Regulation and control

1. Regulation and control

2. Homeostasis • Ability to maintain internal conditions with narrow limits. • Important limits include • pH and temperature for optimal enzyme activity • Water potential optimal for cells

3. Principles of homeostasis • Detectors monitor the output of a system – sensory receptors • Effectors then make small adjustments to return the system to it’s normal state. Eg. Muscles and glands. • Negative feedback – self-adjusting system or corrective mechanism. Eg. A decrease in body temperature causes the body to generate more heat to return to normal.

4. Negative feedback

6. Cholecystokinin- A hormone that stimulates digestion (released in duodenum). Vagal afferent- A part of the Vagus Nerve, that sends impulses to the brain telling it that food is being digested Satiety Centre in Medulla- The Medulla is a part of the brain. After a period of stimulation, it removes the feeling of hunger, so you stop eating. Blue arrows are the negative feedback process

7. Positive Feedback Positive feedback is the body's mechanism to enhance a output needed to maintain homeostasis. Positive feedback mechanisms push levels outof normal ranges. It is rarely used by body because of the risk of the increased stimuli becoming out of control. An example of positive feedback is the release of oxytocin to increase and keep the contractions of child birth happening as long as needed for the child's birth.

8. Excretion • Elimination of metabolic waste, excess substances and toxic substances. • Two main substances that the body needs to excrete: • Carbon dioxide – through lungs • Urea – produced in the liver from deamination of excess amino acids. Removed by kidneys.

9. Kidney

10. Structure:

11. The nephron

13. Ultrafiltration: the formation of the glomerular filtrate • The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron. • A glomerulus is enclosed in the sac. • The basement membrane separates the glomerulus from the capsule. • The Bowman’s capsule has special cells called podocytes with numerous foot-like processes that grip on to the basement membrane.

15. The process of filtration of the blood in the Bowman's capsule is ultrafiltration  • The normal rate of filtration is 125 ml/min, equivalent to 80 times the daily blood volume. • Blood enters the glomerulus at high pressure forcing water and some solutes through the holes in the endothelium of the blood vessels, then through the basement membrane and then through the layer of podocytes into the capsular space. • The endothelium and podocytes act as coarse filters but it is the basement membrane that is the fine filter.

16. Each podocyte is specially shaped so that, when the cells fit together, they create slits which form part of the blood filter. They can also regulate the slit size, to control filtration. • Any small molecules such as water, glucose, salt, amino acids and urea pass freely into Bowman’s space, but cells, platelets and large proteins do not.

17. Selective Reabsorption • Glucose is reabsorbed into the blood by active transport in the proximal convoluted tubule. • Proximal convoluted tubule is lined with a single layer of epithelial cells which have a microvilli border to increase surface area. • Capillaries surround the proximal convoluted tubule allowing efficient transport of substances.

18. A basement membrane covers the outside of the tubule and it contains protein carriers. Other substances are also reabsorbed:amino acidsvitaminshormoneswatersodium ions

19. Osmoregulation • Osmoregulation is the process which regulates the concentration and osmotic pressure of blood by regulating the water contents of blood plasma. • It is an important process as excessive loss of water may cause dehydration whereas excess of water intake may dilute the body fluids.

20. It is an example of negative feedback. • The receptors monitor the amount of water in the body and the kidney is the effector.

21. Water is mainly reabsorbed through the collecting tubules. • The permeability of the wall of the distal convoluted tubule and collecting tubules is controlled by anti - diuretic hormone or ADH which is released from the posterior lobe of the pituitary gland.

23. Excess water in body fluids • Excess of water in the body fluids signals to posterior pituitary to stop the release of ADH. • Deficiency of this hormone lowers the permeability of the cells of the distal convoluted tubule and the collecting duct, decreasing the reabsorption of Na+ from the filtrate. • More filtration combined with less reabsorption of water produces abundant dilute urine and this brings down the volume of body fluids to normal.

24. Shortage of water in body fluids • The rate of ultra filtration decreases due to decreased blood volume and low hydrostatic pressure of blood in the glomerular capillaries. • Rate of reabsorption of water is increased by increasing the permeability of the wall of the distal convoluted tubule and collecting tubules due to increased release of ADH from the posterior pituitary. • Less ultrafiltration and more reabsorption produce small amount of hypertonic urine which increases body fluid volume to normal.

25. Loop of Henle • The loop of Henle plays an important role. • It functions as a countercurrent exchange mechanism that increases the solute concentration in the medulla. • An osmotic gradient is set up to help withdraw water from the collecting duct if circumstances require it.

26. The descending limbs both thick and thin have low permeability to ions and urea, while being highly permeable to water. The loop has a sharp bend in the renal medulla going from descending to ascending thin limb. • Thin ascending limb of loop of Henle • The thin ascending limb is not permeable to water, but it is permeable to ions. • Thick ascending limb of loop of Henle • Sodium (Na+), potassium (K+) and chloride (Cl-) ions are reabsorbed from the urine by active transport

27. Loop of Henle

29. Communication systems • Sensitvity ≡ ability to respond appropriately to external and internal stimuli • Stimuli are detected by receptors in sense organs, and the organs that respond are called effectors. • The body needs to be coordinated in its actions so one part of the body must be able to send information to another part..

30. Nervous System

32. Made of neurones which use electrical nerve impulses. • The central nervous system of vertebrates contains the brain, spinal cord, and retina. • The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system.  • There are three types of neurones: • Sensory neurone (sensor) • Relay neurone (connector) • Motor neurone (effector)

33. Sensory receptors • A sensory receptor receives information from the world and relates it to your nervous system. • activated when they are bent, squished, or disturbed in some way.  Others are activated by chemicals or temperature or light. 

36. Physiologists refer to receptors as selective transducers. • They are called transducers because they 'convert' the energy contained in the stimulus into another form of energy, specifically into some sort of membrane potential. • They are selective because they are highly specific (selective) with respect to the type of stimulus it responds to.

38. When you press on a Pacinian Corpuscle (sensitive to pressure), you deform the lamellae and cause them to press on the tip of the sensory neuron. • That, in turn, physically deforms the neuron's plasma membrane and makes it 'leaky' to sodium ions (Na+) • The increased positive charge inside the axon is called a receptor potential.

39. Motor Neurone • Cell body lies within brain or spinal cord. • Dendrites conduct impulses towards the cell body. • Axon conducts impulse away from cell body to the effector.

40. Schwann cells wrap themselves around the axon. • This forms an enclosing sheath called the myelin sheath. • The small space between the Schwann cells is called the node of Ranvier. • They occur every 1-3mm in human nerves and are about 2-3um wide. • The Schwann cells and nodes of Ranvier affect the speed of the impulse.

43. Sensory Neurone

44. Nervous impulses • Origin of a membrane potential • If the smaller ions are able to diffuse through the membrane but the larger ions cannot, a potential difference will develop between the two solutions. The normal potential difference between the inner and outer parts of nerve cells is about –70 mv. Transmissin of a nerve impulse is initiated by a reduction of this potential difference to about –20 mv.

45. Action Potentials • Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane.  • These channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value. • When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential.

46. This then causes more channels to open, producing a greater electric current, and so on. • The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. • The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. • As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported out of the plasma membrane. 

47. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. • After an action potential has occurred, there is a transient negative shift, called the refractory period, due to additional potassium currents. • This is the mechanism that prevents an action potential from traveling back the way it just came.