Homeostasis and Excretion Notes

Homeostasis

  • Organisms have control systems to maintain constant internal conditions.
  • Homeostasis: Maintaining constant internal body conditions.
  • Importance: Ensures optimal conditions for enzyme action and cell function.
  • Sensory cells detect internal and external conditions.
  • Physiological factors controlled by homeostasis in mammals:
    • Core body temperature
    • Metabolic waste (carbon dioxide and urea)
    • Blood pH
    • Blood glucose concentration
    • Blood water potential
    • Concentration of respiratory gases (carbon dioxide and oxygen)

Key Definition

  • Homeostasis: Regulation of internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.

Principles of Homeostasis

  • Homeostatic control mechanisms use negative feedback.
  • Negative feedback control loops involve:
    • Receptor (or sensor): Detects a stimulus.
    • Coordination system (nervous and endocrine systems): Transfers information.
    • Effector (muscles and glands): Carries out a response.
  • Outcome of negative feedback loop:
    • Factor/stimulus is continuously monitored.
    • Increase in factor: Body responds to decrease it.
    • Decrease in factor: Body responds to increase it.
  • Negative feedback diagram (flow diagram of a negative feedback control loop).

Coordination Systems in Mammals

  • Nervous system: Information transmitted as electrical impulses along neurones.
  • Endocrine system: Information transmitted as hormones in the blood.

Production of Urea

  • Metabolic reactions produce waste products.
  • Excretion: Removal of waste products.
  • Excretory products: Carbon dioxide and urea (in large quantities).

Urea

  • Produced in the liver from excess amino acids.
  • Excess protein cannot be stored.
  • Amino acids from protein can provide energy.

Deamination

  • Amino group removed from amino acid.
  • Process: Deamination.
    • The amino group (NH_2) of an amino acid is removed, together with an extra hydrogen atom.
    • These combine to form ammonia (NH_3).
    • The remaining keto acid may enter the Krebs cycle to be respired, be converted to glucose, or converted to glycogen / fat for storage
  • Deamination of an amino acid diagram with associated chemical formulae.

Ammonia

  • Very soluble and highly toxic compound produced during deamination.
  • Damaging if allowed to build up in the blood.
    • Dissolves in blood to form alkaline ammonium hydroxide, disrupting blood pH.
    • Impacts reactions of cell metabolism such as respiration.
    • Interferes with cell signalling processes.
  • Avoided by converting ammonia to urea.
  • Urea is less soluble and less toxic than ammonia.
  • Ammonia is combined with carbon dioxide to form urea.
  • Formation of urea equation with chemical formulae.

Structure of the Human Kidney

  • Humans have two kidneys.
  • Functions:
    • Osmoregulatory organ: Regulates water content of the blood (vital for maintaining blood pressure).
    • Excretory organ: Excretes toxic waste products (urea) and excess substances (salts).
  • Excretory system diagram showing the position of the kidneys and their associated structures.

Kidney Structure and Function

  • Renal artery: Carries oxygenated blood (containing urea and salts) to the kidneys.
  • Renal vein: Carries deoxygenated blood (that has had urea and excess salts removed) away from the kidney.
  • Kidney: Regulates water content of blood and filters blood.
  • Ureter: Carries urine from the kidneys to the bladder.
  • Bladder: Stores urine (temporarily).
  • Urethra: Releases urine outside of the body.
  • Kidney surrounded by fibrous capsule.
  • Three main areas:
    • Cortex: Contains glomerulus, Bowman’s capsule, proximal convoluted tubule, and distal convoluted tubule of nephrons.
    • Medulla: Contains the loop of Henle and collecting duct of the nephrons.
    • Renal pelvis: Where the ureter joins the kidney.
  • Kidney diagram showing a cross-section of a kidney.

Nephron Structure

  • Each kidney contains thousands of tiny tubes, known as nephrons.
  • Nephron: Functional unit of the kidney – responsible for the formation of urine.
  • The structure of a nephron diagram.

Nephron Location and Blood Vessels

  • Location and structure of a nephron within the kidney across the cortex and the medulla.
  • Network of blood vessels associated with each nephron:
    • Glomerulus: Within the Bowman’s capsule.
    • Afferent arteriole: Supplies blood to the glomerulus from the renal artery.
    • Efferent arteriole: Blood flows from the glomerulus into this.
    • Capillaries: Network running alongside the nephron.
    • Renal vein: Blood from these capillaries eventually flows into this.
  • The blood vessels of the nephron diagram.
  • The blood supply associated with a nephron diagram.

Formation of Urine in the Nephron

  • Nephron: Functional unit of the kidney – responsible for the formation of urine.
  • The process of urine formation in the kidneys occurs in two stages:
    • Ultrafiltration.
    • Selective reabsorption.
  • The Two Stages of Urine Production in the Kidneys Table:
    • Stage 1: Ultrafiltration, occurs in the Bowman's capsule. Small molecules (including amino acids, water, glucose, urea and i ions) are filtered out of the blood capillaries of the glomerulus and i Bowman's capsule to form filtrate known as glomerular filtra
    • Stage 2: Selective reabsorption, occurs in the Proximal convoluted tubule. Useful molecules are taken back (reabsorbed) from the filtrate and to the blood as the filtrate flows along the nephron.

Ultrafiltration overview

  • Ultrafiltration overview from the glomerulus to the Bowman's capsule.

