The testicles

Physiology

The testicles are the reproductive glands (gonads) in males.

They produce and store sperm cells.
They also produce testosterone.

 

Pathophysiology

• Epididymo-orchitis
• Hydrocele 
• Inguinal hernia 
• Testicular torsion 
• Varicocele

The nervous system

Medical specialty: Neurology
Surgical specialty: Neurosurgery

Divisions

• Central nervous system
• Peripheral nervous system

 

Pathology

• Nervous system diseases

[Neurology] The central nervous system

Also known as the CNS.

This is a subdivision of the nervous system.
The CNS is a system composed of the brain and spinal cord.

 

Pathology

• Central nervous system diseases

[Endocrinology] The pancreatic islets

Back to endocrinology

 

Physiology

The islets of Langerhans in the pancreas produce several endocrine hormones to regulate metabolism.
These include:

• Alpha cells producing glucagon (20% of total islet cells) 
• Beta cells producing insulin and amylin (≈70%) 
• Delta cells producing somatostatin (<10%) 
• Epsilon cells producing ghrelin (<1%) 
• PP cells (gamma cells or F cells) producing pancreatic polypeptide (<5%) 

Somatostatin inhibits secretion of glucagon and insulin. 

 

Pathophysiology

• Autoimmune-mediated destruction of pancreatic beta cells leads to diabetes mellitus type 1. 
• Pancreatic neuroendocrine tumours (PNETS) 

 

 Hormones

• Output: Glucagon
• Output: Insulin
• Output: Somatostatin
• Output: Ghrelin
• Output: Pancreatic polypeptide

Biliary tract

Organ system: Digestive system

Also known as the biliary tree or biliary system.

The biliary tract consists of the liver, gallbladder and bile ducts.
They work together to make, store and secrete bile. 

Simplified diagrammatic representation of the biliary tract

 

Biliary tract diseases

[Obstetrics and gynaecology] The ovaries

The ovaries are the reproductive glands (gonads) in females.

 

Physiology

Each ovary contains a supply of oocytes (egg cells) which is sufficient to last from the menarche (first menstruation) until the menopause (final menstruation) around 30 years later.

During every menstrual cycle, a single oocyte is released from one ovary travels along the fallopian
tube to reach the uterus.

Sometimes a second oocyte is released from the other ovary simultaenously and travels along that respective fallopian tube to reach the uterus. If each egg is successfully fertilised by a sperm cell, the result is dizygotic (non-identical) twins.

If one oocyte is successfully fertilised by one sperm cell, and the zygote becomes two embryos, the result is monozygotic (identical) twins.

Pathophysiology

• Polycystic ovary syndrome (PCOS) 
• Premature ovarian failure
• Ovarian cancer  
• Ovarian torsion

[Obstetrics and gynaecology] The cervix

The cervix sits between the vagina and the uterus.

[Obstetrics and gynaecology] The uterus

 Hormones

• Stimulated by: Stretching of the cervix and uterus
• Effect: Stimulates muscular contractions of the uterus

Basal ganglia

This structure is a component of the brain. 

 

Physiology

In the human brain, there is a map of every skeletal muscle, called the motor cortex. It is a collection of upper motor neurons, each of which initiates a signal down the spine to the muscle it corresponds to. The brain is constantly trying to anticipate and prepare for actions, so signals are constantly being sent to these muscles. If there is no inhibition of these signals, purposeless movements would constantly occur. The basal ganglia have evolved to provide a locking mechanism here.

The basal ganglia are a system of neural circuitry which provides logical processing of these signals before they can continue to reach the body. They consist of the striatum, globus pallidus, ventral pallidum, substantia nigra, and subthalamic nucleus. These each behave differently and they interact like components of an electronic microchip. Most of the components have an overall inhibitory effect on outgoing signals. They block unnecessary motor neuron signals.

When a healthy person decides to contract their bicep muscle to flex their arm, the brain sends signals to the basal ganglia, which tell neurons there to release a neurotransmitter called dopamine. The dopamine bind to other neurons and switches the activity "mode" of the cells. This has an effect of removing inhibition on the outgoing signals. The basal ganglia do not only modulate motor signalling. They have connections to the oculo-motor, associative, limbic and orbitofrontal circuits. The motor function is the easiest to assess and the most vital for survival, so it is the most well-researched.

Basal ganglia diseases

[Haematology] The spleen

The spleen is an organ sitting in the left upper quadrant of the abdomen. It is usually small and sits inside the rib case. It is similar to the liver in many ways, which sits a similar position in the right upper quadrant of the abdomen. 

