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Homeostasis in Mammals

• Complex, multicellular organisms evolved specialized cells for specific functions, leading to interdependence.
• This division of labor requires coordination between tissues, organs, and organ systems for efficient performance.
• Coordination is achieved through the nervous system (rapid, specific) and the endocrine system (slower, less specific); both systems work together.
• Multicellularity led to developing an internal environment of extracellular fluids that bathe cells, providing nutrients and removing wastes.
• Maintaining this internal environment protects cells from external changes and gives the organism a degree of independence.

What is Homeostasis?

• Homeostasis maintains a constant internal state, specifically maintaining the chemical make-up, volume, and features of blood and tissue fluid within narrow limits (normal ranges).
• Ensures cells function normally despite external changes.
• Fluctuations occur around a set point, and homeostasis is the ability to return to that set point and maintain equilibrium.

The Importance of Homeostasis

• Enzymes and membrane channel proteins are sensitive to changes in pH and temperature.
• Changes reduce enzyme efficiency and may denature them, while changes to membrane proteins affect substance transport.
• Changes to blood and tissue fluids' water potential cause cells to shrink/expand by osmosis, impairing their function.

Components of a Typical Control System

• Set Point: The desired operating level of a system.

• Receptor: Detects internal and external stimuli, indicating deviations from the set point.

• Central Control: Coordinates information and sends instructions to effectors.

• Effector: (muscle or gland) Brings about changes to return the system to the set point.

• Feedback Loop: Informs the receptor of changes due to the effector's actions.

Control Mechanisms and Feedback

• Biochemical reactions are in dynamic equilibrium, and changes can disrupt this equilibrium to the organism's harm.

• Organisms with constant internal environments are more independent, have a wider geographical range, and increased chances of finding food and shelter.

• Mammals maintain a constant temperature, found in various habitats.

• Maintaining constant blood glucose allows constant energy release.

Negative Feedback

• Most systems, including biological ones, use negative feedback: turns the system off.
• Positive feedback causes deviation from set point that results in an even greater deviation.

Positive Feedback Examples

• A stimulus in neurons causes a small influx of sodium ions, increasing permeability and leading to a rapid, large response.
• Oxytocin causes uterine contractions, which stimulate more oxytocin release, leading to more contractions until birth.

Coordination of Control Mechanisms

• Systems typically have many receptors and effectors.
• Central control analyzes receptor information before action.
• Integrating multiple sources of information allows better control.
• Central control coordinates effectors to operate together effectively.

Excretion and Kidney Structure

• Excretion is the removal of metabolic waste products, distinct from elimination (egestion).
• Humans produce ~500 dm³ of carbon dioxide and 400 cm³ of water daily through respiration.
• Other excretory products include bile pigments, mineral salts, and urea (nitrogenous waste).

Urea Formation

• Urea is produced in the liver from excess amino acids in three stages:
  • Deamination: Amino groups (NH2) are removed from amino acids to form ammonia.
  • Remainder of amino acid can be respired to provide ATP.
  • Ammonia converted to urea (CO(NH2)2) via the ornithine cycle, which requires ATP and adds carbon dioxide.

Other Excretory Substances

- Ammonia (NH3) is easily formed from amino groups (NH2).
    - Requires no ATP, highly soluble, easily dissolved, and washed out of the body.
   -  Extremely poisonous.
  -  Only freshwater fish use ammonia.
- Uric acid is almost insoluble in water and non-toxic.
   -  Requires seven ATP molecules to produce.
  -  Used by organisms in dry conditions and flying organisms.
  -  Used to remove nitrogenous waste in eggs.

Structure of the Mammalian Kidney

• Mammals have two kidneys in the abdominal cavity, protected by fat.
• Weighing ~150 g each, they filter blood plasma every 22 minutes.

Kidney Structure

• Fibrous Capsule: An outer membrane that protects the kidney.
• Cortex: A lighter colored outer region; consisting of renal (Bowman's) capsules, convoluted tubules, and blood vessels.
• Medulla: a Darker colored inner region; consists of loops of Henlé, collecting ducts, and blood vessels.
• Renal Pelvis: a Funnel-shaped cavity collects urine into the ureter.
• Ureter: a Tube that carries urine to the bladder.
• Renal Artery: Supplies blood to the kidney from the heart via the aorta.
• Renal Vein: Returns blood to the heart via the vena cava.
• Microscopically, the cortex and medulla reveal one million tiny tubular structures (nephrons).

