D3.3: Homeostasis/Kidneys

Homeostasis

Maintenance of a constant internal environment between cells within an organism

*kept within a preset range*

Cell's Environment

Immediate area surrounding plasma membrane

Plants: cell wall and its fluid

Animals: Extracellular matrix (collagen, elastin, etc)

Digestion Processes Under Voluntary Control

Swallowing and Defecation

Swallowing

Voluntary action of tongue (striated muscle) which pushes food to back of mouth, touch receptors in pharynx allow swallowing

Defecation

Removal of feces from rectum via anus, sphincter (smooth muscle ring) relaxes allowing rectum wall to push

Enteric Nervous System

Controls digestion without input from CNS, under involuntary control, includes Peristalsis

Peristalsis

Rhythmic waves of vigorous contraction that move food in one direction

Moves food slowly in intestines for digestion and absorption, churns food to mix with pancreatic enzymes

Muscles in Digestive System

Outer layer consisting of longitudinal muscles

Inner layer consisting of circular muscles

*smooth muscle*

Longitudinal Muscles

Pushes food down along digestive track caudally (towards anus)

Circular Muscles

Mixes food with digestive enzymes, prevents food from moving backwards (stops negative pressure from pushing food up)

Feedback Regulation

Uses outcome of processes to control future of that process

Positive Feedback

Increases gap between original and new levels (more out of homeostasis), promotes change

ex. oxytocin

Negative Feedback

Decreases gap between original and new levels (towards homeostasis), promotes stability by returning to the set point

Disadvantage: uses lots of energy

Control of Blood Glucose

done by pancreas

Pancreas

both an endocrine and an exocrine gland

Exocrine: secretes enzymes into small intestines

Endocrine: secretes glucagon and insulin into bloodstream

Islets of Langerhans

Found in pancreas, consists of alpha and beta cells

Alpha Cells

produce glucagon, when blood glucose falls below set point, glucagon signals glycogen to be converted to glucose in liver (adipose) cells

increases concentration of glucose in blood

Beta Cells

Produces insulin, when blood glucose rises above set point, insulin signals glucose to be converted to glycogen in liver and skeletal muscle cells, glucose taken up to be used by almost all cells to be used in cellular respiration (except brain bc it always has access to glucose)

Diabetes Mellitus

Characterized by consistently elevated blood glucose levels during prolonged fasting

Symptoms: presence of glucose in urine and persistent thirst

Diabetes Mellitus impairs water reabsorption during urine production, leading to dehydration as when sugar is excreted, water follows due to osmosis

Type I Diabetes

Early onset, caused by inability to produce insulin when beta cells are destroyed by own immune system, appears during childhood

Type I Diabetes Treatment

Test blood glucose regularly for insulin injections (done before meals to prevent peaks)

Also implanted devices can release exogenous insulin

Type II Diabetes

Late onset, caused by inability to respond to insulin, either deficiency in insulin receptors or glucose transporters

Risk factors: sugary or fatty diets, prolonged obesity, genetic factors that affect metabolism

Type II Diabetes Treatment

Adjust diet to avoid blood glucose peaks and troughs

-small amounts of food eaten frequently

-avoid high sugar/starch foods

-strenuous exercise and weight loss

Hypothalamus

Function: integration of body systems

Space is filled with cerebrospinal fluid, which links nervous system to endocrine system via pituitary gland

Within the hypothalamus, there are specialized areas called nuclei, which control specific systems

-contains sensors for blood glucose [] and osmolarity

-processes signals from sense organs and other parts of brain

Pituitary Gland

located directly below hypothalamus, secretes hormones into blood capillaries

Made of posterior and anterior pituitary

Posterior Pituitary Gland

Osmoreceptors in hypothalamus monitor solute [] in blood (nuclei example), posterior pituitary produces antidiuretic hormone (ADH) for osmoregulation

*faces back*

Anterior Pituitary Gland

Produces Thyroid stimulating hormone (TSH) for thermoregulation

*faces front*

Circadian Rhythm

Internally generated 24-hour, day/night cycle

Regulated by superchiasmatic nuclei (SCN) in hypothalamus, controls the production of melatonin by pineal gland

Without light, circadian rhythm is slightly longer than 24 hours, therefore it is synchronized with diurnal (daily) cycle via light

-retina detects blue light, signals superchiasmatic nuclei to inhibit melatonin at dawn, release at dusk

Melatonin

hormone that controls sleep/wake cycle, levels increase at dusk and decrease at dawn

Promotes drowsiness and sleep, contributes to nighttime drop in core temperature and decrease in urine production

Pineal Gland

releases melatonin

Blue light

460-480 nm, most from dawn to midday, decreases at dusk

Thermoregulation

Control of core body temp to keep close to set point, regulated by negative feedback and monitored by thermoreceptors (sensory nerve endings, both cold and warm receptors)

has peripheral and central elements

Peripheral Thermoregulation

Thermoreceptors located in skin to detect external temperatures, anticipates heat loss

Central Thermoregulation

Thermoreceptors located in core of body like hypothalamus

Generation of Heat

Done by metabolism in cells

1. hypothalamus releases thyrotropin releasing hormone

2. Signals pituitary gland to release thyroid stimulating hormone

3. Signals thyroid gland to release thyroxin

4. Increases metabolic rate of cells

How does increase in metabolic rate generate heat?

