Inspired and Expired Air
Inspired and Expired Air Comparison |
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Gas + % |
Inspired Air |
Expired Air |
Alteration |
Nitrogen |
78% |
76% |
No real change. |
Oxygen |
20.8% |
15.3% |
Reduced by about a quarter |
Carbon Dioxide |
0.04% |
4.2% |
Increased by about a hundred and five times |
Water Vapour |
1.2% |
6.1% |
Increased about five times |
Note: a lot of water is lost from the body each day due to breathing. |
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The Breathing Mechanism
Inspiration
An active process because it involves muscle contraction.
The diaphragm and intercostal muscles contract.
The contracting diaphragm flattens and stretches the elastic lungs downward.
The contracting intercostals pull the ribcage up and out causing the elastic lungs to stretch.
The expanding lungs cause the air inside to expand (a gas will always fill its container).
The expansion of air causes a drop in air pressure in the lungs.
The air in the lungs is at a lower pressure than the air outside.
Air flows from higher to lower pressure so air flows into the lungs from outside.
Expiration
A passive process because it does not involve muscle contraction.
The diaphragm and the intercostal muscles relax.
The deforming force on the elastic lungs has been removed.
The lungs recoil elastically reducing their volume – a passive process.
The volume of air in the lungs decreases causing an increase in the air pressure.
The air in the lungs is at a higher pressure than the air outside.
Air flows from higher to lower pressure so the air flows out of the lungs.
Note: the elastic recoil of the lungs pulls up the adhering diaphragm and drags in the adhering ribcage.
Carbon Dioxide and Breathing
Carbon dioxide blood level controls the rate and depth of breathing.
Normal relaxed rhythmical breathing is controlled unconsciously by the medulla oblongata of the brain.
A rise in blood CO2 stimulates the medulla to fire, sending nerve impulses to the breathing muscles.
The diaphragm and intercostal muscles contract and so air is breathed in.
Nervous feedback from the inflating alveoli causes the medulla to switch of its stimulation.
Inspiration stops and the lungs recoil causing expiration.
Rapidly rising levels of CO2 increase the rate of breathing.
Exercise increases the production of CO2 leading to an increase in the breathing rate.
Gas Exchange Adaptations
Textbook Diagram: alveolus and its capillaries.
Gas exchange is by diffusion so the adaptations are such to make diffusion efficient.
Oxygen diffuses from the air in the alveolus into the blood of the capillary.
Carbon dioxide diffuses from the blood into the air of the alveolus.
Large Surface Area: 90m2 — 700 million alveoli and 40 billion capillaries.
Permeable Surfaces: the cell membranes are freely permeable to O2 and CO2.
Thin Barrier: the distance between the air and the blood is two cells wide.
Moist Surface of Alveolus: enhances the uptake of O2.
Elastic Alveoli Walls: efficient filling with air and recoil enhances emptying.
Slow Capillary Blood Flow: time for complete oxygenation and excretion of CO2.
Asthma
Asthma is a breathing disorder.
Symptoms: coughing, wheezing, tightness of chest and breathlessness.
Cause: allergic reaction to materials in the environment e.g. pollen, cigarette smoke, house dust and pet dander.
Prevention: avoid the allergen
Treatment: inhalation of bronchodilators to widen the narrowed air passages.
Digestive System
The human digestive system is adapted to deal successfully with a mixed diet of plant and animal material.
Major Functions
Digestion: breakdown of complex food into their simple soluble absorbable sub-units. Absorption: the passage of the products of digestion into the blood or lymph. Movement of Food: controlled by sphincter muscles, longitudinal and circular muscles in the gut wall.
Five Stages of Human Nutrition
Ingestion: placing food into the alimentary canal at the mouth.
Digestion: breakdown of complex food into their simple soluble absorbable subunits.
Absorption: the passage of the products of digestion into the blood or lymph.
Assimilation: conversion of the absorbed nutrients into complex molecules for growth, repair and defence.
Egestion: expulsion of the undigested and unabsorbed material from the alimentary canal.
Structure of the Human Digestive System: Textbook Diagram: Human Digestive System
Digestion
Physical Digestion
The food is physically broken into smaller pieces.
This speeds up chemical digestion by increasing the food’s surface area.
The teeth, muscles of the stomach wall and the liver are the structures involved in physical digestion.
Chemical Digestion Specific enzymes ‘cut’ the large complex food molecules into new simpler molecules.
Passage of ‘Food’ from Mouth to Anus
Mouth Cavity Physical Digestion: by the teeth.
Textbook Diagram: Shapes of Teeth.
Four types of teeth:
incisors — cutting and biting;
canines — tearing;
premolars —crush and grind.
molars —crush and grind.
General Tooth Structure
Textbook Diagrams: external and internal structure of a tooth.
Crown — above the gum.
Neck — surrounded by the gum.
Root — embedded into jawbone.
Details of Tooth Structure Enamel: to protect the teeth from physical and chemical damage and it gives strength to break the food. Dentine: gives the shape and rigidity to the tooth. Pulp Cavity: contains nerves, blood and lymph vessels. Cement: glues the tooth to the jawbone.
Human Dentition: omnivorous — adapted for physical digestion of plant material and meat. Dental Formula: I 2/2; C 1/1; PM 2/2; M 3/3 The dental formula indicates the number of each tooth type on one side of the upper and corresponding lower jaw. The total number of teeth is twice the number in the dental formula.
Chemical Digestion in the Mouth
Salivary glands secrete a digestive juice containing amylase.
Amylase converts starch to maltose.
