NEET MDS Lessons
Physiology
The nephron of the kidney is involved in the regulation of water and soluble substances in blood.
A Nephron
A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.
Its function is vital for homeostasis of blood volume, blood pressure, and plasma osmolarity.
It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.
The Glomerulus
The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman's capsule.
A group of specialized cells known as juxtaglomerular apparatus (JGA) are located around the afferent arteriole where it enters the renal corpuscle. The JGA secretes an enzyme called renin, due to a variety of stimuli, and it is involved in the process of blood volume homeostasis.
The Bowman's capsule surrounds the glomerulus. It is composed of visceral (simple squamous epithelial cells; inner) and parietal (simple squamous epithelial cells; outer) layers.
Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances. However, in some injuries they may be able to pass through and can cause blood and protein content to enter the urine, which is a sign of problems in the kidney.
Proximal Convoluted Tubule
The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place. Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na+/K+/ATPase pumps that actively transports sodium across the basolateral membrane.
Water and glucose follow sodium through the basolateral membrane via an osmotic gradient, in a process called co-transport. Approximately 2/3rds of water in the nephron and 100% of the glucose in the nephron are reabsorbed by cotransport in the proximal convoluted tubule.
Fluid leaving this tubule generally is unchanged due to the equivalent water and ion reabsorption, with an osmolarity (ion concentration) of 300 mOSm/L, which is the same osmolarity as normal plasma.
The Loop of Henle
The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed, which increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle's loop is impermeable to water but highly permeable to ions, which causes a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L.
Distal Convoluted Tubule and Collecting Duct
The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH.
Normally, it is impermeable to water and permeable to ions, driving the osmolarity of fluid even lower. However, anti-diuretic hormone (secreted from the pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water to increase water reabsorption. This example results in increased blood volume and increased blood pressure. Many other hormones will induce other important changes in the distal convoluted tubule that fulfill the other homeostatic functions of the kidney.
The collecting duct is similar in function to the distal convoluted tubule and generally responds the same way to the same hormone stimuli. It is, however, different in terms of histology. The osmolarity of fluid through the distal tubule and collecting duct is highly variable depending on hormone stimulus. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.
1 - Passive processes - require no expenditure of energy by a cell:
- Simple diffusion = net movement of a substance from an area of high concentration to an area of low concentration. The rate of diffusion is influenced by:
- concentration gradient
- cross-sectional area through which diffusion occurs
- temperature
- molecular weight of a substance
- distance through which diffusion occurs
- Osmosis = diffusion of water across a semi permeable membrane (like a cell membrane) from an area of low solute concentration to an area of high solute concentration
- Facilitated diffusion = movement of a substance across a cell membrane from an area of high concentration to an area of low concentration. This process requires the use of 'carriers' (membrane proteins). In the example below, a ligand molecule (e.g., acetylcholine) binds to the membrane protein. This causes a conformational change or, in other words, an 'opening' in the protein through which a substance (e.g., sodium ions) can pass.
2 - Active processes - require the expenditure of energy by cells:
- Active transport = movement of a substance across a cell membrane from an area of low concentration to an area of high concentration using a carrier molecule
- Endo- & exocytosis - moving material into (endo-) or out of (exo-) cell in bulk form
Function of Blood
- transport through the body of
- oxygen and carbon dioxide
- food molecules (glucose, lipids, amino acids)
- ions (e.g., Na+, Ca2+, HCO3−)
- wastes (e.g., urea)
- hormones
- heat
- defense of the body against infections and other foreign materials. All the WBCs participate in these defenses
DNA (Deoxyribonucleic acid) - controls cell function via transcription and translation (in other words, by controlling protein synthesis in a cell)
Transcription - DNA is used to produce mRNA
Translation - mRNA then moves from the nucleus into the cytoplasm & is used to produce a protein . requires mRNA, tRNA (transfer RNA), amino acids, & a ribosome
tRNA molecule
- sequence of amino acids in a protein is determined by sequence of codons (mRNA). Codons are 'read' by anticodons of tRNAs & tRNAs then 'deliver' their amino acid.
- Amino acids are linked together by peptide bonds (see diagram to the right)
- As mRNA slides through ribosome, codons are exposed in sequence & appropriate amino acids are delivered by tRNAs. The protein (or polypeptide) thus grows in length as more amino acids are delivered.
- The polypeptide chain then 'folds' in various ways to form a complex three-dimensional protein molecule that will serve either as a structural protein or an enzyme.
The Cardiac Cycle: the sequence of events in one heartbeat.
systole - the contraction phase; unless otherwise specified refers to left ventricle, but each chamber has its own systole.
diastole - the relaxation phase; unless otherwise specified refers to left ventricle, but each chamber has its own diastole.
1) quiescent period - period when all chambers are at rest and filling. 70% of ventricular filling occurs during this period. The AV valves are open, the semilunar valves are closed.
2) atrial systole - pushes the last 30% of blood into the ventricle.
3) atrial diastole - atria begin filling.
4) ventricular systole - First the AV valves close causing the first heart sound, then after the isovolumetric contraction phase the semilunar valves open permitting ventricular ejection of blood into the arteries.
5) ventricular diastole - As the ventricles relax the semilunar valves close first producing the second heart sound, then after the isovolumetric relaxation phase the AV valves open allowing ventricular filling.
Lung volumes and capacities:
I. Lung`s volumes
1. Tidal volume (TV) : is the volume of air m which is inspired and expired during one quiet breathing . It equals to 500 ml.
2. Inspiratory reserve volume (IRV) : The volume of air that could be inspired over and beyond the tidal volume. It equals to 3000 ml of air.
3. Expiratory reserve volume (ERV) : A volume of air that could be forcefully expired after the end of quiet tidal volume. It is about 1100 ml of air.
4. Residual volume (RV) : the extra volume of air that may remain in the lung after the forceful expiration . It is about 1200 ml of air.
5. Minute volume : the volume of air that is inspired or expired within one minute. It is equal to multiplying of respiratory rate by tidal volume = 12X500= 6000 ml.
It is in female lesser than that in male.
II. Lung`s capacities :
1. Inspiratory capacity: TV + IRV
2. Vital capacity : TV+IRV+ERV
3. Total lung capacity : TV+IRV+ERV+RV
Structural Divisions of the nervous system:
1) Central Nervous System (CNS) - the brain and spinal cord.
2) Peripheral Nervous System (PNS) - the nerves, ganglia, receptors, etc