All cells respond to thyroxin but main targets are liver, muscle, and brain

-muscle contractions generate heat

-subcutaneous adipose tissue acts as insulation

-brown adipose tissue produces heat at a rapid rate (lots of mitochondria, decouples ATP synthesis)

Responses to Cold

Vasoconstriction, Shivering, Uncoupled respiration of brown adipose tissue, hair erection

Vasoconstriction

Arteriole walls contract, lumen (area in arteriole) narrows, less blood flows to skin so less heat loss

Arteriole

Smaller than artery, larger than capillary

Shivering

Involuntary response that causes striated muscles to contract and relax at a rapid rate, solely for heat generation

Uncoupled respiration of brown adipose tissue

Brown color due to lots of mitochondria, produces heat during cellular respiration, not ATP

Hair Erection

erector muscles raise thick coats of hair to use the air in between as insulation

Humans have lost most of body hair during evolution, our ineffective response is goosebumps

Responses to Heat

Vasodilation and Sweating

Vasodilation

Arteriole walls relax, lumen widens, causing more blood to flow to skin, resulting in more heat loss

Sweating

Hypothalamus activates sweat glands (exocrine), H2O evaporates, leaving behind solutes like Na+

Sweating also activated by adrenaline, in anticipation of intense activity

Kidneys

major organ for osmoregulation and excretion

Outer region is cortex, inner is medulla

Nephron is basic functional unit (1-2 million per kidney)

Functions of Kidneys

Osmoregulation and Excretion

Osmoregulation

maintenance of osmotic concentration (water balance), units: osm/L

Total solute concentration that affects water movement via osmosis, osmoregulation regulates amount of H2O and saltes removed from urine

Excretion

Removal of toxic waste products of metabolism from digestion of nitrogen-containing foods like proteins and nucleic acids (main waste: urea).

Also removal of drugs and food pigments

*different from defecation, defecation focuses on removal of undigestables*

Ultrafiltration

non-specific filtration of blood using hydrostatic pressure

Location: glomerulus and Bowman's capsule

Function: Filters out most of blood plasma except cells (red/white blood cells) and proteins

Afferent vs. Efferent Arterioles

carry blood TOWARD glomerulus vs carrying blood AWAY from glomerulus

Afferent wider than efferent, creates pressure in glomerulus

Glomerulus

located in cortex, consists of ball-shaped network of capillaries

blood moves from renal artery-->afferent arterioles-->glomerulus (capillaries)-->efferent arterioles-->renal vein

Cause of Ultrafiltration

-Afferent arterioles diameter greater than efferent arterioles diameter (creates pressure)

-clumping, or twisted route blood must take in glomerulus increases pressure and increases SA

-capillaries are fenestrated

Fenestrated Capillaries

Capillaries are highly porous, allowing for solutes to flow through quite easily (except for proteins/blood cells)

Bowman's Capsule

located in cortex, have walls made of podocytes (cells) w/ pedicels (extensions that wrap around glomerulus to increase SA

Between glomerulus and podocytes is the basement membrane

**Filtration at this stage occurs solely based on size**

Podocytes

cells in the Bowman's capsule in the kidneys that wrap around capillaries of the glomerulus

Pedicels

finger-like projections of podocytes surrounding glomerular capillaries, increase SA. There are also gaps btwn pedicels

Basement Membrane

Made of negatively charged glycoproteins, prevents blood proteins from passing based on size and charge

Filtrant Travel Process in Ultrafiltration

Filtrants go through fenestrated capillaries, then through an interstitial fluid, through the basement membrane, through more interstitial fluid, then finally through the podocytes into the Bowman's Capsule.

What does ultrafiltration produce?

glomerular filtrate (basically blood) which contains everything except proteins and blood cells

Fluid forced out of glomerulus into nephron (bowman's capsule through pedocytes)

Only blood cells and proteins remain totally in blood, other things (urea, glucose, etc) have equal concentrations in blood and glomerular filtrate

Selective Reabsorption

Location: proximal convoluted tubules (fine adjustment made later in distal convoluted tubules)

Function: Reuptake of necessary materials from filtrate into blood

Proximal Convoluted Tubule

located in cortex

The tubule itself is made of epithelium with a lumen (inner space) inside.