Blood supply

The spleen receives blood from the splenic artery, which is a branch of the celiac artery. The celiac artery branches from the abdominal aorta. 

The spleen is drained by the splenic vein, into the portal hepatic vein. This blood then moves through the liver and eventually returns to the heart through the inferior vena cava.

The spleen usually contains a large volume of blood and consequently, a rupture of the capsule is a medical emergency.

Systemic functions

• The mononuclear phagocyte system (also called the Reticuloendothlial system).

The spleen is involved because it contains a huge reservoir of monocytes in red pulp. Red pulp macrophages eat cellular debris to remove badly damaged or dysfunctional erythrocytes (red blood cells) from the body.

• The lymphatic system

The spleen is involved because it contains specialised lymphoid tissues in which lymphocytes are created, trained, and organised. A large proportion of serum immunoglobulin type M (IgM) is produced in the spleen. 

There are two distinct tissues within the spleen which are specialised to carry out these functions: red pulp and white pulp. 

Histology

Simplified representation of the functional organisation of the spleen

The splenic artery branches into many trabecular arteries. These pass through both the white pulp and the red pulp. The tissues are drained by trabecular veins which drain into the splenic vein.

White pulp

White pulp comprises approximately 25% of splenic tissue. It is not truly white but it appears less red than red pulp. Blood reaches the tissue through a central arteriole. Each arteriole is surrounded by Periarteriolar Lymphoid Sheaths (PALS). These contain T lymphocytes. 

The PALS are surrounded by lymph follicles with dividing B lymphocytes. IgM and IgG2 are produced here. These are antibodies which are particularly effective against encapsulated bacteria. 

Furthest from the central arteriole you can find marginal zone tissue. It lies between white pulp and red pulp. It contains antigen presenting cells (including dendritic cells, macrophages, and metallophillic macrophages). It appears to be specialised for the trapping of particles from the systemic circulation. Immuno-stimulatory antigens are then trafficked into the white pulp for processing. It also appears to be proficient at the removal of viral particles from the blood stream.

Red pulp

Red pulp is engorged with blood, giving it a red appearance. It is made of connective tissue organised into cords of Bilroth and sinusoids. Incoming blood must pass these cords and is forced through narrow gaps in the endothelium (the lining) of the sinusoids. Fragile, inflexible erythrocytes are trapped or destroyed by the strain. Healthy erythrocytes should pass intact into the sinusoids, which drain into the trabecular veins, into the splenic vein and back into systemic circulation. The red pulp contains a huge reservoir of monocytes and specialised macrophages which consume the debris.

Splenic capsule

Finally the spleen is surrounded by a tough capsule made of connective tissue.

Comparison to other organs

• The liver 

Both organs are part of the reticuloendothelial system. They both contain a huge volume of blood and remove waste from the circulation. They sit in equivalent positions in the upper abdomen/lower thorax. They both receive blood from the same branch of the systemic circulation (the celiac artery and abdominal aorta). The splenic vein drains into the liver, allowing the venous blood to be processed a second time before it returns to the heart.

• Lymph nodes 

The spleen and lymph nodes are useful sites of activity for the lymphatic system. The microscopic organisation of the white pulp is very similar to the microscopic organisation of lymph nodes. Within both structures, immune cells can share antigens and undergo priming. Cells within lymph nodes and within the spleen are able to express various chemokines. These are immunological proteins which guide chemotaxis (movement) of specific immune cells towards the relevant tissues. The state of an immune cell determines which chemokines it will migrate towards and the ability of the cell to detach itself from a tissue or migrate through a tissue.

Splenic diseases

 

Summary of the spleen 

The spleen houses and organises white blood cells, and it provides a filter for fragile red blood cells and infectious agents to be removed from the blood. The essential theme of the spleen is mechanical blood filtration.

[Opthalmology] The eye

The eye is a sensory organ which provides us with vision. It contains several fluid-filled chambers and layers of specialised tissue. Sensory information is detected by the retina and carried to the brain through the optic nerve.

The liver

Organ system: Digestive system

The liver is a massive collection of highly active cells called hepatocytes.

The biliary system

The hepatocytes produce components of bile, this flows through an intricate network of drainage vessels into the biliary (bile system) and ultimately into the intestines where it aids digestion and protects the tissues from excessive stomach acid.

Metabolism

The hepatocytes also produce an arsenal of enzymes to breakdown waste and toxins. For this purpose, the liver has a rich blood supply and accepts a large proportion of the blood returning from the intestines through the venous system. At any moment, you can find a large proportion of your blood volume in your liver. 