Amylase optimum pH is 8 and optimum temperature 37ºC.
Saliva moistens the food making it easier to chew and swallow.
Note: for chemical digestion you must choose an amylase, a protease (an enzyme that digests protein) and a lipase – for each ensure you know the production site, the pH at a named location of its action and the products.
Pharynx (Throat)
Special muscles in the wall of the pharynx lift the trachea so the epiglottis during swallowing.
This closes the opening to the windpipe ensuring that the food or drink enters the oesophagus.
Oesophagus Muscles in the wall of the oesophagus propel the swallowed material to the stomach by peristalsis.
Stomach
The food is temporarily trapped here by the action of sphincter muscles.
Gastric glands secreted a gastric juice into the lumen of the stomach – pH 2.
Muscles in the wall of the stomach work to mix the juice throughout the food and physically break it down.
The food is also warmed to body temperature, 37°C.
Hydrochloric acid sterilises the food killing micro-organisms.
Pepsin, a protease, converts proteins into peptides and amino acids.
Pepsin’s optimum pH is 8 and temperature 37ºC.
Salivary amylase activity stops when it is denatured by hydrochloric acid.
Mucus lines the inner surface of the stomach protecting it from its acid and digestive enzymes.
The stomach absorbs water, glucose and salts.
Small Intestine – duodenum, jejenum and ileum Duodenum (2 + 10 = 12 finger-widths in length) Receives bile from the liver, pancreatic juice from the pancreas and itself produces many digestive enzymes.
Bile
Delivered to the duodenum by way of the bile duct.
Alkaline salts in the bile assist to raise the pH from pH 2 to pH 8.
Bile salts emulsify fats – physical digestion – this increases in its surface area of the fat.
Emulsification greatly speeds up chemical digestion of lipid by lipase.
Bile also carries excretory product, bile pigments and excess cholesterol.
Pancreatic Juice
Delivered to the duodenum by way of the pancreatic duct.
Alkaline salts in the pancreatic raise the pH of the food to pH 8.
Pancreatic amylase converts starch to maltose.
Pancreatic lipase converts lipids to glycerol and fatty acids.
Trypsin, a protease, converts protein to peptides.
Intestinal Juice
Mucus: protects the wall of the duodenum from the acid food passed from the stomach.
Amylase: converts starch to maltose.
Lipase: converts lipids to glycerol and fatty acids.
Erepsin: a protease converts protein and peptides to amino acids.
Jejunum and Ileum follow the duodenum — digestion and absorption continue. Muscles in the wall of the small intestine push the food along by peristalsis.
Absorption of Nutrients
Over 90% of the food is digested and absorbed.
Most of the digestion and absorption occurs in the small intestine.
Adaptations of Small Intestine for Absorption
Textbook Diagrams: transverse and longitudinal of small intestine; villus diagram.
Enormous surface area due to its length, villi and microvilli: for efficient absorption.
Rich network of blood capillaries: collect the absorbed amino acids and simple sugars.
Lymphatic capillaries: collect the absorbed ‘lipid’.
The absorbing surface is very thin: — only one membrane ‘thick’.
The absorbed glycerol and fatty acids are recombined in the absorbing cells forming lipid globules. These globules are passed into the lymphatic capillary (lacteal) of the villus. The lacteals drain their contents into the branches of the thoracic lymphatic duct. T he fat-laden lymph is delivered to the blood at the left subclavian vein.
Large Intestine
Textbook Diagram: transverse section of large intestine — caecum, appendix, colon, rectum, anal canal.
The appendix produces defensive lymphocytes and antibodies.
Symbiotic bacterial in the caecum and colon protect us from pathogenic micro-organisms and supply us with vitamins B and K.
The colon’s main function is to reabsorb water preventing excessive water loss from the body.
The colon also absorbs glucose, vitamins and salts.
Benefits of Dietary Fibre
Stimulates peristalsis.
Reduces the risk of colon cancer, heart disease and middle age diabetes.
Role in the formation of soft faeces so reduced risk of constipation.
The Liver
The liver is dark red and lies in the abdominal cavity just below the diaphragm close to the stomach. It is the largest internal organ of the human body.
Liver Blood Supply
Textbook Diagram: liver and its associated blood vessels.
Oxygenated blood is delivered to it by the hepatic artery — a branch of the dorsal aorta. Hepatic portal veins supplies the liver with glucose and amino acids absorbed from the gut.
Functions of the Liver
Regulates the chemical composition of the blood — homeostatic organ.
Production of:
Bile: bile salts emulsify fat in the small intestine speeding up their chemical digestion, alkaline salts help neutralise the food from the stomach providing a suitable pH for digestive enzymes.
Plasma proteins to maintain the blood at the correct concentration.
Cholesterol — essential for the cell membrane structure.
Amino acids — amino acids that the liver cannot make are called ‘essential amino acids’.
Storage
Excess glucose in the blood is absorbed and stored as glycogen and fat.
The fat-soluble vitamins are held in reserve in the liver.
Excretion
Expels excess cholesterol.
Makes urea waste from excess amino acids in the diet.
Detoxification
Renders toxic chemicals harmless.
Breaks down hydrogen peroxide, alcohol, nicotine and other drugs.
Recycling — red blood corpuscles are chemically destroyed and their material recycled.
Immunity — protection against pathogens.
Special white cells are located in the liver to detect and destroy pathogens in the blood supplied to the liver from the gut. Plays a role in the regulation of the body temperature: the liver’s metabolism can be altered to vary its production of heat depending on the homeostatic requirement of the body.