The Structure of the Nephron

• The nephron is the functional unit of the kidney: a narrow tube, closed at one end, with two twisted regions separated by a long hairpin loop.
  • The nephron consists of:
    • Renal (Bowman's) capsule: a Cup-shaped, closed end that contains the glomerulus (mass of blood capillaries).
      • Inner layer made of podocytes.
    • Proximal convoluted tubule: a Series of loops surrounded by blood capillaries; walls are cuboidal epithelial cells with microvilli.
    • Loop of Henlé Long, hairpin loop extending from cortex into the medulla; surrounded by blood capillaries.
    • Distal convoluted tubule: a Series of loops surrounded by blood capillaries; walls are cuboidal epithelial cells with fewer capillaries than the proximal tubule.
    • Collecting duct: a Tube collecting distal convoluted tubules; lined by cuboidal epithelial cells; widens to the kidney's pelvis.

Blood Vessels Associated with Nephrons

 - include:
    - Afferent arteriole: Supplies the nephron with blood.
    - Glomerulus: a Many-branched knot of capillaries where fluid is forced out.
    - Efferent arteriole: Vessel leaving the renal capsule, smaller in diameter, raises the blood pressure.

More Blood Vessels

  - Peritubular capillaries: Network surrounding tubules, reabsorbing mineral salts, glucose, water merge into venules to renal vein.
  - Unique structure: the afferent arteriole supplies blood, and an efferent arteriole drains the blood.

Kidney Function – Ultrafiltration and Selective Reabsorption

• The kidney regulates blood composition through ultrafiltration, selective reabsorption, and reabsorption of water/minerals.

Table 1 The Functions of the Kidneys

• Regulating blood composition and water potential, maintaining water volume, removing wastes, maintaining ion concentration.
• Regulating blood pressure and maintaining calcium levels.
• Stimulating red blood cell production.

Ultrafiltration Process

• Blood enters the kidneys through the renal artery.
• Branches to enter the nephron's renal (Bowman's) capsule.
• Smaller diameter efferent arteriole creates high hydrostatic pressure in the glomerulus.
• Forces the smallest particles out of the capillary to form filtrate.
• Water, glucose, mineral ions and other ≤68000 relative molecular mass substances are squeezed out of the capillary to form the glomerular filtrate.
• Movement of filtrate is resisted by the capillary endothelium, membrane of the renal capsule, intracapsular pressure, and low blood's water potential.

Glomerular Adaptations

• Bowman's capsule's inner layer consists of Podocytes
• Renal walls have spaces of up to 100 nm to allow fluids to flow in between the cells rather than through.

Selective Reabsorption

• In the Convoluted tubule returns about 85% of the filtrate back into the bloodstream.
• Ultrafiltration is based on molecule size.
• Filtrate similar to blood plasma (without plasma proteins).

Selective Reabsorption Breakdown

• 180 dm³ of water enters the kidney nephrons each day; 1dm³ is released as urine.
• Aquaporins exist which Reabsorb 85% of water in the Proximal Convoluted Tubule.

Podocyte and Ultrafiltration

• Cells of proximal convoluted tubules are adapted to reabsorb substances into the blood.
• Microvilli increase surface area, while many mitochondria supply ATP for active transport of sodium ions.

Reabsorption Process:

• Sodium ions (Na+) are actively transported out of tubule cells, decreasing the Na+ concentration and creating a concentration gradient.
• Na+ diffuses back into cells via special carrier proteins, co-transporting another molecule such as glucose, amino acids, or chloride ions.

After Co-Transport Occurs

 - Water follows osmotically down the water potential gradient created by the increase in Na+, Cl-, etc. concentration.
 -  As a result, all the glucose, amino acids, chloride ions plus water, and other valuable molecules reabsorbed.

Kidney Function – Loop of Henlé and Reabsorption of Water

• The Loop of Henlé is important for the reabsorption of water and creates conditions that lead to it being more concentrated.
• Has a hairpin shaped tubule to create interstitial fluid that will lead to distal convoluted tubule and the collecting duct.
• The concentration is related to Loop of Henlé length (short vs. long).

The Loop of Henlé Has two regions:

• Descending limb narrow and water permeable.
• Ascending limb Wider with impermeable to water.
• Acts as a counter current multiplier.

Counter Current Multiplier

• Sodium chloride actively pumped out of ascending limb using ATP (produced by lots of mitochondria).
  • Creates a low potential in the interstitial region.
• Walls impermeable to water
• Water passively, water out, blood capillaries.
• It Continually loses water, reaching lowest concentration at the tip.

The Loop of Henlé Continued

• Na and Cl ions diffuse, and is actively pumped, out as the filtrate moves up, reaching progressively higher concentration.
• Creating a pressure difference gradient from the descending limb, creating a flow of H2O out of the tubule to maintain levels.
• Also uses the ADH Hormone to help retain H2O Levels and electrolytes.