 

Liver diseases

[Nephrology] The kidneys

The kidney is shaped by a need to regulate the levels of fluid in our bodies. It responds to feedback and changes how much water it releases into urine according to how much water we need. It has a unique toolkit for the removal of toxins and waste from the blood. It's involved in bone maintenance and red blood cell production.

The stomach

Organ system: The digestive system

The stomach is an organ of digestion in the gastrointestinal tract.

 

Acid production

Specialised cells known as parietal cells are able to actively transport protons from the cell cytoplasm out into canaliculi to form a strong acid. This diffuses into the gastric lumen. These cells can produce an acid with a pH as low as 0.8, but this is diluted to reach a normal pH of between 1.0 and 3.0. Acid has two major functions in the stomach:

1. Denaturation of proteins
2. Destruction of any swallowed pathogens or toxins

 

Enzyme production

Gastric chief cells produce a pro-enzyme called pepsinogen. This is a precursor to a powerful protease enzyme called pepsin. Pepsinogen is activated by the highly acidic environment in the gastric lumen. It is capable of catalysing the breakdown of proteins (long chains of amino acids) into peptides (short chains of amino acids).

 

Intrinsic factor production

Parietal cells also produce a glycoprotein called intrinsic factor. This binds to vitamin B12 to facilitate absorption into the body. The IF-B12 complex is absorbed by cells in the ileum later in the gastrointestinal tract. B12 is an essential nutrient which allows DNA synthesis. Blood cells are rapidly produced in the body and so these cell lines require a rapid rate of DNA synthesis. B12 deficiency is often detected as impaired production of new functional blood cells.

Simplified representation of the stomach

 

Stomach diseases

The skin

Organ system: Integumentary system

The skin is an organ of protection, which forms a physical barrier between the atmosphere and the vulnerable contents of the human body. It is the largest organ of the human body (by surface area).
The integumentary system consists of the skin and its appendages.

• Protection
• Sensation
• Thermoregulation
• Control of evaporation
• Storage and synthesis
• Absorption through the skin
• Water resistance

[Gastroenterology] The pancreas

Organ system: The digestive system 

Organ system: The endocrine system

The pancreas is both an endocrine and an exocrine gland.

 

Endocrine function

The pancreas plays a major role in the regulation of blood glucose by releasing several regulatory hormones into the blood.

 

Exocrine function

The pancreas releases powerful digestive enzymes into the digestive tract. This is a vital feature of successful digestion.

 

Pancreatic diseases

 

Pathology

• Acute pancreatitis
• Chronic pancreatitis
• Exocrine insufficiency  
• Cancer

[Gastroenterology] The small intestine

Organ system: The digestive system

The small intestine is a very long tract which is folded into the abdominal cavity. Its primary role is digestion and it has a massive surface area to achieve this efficiently. Food enters from the stomach, passes through the small intestine and passes into the cecum of the large intestine.

Gross anatomy

• Duodenum
• Jejunum
• Ileum

[Gastroenterology] The large intestine

Organ system: The digestive system

The large intestine plays a role in digestion, stool formation, and water reabsorption.

Reabsorption of water

When digested matter reaches the large intestine it has already lost a lot of water and nutrients. Along the path from the large intestine to the anus it will lose a lot more water. This allows faeces to form with the correct consistency. Too much water removal and the faeces become hard, causing constipation. Too little water removal and the faeces remain soft, causing diarrhea and dehydration. 

 

The microbiome

The large intestine is a habitat for an enormous number of micro-organisms. This is not a pathological contamination, but it is more similar to a symbiotic relationship. It is a healthy and natural feature of the human body. These organisms have evolved to metabolise nutrients which pass through our intestines, releasing usable energy for them, and producing usable nutrients for us. A large volume of evidence has accumulated over the years to suggest that the microbiome influences obesity, diabetes, and even mood.

The brain

The brain and spinal cord together form the central nervous system (CNS). 

 

Components  

• Pineal gland 
• Hypothalamus and pituitary 
• Basal ganglia 

 

Microscopic structure

Cognition and memory rely on neurons (nerve cells) and the synapses (connections) between them. The genetic (the combination of genes in the DNA) and epigenetic (the regulation of which genes are active or inactive) mechanisms in each neurone regulate its behaviour. On top of this we have the global features of the brain such as the number of neurons, the number of glial cells (support cells), and the relationship between neurons. Every brain has a unique "network" or "web" of neuronal connections. Your memories, your skills, your behaviours, your processing patterns, are the gross product of information stored in the DNA of each microscopic neurone, and in the macroscopic network of the brain.

 

Pathology

• Brain diseases

[Urology] The bladder

The urinary bladder is an organ of storage in the urinary tract.