Excretion, Osmoregulation and Homeostasis
Kidneys
Functions
Excretion: nitrogenous wastes (urea, uric acid), excess salts, excess water.
Osmoregulation: maintaining the blood at a suitable constant concentration.
Homeostasis: maintaining a suitable constant internal environment to sustain efficient metabolism.
Urinary System
Textbook Diagram: urinary system.
The kidneys are a pair of fist-sized red-brown bean-shaped structures.
The kidneys are attached to the back wall of the abdominal cavity.
They lie on either side of the backbone just above the pelvis.
Each kidney receives a good supply of oxygenated blood from the renal artery, a branch of the dorsal aorta.
The renal vein takes the deoxygenated blood from the kidneys to the inferior vena cava.
The blood in the renal vein has less oxygen, salt, urea and uric acid than the renal artery.
Urine is carried to the bladder along the ureter by peristalsis for temporary in the bladder.
A sphincter muscle at the junction of the bladder and urethra regulates the retention and release of urine.
Urine is channelled to the exterior along the urethra.
Kidney Structure
Textbook Diagram: longitudinal section of a kidney showing its internal structure.
A smooth thin protective cover called the capsule surrounds each kidney.
Below the capsule is a thick reddish granular layer, the cortex.
The central part of the series of triangular structures is reddish-brown, the renal pyramids, the tips of which project into the upper expanded end of the ureter known as the pelvis.
Nephron
Textbook diagram: structure and blood supply of the nephron.
The nephron or renal tubule is the functional unit of the kidney.
The nephron has a number of functionally distinct parts.
Each human kidney has about one million nephrons.
Urine is manufactured by the nephrons.
Production of Urine: Filtration and Selective Reabsorption
Details of Urine Formation
Filtration
The glomerulus functions as a filter.
The glomerular capillary walls are porous.
The red blood cells, white cells, platelets and plasma proteins are too big to pass through the pores.
Therefore the glomerulus filters the blood.
The filtrate passing into Bowman’s Capsule.
Glomerular filtrate composition is water, glucose, amino acids, vitamins, salts, urea, and uric acid.
About 20% of the plasma volume passes out of the glomerulus.
The filtration is much higher than expected.
The blood pressure is unusually high in the glomerulus.
The blood pressure is generated by the pumping action of the heart.
The high blood pressure in the glomerulus is due to:
The arrangement of blood vessels: arteriole —> capillaries —> arteriole
This arrangement is unusual - normally low-pressure venules follow capillaries.
The efferent arteriole is narrower than the afferent arteriole.
The higher than normal filtration at the glomerular capillaries is known as ultrafiltration.
Selective Reabsorption
Much useful material was lost from the blood into Bowman’s Capsule.
The ‘useful’ materials are taken back into the blood from the nephron.
By de-selection, urea and uric acid remain in the nephron and are excreted in the urine.
Urine is the unabsorbed glomerular filtrate.
Proximal Convoluted Tubule (PCT)
Total reabsorption of glucose and amino acids.
Four fifths of the salts and water are reabsorbed.
Glucose, amino acids and salts are reabsorbed by active transport.
Water is reabsorbed by osmosis.
The cells lining the PCT are rich in mitochondria, which supply the ATP for active transport.
The Loop of Henle
This structure allows the kidney to reabsorb extra water in times of water stress. As a result it is possible for the kidney to produce hypertonic urine, i.e., more concentrated than blood plasma. A Loop of Henle is only present in mammals and birds — the only animals able to produce hypertonic urine.
About 5% of the water from the glomerular filtrate is reabsorbed from the Loop of Henle by osmosis.
The main function of the Loop is to develop an increasingly concentrated medulla. It accomplishes this by acting as a ‘hairpin counter current multiplier’. This allows extra water, if needed, to be absorbed from the collecting duct under the influence of ADH hormone.
Distal Convoluted Tubule (DCT)
Reabsorption of water is by osmosis.
The amount varies depending on the need of the body.
Water reabsorption by the DCT is under the influence of ADH (antidiuretic hormone).
Reabsorption of salt is by active transport.
The amount of salt reabsorbed depends on the needs of the body.
The role of the DCT is crucial in osmoregulation.
Osmoregulation is a major process in homeostasis.
Osmoregulation
Blood concentration is kept in check by varying the amount of water and salt reabsorbed by the kidneys nephrons.
Blood Concentration Rising
Cause: salty food, water loss due to sweating, inadequate water intake.
Response: increases water reabsorption, decreases salt reabsorption.
Blood Concentration Falling
Cause: excessive water intake, cold weather (sweating less than usual), diet very low on salt.
Response: decreases water reabsorption, increases salt reabsorption.
Note: the greater the excess protein in the diet the greater is the urea content of the urine.
Regulation of Body Fluids by the Kidney The kidney maintains the blood at the correct composition and concentration by excretion and osmoregulation.
As a result all the other body fluids are kept at optimum condition i.e. tissue fluid and cell cytoplasm.
Role of ADH (antidiuretic hormone)
ADH is secreted to increase water reabsorption by the DCT and collecting duct when blood concentration rises.
Osmoreceptors in the brain’s hypothalamus detect the increase in osmotic pressure of the blood.
This stimulates the pituitary to increase the secretion of ADH into the blood.
ADH is transported everywhere throughout the body in the blood.
The DCT and collecting duct are the target tissues of this hormone.
ADH causes these parts of the nephron to become more permeable to water.
Extra water is now reabsorbed into the blood reducing its concentration back to normal.
