NEET MDS Lessons
Physiology
The cell membrane is called the sarcolemma. This membrane is structured to receive and conduct stimuli. The sarcoplasm of the cell is filled with contractile myofibrils and this results in the nuclei and other organelles being relegated to the edge of the cell.
Myofibrils are contractile units within the cell which consist of a regular array of protein myofilaments. Each myofilament runs longitudinally with respect to the muscle fiber. There are two types: the thick bands and the thin bands. Thick bands are made of multiple molecules of a protein called myosin. The thin bands are made of multiple molecules of a protein called actin. The thin actin bands are attached to a Z-line or Z-disk of an elastic protein called titin. The titin protein also extends into the myofibril anchoring the other bands in position. From each Z-line to the next is a unit called the
The sarcomere is the smallest contractile unit in the myofibril. Sarcomeres contract because the Z-lines move closer together. As the sarcomeres contract the myofibrils contract. As the myofibrils contract the muscle cell contracts. And as the cells contract the entire muscle contracts.
The arrangement of the thick myosin filaments across the myofibrils and the cell causes them to refract light and produce a dark band known as the A Band. In between the A bands is a light area where there are no thick myofilaments, only thin actin filaments. These are called the I Bands. The dark bands are the striations seen with the light microscope.
Factors , affecting glomerular filtration rate :
Factors that may influence the different pressure forces , or the filtration coefficient will affect the glomerular filtration rate .
1. Dehydration : Causes decrease hydrostatic pressure , and thus decreases GFR
2- Liver diseases that may decrease the plasma proteins and decrease the oncotic pressure , and thus increases glomerular filtration rate .
3- Sympathetic stimulation : will decrease the diameter of afferent arteriole and thus decreases glomerular filtration rate.
4- Renal diseases : Nephrotic syndrome for example decreases the number of working nephrons and thus decreases the filtration coefficient and thus decreases the glomerular filtration rate.
Glomerulonephritis will causes thickening of the glomerular basement membrane and thus decreases the glomerular filtration rate by decreasing the filtration coefficient too.
Oxygen Uptake in the Lungs is Increased About 70X by Hemoglobin in the Red Cells
- In the lungs oxygen must enter the blood
- A small amount of oxygen dissolves directly in the serum, but 98.5% of the oxygen is carried by hemoglobin
- All of the hemoglobin is found within the red blood cells (RBCs or erythrocytes)
- The hemoglobin content of the blood is about 15 gm/deciliter (deciliter = 100 mL)
- Red cell count is about 5 million per microliter
Each Hemoglobin Can Bind Four O2 Molecules (100% Saturation)
- Hemoglobin is a protein molecule with 4 protein sub-units (2 alphas and 2 betas)
- Each of the 4 sub-units contains a heme group which gives the protein a red color
- Each heme has an iron atom in the center which can bind an oxygen molecule (O2)
- The 4 hemes in a hemoglobin can carry a maximum of 4 oxygen molecules
- When hemoglobin is saturated with oxygen it has a bright red color; as it loses oxygen it becomes bluish (cyanosis)
The Normal Blood Hematocrit is Just Below 50%
- Blood consists of cells suspended in serum
- More than 99% of the cells in the blood are red blood cells designed to carry oxygen
- 25% of all the cells in the body are RBCs
- The volume percentage of cells in the blood is called the hematocrit
- Normal hematocrits are about 40% for women and 45% for men
At Sea Level the Partial Pressure of O2 is High Enough to Give Nearly 100% Saturation of Hemoglobin
- As the partial pressure of oxygen in the alveoli increases the hemoglobin in the red cells passing through the lungs rises until the hemoglobin is 100% saturated with oxygen
- At 100% saturation each hemoglobin carries 4 O2 molecules
- This is equal to 1.33 mL O2 per gram of hemoglobin
- A person with 15 gm Hb/deciliter can carry:
- Max O2 carriage = 1.33 mL O2/gm X 15 gm/deciliter = 20 mL O2/deciliter
- A plot of % saturation vs pO2 gives an S-shaped "hemoglobin dissociation curve"
- At 100% saturation each hemoglobin binds 4 oxygen molecules
At High Altitudes Hemoglobin Saturation May be Well Below 100%
