The small intestine

Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi, which line the inner surface of the small intestine.

This scanning electron micrograph (courtesy of Keith R. Porter) shows the villi carpeting the inner surface of the small intestine.

The crypts at the base of the villi contain stem cells that continuously divide by mitosis producing

• more stem cells
• cells that migrate up the surface of the villus while differentiating into
  1. columnar epithelial cells (the majority). They are responsible for digestion and absorption.
  2. goblet cells, which secrete mucus;
  3. endocrine cells, which secrete a variety of hormones;
• Paneth cells, which secrete antimicrobial peptides that sterilize the contents of the intestine.

All of these cells replace older cells that continuously die by apoptosis.

The villi increase the surface area of the small intestine to many times what it would be if it were simply a tube with smooth walls. In addition, the apical (exposed) surface of the epithelial cells of each villus is covered with microvilli (also known as a "brush border"). Thanks largely to these, the total surface area of the intestine is almost 200 square meters, about the size of the singles area of a tennis court and some 100 times the surface area of the exterior of the body.

Incorporated in the plasma membrane of the microvilli are a number of enzymes that complete digestion:

• aminopeptidases attack the amino terminal (N-terminal) of peptides producing amino acids.
• disaccharidasesThese enzymes convert disaccharides into their monosaccharide subunits.
  • maltase hydrolyzes maltose into glucose.
  • sucrase hydrolyzes sucrose (common table sugar) into glucose and fructose.
  • lactase hydrolyzes lactose (milk sugar) into glucose and galactose.

Fructose simply diffuses into the villi, but both glucose and galactose are absorbed by active transport.

• fatty acids and monoglycerides. These become resynthesized into fats as they enter the cells of the villus. The resulting small droplets of fat are then discharged by exocytosis into the lymph vessels, called lacteals, draining the villi.

Membrane Structure & Function

Cell Membranes

• Cell membranes are phospholipid bilayers (2 layers)
• Bilayer forms a barrier to passage of molecules in an out of cell
• Phospholipids = glycerol + 2 fatty acids + polar molecule (i.e., choline) + phosphate
• Cholesterol (another lipid) stabilizes cell membranes
• the hydrophobic tails of the phospholipids (fatty acids) are together in the center of the bilayer. This keeps them out of the water

Membranes Also Contain Proteins

• Proteins that penetrate the membrane have hydrophobic sections ~25 amino acids long
• Hydrophobic = doesn't like water = likes lipids
• Membrane proteins have many functions:
  • receptors for hormones
  • pumps for transporting materials across the membrane
  • ion channels
  • adhesion molecules for holding cells to extracellular matrix

cell recognition antigens

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.

There are three types of muscle tissue, all of which share some common properties:

• Excitability or responsiveness - muscle tissue can be stimulated by electrical, physical, or chemical means.
• contractility - the response of muscle tissue to stimulation is contraction, or shortening.
• elasticity or recoil - muscles have elastic elements (later we will call these their series elastic elements) which cause them to recoil to their original size.
• stretchability or extensibility - muscles can also stretch and extend to a longer-than-resting length.

The three types of muscle: skeletal, cardiac, and visceral (smooth) muscle.

Skeletal muscle

It is found attached to the bones for movement.

cells are long multi-nucleated cylinders.

 The cells may be many inches long but vary in diameter, averaging between 100 and 150 microns.

 All the cells innervated by branches from the same neuron will contract at the same time and are referred to as a motor unit.

 Skeletal muscle is voluntary because the neurons which innervate it come from the somatic or voluntary branch of the nervous system.

That means you have willful control over your skeletal muscles.

 Skeletal muscles have distinct stripes or striations which identify them and are related to the organization of protein myofilaments inside the cell.

Cardiac muscle

This muscle found in the heart.

 It is composed of much shorter cells than skeletal muscle which branch to connect to one another.

 These connections are by means of gap junctions called intercalated disks which allow an electrochemical impulse to pass to all the connected cells.

 This causes the cells to form a functional network called a syncytium in which the cells work as a unit. Many cardiac muscle cells are myogenic which means that the impulse arises from the muscle, not from the nervous system. This causes the heart muscle and the heart itself to beat with its own natural rhythm.

But the autonomic nervous system controls the rate of the heart and allows it to respond to stress and other demands. As such the heart is said to be involuntary.

Visceral muscle is found in the body's internal organs and blood vessels.

 It is usually called smooth muscle because it has no striations and is therefore smooth in appearance. It is found as layers in the mucous membranes of the respiratory and digestive systems.

It is found as distinct bands in the walls of blood vessels and as sphincter muscles.

Single unit smooth muscle is also connected into a syncytium similar to cardiac muscle and is also partly myogenic. As such it causes continual rhythmic contractions in the stomach and intestine. There and in blood vessels smooth muscle also forms multiunit muscle which is stimulated by the autonomic nervous system. So smooth muscle is involuntary as well

Platelets

Platelets are cell fragments produced from megakaryocytes.

Blood normally contains 150,000 to 350,000 per microliter (µl). If this value should drop much below 50,000/µl, there is a danger of uncontrolled bleeding. This is because of the essential role that platelets have in blood clotting.

When blood vessels are damaged, fibrils of collagen are exposed.

• von Willebrand factor links the collagen to platelets forming a plug of platelets there.
• The bound platelets release ADP and thromboxane A2 which recruit and activate still more platelets circulating in the blood.
• (This role of thromboxane accounts for the beneficial effect of low doses of aspirin a cyclooxygenase inhibitor in avoiding heart attacks.)

ReoPro is a monoclonal antibody directed against platelet receptors. It inhibits platelet aggregation and appears to reduce the risk that "reamed out" coronary arteries (after coronary angioplasty) will plug up again.

Gonadotropin-releasing hormone (GnRH)

GnRH is a peptide of 10 amino acids. Its secretion at the onset of puberty triggers sexual development.

Primary Effects

FSH and LH Relaese

Secondary Effects

Increases estrogen and progesterone (in females)

testosterone Relaese (in males)

Growth hormone-releasing hormone (GHRH)

GHRH is a mixture of two peptides, one containing 40 amino acids, the other 44.  GHRH stimulates cells in the anterior lobe of the pituitary to secrete growth hormone (GH).

Corticotropin-releasing hormone (CRH)

CRH is a peptide of 41 amino acids. Its acts on cells in the anterior lobe of the pituitary to release adrenocorticotropic hormone (ACTH) CRH is also synthesized by the placenta and seems to determine the duration of pregnancy.  It may also play a role in keeping the T cells of the mother from mounting an immune attack against the fetus

Somatostatin

Somatostatin is a mixture of two peptides, one of 14 amino acids, the other of 28. Somatostatin acts on the anterior lobe of the pituitary to

• inhibit the release of growth hormone (GH)
• inhibit the release of thyroid-stimulating hormone (TSH)

Somatostatin is also secreted by cells in the pancreas and in the intestine where it inhibits the secretion of a variety of other hormones.

Antidiuretic hormone (ADH) and Oxytocin

These peptides are released from the posterior lobe of the pituitary