CH 14 Homeostasis

• Homeostasis is essential for the proper functioning of organisms.
• Enzymes and membrane proteins are sensitive to changes in pH and temperature, which can reduce their efficiency or denature them.
• Changes in water potential can cause cells to shrink or burst due to osmosis, disrupting their normal function.

Coordination

• Multicellular organisms coordinate functional systems for efficient performance.
• Nervous system enables rapid communication.
• Endocrine system enables slower, less specific communication.
• Both systems work together.

Internal Environment

• Multicellular organisms develop an internal environment comprised of extracellular fluids that bathe cells, providing nutrients and removing wastes.
• Cells are protected from external changes, giving organisms independence.

Definition

• Homeostasis is the maintenance of a constant internal state within narrow limits to ensure normal cell function.
• Continuous fluctuations occur around a set point, and homeostasis is the ability to return to that set point.

Control System

• The following stages are involved in any self regulating system:
• Set point is the desired level at which the system operates.
• Receptor detects internal and external stimuli, indicating deviation from set point.
• Central control coordinates information and sends instructions to effectors.
• Effector (muscle or gland) brings about necessary change to return the system to set point.
• Feedback loop informs the receptor about changes brought about by the effector.

Feedback

• Negative feedback turns the system off.
• Positive feedback causes changes that result in even greater deviation from the norm.
• Coordination of control mechanisms involves analyzing information from many receptors before action is taken.

Topics Related to Homeostasis

• Regulation of blood glucose
• Role of cyclic AMP as a second messenger
• Role of hypothalamus in osmoregulation
• Role of posterior pituitary in osmoregulation
• Role of ADH in osmoregulation

Excretion

• Excretion removes the waste products of metabolism from the body.
• Elimination (egestion) removes substances that have never been involved in metabolic activity.

Excretory Substances

• Adult humans produce carbon dioxide and water as a result of respiration.
• Other excretory products include bile pigments, mineral salts, and urea.

Urea Production

• Urea is the nitrogenous excretory product of organisms that have some access to water.
• It is produced in the liver from excess amino acids in three stages: deamination, respiration, and the ornithine cycle.
• Amino groups are removed from amino acids to form ammonia during deamination.
• The remainder of the amino acid is respired to give ATP.
• Ammonia is converted to urea by adding carbon dioxide through the ornithine cycle, which requires ATP.

Other Substances

• Ammonia is the easiest product to form and requires no ATP.
• It is very soluble in water; only organisms with access to large volumes of water can use ammonia as their nitrogenous excretory product.
• Uric acid is almost insoluble and requires seven ATP molecules to produce.
• It is used by organisms living in dry conditions and flying organisms as its low mass when stored.

Mammalian Kidney

• Mammals have two kidneys located at the back of the abdominal cavity.
• Each kidney is surrounded by fat and weighs only 150g.
• Blood plasma is filtered every 22 minutes.

Kidney Structure

• Different sections of the kidney are:
• Fibrous capsule- the outer membrane that protects the kidney.
• Cortex- a lighter colored outer region made up of renal (Bowman's) capsules, convoluted tubules, and blood vessels.
• Medulla- a darker colored inner region made up of loops of Henlé, collecting ducts, and blood vessels.
• Renal pelvis- a funnel-shaped cavity that collects urine into the ureter.
• Ureter- a tube that carries urine to the bladder.
• Renal artery- supplies the kidney with blood from the heart via the aorta.
• Renal vein- returns blood to the heart via the vena cava.

Nephrons

• The microscopic examination of the cortex and medulla reveals millions of tubular structures in each kidney.
• These are the functional units of the kidney.

Components of a Nephron

• The nephron is the functional unit of the kidney, consisting of a narrow tube closed at one end with twisted regions separated by a hairpin loop.
• Renal (Bowman's) capsule which is the closed end, cup-shaped, contains the glomerulus, and has inner cells called podocytes.
• Proximal convoluted tubule: series of loops with cuboidal epithelial cells and microvilli, surrounded by blood capillaries.
• Loop of Henlé: hairpin loop extending into the medulla, surrounded by blood capillaries.
• Distal convoluted tubule: series of loops with cuboidal epithelial cells, surrounded by fewer capillaries than the proximal tubule.
• Collecting duct: tube into which distal tubules empty, lined with cuboidal epithelium, becomes wider towards the kidney's pelvis.