- At the alveolar pO2 of 105 mm Hg at sea level the hemoglobin will be about 97% saturated, but the saturation will fall at high altitudes
- At 12,000 feet altitude alveolar pO2 will be about 60 mm Hg and the hemoglobin will be 90% saturated
- At 29,000 feet (Mt. Everest) alveolar pO2 is about 24 mm Hg and the hemoglobin will be only 42% saturated
- At very high altitudes most climbers must breath pure oxygen from tanks
- During acclimatization to high altitude the hematocrit can rise to about 60%- this increases the amount of oxygen that can be carried
- Hematocrits above 60% are not useful because the blood viscosity will increase to the point where it impairs circulation
The defecation reflex:
As a result of the mass movements, pressure is exerted on the rectum and on the internal anal sphincter, which is smooth muscle, resulting in its involuntary relaxation. Afferent impulses are sent to the brain indicating the need to defecate. The external sphincter is voluntary muscle and is controlled by the voluntary nervous system. This sphincter is relaxed along with contraction of the rectal and abdominal muscles in the defecation reflex
1) Storage - the stomach allows a meal to be consumed and the materials released incrementally into the duodenum for digestion. It may take up to four hours for food from a complete meal to clear the stomach.
2) Chemical digestion - pepsin begins the process of protein digestion cleaving large polypeptides into shorter chains .
3) Mechanical digestion - the churning action of the muscularis causes liquefaction and mixing of the contents to produce acid chyme.
4) Some absorption - water, electrolytes, monosaccharides, and fat soluble molecules including alcohol are all absorbed in the stomach to some degree.
1. Automatic control (sensory) of respiration is in - brainstem (midbrain)
2. Behavioral/voluntary control is in - the cortex
3. Alveolar ventilation -the amount of atmospheric air that actually reaches the alveolar per breath and that can participate in the exchange of gasses between alveoli and blood
4. Only way to increase gas exchange in alveolar capillaries - perfusion-limited gas exchange
5. Pulmonary ventiliation not effected by - concentration of bicarbonate ions
6. Central chemoreceptors - medulla - CO2, O2 and H+ concentrations
7. Peripheral chemoreceptors - carotid and aortic bodies- PO2, PCO2 and pH
8. Major stimulus for respiratory centers - arterial PCO2
9. Rhythmic breathing depends on
1. continuous (tonic) inspiratory drive from DRG (dorsal respiratory group)
2. intermittent (phasic) expiratory input from cerebrum, thalamus, cranial nerves and ascending spinal cord sensory tracts
10. Primary site for gas exchange - type I epithelial cells for alveoli
Hormones are carried by the blood throughout the entire body, yet they affect only certain cells. The specific cells that respond to a given hormone have receptor sites for that hormone.
This is sort of a lock and key mechanism. If the key fits the lock, then the door will open. If a hormone fits the receptor site, then there will be an effect. If a hormone and a receptor site do not match, then there is no reaction. All of the cells that have receptor sites for a given hormone make up the target tissue for that hormone. In some cases, the target tissue is localized in a single gland or organ. In other cases, the target tissue is diffuse and scattered throughout the body so that many areas are affected.
Hormones bring about their characteristic effects on target cells by modifying cellular activity. Cells in a target tissue have receptor sites for specific hormones. Receptor sites may be located on the surface of the cell membrane or in the interior of the cell.
In general those protein hormones are unable to diffuse through the cell membrane and react with receptor sites on the surface of the cell. The hormone receptor reaction on the cell membrane activates an enzyme within the membrane, called adenyl cyclase, which diffuses into the cytoplasm. Within the cell, adenyl cyclase catalyzes or starts the process of removal of phosphates from ATP to produce cyclic adenosine monophosphate or c AMP. This c AMP activates enzymes within the cytoplasm that alter or change the cellular activity. The protein hormone, which reacts at the cell membrane, is called the first messenger. c Amp that brings about the action attributed to the hormone is called the second messenger. This type of action is relatively rapid because the precursors are already present and they just needed to be activated in some way.