Selective reabsorption overview

  • Selective reabsorption overview
  • After the necessary reabsorption of amino acids, water, glucose and inorganic ions is complete (even some urea is reabsorbed), the filtrate eventually leaves the nephron and is now referred to as urine.
  • This urine then flows out of the kidneys, along the ureters and into the bladder, where it is temporarily stored

Ultrafiltration Process

  • Arterioles branch off the renal artery, leading to the glomerulus in Bowman’s capsule.
  • Capillaries narrow, increasing blood pressure.
  • Smaller molecules forced out of capillaries into Bowman’s capsule, forming filtrate.
  • Blood in glomerular capillaries separated from Bowman’s capsule lumen by:
    • Endothelium of the capillary: Perforated by tiny membrane-lined holes.
    • Basement membrane: Network of collagen and glycoproteins.
    • Epithelium of Bowman’s capsule: Podocytes with finger-like projections and gaps.
  • Substances dissolved in blood plasma pass into Bowman’s capsule through holes and gaps.
  • Fluid that filters through is the glomerular filtrate.
  • Main substances in glomerular filtrate: amino acids, water, glucose, urea, inorganic ions (Na+, K+, Cl-).
  • Red and white blood cells and platelets remain in the blood.
  • Basement membrane acts as a filter, stopping large protein molecules.
  • Ultrafiltration diagram.

Ultrafiltration Defined

  • Ultrafiltration occurs when small molecules (such as amino acids, water, glucose, urea and inorganic ions) filter out of the blood and into the Bowman’s capsule to form glomerular filtrate. These molecules must pass through three layers during this process: the capillary endothelium, the basement membrane and the Bowman’s capsule epithelium.

How Ultrafiltration Occurs

  • Ultrafiltration occurs due to differences in water potential between plasma in glomerular capillaries and filtrate in Bowman’s capsule.
  • Water moves down a water potential gradient (from high to low).
  • Water potential is increased by high pressure and decreased by solutes.
    Factor Affecting Water Potential Table:
    Factor affecting water potential How factor affects water potential in the glomerulus and Bowman’s capsule Resulting movement of:
    Blood pressure is relatively high in the glomerular capillaries due to the fact that the afferent arteriole is wider than the efferent arteriole. This raises the water potential of the blood plasma in the glomerular capillaries above the water potential of the filtrate in the Bowman’s capsule. Water moves down the wat potential gradient, from the plasma in the glomerular c into the Bowman’s capsule
    Solute Concentration: Plasma protein molecules are too big to get through basement membrane. As a result, the solute concentration in the blood plasma in the glomerular capillaries is higher than that in the filtrate in the Bowman’s capsule. This makes the water potential of the blood plasma lower than that of the filtrate in the Bowman’s capsule. Water moves down the wat potential gradient from the Bowman’s capsule into the plasma in the glomerular c
    Overall, the effect of the pressure gradient outweighs the effect of solute gradient. Therefore, the water potential of the blood plasma in the glomerulus is higher than the water potential of the filtrate in the Bowman’s capsule. This means that as blood flows through the glomerulus, there is an overall movement of water down the water potential gradient from the blood into the Bowman’s capsule.
  • Water movement during ultrafiltration diagram.
  • As blood flows through the glomerulus, there is an overall movement of water down the water potential gradient from the blood plasma (region of higher water potential) into the Bowman’s capsule (region of lower water potential)

Selective Reabsorption

  • Many substances in glomerular filtrate need to be kept by the body.
  • These substances are reabsorbed into the blood as the filtrate passes along the nephron.
  • Selective reabsorption: only certain substances are reabsorbed.
  • Glucose reabsorption occurs in the proximal convoluted tubule.
  • Lining of proximal convoluted tubule: single layer of epithelial cells adapted for reabsorption:
    • Microvilli
    • Co-transporter proteins
    • High number of mitochondria
    • Tightly packed cells
  • Water and salts are reabsorbed via the Loop of Henle and collecting duct
    Adaptations for selective reabsorption table:
    Many microvilli present on the luminal membrane (the cell surface membrane that faces the lumen) This increases the surface area for reabsorption
    Many co-transporter proteins in the luminal membrane Each type of co-transporter protein transports a specifi (e.g. glucose or a particular amino acid) across the lu membrane.
    Many mitochondria These provide energy for sodium-potassium (Na+-K+) proteins in the basal membranes of the cells.
    Cells tightly packed together This means that no fluid can pass between the cells (all s reabsorbed must pass through the cells)

Solute Reabsorption

  • Blood capillaries located close to the outer surface of the proximal convoluted tubule.
  • Blood in capillaries has little plasma and has lost much of its water, inorganic ions and other small solutes.
  • Basal membranes (of proximal convoluted tubule epithelial cells) closest to blood capillaries.
  • Sodium-potassium pumps in basal membranes move sodium ions out of epithelial cells into the blood.
  • Lowers sodium ion concentration inside epithelial cells.
  • Sodium ions in filtrate diffuse into epithelial cells through luminal membranes via co-transporter proteins.
  • Each co-transporter protein transports a sodium ion and another solute from the filtrate (e.g. glucose or a particular amino acid).
  • Solutes diffuse down concentration gradients into the blood through transport proteins in the basal membranes of the epithelial cells.

Molecules Reabsorbed

  • All glucose in glomerular filtrate is reabsorbed into the blood.
  • No glucose should be present in the urine.
  • Amino acids, vitamins and inorganic ions are reabsorbed.
  • Movement of solutes increases the water potential of the filtrate and decreases the water potential of the blood in the capillaries.
  • Creates a steep water potential gradient.
  • Water moves into the blood by osmosis.
  • Significant amount of urea is reabsorbed too.
  • Urea concentration in the filtrate is higher than in the capillaries.
  • Urea diffuses from the filtrate back into the blood.
  • Selective reabsorption diagram.

Selective Reabsorption Illustration

  • Selective reabsorption in the proximal convoluted tubule

Water and Salts Reabsorption

  • As the filtrate drips through the Loop of Henle necessary salts are reabsorbed back into the blood by diffusion.
  • As salts are reabsorbed back into the blood, water follows by osmosis.
  • Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time