The loss of extra water from the filtrate reduces the volume but increases the concentration of the urine.
Major Homeostatic Organs: kidneys, liver, lungs, skin and brain.
Skeleton
The human skeleton is an endoskeleton of bone and cartilage.
Major Functions
Support for the soft tissues and largely responsible for the shape of the human body.
Movement: the strong rigid bones function as levers moved by the contraction of muscles.
Protection: the tough bones protect delicate vital organs
the brain is protected by the skull
heart and lungs are protected by the rib cage
spinal cord is protected by the backbone
Axial Skeleton: skull, backbone, ribs and sternum.
Appendicular Skeleton: pectoral girdle, pelvic girdle and limbs.
Textbook Diagram: human skeleton.
Axial Skeleton
Skull: composed of fused bones except for the mandible (lower jaw).
Backbone: Also known as the vertebral column or spine.
Textbook Diagram: Vertebrae
Composed of 33 small bones in a line - cervical (7), thoracic (12), lumbar (5), sacrum (5), coccyx (4).
The vertebrae of the sacrum and coccyx are fused.
The vertebrae of the other regions can articulate slightly giving flexibility to the backbone.
Cartilage discs between these act as shock absorbers.
Spinal nerves emerge in pairs from the spinal cord between the vertebrae.
The vertebrae are held in position by ligaments.
The vertebrae are also supported by muscles attached to their surfaces.
Ribs
12 pairs all attached to the backbone.
First seven are true ribs joining also to the sternum.
Next three are the false ribs, which also join to the seventh rib.
Last two are the floating ribs — only joined at one end to the backbone.
The sternum or breastbone is a flat thin bone at the centre of the chest wall.
Appendicular Skeleton
Pectoral Girdle
Composed of four bones — two clavicles (collar bones) and two scapulae (shoulder blades).
The arms articulate with the scapulae at a ball and socket joint.
Pelvic Girdle
Appears to be one large cylindrical bone but are actually six fused bones.
The legs articulate with the pelvis at a ball and socket joint.
The pelvis is securely attached to the backbone at the sacrum.
Limbs
The arms and legs have similar design.
Long upper bone: humerus in the arm, femur in the leg.
Two long bones in the medial region: ulna and radium in the arm, tibia and fibula in the leg.
Carpals are in the wrist of the arm, tarsals in the ankle of the leg.
Metacarpals are in the palm of the hand, metatarsals in the hind foot.
Phalanges are in the fingers and toes.
Long Bone Structure
Textbook Diagram: longitudinal section of long bone
Compact Bone: solid bone that at microscopic level has concentric ring structure.
Spongy Bone: irregular openwork of thin plates of bone.
Marrow Cavity: the space within the diaphysis containing the marrow.
Epiphysis: one at end of the long bone and is formed separately of the dipphysis.
Diaphysis: the shaft or long main portion of the bone.
Articular Cartilage: covers the epiphyses protecting them from friction and shock at freely moveable joints.
Red Bone Marrow: formation of red blood cells, white blood cells and platelets.
Yellow Bone Marrow: energy storage in the form of lipid (fat).
Osteoblasts
Osteoblasts are bone secreting cells.
The original cartilage skeleton of the human embryo was replaced by bone secreted by osteoblasts.
Bone is continually being broken down and replaced.
Bone renewal is carried out by osteoblasts.
Osteoblasts, tapped in their own bone secretion are called osteocytes.
Role of Calcium in Bone
Bone is composed of organic and inorganic material
The organic material is mostly collagen protein.
The inorganic material is chiefly calcium phosphate salts.
Calcium is needed in the diet for strong bones.
Lack of calcium causes bones to be soft and weak; this condition is known as osteomalacia.
Terminating Development of Adult Height
Increase in height ceases when the long bones of the legs stop growing.
There are two growing areas in the long bones.
These areas lie between the diaphysis and the two epiphyses.
Each area is a layer of actively growing cartilage.
New cartilage is converted to bone.
When this cartilage layer stops growing increase in height terminates.
Termination usually occurs at the end of adolescence.
Joints
A joint is the junction between two or more bones.
There are three major types of joints.
Fused Joints: skull, sacrum, pelvis, and coccyx.
Slightly Moveable: between the vertebrae.
Freely Moveable or Synovial: the movement is lubricated by synovial fluid.
Structure of a Freely Moveable Joint
Textbook Diagram: vertical section of a synovial joint.
The two bones are joined together by sleeve-like capsule.
The capsule encloses the synovial cavity.
The outer layer of the capsule is composed of ligaments.
The ligaments keep bones together preventing dislocation and control the range of movement.
The inner layer of the capsule is the synovial membrane.
The synovial membrane secretes the lubricating synovial fluid.
Lubrication is essential to prevent frictional wear and tear.
The cartilage at the contact ends of the bones also reduces friction.
The cartilage pads also acts as shock absorbers against mechanical damage.
Classes of Synovial Joint
Gliding: the bones move across each other, back-and-forth and side-to-side. Examples: between the carpals of the wrist and tarsals of the ankle.
Pivot: allows a turning movement. Examples: between the first and second vertebras when turning the head, between the ulna and the radius of the lower arm when turning the palm of the hand up or down.
Hinge: movement in one plane during flexion and extension. Examples: bending the elbow or knee.
Ball and Socket: permits movement in three planes, i.e., in all directions. Examples: the shoulder and hip joints.
Skeletal Muscles
Muscle is extremely contractile connective tissue.