Associated Blood Vessels

• Afferent arteriole: branch of the renal artery, supplies blood to the nephron.
• Glomerulus: knot of capillaries where fluid is forced out of the blood.
• Efferent arteriole: vessel leaving the renal capsule, smaller diameter than afferent, increases pressure within the glomerulus.
• Peritubular capillaries: network surrounding the proximal tubule, loop of Henlé, and distal tubule, reabsorb mineral salts, glucose, and water.

Kidney Functions

• Nephrons in the kidneys regulate the composition of blood through a series of stages.
• The series of stages: ultrafiltration, selective reabsorption, and the reabsorption of water and minerals.

Ultrafiltration

• The incoming renal artery branches to give rise to the afferent arteriole, which enters a renal Bowman's capsule of a nephron
• The arteriole then subdivides into the glomerulus containing a complex of capillaries
• Blood leaves through the efferent arterioles which in turn, subdivides into capillaries (peritubular capillaries) winded around the tubules of the nephron.
• Due to the larger diameter of incoming afferent arteriole relative to the efferent arteriole exiting, leads to a hydrostatic pressure build up in the glomerlus.
• This causes water, glucose, mineral ions and certain substances to be squeezed out of the capillary.
• The movement is resisted by factors like the capillary endothelium, epithelial and capsule layers, intracapsular pressure and low water potential of blood.

Resistance Reduction

• Resistance can be lowered by specific modifications of the structure.
• Podocytes are specialized cells lifted off the surface membrane allowing filtrate to pass beneath and through its branches.
• Glomerular capillary endothelium has spaces up to 100nm wide.
• Thus, the overall pressure is enough to counter resistance and the process occurs, a known as ultrafiltration.

Selective Reabsorption

• Nearly 85% of the filtrate is reabsorbed back into the blood.
• The goal of ultrafiltration is to remove any molecule less than 68000 Da where some are wastes but most are useful.
• Most water (85%) reabsorption occurs in the proximal convoluted tubule.

Podocytes and Ultrafiltration

• Cells of proximal convoluted tubules are adapted to reabsorb substances with microvilli to increase surface area and mitochondria for active transport.
• Co-transport is also important where substances are transported together e.g. with sodium ions.

Loop of Henlé

• The Loop of Henlé maintains conditions in surrounding interstitial fluid to draw water out.

Regions of the Loop of Henle

• The Loop consists of the descending (thin walls, water permeable) and ascending limb ( after short distances, thicker with water impermeable)
• Hence the Loop of Helé acts as counter-current multiplier.

Stomata

• Plants regulate carbon dioxide via stomata.
• Stomata opening allows carbon dioxide influx but also causes water loss from transpirtation.
• Plants close stomata at night and in dry conditions.

Guard Cells

• Each stoma is flanked and surrounded by guard cells with a minute pore (opening).
• Unlike the epidermal cells, guard cells are cytoplasmic and dense, and possess chloroplasts.
• Uneven thickness of cell walls, means guard cells can control the rate of gaseous exchange

Stomatal Opening

• One proposed mechanism for stomatal openings are:
• Illumination leads to activation of ATP synthase and then ATP production.
• The increased ATP availability reduces levels of intracellular H+ as they exit guard cells.
• High negativity intracellularly causes potassium channels to open bringing them inside the cell, thereby reducing water potential and also bringing chloride ions in.
• Osmosis leads to water influx, extra turgor, bending of the outer wall leading to the overall pore to open.

Abscisic Acid

• Increase in Abscisic acid results in closure of stomata via various cell surface mechanisms.
• Increased Calcium concentration affects the ability of the guard cell, causing osmosis leading to cells decreasing in size and pore closing.

Detection of Blood and Urine

• People with various diseases can frequently use simple dipsticks or complex biosensors to measure concentration of glucose in the blood or urine.
• Health care professionals routinely use dipsticks as a quick and reliable method to detect the abnormal presence of glucose in urine.
• To determine the presence of glucose, immobilised enzymes such as use glucose oxidase catalyses the conversion of glucose to gluconic acid and hydrogen peroxide.
• Peroxidase catalyses the breakdown of hydrogen peroxide to water and oxygen
• Glucose dipsticks are a convenient way to detect glucose in a sample.

Blood Analysis

The followiing can be identified:

• Proteins. These molecules do not usually ultrafilter but their presence can indicate glomerulus damage.
• Glucose presence (glucosuria) can indicate diabetes.
• Ketone body presence can be indicitive of use of fat or uncontrolled diabetes.