T14 Processes and Key Points Flashcards
Homeostasis
maintaining a relatively constant environment for the cells within the body, despite changes in external environment
Controlled by the composition of blood, and hence the tissue fluid
Features of tissue fluid influences cell activity in many ways, such as:
Temperature- low temp, slow metabolic reactions. High temp however denatures enzymes and proteins
Water potential- low water potential causes water to enter a cell and burst. High water potential causes water to leave and cell to shrink.
Water potential is the ability of water to move freely so if it is high water will leave not enter.
Conc. of glucose- too less, no energy for cell to respire, too high would affect osmotic balance and disturb cells
Homeostatic Control
Involves receptor that detects stimuli
A stimuli is a change in physiological factors, such as temperature, pH of blood, water potential etc
Receptor sends information to the central control in the brain or the spinal cord
The input is processed and instructions are sent to the effector
Effectors such as muscles and glands cause the factor to return to its ideal value or set point
This is done through two coordination systems:
Nervous system, electrical impulse along neurons
Endocrine system, in the form of chemical messengers (Hormones) that travel in the blood
Negative feedback
Negative feedback keeps factors within narrow limits, making it close to set point as possible
When a factor is increased, an effector is stimulated that makes the factor decrease, and vice versa
Positive feedback
Is not used in keeping conditions constant as it increases effect when stimulus is increased
This is useful in other areas such as transmission of nerve impulses where the factor must be increased
Excretion
Deamination is the removal of an amino group (NH2) from a molecule. This is done in the liver when there is an excess of protein, rather than wasting a useful energy source
The –NH2 and a hydrogen atom are removed leaving behind a keto acid
Urea formation
Since ammonia is very soluble and highly toxic compound it is converted immediately to urea
2NH3 + CO2 CO(NH2)2+ H2O
Urea is the main nitrogenous excretory product, however we also produce creatinine and uric acid
Creatine is made in the liver from amino acids that is used as an energy store in muscles
Uric acid is made from the breakdown of purines
Ultrafiltration
Involves filtering small molecules out of the glomerulus and into the Bowman’s capsule due to hydrostatic pressure build up
How it happens:
Hydrostatic pressure builds up in the glomerulus due to the wider afferent and narrower efferent arterioles
This causes the hydrostatic blood pressure in the glomerulus to rise above that of the Bowman’s capsule.
Water from blood therefore goes down its water potential gradient through the endothelium of the capillary walls, the basement membrane and podocytes, thus filtering substances.
Endothelium: one cell thick cell with many holes
Basement Membrane: makes up inner lining of bowman’s capsule and acts as filter for large molecules eg large Mr proteins, WBC and RBC
Podocytes: inner lining of bowman’s capsule with large holes
Reabsorption in Proximal Convoluted Tubule (PCT)
The glomerular filtrate is almost identical to the plasma’s composition except it has no plasma proteins
Many of the substances however are needed to be kept in the body, thus most reabsorption occurs in the PCT
Adaptation of cuboidal epithelial cells:
Microvilli to increase surface area for many co-transporters for maximum reabsorption
Tight junction between cells so that fluid can’t pass between them
Many mitochondria to provide ATP for (Na+–K+) pump on basal membrane
Folded basal membrane providing large surface area for (Na+–K+) pump
1. Removal of Na+ from the cell: Na+–K+ pumps in the basal membrane use ATP pumping 3 Na+ out and 2K+ in, lowering its concentration inside the cell
- Passive movement of Na+/glucose/amino acid inside the cell:
Na+ goes down its concentration gradient via a cotransporter that brings along glucose/amino acids.
This a secondary active transport as ATP was not used for pumping Na+ into the PCT cell but has occurred as a result of actively transporting Na+ out of the cell
Glucose and amino acids diffuse down their gradient from cell into blood via transport proteins in the basal membrane
3. Reabsorption of water:
Removal of ions from the tubule increases its water potential, and increases the solute potential of the cell
Thus, water diffuses down its gradient into the cell, and is reabsorbed in the blood via osmosis
4. Reabsorption of urea: urea is a small molecule and passively gets reabsorbed
Reabsorption in Loop of Henle
Na+ & Cl- are actively transported out of higher end of ascending limb into the tissue fluid
This increases concentration of ions in tissue fluid
Water is therefore lost from the descending limb
Loss of water concentrates Na+ and Cl− along the descending limb.
This concentrates the fluid inside the loop, so ions passively move down their concentration gradient, into the tissue fluid
Distal Convoluted Tubule (DCT)
First part functions the same way as ascending limb
Second part functions the same way as collecting duct
Reabsorption of water in DCT and Collecting duct
Fluid in ascending limb is dilute due to loss of ions and urea, concentrating tissue fluid in medulla
When fluid enters collecting duct from DCT, it returns to the concentrated medulla region, thus water moves out by osmosis into the tissue fluid and is reabsorbed, concentrating urine
Osmoregulation
Involves the control of water potential in body fluid
Osmoreceptors in the hypothalamus constantly monitor water potential in the blood
Decrease in blood water level
Osmoreceptors detect and send impulses to the posterior pituitary gland to secrete antidiuretic hormone (ADH)
ADH in the blood binds to receptors on the cells of collecting duct, activating intracellular enzymes
Vesicles that contain aquaporin in the cell are stimulated to fuse to membrane
This causes duct to become permeable to water hence water moves out, down its conc. gradient
Note: volume of urine decreases and becomes more conc.
Increase in blood water level
Osmoreceptors no longer stimulate ADH production, so aquaporins moved back into cytoplasm as vesicles, making cells impermeable to water again
This process is very slow because ADH molecules take 15-20 mins to be broken down in the blood and another 15-20 mins for aquaporins to be removed from the membrane
Control of Blood Glucose
When glucose is in low concentration our cells may not have enough glucose for respiration, hence might not be able to carry out its normal function
On the contrary, high concentrations can effect normal behaviour of cells as they may lose water due to the concentration gradient built (cells become flaccid)
The homeostatic control is carried out in the pancreas by a tissue called the islets of Langerhans which consisting two types of cells:
\alphaα cells which secrete glucagon
\betaβ cells which secrete insulin
After a meal containing carbohydrates, glucose is digested and passed into the blood
When Blood glucose levels rise
The \alphaα and \betaβ cells detect the change
\alphaα responds by stopping secretion of glucagon
\betaβ responds by secreting insulin into the blood
Insulin is a signalling molecule that targets the liver and muscle cells and binds to a receptor
This stimulates the cells to increase rate of glucose absorption by making vesicles carrying glucose transporter proteins (GLUT) to bind onto cell membrane