The contraction of muscle performs four important functions:
movement
posture maintenance
support the joints
heat production
Skeletal muscle is connected to the skeleton.
It is under conscious control.
Its contraction is fast and strong.
Skeletal muscle tires easily.
Antagonistic Skeletal Muscles
Textbook Diagram: antagonistic muscles – biceps and triceps.
Antagonistic muscles are pairs of muscles the action of one member is opposite to that of the other member.
Biceps: two tendons connect it to the shoulder.
Triceps: three tendons connect it to the shoulder.
Flexing the Arm
Bending a straight arm.
Relax the triceps and contract the biceps.
The lower arm now moves towards the shoulder.
Extending the Arm
Straightening a bent arm.
Relax the biceps and contract the triceps.
The lower arm now moves away from the shoulder.
Tendons
Tendons are strong inelastic cords or bands of connective tissue that connect muscle to bone.
The inelastic tendon will not stretch when the muscle contracts.
Therefore the full pull is transmitted to the bone and the full range of motion is realised.
Ligaments
Ligaments connect bone to bone.
Ligaments preventing dislocation of the joint.
Ligaments control the range of movement of the bones at the joint.
Osteoporosis
Description: a bone disorder – the bone is more spongy, weaker and lighter.
Cause: lifestyle lacking load-bearing physical exercise, protein deficient diet, hormone changes.
Treatment: oestrogen for menopausal females, vitamin D and calcium rich diet.
Prevention: continuous physical activity from a young age, calcium rich diet in youth.
Nervous System
The nervous system allows the animal to quickly detect, communicate and co-ordinate information about its external and internal environment so it can make efficient appropriate responses for survival and/or reproduction.
The two major parts of our nervous system are the central nervous system (CNS) and peripheral nervous system (PNS).
The CNS is made of the brain and spinal cord.
The cranial nerves, spinal nerves and ganglia make up the PNS.
The cranial nerves connect to the brain.
The cranial and spinal nerves contain the axons (fibres) of sensory and motor nerve cells.
Nerve cells area also known as neurons.
Sensory Neurons
Sensory neurons carry electrical signals (impulses) from receptors or sense organs to the CNS.
Sensory neurons are also called afferent neurons.
The cell body of sensory neurons is outside the CNS in ganglia.
Motor Neurons
Motor neurons carry impulses from the CNS to effector organs.
Motor neurons are also called efferent neurons.
The cell bodies of motor neurons are inside the CNS.
Motor Neuron Structure
Textbook Diagram: motor neuron structure.
Cell Body: contains specific neurotransmitter receptors in its outer membrane, produces neurotransmitters that will be sent to the synaptic knobs.
Cell Body Dendrites: increase the number of nerve cells with which it makes contact.
Axon: fast uninterrupted transfer of impulses from the cell body to the ‘distant’ target.
Schwann Cells (Myelin Sheath): protects the axon and electrically insulates it maintaining impulse strength and speed.
Nodes of Ranvier: role in ‘saltation conduction’ of impulses, which is ten times faster than normal.
Terminal Dendrites: increase the stimulation of the effector organ.
Synaptic Knobs: transfer the signal to the effector by releasing a specific neurotransmitter.
Definite ‘poles’: cell body and its dendrites receive information; terminal dendrites transfer the information to other cells.
Nerve Impulse
Impulse: a wave of electrical change along the nerve cell.
Stimulus: a change in the environment of the nerve cell that, if strong enough, will generate an impulse.
Threshold: the minimum intensity of stimulus needed to bring about an impulse.
All-or-Nothing: if the stimulus is at or greater than threshold an impulse is generated, but if the stimulus is less than threshold there is no impulse.
Characteristics of Impulses
Impulses are identical in all nerve cells.
Impulses are waves of electrical change along the membrane of the neuron.
The electrical changes involve the flow of ions into the neuron.
The stimulus must be at or above threshold to generate an impulse.
The impulse is at a set strength.
The impulse is self-propagating along the neuron to the terminal dendrites.
Impulses cause the release of neurotransmitters that carry the signal across to the target cell.
Synapse
Textbook Diagram: the synapse.
A synapse is a specialised junction between a neuron and its target cell.
Transmission of Impulses from Nerve Cell to Target Cell at the Synapse:
The ‘impulse’ is carried across from the pre-synaptic neuron to post-synaptic neuron.
Vesicles filled with neurotransmitter e.g. acetylcholine are in the synaptic knobs.
The arrival of an impulse causes the release of neurotransmitter into the synaptic cleft.
The neurotransmitter diffuses rapidly across the synaptic cleft.
The neurotransmitter combines with the receptors in the target cell membrane.
The neurotransmitter-receptor complex generates a response in the target cell.
An enzyme, e.g. cholinesterase, in the synaptic cleft destroys the neurotransmitter.
Neurotransmitter-receptor complexes are broken and the target cell stops responding.
The neurotransmitter breakdown products are reabsorbed into the synaptic knob.
New neurotransmitter is reformed.
Advantages of Chemical Synapses
Chemical transmission is more versatile than electrical transmission, allows a greater range of interactions.
Allows amplification of signals - a small signal can cause a big response, e.g., muscle contraction.
Acts like a valve – the signal only travel in one direction. Allows specific transmission pathways.
Permits a specific local responses rather than general full body response.
The major disadvantage is the delay of five milliseconds (one two-hundredth of a second) to transfer the signal.
Central Nervous System
The brain and spinal cord make up the central nervous system.
Textbook Diagram: Brain and Spinal Cord - the central nervous system.