Reabsorbed materials are transported from lumen of proximal convoluted tubule-->epithelial cell that makes up lining-->interstitial fluid-->peritubular capillaries surrounding nephron

Epithelium

The material of the proximal convoluted tubules (lining)

It is one cell thick, joined by tight junctions (no gaps), and lined with microvilli which increase SA for absorption

Also contains many mitochondria for active transport of solutes

Peritubular Capillaries

The network of tiny blood vessels that surrounds the proximal and distal tubules in the kidney

Reabsorbed Materials

Sodium ions, chloride ions, glucose/AA, H2O

All moved to high concentration in interstitial fluid, then passively diffuse into peritubular capillaries

Sodium Ions

Diffuses (passive transport) via cotransport (Na+/Glucose cotransporter) into the epithelium. Then actively transported into interstitial fluid via pump proteins embedded in outer membrane of epithelium.

Chloride Ions

Facilitated diffusion into interstitial fluid, follows charge gradient set by Na+

Glucose/AA

Cotransported against concentration gradient into epithelial cells as Na+ moves down its concentration gradient using Na+/Glucose cotransporter. Then glucose undergoes facilitated diffusion into the interstitial fluid.

H2O

Follows established concentration gradient into epithelial cells and interstitial fluid via osmosis

Osmoregulation Nephron Function

Location: Loop of Henle

Function: establishes and maintains the osmotic gradient in kidney (interstitial fluid)

Flow: loop of Henle (nephron) --> "kidney" (interstitial fluid) --> vasa recta (blood = body)

Vasa Recta

the capillary system in the kidney that serves the loop of Henle

Loop of Henle

Located in the medulla, contains both descending and ascending limbs

Maintains osmotic gradient in kidney

Descending Limb of the Loop of Henle

Mostly permeable to H2O, NaCl in very small amounts

-H2O moves out of kidney (interstitial fluid), returns to blood (via vasa recta), uses aquaporins

*increases filtrate concentration*

Ascending Limb of the Loop of Henle

Permeable to NaCl but not H2O

-Na+ is pumped out actively (Cl- follows)

-osmolarity (amt solute) of kidney increases

*decreases filtrate concentration*

Countercurrent Multiplier System

Establishes a salt gradient (so we can move H2O via osmosis)

Salt [] in interstitial fluid of kidney becomes increasingly greater (more hypertonic, higher [salt]) from cortex to medulla (going "down")

Loop of Henle and Vasa Recta flow in opposite directions (counter-current), aids in establishment of salt gradient

Overall process creates a salt gradient which allows H2O to be retained in body (blood) more efficiently

*creates hypertonic urine*

General Loop of Henle Process

Na+ pumped out of ascending limb, creating high [Na+] in interstitial fluid

This then allows the H2O in descending limb to flow into interstitial fluid via osmosis, further strengthening the salt gradient

Distal Convoluted Tubule

Located in Cortex (after loop of Henle)

Same process as proximal convoluted tubule, but makes fine adjustments to filtrate.

**at this point, filtrate [] is hypotonic (low solute)**

Osmoregulation Nephron Function Part II

Location: collecting duct

Function: maintains H2O balance and collects urine

Collecting Duct

Located in the Medulla

Moves filtrate (urine) to renal pelvis, which empties via ureter into bladder

Collecting Duct is Permeable to H2O, amt H2O that moves out (into interstitial space) depends on ADH

Impermeable to Urea in cortex, but permeable to urea in medulla

-urea released from collecting duct in medulla increases osmolarity of interstitial fluid (more solute)

-this, in turn, increases [] of filtrate bc movement of urea out of collecting duct causes more H2O to move out of collecting duct

Antidiuretic Hormone (ADH)

Osmoreceptors (neurons) in hypothalamus detect osmolarity of blood to determine degree of H2O reabsorption

-varies the permeability of plasma membranes of distal convoluted tubule and collecting duct to H2O, done via number of aquaporins

If dehydrated...

Posterior pituitary releases ADH

ADH increases H2O reabsorption by increasing aquaporin production

-results in smaller amount of more concentrated urine being produced

-ADH also creates "thirst" (bc dehydrated)

Once hydrated, ADH decreases and less water is reabsorbed

Osmolarity

The osmotic of solute concentration, measured in mOsm/L (milliosmoles/Liter)

- mOsm/L = 1/1000 moles of solute in 1L solution

Greater osmolarity = more hypertonic solution (more solute)

Osmolarity Stats

-Ascending limb tires to set about 200 mOsm gradient difference

-maximum kidney osmolarity is about 1,200 mOsm

-blood is about 300 mOsm