doesn't filter much 

• The basement membrane is the effective filter and it prevents the blood cells and plasma proteins from leaving the blood

• Podocytes are cells that have extensions that allow the filtered material to pass through easily. They also have spacious gaps called filtration silts making them an ineffective filter.

Ultrafiltration

Due to filtration forces of a more negative solute potential in the blood, a high hydrostatic pressure is required to push the plasma out of the capillaries. The pressure comes from:

• The short distance from the heart down into the renal artery.
• The pressure difference from the wide afferent arteriole and the thin efferent arteriole.
• The restricted blood flow in the glomerulus due to it being tightly coiled.

The high pressure forces the plasma out of the capillaries the single layer of flattened squamous endothelial cells of the capillary walls let out useful substances through the pores.

This then passes through the basement membrane were effective filtration takes place and the glomerular filtrate which passes through the podocytes and continues through the PCT

Glomerular Filtrate is similar to blood but does not contain large plasma proteins and blood cells that are too large to penetrate the membrane.

Reabsorption

Reabsorption occurs in the proximal convoluted tubule made from cuboidal epithelium cells. 

This is where most selective reabsorption through active transport and facilitated diffusion of glucose, amino acids and some salts and pinocytosis also occurs as small plasma proteins are reabsorbed.

Adaptions

• Microvilli increase surface area and protein carriers needed for AT and FD.
• Infolding of membrane further increases surface area 
• Many mitochondria supply extra ATP needed for AT.
• Close to capillaries with thin squamous walls for short diffusing distance.
• The rate of blood flood maintains concertation gradient.

Loop of Henlé

The loop of Henlé is the part of the nephron that enables mammals to have hypertonic urine. The walls of the descending limb are thin and permeable to water. The walls of the acceding limb are thicker and impermeable to water.

The Ascending Limb

The ascending limb secretes Na⁺ ions and Cl^- ions into the medulla by active transport. This builds up a salt gradient in the interstitial fluid in the medulla, as a result of the ions leaving the ascending limb the filtrate in the limb becomes progressively more dilute and is hypotonic by the time it reaches the top of the ascending limb.

The Descending Limb

The descending limb has water removed by osmosis due to the very negative water potential in the medulla and the permeability of the walls. Therefore, the filtrate becomes progressively more concentrated with distance down the limb.

Osmoregulation

Osmoregulation is a homeostatic process that controls water balance in the body. This is done in the collecting duct is where water regulation takes place although most water is reabsorbed in the proximal convoluted tubule this is a passive process and cannot be controlled. 

Whereas in the collecting duct water absorption can be controlled in the collecting duct by changing the permeability of the collecting duct walls this is caused by the antidiuretic hormone.

• The solute potential of the blood becomes more negative.

• This detected by osmoreceptors in the hypothalamus.

• The posterior lobe of the pituitary body releases more ADH into the blood.

• Therefore, the walls of the collecting duct are more permeable

• More water is reabsorbed from the collecting duct back into the blood.