Functions of Brain Parts
Cerebrum: centre for sensory and motor control, language, memory, intelligence, and consciousness.
Cerebellum: muscle co-ordination, movement and balance.
Medulla Oblongata: controls breathing and regulates heart rate.
Pons: interconnection between cerebrum and cerebellum.
Hypothalamus: osmoregulation and temperature control.
Pituitary Gland: secretes a wide variety of hormones – a major endocrine gland.
Reflex Action
A reflex action is a very fast unconscious response to an unexpected and potentially dangerous stimulus.
Examples of reflex action: knee jerk, eye blink, pupil size alteration, closure of the glottis on swallowing.
Reflex Arc A reflex arc is a specific nerve pathway involved in a fast, unconscious response to an unexpected stimulus.
Spinal Reflex Action E.g., withdrawal of hand when finger jabbed unexpectedly with a pin.
Textbook Diagram: transverse section of spinal cord showing the reflex arc pathway.
Pain and pressure receptors in the skin are stimulated.
Sensory (afferent) neurons carry the impulses to the spinal cord by way of the dorsal root.
An interneuron picks up the impulse from the sensory and transmits it to the motor (efferent) neuron.
At the same time the impulse is also transmitted to the brain.
The motor neuron stimulates the specific effector organ to make the appropriate response, muscles in the arm are stimulated to contract pulling the hand away from the pin.
The response is very fact – occur before the brain is aware of the situation.
The brain, having received the information, may transmit impulses to other muscles to modify the response.
Sense Organs
Eye, Ear, Nose, Tongue and Skin
The Eye
Textbook Diagram: vertical section of the eye.
Sclera: tough outer white layer of the wall of the eye.
Cornea: transparent ‘window’ of the eye, focussing of light on the retina.
Iris: the coloured sheet of muscle, controls the pupil size so controls entry of light.
Pupil: a hole in the iris letting light into the back of the eye.
Ciliary Body: a ring of muscle controlling the shape of the lens.
Suspensory Ligaments: transfer the pull of the ciliary body to the lens.
Lens: accommodation — the fine adjustment to the focussing of light onto the retina.
Retina: light sensitive layer of rods and cones converting light into nerve impulses.
Fovea or Yellow Spot: a tiny area of densely packed cones for detailed and coloured vision.
Choroid: a black-pigmented layer preventing internal reflection of light.
Blind Spot: exit point of the optic nerve cutting through the retina so no rods or cones here.
Optic Nerve: carries the impulses from the rods and cones to the visual centre of the brain.
Aqueous Humour: a clear liquid in front of the lens maintaining the shape of the cornea.
Vitreous Humour: a clear jelly offering support and shape to the back of the eye.
Note: you have a choice – correction of long and short sight or correction for a hearing defect.
Eye Defects and Correction
Textbook Diagrams: short sight and correction; long sight and correction.
Short Sight (myopia)
Condition: can view close objects clearly but distant objects are out of focus.
Light rays are focussed short of the retina.
Cause: eyeball is too long or the focussing elements of the eye are too strong.
Correction: use a concave (divergent) lens to widen the angle over which the light rays have to be refracted.
Long Sight (hypermetrpia)
Condition: can view distant objects clearly but close objects are out of focus,
The focal point is long of the retina i.e. is behind retina.
Cause: eyeball is too short or the focussing elements of the eye are too weak.
Correction: use a convex (convergent) lens to reduce the angle over which the light rays have to be refracted.
Ear
The ear has two major functions:
Hearing: detection of vibrations, their frequency (pitch) and amplitude (loudness).
Balance: detection of direction of motion, acceleration and head position related to gravity.
Textbook Diagram: Ear Structure
Pinna: said to collect sound waves and channel them into the external auditory canal.
External Auditory Canal: earwax and hairs prevent foreign material entering the ear.
Eardrum (tympanum): vibrates with the same frequency as the sound waves beating against it.
Ear Ossicles (hammer, anvil, stirrup): transmit and amplify the vibrations of the eardrum to the inner ear.
Eustachian Tube: when it opens it equalises the air pressure on both sides of the eardrum allowing the eardrum to vibrate freely so accurate sound sensations are generated.
Oval Window: a flexible partition allowing transfer of vibrations to inner ear.
Round Window: a flexible partition to deaden the pressure changes in the inner ear.
Cochlea: conversion of sound waves into nerve impulses — ‘hearing’ apparatus.
Auditory Nerve: carries impulses from the cochlea to the brain’s hearing centre.
Semicircular Canals: detection of direction of motion and acceleration — balance.
Utricle and Saccule: detection of acceleration and head position related to gravity — balance.
Vestibular Nerve: carries impulses from the balance organs to the cerebellum.
Correction of a Hearing Defect
In children middle ear inflammation, called otitis media, is a common cause of hearing loss.
Cause: food allergy or bacterial infection.
Treatment: antibiotics or avoidance of the allergen.
Skin
The skin is the outer layer of vertebrate animals. Its major functions are protection, temperature regulation and to act as a sense organ.
Textbook Diagram: vertical section of the skin showing its structure.
Two major layers of the skin: epidermis and dermis.
Epidermis
The epidermis is the outer renewable layer of the skin. As it wears away at the surface it grows at the base.
Malpighian Layer: This is the base layer, which is constantly producing new cells by mitosis. The new cells are pushed towards the surface. The dark pigment melanin is produced here.
Granular Layer: The protein keratin accumulates in the cells giving them a granular appearance. The cells finally die.
Cornified Layer: This is the surface layer of dead keratinised cells, which is constantly being eroded.
Dermis
The dermis is the inner layer of the skin, above it is the epidermis and below it is adipose tissue.
Many different structures are present in the dermis: hair follicles, sebaceous glands, sweat glands, blood vessels, sensory nerve endings all of which are embedded in a dense matrix of connective tissue.
Functions of the Skin and Related Structures
Protection
Prevents excessive loss of water — the cornified layer of the epidermis is waterproof.
Prevents the entry of pathogens — the cornified layer is made of dead cells.
Sebum from the sebaceous glands contain anti-microbial chemicals.
‘Sebum oil’ keeps the skin intact preventing it from ‘cracking’.
Melanin gives protection against the damaging UV rays of sunlight.
The dermis and adipose tissue protect against mechanical injury.
Temperature Regulation: maintains blood at 37°C
Threat of Temperature Rise: increases heat loss.
Sweat glands secrete water onto the skin surface. Evaporation of water removes heat from the body.
Arterioles dilate increasing blood flow through the capillaries close to the skin’s surface.
The hair erector muscles relax, the hair lies flatter reducing the depth of insulation.
Threat of Temperature Fall: decreases heat loss.
Arterioles contract reducing blood flow into the capillaries lying close to the skin.
The hair erector muscles contract, the hair rises increasing the depth of the insulation.
Sense Organ
The skin contains receptors for touch, pressure, pain, temperature rise and temperature decrease. The skin supplies information about a variety of external environment conditions.
Vitamin D Production
Made when ultraviolet light penetrates the skin converting a chemical in the blood to vitamin D. As a result vitamin D is often called the ‘sunshine vitamin’.
Excretion
The skin has about 2.5 million sweat glands.
Sweat is a dilute solution of water, sodium chloride, urea, ammonia, uric acid and lactic acid. Urea, ammonia and uric acid are nitrogenous wastes.
Energy Storage
There is a layer of fat storage adipose tissue below the dermis of the skin. Fat is also a poor conductor of heat and so the skin acts as a heat insulator. This fat layer also acts as a shock absorber protecting against mechanical damage.
Homeostatic Organ
Many of the functions of the skin play an important role in maintaining the internal environment of the body in a constant suitable condition for efficient metabolism – temperature control, protection against pathogens, vitamin D formation, excretion and role in response to changes in environmental conditions.
The Senses of Smell and Taste
Smell and taste are chemical senses. They allow us to distinguish between different kinds of molecules in our environment.
Smell is a distant sense allowing the distinction between safe and enjoyable substances from ones that may be dangerous and unpleasant
Smell
The olfactory region is high up in the nasal cavity about 5 cm2 in area with about 20,000 receptors. Stimulatory chemicals must be volatile and soluble in water.
There are 50 primary smell qualities but in combination they can produce over 3,000 different odours. Prolonged exposure to a particular chemical causes fatigue of that sensation.
Taste
Textbook Diagram: taste regions of the tongue.
There are four primary taste qualities: bitter, sour, salty, sweet.
Most tastes are combinations of these.
The taste receptors are collected in groups of about ten in the taste buds.
Taste buds are present on the tongue’s edges and upper surface.
Taste buds are also present on the soft palate and on the back of the pharynx.
Certain regions of the tongue are more sensate to a particular primary tastes than others.
Nervous System Disorder
Example: Parkinson’s Disease
Description: loss of muscle co-ordination, constant tremor and loss of balance.
Cause: degeneration of dopamine secreting cells in a particular part of the brain.
Prevention: none.
Treatment: incurable; drugs to replace the dopamine; electrical stimulation of specific co-ordination centres of the brain; stem cell tissue transplant.
Endocrine System
The endocrine system is a set of hormone secreting glands within the body of an animal.
The function of the endocrine system is homeostasis, communication and response to stimuli.
The endocrine system regulates the internal environment of the animal for growth, survival and reproduction as well as allowing it to respond to changes in its external environment.
An endocrine gland is a ductless organ that makes and secretes specific chemical messengers, called hormones, into the blood.
An exocrine gland has a duct that is used to carry the exocrine gland’s section to the outside of the body. Examples: sweat gland, tear gland, salivary gland.
A hormone is a chemical messenger secreted by an endocrine gland into the blood in which it is transported to specific target organs where an adaptive response is stimulated.
The other communication method in the body is the nervous system. Although there are differences between them, they complement each other in many responses, e.g., response to danger.
Difference between Nervous and Endocrine Control
Nervous response is faster.
Nervous response is shorter in duration.
Nervous response stops quicker.
Nervous response is much more local.
Nerve ‘messages’ are conducted electrically; endocrine ‘messages’ are carried chemically.
Location of Endocrine Glands
Textbook Diagram: location of the endocrine glands in the human body.
Note: you only need to know one hormone example from each gland.
Pituitary Gland
Growth Hormone: stimulates growth and long bone elongation.
Follicle Stimulating Hormone (FSH): stimulates the formation of Graafian follicles in the ovary, stimulates sperm formation in the testes.
Luteinising Hormone (LH): stimulates ovulation, stimulates testes to produce testosterone.
Prolactin: stimulates milk formation by the breast after the birth of the baby.
Oxytocin: stimulates muscle contraction of uterus during birth, stimulates muscle contraction in the milk ducts during breast-feeding.
Antidiuretic Hormone (ADH): causes increased water reabsorption by kidneys.
Thyroid: secretes thyroxine that influences growth, development, and the rate of metabolism.
Islets of Langerhans of Pancreas: secrete insulin for tissue cells to take up glucose from the blood.
Adrenal Glands: located on top of each kidney, secrete adrenaline, prepares the body for stress — increases heart rate, blood pressure, breathing rate.
Ovaries: on the back wall of the lower abdomen in the pelvic region.
Oestrogen: sexual maturation of females; repair of uterine lining after menstruation.
Progesterone: maturation and maintenance of uterine lining.
Testes: outside the body in the scrotal sacs of the groin.
Testosterone: sexual maturation of males and has a role in sperm development.
Thyroxine
Thyroxine is secreted by the thyroid gland.
Thyroxine’s major functions are regulation of metabolic rate, growth and development.
Iodine is needed for thyroxine formation.
Iodine is absorbed from the blood by active transport.
Thyroxine Deficiency (Hypothryroidism)
Thyroxine secretion is below normal. Causes cretinism in childhood — poor growth and poor brain development. Causes myxoedema in adults — fatigue, low energy, reduced resistance to disease. A severe lack of iodine in the diet can cause hypothyroidism.
Corrective Measures: hormone replacement therapy or iodine-rich diet.
Thyroxine Excess (Hyperthyroidism)
Thyroxine secretion is above normal. Causes raised metabolism, greater appetite, weight loss, general restlessness.
Corrective Measures: drugs to suppress thyroid activity, surgically remove part of the gland, use radioactive iodine to destroy some of the gland.
Hormone Supplements
A specific hormone may be given to a patient if their below normal of the hormone is causing problems.
Examples:
Insulin for the treatment of diabetes.
Growth hormone for the treatment of potential dwarfism.
Thyroxine for the treatment of potential cretinism.
HRT (oestrogen) to reduce the severity of menopause symptoms.
Many hormones are relatively small water-soluble proteins.
These can be made by genetic engineering and use in medical treatment of patients e.g. insulin, thyroxine and growth hormone.
Regulation of Thyroid Action
Example: role of thyroid in maintaining body temperature at 37°C.
The hypothalamus of the brain detects a drop in blood temperature.
The hypothalmus stimulates the pituitary to secrete TSH (thyroid-stimulating hormone).
This hormone stimulates the thyroid to increase its secretion of thyroxine.
The higher concentration of thyroxine increases metabolism and heat production increases.
The blood is warmed back to normal temperature.
Thyroxine secretion by reduced activity of the thyroid caused by:
Hypothalamus detecting raised blood temperature and reducing its stimulation of the pituitary.
High thyroxine levels inhibiting the release of TSH from the pituitary.
This form of control is called ‘negative feedback’ — the output of the system brings about a response that that limits the response to a level to maintain homeostasis. The increased level of thyroxine leads to the limitation or reduction of its secretion.
Defence System
We are constantly under attack by bacteria, fungi, viruses and other organisms.
Our body is a rich source of nutrients and water.
Amazingly we remain healthy most of the time. We are obviously very good at protecting ourselves.
There are two major aspects to our defence system – general and specific.
General Defence System
Mechanical Barriers
Non-specific protective cells
Mechanical Barriers
Skin
Outer dead layer of the epidermis.
Sweat and sebum contain anti-microbial agents.
Mutualistic bacterial and fungi inhibit pathogens colonisation of our skin.
Breathing System
Nasal hairs remove suspended micro-organisms from the air.
Mucous membranes secrete mucus trapping micro-organisms.
Lysozyme in mucus kills bacteria.
Cilia move the mucus to the pharynx for swallowing to the stomach.
Digestive System
Lysozyme is in saliva and kills bacteria.
Hydrochloric acid in the stomach kills micro-organisms.
Mucous membranes secrete mucus trapping and killing bacteria.
Reproductive System
The low pH of the vagina kills micro-organisms.
Mucous membranes in the vagina secrete mucus that inhibits micro-organisms.
Tear Gland
Lysozyme in tears keeps the surface of the eye free of bacterial infection.
Non-specific Protective Cells
Phagocytes
Phagocytes are white blood cells.
They have the ability to engulf cellular debris and infectious material like bacteria and viruses.
Phagocytes ‘feed’ like Amoeba.
Free phagocytes wander throughout tissues ‘searching’ for ‘foreign invaders’.
Fixed phagocytes reside in a particular area destroying pathogens that enter their space.
Certain phagocytes are particularly effective against parasitic worms.
Natural Killer Cells
Present in blood and lymph destroying cancer cells and virus-infected cells.
Specific Defence System
This defence strategy uses very precise unique defensive proteins against a particular pathogen. The defence proteins are called antibodies.
The pathogen is identified as its surface has a chemical that is ‘foreign’ – to us it is a ‘non-self’ chemical. This non-self chemical is called an antigen.
Antigen: a non-self chemical that on detection by the specific defence system causes the production of specific complementary antibodies.
Antibody: a specific defence protein produced by the specific defence system on detection of an antigen.
Induced Immunity: protection gained against a particular pathogen by the production of specific antibodies after the antigen on the pathogen has been detected.
Passive Induced Immunity
The antibodies are produced and supplied by another organism.
Natural: mother’s antibodies to her developing baby in the womb and antibodies supplied in mother’s milk during lactation.
Artificial (Medical Science): injection of specific antibodies against a particular pathogen – anti-tetanus and anti-hepatitis treatment.
Passive induced immunity is short-lived.
Active Induced Immunity
The patient produces the antibodies in response to antigen detection.
Natural: normal natural infection.
Artificial (Medical Science): vaccination
Active induced immunity is long term protection because of the long life of memory T cells.